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Internet Engineering Task Force (IETF) M. Pritikin, Ed.
Request for Comments: 7030 Cisco Systems, Inc.
Category: Standards Track P. Yee, Ed.
ISSN: 2070-1721 AKAYLA, Inc.
D. Harkins, Ed.
Aruba Networks
October 2013
Enrollment over Secure Transport
Abstract
This document profiles certificate enrollment for clients using
Certificate Management over CMS (CMC) messages over a secure
transport. This profile, called Enrollment over Secure Transport
(EST), describes a simple, yet functional, certificate management
protocol targeting Public Key Infrastructure (PKI) clients that need
to acquire client certificates and associated Certification Authority
(CA) certificates. It also supports client-generated public/private
key pairs as well as key pairs generated by the CA.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7030.
Pritikin, et al. Standards Track [Page 1]
RFC 7030 EST October 2013
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
1.1. Terminology ................................................4
2. Operational Scenario Overviews ..................................5
2.1. Obtaining CA Certificates ..................................6
2.2. Initial Enrollment .........................................7
2.2.1. Certificate TLS Authentication ......................8
2.2.2. Certificate-Less TLS Authentication .................8
2.2.3. HTTP-Based Client Authentication ....................8
2.3. Client Certificate Reissuance ..............................8
2.4. Server Key Generation ......................................9
2.5. Full PKI Request Messages ..................................9
2.6. Certificate Signing Request (CSR) Attributes Request .......9
3. Protocol Design and Layering ...................................10
3.1. Application Layer .........................................13
3.2. HTTP Layer ................................................14
3.2.1. HTTP Headers for Control ...........................15
3.2.2. HTTP URIs for Control ..............................16
3.2.3. HTTP-Based Client Authentication ...................17
3.2.4. Message Types ......................................19
3.3. TLS Layer .................................................20
3.3.1. TLS-Based Server Authentication ....................20
3.3.2. TLS-Based Client Authentication ....................21
3.3.3. Certificate-Less TLS Mutual Authentication .........21
3.4. Proof-of-Possession .......................................22
3.5. Linking Identity and POP Information ......................22
3.6. Server Authorization ......................................23
3.6.1. Client Use of Explicit TA Database .................24
3.6.2. Client Use of Implicit TA Database .................24
3.7. Client Authorization ......................................24
4. Protocol Exchange Details ......................................25
4.1. Distribution of CA Certificates ...........................25
Pritikin, et al. Standards Track [Page 2]
RFC 7030 EST October 2013
4.1.1. Bootstrap Distribution of CA Certificates ..........25
4.1.2. CA Certificates Request ............................26
4.1.3. CA Certificates Response ...........................26
4.2. Client Certificate Request Functions ......................27
4.2.1. Simple Enrollment of Clients .......................28
4.2.2. Simple Re-enrollment of Clients ....................29
4.2.3. Simple Enroll and Re-enroll Response ...............29
4.3. Full CMC ..................................................30
4.3.1. Full CMC Request ...................................30
4.3.2. Full CMC Response ..................................30
4.4. Server-Side Key Generation ................................31
4.4.1. Server-Side Key Generation Request .................32
4.4.1.1. Requests for Symmetric Key
Encryption of the Private Key .............32
4.4.1.2. Requests for Asymmetric Encryption
of the Private Key ........................33
4.4.2. Server-Side Key Generation Response ................33
4.5. CSR Attributes ............................................35
4.5.1. CSR Attributes Request .............................35
4.5.2. CSR Attributes Response ............................35
5. IANA Considerations ............................................37
6. Security Considerations ........................................39
7. References .....................................................41
7.1. Normative References ......................................41
7.2. Informative References ....................................43
Appendix A. Operational Scenario Example Messages .................45
A.1. Obtaining CA Certificates ..................................45
A.2. CSR Attributes .............................................47
A.3. Enroll/Re-enroll ...........................................47
A.4. Server Key Generation ......................................50
Appendix B. Contributors and Acknowledgements .....................52
1. Introduction
This document profiles certificate enrollment for clients using
Certificate Management over CMS (CMC) [RFC5272] messages over a
secure transport. Enrollment over Secure Transport (EST) describes
the use of Transport Layer Security (TLS) 1.1 [RFC4346], 1.2
[RFC5246], or any future version) and Hypertext Transfer Protocol
(HTTP) [RFC2616] to provide an authenticated and authorized channel
for Simple Public Key Infrastructure (PKI) Requests and Responses
[RFC5272].
Architecturally, the EST service is located between a Certification
Authority (CA) and a client. It performs several functions
traditionally allocated to the Registration Authority (RA) role in a
PKI. The nature of communication between an EST server and a CA is
not described in this document.
Pritikin, et al. Standards Track [Page 3]
RFC 7030 EST October 2013
EST adopts the Certificate Management Protocol (CMP) [RFC4210] model
for CA certificate rollover, but it does not use the CMP message
syntax or protocol. EST servers are extensible in that new functions
may be defined to provide additional capabilities not specified in
CMC [RFC5272], and this document defines two such extensions: one for
requesting Certificate Signing Request attributes and another for
requesting server-generated keys.
EST specifies how to transfer messages securely via HTTP over TLS
(HTTPS) [RFC2818], where the HTTP headers and media types are used in
conjunction with TLS. HTTPS operates over TCP; this document does
not specify EST over HTTP/Datagram Transport Layer Security/User
Datagram Protocol (HTTP/DTLS/UDP). With a suitable specification for
combining HTTP, DTLS, and UDP, there are no EST requirements that
would prevent it from working over such a stack. Figure 1 shows how
the layers build upon each other.
EST Layering:
Protocols:
+--------------------------------------------+
| |
| EST request/response messages |
| |
+--------------------------------------------+
| |
| HTTP for message transfer and signaling |
| |
+--------------------------------------------+
| |
| TLS for transport security |
| |
+--------------------------------------------+
| |
| TCP for transport |
| |
+--------------------------------------------+
Figure 1
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
Pritikin, et al. Standards Track [Page 4]
RFC 7030 EST October 2013
It is assumed that the reader is familiar with the terms and concepts
described in Public Key Cryptography Standard (PKCS) #10 [RFC2986],
HTTPS [RFC2818], CMP [RFC4210], CMC [RFC5272][RFC5273][RFC5274], and
TLS [RFC4346].
In addition to the terms defined in the terminology section of CMC
[RFC5272], the following terms are defined for clarity:
EST CA: For certificate issuing services, the EST CA is reached
through the EST server; the CA could be logically "behind" the EST
server or embedded within it.
Third-Party Trust Anchor: Any trust anchor (TA) that is not
authoritative for the PKI hierarchy for which the EST server is
providing services.
Explicit Trust Anchor: Any TA that is explicitly configured on the
client or server for use during EST TLS authentication; for
example, a TA that is manually configured on the EST client or
bootstrapped as described in Section 4.1.1. (See more details in
Sections 3.6 and 6.)
Implicit Trust Anchor: Any third-party TA that is available on the
client or server for use during TLS authentication but is not
specifically indicated for use during EST TLS authentication; for
example, TAs commonly used by web browsers to authenticate web
servers or TAs used by servers to authenticate manufacturer-
installed client credentials (such as certificates populated into
cable modems or routers in the factory). The authorization model
for these TAs is different from the authorization model for
Explicit Trust Anchors. (See more details in Sections 3.6.1,
3.6.2, and 6).
Certificate-Less TLS: Certificate-less TLS cipher suites provide a
way to perform mutual authentication in situations where neither
the client nor server have certificates or are willing to use
them. The credential used for authentication is a word, phrase,
code, or key that is shared between the client and server. The
credential must be uniquely shared between the client and server
in order to provide authentication of an individual client to an
individual server.
2. Operational Scenario Overviews
This section provides an informative overview of the operational
scenarios to better introduce the reader to the protocol discussion.
Pritikin, et al. Standards Track [Page 5]
RFC 7030 EST October 2013
Both the EST clients and server are configured with information that
provides the basis for mutual authentication and for authorization.
The specific initialization data depends on the methods available in
the client and server, but it can include shared secrets, network
service names and locations (e.g., a Uniform Resource Identifier
(URI) [RFC3986]), trust anchor information (e.g., a CA certificate or
a hash of a TA's certificate), and enrollment keys and certificates.
Depending on an enterprise's acquisition and network management
practices, some initialization may be performed by the vendor prior
to delivery of client hardware and software. In that case, the
client vendor may provide data, such as trust anchors, to the
enterprise via a secure procedure. The distribution of this initial
information is out of scope.
Distribution of trust anchors and other certificates can be effected
via the EST server. However, nothing can be inferred about the
authenticity of this data until an out-of-band mechanism is used to
verify them.
Sections 2.1-2.3 very closely mirror the text of the Scenarios
Appendix of [RFC6403] with such modifications as are appropriate for
this profile. Sections 2.1-2.6, below, enumerate the set of EST
functions (see Figure 5) and provide an informative overview of EST's
capabilities.
The general client/server interaction proceeds as follows:
The client initiates a TLS-secured HTTP session with an EST
server.
A specific EST service is requested based on a portion of the URI
used for the session.
The client and server authenticate each other.
The client verifies that the server is authorized to serve this
client.
The server verifies that the client is authorized to make use of
this server and the request that the client has made.
The server acts upon the client request.
2.1. Obtaining CA Certificates
The EST client can request a copy of the current EST CA
certificate(s) from the EST server. The EST client is assumed to
perform this operation before performing other operations.
Pritikin, et al. Standards Track [Page 6]
RFC 7030 EST October 2013
Throughout this document we assume the EST CA has a certificate that
is used by the client to verify signed objects issued by the CA,
e.g., certificates and certificate revocation lists (CRLs), and that
a different certificate than the one used to verify signatures on
certificates and CRLs is used when EST protocol communication
requires additional encryption.
The EST client authenticates and verifies the authorization scope of
the EST server when requesting the current CA certificate(s). As
detailed in Sections 3.3.1 and 3.3.3, available options include:
o Verifying the EST server's HTTPS URI against the EST server's
certificate using Implicit TAs (similar to a common HTTPS
exchange). This allows the EST server and client to leverage
existing TAs that might be known to the EST client.
o The client can leverage a previously distributed trust anchor
specific to the EST server. This allows the EST client to use an
existing, potentially older, CA certificate to request a current
CA certificate.
o For bootstrapping, the EST client can rely upon manual
authentication performed by the end-user as detailed in
Section 4.1.1.
o The client can leverage the binding of a shared credential to a
specific EST server with a certificate-less TLS cipher suite.
Client authentication is not required for this exchange, so it is
trivially supported by the EST server.
2.2. Initial Enrollment
After authenticating an EST server and verifying that it is
authorized to provide services to the client, an EST client can
acquire a certificate for itself by submitting an enrollment request
to that server.
The EST server authenticates and authorizes the EST client as
specified in Sections 3.3.2, 3.3.3, and 3.7. The methods described
in the normative text that are discussed in this overview include:
o TLS with a previously issued client certificate (e.g., an existing
certificate issued by the EST CA);
o TLS with a previously installed certificate (e.g., manufacturer-
installed certificate or a certificate issued by some other
party);
Pritikin, et al. Standards Track [Page 7]
RFC 7030 EST October 2013
o Certificate-less TLS (e.g., with a shared credential distributed
out-of-band);
o HTTP-based with a username/password distributed out-of-band.
2.2.1. Certificate TLS Authentication
If the EST client has a previously installed certificate issued by a
third-party CA, this certificate can be used to authenticate the
client's request for a certificate from the EST server (if that CA is
recognized by the EST server). An EST client responds to the EST
server's TLS certificate request message with the existing
certificate already held by the client. The EST server will verify
the client's existing certificate and authorize the client's request
as described in Section 3.3.2.
2.2.2. Certificate-Less TLS Authentication
The EST client and EST server can be mutually authenticated using a
certificate-less TLS cipher suite (see Section 3.3.3).
2.2.3. HTTP-Based Client Authentication
The EST server can optionally also request that the EST client submit
a username/password using the HTTP Basic or Digest authentication
methods (see Section 3.2.3). This approach is desirable if the EST
client cannot be authenticated during the TLS handshake (see
Section 3.3.2) or the EST server policy requires additional
authentication information; see Section 3.2.3. In all cases,
HTTP-based client authentication is only to be performed over a
TLS-protected transport (see Section 3.3).
2.3. Client Certificate Reissuance
An EST client can renew/rekey its existing client certificate by
submitting a re-enrollment request to an EST server.
When the current EST client certificate can be used for TLS client
authentication (Section 3.3.2), the client presents this certificate
to the EST server for client authentication. When the to be reissued
EST client certificate cannot be used for TLS client authentication,
any of the authentication methods used for initial enrollment can be
used.
For example, if the client has an alternative certificate issued by
the EST CA that can be used for TLS client authentication, then it
can be used.
Pritikin, et al. Standards Track [Page 8]
RFC 7030 EST October 2013
The certificate request message includes the same Subject and
SubjectAltName as the current certificate. Name changes are
requested as specified in Section 4.2.2.
2.4. Server Key Generation
The EST client can request a server-generated certificate and key
pair (see Section 4.4).
2.5. Full PKI Request Messages
Full PKI Request [RFC5272] messages can be transported via EST using
the Full CMC Request function. This affords access to functions not
provided by the Simple Enrollment functions. Full PKI Request
messages are defined in Sections 3.2 and 4.2 of [RFC5272]. See
Section 4.3 for a discussion of how EST provides a transport for
these messages.
2.6. Certificate Signing Request (CSR) Attributes Request
Prior to sending an enrollment request to an EST server, an EST
client can query the EST server for a set of additional attributes
that the client is requested to use in a subsequent enrollment
request.
These attributes can provide additional descriptive information that
the EST server cannot access itself, such as the Media Access Control
(MAC) address of an interface of the EST client. Alternatively,
these attributes can indicate the kind of enrollment request, such as
a specific elliptic curve or a specific hash function that the client
is expected to use when generating the CSR.
Pritikin, et al. Standards Track [Page 9]
RFC 7030 EST October 2013
3. Protocol Design and Layering
Figure 2 provides an expansion of Figure 1, describing how the layers
are used. Each aspect is described in more detail in the sections
that follow.
EST Layering:
Protocols and uses:
+----------------------------------------------------+
| |
| Message types: |
| - "Simple PKI" messages |
| (incorporates proof-of-possession) |
| - CA certificate retrieval |
| - "Full PKI" messages (OPTIONAL) |
| (incorporates proof-of-possession) |
| - CSR Attributes Request (OPTIONAL) |
| - Server-generated key request (OPTIONAL) |
| |
+----------------------------------------------------+
| |
| HTTP: |
| - HTTP headers and URIs for control |
| - Content-Type headers specify message type |
| - Headers for control/error messages |
| - URIs for selecting functions |
| - Basic or Digest authentication (OPTIONAL) |
| |
+----------------------------------------------------+
| |
| TLS for transport security: |
| - Authentication of the EST server |
| - Authentication of the EST client (OPTIONAL) |
| - Provides communications integrity |
| and confidentiality |
| - Supplies channel-binding [RFC5929] information |
| to link proof-of-identity with message-based |
| proof-of-possession (OPTIONAL) |
| |
+----------------------------------------------------+
Figure 2
Specifying HTTPS as the secure transport for enrollment messages
introduces two "layers" to communicate authentication and control
messages: TLS and HTTP.
Pritikin, et al. Standards Track [Page 10]
RFC 7030 EST October 2013
The TLS layer provides integrity and confidentiality during
transport. The proof-of-identity is supplied by TLS handshake
authentication and optionally also by the HTTP layer headers. The
message type and control/error messages are included in the HTTP
headers.
CMC ([RFC5272], Section 3.1) notes that "the Simple PKI Request MUST
NOT be used if a proof-of-identity needs to be included". Since the
TLS and HTTP layers can provide proof-of-identity for EST clients and
servers, the Simple PKI message types are used.
The TLS layer certificate exchange provides a method for authorizing
client enrollment requests using existing certificates. Such
certificates may have been issued by the CA (from which the client is
requesting a certificate), or they may have been issued under a
distinct PKI (e.g., an IEEE 802.1AR Initial Device Identifier
(IDevID) [IDevID] credential).
Proof-of-possession (POP) is a distinct issue from proof-of-identity
and is included in the Simple PKI message type as described in
Section 3.4. A method of linking proof-of-identity and
proof-of-possession is described in Section 3.5.
This document also defines transport for CMC [RFC5272] that complies
with the CMC Transport Protocols [RFC5273]. CMC's POP and
proof-of-identity mechanisms are defined in CMC, but the mechanisms
here can also be used in conjunction with those mechanisms in "Full
PKI" messages.
During protocol exchanges, different certificates can be used. The
following table provides an informative overview. End-entities can
have one or more certificates of each type listed in Figure 3 and use
one or more trust anchor databases of each type listed in Figure 4.
Pritikin, et al. Standards Track [Page 11]
RFC 7030 EST October 2013
Certificates and their corresponding uses:
+--------------+--------------------+-------------------------------+
| Certificate | Issuer | Use and section references |
+==============+====================+===============================+
| EST server | The CA served by | Presented by the EST server |
| certificate | the EST server | during the TLS handshake. |
| | | |
| | | Section 3.3.1 |
+--------------+--------------------+-------------------------------+
| EST server | A CA | Presented by the EST server |
| certificate | authenticatable by | during the TLS handshake. |
| | a third-party TA, | |
| | e.g., a web server | Section 3.3.1 and |
| | CA | Security Considerations |
+--------------+--------------------+-------------------------------+
| Third-party | A CA | Presented by the EST client |
| EST client | authenticatable by | to the EST server by clients |
| certificate | a third-party TA, | that have not yet enrolled. |
| | e.g., a device | |
| | manufacturer | Section 3.3.2 |
+--------------+--------------------+-------------------------------+
| EST client | The CA served by | Presented to the EST server |
| certificate | the EST server | during future EST operations. |
| | | |
| | | Section 3.3.2 |
+--------------+--------------------+-------------------------------+
| End-entity | The CA served by | Clients can obtain certs |
| certificate | the EST server | that are intended for |
| | | non-EST uses. This includes |
| | | certs that cannot be used |
| | | for EST operations. |
| | | |
| | | Section 4.2.3 |
+--------------+--------------------+-------------------------------+
Figure 3
Pritikin, et al. Standards Track [Page 12]
RFC 7030 EST October 2013
Trust anchor databases and their corresponding uses:
+--------------+----------------------------------------------------+
| TA database | Use and section references |
+==============+====================================================+
| EST server | EST servers use this TA database to authenticate |
| Explicit | certificates issued by the EST CA, including EST |
| TA database | client certificates during enroll/re-enroll |
| | operations. |
| | |
| | Section 3.3.2 |
+--------------+----------------------------------------------------+
| EST server | EST servers use this TA database to authenticate |
| Implicit | certificates issued by third-party TAs; |
| TA database | e.g., EST client certificates issued by a device |
| | manufacturer. |
| | An Implicit TA database can be disabled. |
| | |
| | Section 3.3.2 |
+--------------+----------------------------------------------------+
| EST client | EST clients use this TA database to authenticate |
| Explicit | certificates issued by the EST CA, including EST |
| TA database | server certificates. |
| | |
| | Sections 3.1, 3.3.1, 3.6.1, and 4.1.1 |
+--------------+----------------------------------------------------+
| EST client | EST clients use this TA database to |
| Implicit | authenticate an EST server that uses an externally |
| TA database | issued certificate. |
| | An Implicit TA database can be disabled. |
| | |
| | Sections 3.1, 3.3.1, 3.6.2, and |
| | Security Considerations |
+--------------+----------------------------------------------------+
Figure 4
3.1. Application Layer
The EST client MUST be capable of generating and parsing Simple PKI
messages (see Section 4.2). Generating and parsing Full PKI messages
is OPTIONAL (see Section 4.3). The client MUST also be able to
request CA certificates from the EST server and parse the returned
"bag" of certificates (see Section 4.1). Requesting CSR attributes
and parsing the returned list of attributes is OPTIONAL (see
Section 4.5).
Pritikin, et al. Standards Track [Page 13]
RFC 7030 EST October 2013
Details of the EST client application configuration are out of scope
of the protocol discussion but are necessary for understanding the
prerequisites of initiating protocol operations. The EST client is
RECOMMENDED to be configured with TA databases for Section 3.3.1 or
with a secret key for Section 3.3.3. Implementations conforming to
this standard MUST provide the ability to designate Explicit TAs.
For human usability reasons, a "fingerprint" of an Explicit TA
database entry can be configured for bootstrapping as discussed in
Section 4.1.1. Configuration of an Implicit TA database, perhaps by
its inclusion within the EST client distribution or available from
the operating system, provides flexibility along with the caveats
detailed in Section 6. Implementations conforming to this standard
MUST provide the ability to disable use of any Implicit TA database.
The EST client is configured with sufficient information to form the
EST server URI. This can be the full operation path segment (e.g.,
https://www.example.com/.well-known/est/ or
https://www.example.com/.well-known/est/arbitraryLabel1), or the EST
client can be configured with a tuple composed of the authority
portion of the URI along with the OPTIONAL label (e.g.,
"www.example.com:80" and "arbitraryLabel1") or just the authority
portion of the URI.
3.2. HTTP Layer
HTTP is used to transfer EST messages. URIs are defined for handling
each media type (i.e., message type) as described in Section 3.2.2.
HTTP is also used for client authentication services when TLS client
authentication is not available, due to the lack of a client
certificate suitable for use by TLS (see Section 3.2.3). HTTP
authentication can also be used in addition to TLS client
authentication if the EST server wishes additional authentication
information, as noted in Section 2.2.3. Registered media types are
used to convey EST messages as specified in Figure 6.
HTTP 1.1 [RFC2616] and above support persistent connections. As
described in Section 8.1 of RFC 2616, persistent connections may be
used to reduce network and processing loads associated with multiple
HTTP requests. EST does not require or preclude persistent HTTP
connections.
Pritikin, et al. Standards Track [Page 14]
RFC 7030 EST October 2013
3.2.1. HTTP Headers for Control
The HTTP Status value is used to communicate success or failure of an
EST function. HTTP authentication is used by a client when requested
by the server.
The media types specified in the HTTP Content-Type header indicate
which EST message is being transferred. Media types used by EST are
specified in Section 3.2.4.
HTTP redirections (3xx status codes) to the same web origin (see
[RFC6454]) SHOULD be handled by the client without user input so long
as all applicable security checks (Sections 3.3 and 3.6) have been
enforced on the initial connection. The client initiates a new TLS
connection and performs all applicable security checks when
redirected to other web origin servers. Redirections to other web
origins require the EST client to obtain user input for non-GET or
HEAD requests as specified in [RFC2616]. Additionally, if the client
has already generated a CSR that includes linking identity and POP
information (Section 3.5), then the CSR will need to be recreated to
incorporate the tls-unique from the new, redirected session. Note:
the key pair need not be regenerated. These are processing and
interface burdens on the client. EST server administrators are
advised to take this into consideration.
Pritikin, et al. Standards Track [Page 15]
RFC 7030 EST October 2013
3.2.2. HTTP URIs for Control
The EST server MUST support the use of the path-prefix of "/.well-
known/" as defined in [RFC5785] and the registered name of "est".
Thus, a valid EST server URI path begins with
"https://www.example.com/.well-known/est". Each EST operation is
indicated by a path-suffix that indicates the intended operation:
Operations and their corresponding URIs:
+------------------------+-----------------+-------------------+
| Operation |Operation path | Details |
+========================+=================+===================+
| Distribution of CA | /cacerts | Section 4.1 |
| Certificates (MUST) | | |
+------------------------+-----------------+-------------------+
| Enrollment of | /simpleenroll | Section 4.2 |
| Clients (MUST) | | |
+------------------------+-----------------+-------------------+
| Re-enrollment of | /simplereenroll | Section 4.2.2 |
| Clients (MUST) | | |
+------------------------+-----------------+-------------------+
| Full CMC (OPTIONAL) | /fullcmc | Section 4.3 |
+------------------------+-----------------+-------------------+
| Server-Side Key | /serverkeygen | Section 4.4 |
| Generation (OPTIONAL) | | |
+------------------------+-----------------+-------------------+
| CSR Attributes | /csrattrs | Section 4.5 |
| (OPTIONAL) | | |
+------------------------+-----------------+-------------------+
Figure 5
The operation path (Figure 5) is appended to the path-prefix to form
the URI used with HTTP GET or POST to perform the desired EST
operation. An example valid URI absolute path for the "/cacerts"
operation is "/.well-known/est/cacerts". To retrieve the CA's
certificates, the EST client would use the following HTTP
request-line:
GET /.well-known/est/cacerts HTTP/1.1
Likewise, to request a new certificate in this example scheme, the
EST client would use the following request-line:
POST /.well-known/est/simpleenroll HTTP/1.1
Pritikin, et al. Standards Track [Page 16]
RFC 7030 EST October 2013
The use of distinct operation paths simplifies implementation for
servers that do not perform client authentication when distributing
/cacerts responses.
An EST server MAY provide service for multiple CAs as indicated by an
OPTIONAL additional path segment between the registered application
name and the operation path. To avoid conflict, the CA label MUST
NOT be the same as any defined operation path segment. The EST
server MUST provide services regardless of whether the additional
path segment is present. The following are three example valid URIs:
1. https://www.example.com/.well-known/est/cacerts
2. https://www.example.com/.well-known/est/arbitraryLabel1/cacerts
3. https://www.example.com/.well-known/est/arbitraryLabel2/cacerts
In this specification, the distinction between enroll and renew/rekey
is explicitly indicated by the HTTP URI. When requesting /fullcmc
operations, CMC [RFC5272] uses the same messages for certificate
renewal and certificate rekey.
An EST server can provide additional services using other URIs.
3.2.3. HTTP-Based Client Authentication
The EST server MAY request HTTP-based client authentication. This
request can be in addition to successful TLS client authentication
(Section 3.3.2) if EST server policy requires additional
authentication. (For example, the EST server may require that an EST
client "knows" a password in addition to "having" an existing client
certificate.) Or, HTTP-based client authentication can be an EST
server policy-specified fallback in situations where the EST client
did not successfully complete the TLS client authentication. (This
might arise if the EST client is enrolling for the first time or if
the certificates available to an EST client cannot be used for TLS
client authentication.)
HTTP Basic and Digest authentication MUST only be performed over TLS
1.1 [RFC4346] or later versions. NULL and anon cipher suites MUST
NOT be used because they do not provide confidentiality or support
mutual certificate-based or certificate-less authentication,
respectively. As specified in "Certificate Management over CMS
(CMC): Transport Protocols" [RFC5273], the server "MUST NOT assume
client support for any type of HTTP authentication such as cookies,
Basic authentication, or Digest authentication". Clients SHOULD
support the Basic and Digest authentication mechanism.
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Servers that wish to use Basic and Digest authentication reject the
HTTP request using the HTTP-defined WWW-Authenticate response-header
([RFC2616], Section 14.47). The client is expected to retry the
request, including the appropriate Authorization Request header
([RFC2617], Section 3.2.2), if the client is capable of using the
Basic or Digest authentication. If the client is not capable of
retrying the request or it is not capable of Basic or Digest
authentication, then the client MUST terminate the connection.
A client MAY set the username to the empty string ("") if it is
presenting a password that is not associated with a username.
Support for HTTP-based client authentication has security
ramifications as discussed in Section 6. The client MUST NOT respond
to the server's HTTP authentication request unless the client has
authorized the EST server (as per Section 3.6).
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3.2.4. Message Types
This document uses existing media types for the messages as specified
by FTP and HTTP [RFC2585], application/pkcs10 [RFC5967], and CMC
[RFC5272].
For consistency with [RFC5273], each distinct EST message type uses
an HTTP Content-Type header with a specific media type.
The EST messages and their corresponding media types for each
operation are:
+--------------------+--------------------------+-------------------+
| Message type | Request media type | Request section(s)|
| | Response media type(s) | Response section |
| (per operation) | Source(s) of types | |
+====================+==========================+===================+
| Distribution of CA | N/A | Section 4.1 |
| Certificates | application/pkcs7-mime | Section 4.1.1 |
| | [RFC5751] | |
| /cacerts | | |
+--------------------+--------------------------+-------------------+
| Client Certificate | application/pkcs10 | Sections 4.2/4.2.1|
| Request Functions | application/pkcs7-mime | Section 4.2.2 |
| | [RFC5967] [RFC5751] | |
| /simpleenroll | | |
| /simplereenroll | | |
+--------------------+--------------------------+-------------------+
| Full CMC | application/pkcs7-mime | Section 4.3.1 |
| | application/pkcs7-mime | Section 4.3.2 |
| /fullcmc | [RFC5751] | |
+--------------------+--------------------------+-------------------+
| Server-Side Key | application/pkcs10 | Section 4.4.1 |
| Generation | multipart/mixed | Section 4.4.2 |
| | (application/pkcs7-mime &| |
| | application/pkcs8) | |
| | [RFC5967] [RFC5751] | |
| /serverkeygen | [RFC5958] | |
+--------------------+--------------------------+-------------------+
| CSR Attributes | N/A | Section 4.5.1 |
| | application/csrattrs | Section 4.5.2 |
| | (This document) | |
| /csrattrs | | |
+--------------------+--------------------------+-------------------+
Figure 6
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3.3. TLS Layer
TLS provides authentication, which in turn enables authorization
decisions. The EST server and EST client are responsible for
ensuring that an acceptable cipher suite is negotiated and that
mutual authentication has been performed. TLS authentication is most
commonly enabled with the use of certificates [RFC5280].
Alternately, certificate-less TLS authentication, where neither the
client nor server present a certificate, is also an acceptable method
for EST mutual authentication (Section 3.3.3). The EST server MUST
be authenticated during the TLS handshake unless the client is
requesting Bootstrap Distribution of CA certificates (Section 4.1.1)
or Full CMC (Section 4.3).
HTTPS [RFC2818] specifies how HTTP messages are carried over TLS.
HTTPS MUST be used. TLS 1.1 [RFC4346] (or a later version) MUST be
used for all EST communications. TLS session resumption [RFC5077]
SHOULD be supported.
TLS channel-binding information can be inserted into a certificate
request, as detailed in Section 3.5, in order to provide the EST
server with assurance that the authenticated TLS client has access to
the private key for the certificate being requested. The EST server
MUST implement Section 3.5.
3.3.1. TLS-Based Server Authentication
TLS server authentication with certificates MUST be supported.
The EST client authenticates the EST server as defined for the cipher
suite negotiated. The following text provides details assuming a
certificate-based cipher suite, such as the TLS 1.1 [RFC4346]
mandatory cipher suite (TLS_RSA_WITH_3DES_EDE_CBC_SHA).
Certificate validation MUST be performed as per [RFC5280]. The EST
server certificate MUST conform to the [RFC5280] certificate profile.
The client validates the TLS server certificate using the EST client
Explicit and, if enabled, Implicit TA database(s). The client MUST
maintain a distinction between the use of Explicit and Implicit TA
databases during authentication in order to support proper
authorization. The EST client MUST perform authorization checks as
specified in Section 3.6.
If certificate validation fails, the client MAY follow the procedure
outlined in Section 4.1.1 for Bootstrap Distribution of CA
certificates.
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3.3.2. TLS-Based Client Authentication
TLS client authentication is the RECOMMENDED method for identifying
EST clients. HTTP-based client authentication (Section 3.2.3) MAY be
used.
The EST server authenticates the EST client as defined for the cipher
suite negotiated. The following text provides details assuming a
certificate-based cipher suite such as the TLS 1.1 [RFC4346]
mandatory cipher suite (TLS_RSA_WITH_3DES_EDE_CBC_SHA). The EST
server MUST support certificate-based client authentication.
Generally, the client will use an existing certificate for renew or
rekey operations. If the certificate to be renewed or rekeyed is
appropriate for the negotiated cipher suite, then the client MUST use
it for the TLS handshake, otherwise the client SHOULD use an
alternate certificate that is suitable for the cipher suite and
contains the same subject identity information. When requesting an
enroll operation, the client MAY use a client certificate issued by a
third party to authenticate itself.
Certificate validation MUST be performed as per [RFC5280]. The EST
client certificate MUST conform to the [RFC5280] certificate profile.
The server validates the TLS client certificate using the EST server
Explicit and, if enabled, Implicit TA database(s). The server MUST
maintain a distinction between the use of Explicit and Implicit TA
databases during authentication in order to support proper
authorization.
The EST server MUST perform authorization checks as specified in
Section 3.7.
If a client does not support TLS client authentication, then it MUST
support HTTP-based client authentication (Section 3.2.3) or
certificate-less TLS authentication (Section 3.3.3).
3.3.3. Certificate-Less TLS Mutual Authentication
Certificate-less TLS cipher suites provide a way to perform mutual
authentication in situations where neither the client nor server have
certificates, do not desire to use certificates, or do not have the
trust anchors necessary to verify a certificate. The client and
server MAY negotiate a certificate-less cipher suite for mutual
authentication.
When using certificate-less mutual authentication in TLS for
enrollment, the cipher suite MUST be based on a protocol that is
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resistant to dictionary attack and MUST be based on a zero knowledge
protocol. Transport Layer Security-Secure Remote Password (TLS-SRP)
cipher suites, i.e., those with _SRP_ in the name, listed in
Section 2.7 of [RFC5054] are suitable for this purpose. Section 6
lists the characteristics of a cipher suite that are suitable for use
in certificate-less mutual authentication for enrollment.
Successful authentication using a certificate-less cipher suite
proves knowledge of a pre-shared secret that implicitly authorizes a
peer in the exchange.
3.4. Proof-of-Possession
As defined in Section 2.1 of CMC [RFC5272], proof-of-possession (POP)
"refers to a value that can be used to prove that the private key
corresponding to the public key is in the possession of and can be
used by an end-entity".
The signed enrollment request provides a signature-based
proof-of-possession. The mechanism described in Section 3.5
strengthens this by optionally including "Direct"-based
proof-of-possession [RFC5272] by including TLS session-specific
information within the data covered by the enrollment request
signature (thus linking the enrollment request to the authenticated
end point of the TLS connection).
3.5. Linking Identity and POP Information
Server policy will determine whether clients are required to use the
mechanism specified in this section. This specification provides a
method of linking identity and proof-of-possession by including
information specific to the current authenticated TLS session within
the signed certification request. The client can determine if the
server requires the linking of identity and POP by examining the CSR
Attributes Response (see Section 4.5.2). Regardless of the CSR
Attributes Response, clients SHOULD link identity and POP by
embedding tls-unique information in the certification request. If
tls-unique information is included by the client, the server MUST
verify it. The EST server MAY reject requests without tls-unique
information as indicated by server policy.
Linking identity and proof-of-possession proves to the server that
the authenticated TLS client has possession of the private key
associated with the certification request, and that the client was
able to sign the certification request after the TLS session was
established. This is an alternative to the "Linking Identity and POP
Information" method defined by Section 6 of [RFC5272] that is
available if Full PKI messages are used.
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The client generating the CSR obtains the tls-unique value from the
TLS subsystem as described in Channel Bindings for TLS [RFC5929].
The EST client operations between obtaining the tls-unique value
through generation of the CSR that contains the current tls-unique
value and the subsequent verification of this value by the EST server
are the "phases of the application protocol during which application-
layer authentication occurs"; these operations are protected by the
synchronization interoperability mechanism described in the "Channel
Bindings for TLS" interoperability notes in Section 3.1 of [RFC5929].
When performing renegotiation, TLS "secure_renegotiation" [RFC5746]
MUST be used.
The tls-unique value is base64 encoded as specified in Section 4 of
[RFC4648], and the resulting string is placed in the certification
request challenge-password field ([RFC2985], Section 5.4.1). The
challenge-password field is limited to 255 bytes (Section 7.4.9 of
[RFC5246] indicates that no existing cipher suite would result in an
issue with this limitation). If the challenge-password attribute is
absent, the client did not include the optional channel-binding
information (the presence of the challenge-password attribute
indicates the inclusion of tls-unique information).
If the EST server makes use of a back-end infrastructure for
processing, it is RECOMMENDED that the results of this verification
be communicated. (For example, this communication might use the CMC
[RFC5272] "RA POP Witness Control" in a CMC Full PKI Request message.
Or, an EST server might TLS-authenticate an EST client as being a
trusted infrastructure element that does not forward invalid
requests. A detailed discussion of back-end processing is out of
scope.)
When rejecting requests, the EST server response is as described for
all enroll responses (Section 4.2.3). If a Full PKI Response is
included, the CMCFailInfo MUST be set to popFailed. If a human-
readable reject message is included, it SHOULD include an informative
text message indicating that the linking of identity and POP
information is required.
3.6. Server Authorization
The client MUST check EST server authorization before accepting any
server responses or responding to HTTP authentication requests.
The EST client authorization method depends on which method was used
to authenticate the server. When the Explicit TA database is used to
authenticate the EST server, then Section 3.6.1 applies. When the
Implicit TA database is used to authenticate the EST server, then
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Section 3.6.2 applies. Successful authentication using a
certificate-less cipher suite implies authorization of the server.
The client MAY perform bootstrapping as specified in Section 4.1.1
even if these checks fail.
3.6.1. Client Use of Explicit TA Database
When the EST client Explicit TA database is used to validate the EST
server certificate, the client MUST check either the configured URI
or the most recent HTTP redirection URI against the server's identity
according to the rules specified in [RFC6125], Section 6.4, or the
EST server certificate MUST contain the id-kp-cmcRA [RFC6402]
extended key usage extension.
3.6.2. Client Use of Implicit TA Database
When the EST client Implicit TA database is used to validate the EST
server certificate, the client MUST check the configured URI and each
HTTP redirection URI according to the rules specified in [RFC6125],
Section 6.4. The provisioned URI or the most recent HTTP redirection
URI provides the basis for authorization, and the server's
authenticated identity confirms it is the authorized server.
3.7. Client Authorization
The decision to issue a certificate to a client is always controlled
by local CA policy. The EST server configuration reflects this CA
policy. This document does not specify any constraints on such
policy. EST provides the EST server access to each client's
authenticated identity -- e.g., the TLS client's certificate in
addition to any HTTP user authentication credentials -- to help in
implementing such policy.
If the client's certificate was issued by the EST CA, and it includes
the id-kp-cmcRA [RFC6402] extended key usage extension, then the
client is a Registration Authority (RA) as described in [RFC5272] and
[RFC6402]. In this case, the EST server SHOULD apply authorization
policy consistent with an RA client. For example, when handling
/simpleenroll requests, the EST server could be configured to accept
POP linking information that does not match the current TLS session
because the authenticated EST client RA has verified this information
when acting as an EST server (as specified in Section 3.5). More
specific RA mechanisms are available if the EST client uses /fullcmc
methods.
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4. Protocol Exchange Details
Before processing a request, an EST server determines if the client
is authorized to receive the requested services. Likewise, the
client determines if it will make requests to the EST server. These
authorization decisions are described in the next two sections.
Assuming that both sides of the exchange are authorized, then the
actual operations are as described in subsequent sections.
4.1. Distribution of CA Certificates
The EST client can request a copy of the current CA certificates.
This function is generally performed before other EST functions.
4.1.1. Bootstrap Distribution of CA Certificates
It is possible that the client was not configured with an Implicit TA
database that allows a bootstrap installation of the Explicit TA
database as described in 4.1.3. This section describes an alternate
method by which minimally configured EST clients can populate their
Explicit TA database.
If the EST client application does not specify either an Explicit TA
database or an Implicit TA database, then the initial TLS server
authentication and authorization will fail. The client MAY
provisionally continue the TLS handshake to completion for the
purposes of accessing the /cacerts or /fullcmc method. If the EST
client continues with an unauthenticated connection, the client MUST
extract the HTTP content data from the response (Sections 4.1.3 or
4.3.2) and engage a human user to authorize the CA certificate using
out-of-band data such as a CA certificate "fingerprint" (e.g., a
SHA-256 or SHA-512 [SHS] hash on the whole CA certificate). In a
/fullcmc response, it is the Publish Trust Anchors control (CMC
[RFC5272], Section 6.15) within the Full PKI Response that must be
accepted manually. It is incumbent on the user to properly verify
the TA information, or to provide the "fingerprint" data during
configuration that is necessary to verify the TA information.
HTTP authentication requests MUST NOT be responded to if the server
has not been authenticated as specified in Section 3.3.1 or if the
optional certificate-less authentication is used as specified in
Section 3.3.3.
The EST client uses the /cacerts response to establish an Explicit
Trust Anchor database for subsequent TLS authentication of the EST
server. EST clients MUST NOT engage in any other protocol exchange
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until after the /cacerts response has been accepted and a new TLS
session has been established (using TLS certificate-based
authentication).
4.1.2. CA Certificates Request
EST clients request the EST CA TA database information of the CA (in
the form of certificates) with an HTTPS GET message using an
operation path of "/cacerts". EST clients and servers MUST support
the /cacerts function. Clients SHOULD request an up-to-date response
before stored information has expired in order to ensure the EST CA
TA database is up to date.
The EST server SHOULD NOT require client authentication or
authorization to reply to this request.
The client MUST authenticate the EST server, as specified in
Section 3.3.1 if certificate-based authentication is used or
Section 3.3.3 if the optional certificate-less authentication is
used, and check the server's authorization as given in Section 3.6,
or follow the procedure outlined in Section 4.1.1.
4.1.3. CA Certificates Response
If successful, the server response MUST have an HTTP 200 response
code. Any other response code indicates an error and the client MUST
abort the protocol.
A successful response MUST be a certs-only CMC Simple PKI Response,
as defined in [RFC5272], containing the certificates described in the
following paragraph. The HTTP content-type of
"application/pkcs7-mime" is used. The Simple PKI Response is sent
with a Content-Transfer-Encoding of "base64" [RFC2045].
The EST server MUST include the current root CA certificate in the
response. The EST server MUST include any additional certificates
the client would need to build a chain from an EST CA-issued
certificate to the current EST CA TA. For example, if the EST CA is
a subordinate CA, then all the appropriate subordinate CA
certificates necessary to build a chain to the root EST CA are
included in the response.
The EST server SHOULD include the three "Root CA Key Update"
certificates OldWithOld, OldWithNew, and NewWithOld in the response
chain. These are defined in Section 4.4 of CMP [RFC4210]. The EST
client MUST be able to handle these certificates in the response.
The EST CA's most recent self-signed certificate (e.g., NewWithNew
certificate) is self-signed and has the latest NotAfter date. If the
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EST server does not include these in the response, then after the
current EST CA certificate expires, the EST clients will need to be
reinitialized with the PKI using the Bootstrap Distribution of CA
certificates (Section 4.1.1) method, which involves user interaction.
After out-of-band validation occurs, all the other certificates MUST
be validated using normal [RFC5280] certificate path validation
(using the most recent CA certificate as the TA) before they can be
used to build certificate paths during certificate validation.
The EST client MUST store the extracted EST CA certificate as an
Explicit TA database entry for subsequent EST server authentication.
The EST client SHOULD disable use of Implicit TA database entries for
this EST server now that an Explicit TA database entry is available.
If the client disables the Implicit TA database, and if the EST
server certificate was verified using an Implicit TA database entry,
then the client MUST include the "Trusted CA Indication" extension in
future TLS sessions [RFC6066]. This indicates to the server that
only an EST server certificate authenticatable by the Explicit TA
database entry is now acceptable (otherwise, the EST server might
continue to use a server certificate that is only verifiable by a now
disabled Implicit TA).
The EST client SHOULD also make the CA Certificate response
information available to the end-entity software for use when
validating peer certificates.
4.2. Client Certificate Request Functions
EST clients request a certificate from the EST server with an HTTPS
POST using the operation path value of "/simpleenroll". EST clients
request a renew/rekey of existing certificates with an HTTP POST
using the operation path value of "/simplereenroll". EST servers
MUST support the /simpleenroll and /simplereenroll functions.
It is RECOMMENDED that a client obtain the current CA certificates,
as described in Section 4.1, before performing certificate request
functions. This ensures that the client will be able to validate the
EST server certificate. The client MUST authenticate the EST server
as specified in Section 3.3.1 if certificate-based authentication is
used or Section 3.3.3 if the optional certificate-less authentication
is used. The client MUST verify the authorization of the EST server
as specified in Section 3.6.
The server MUST authenticate the client as specified in Section 3.3.2
if certificate-based authentication is used or Section 3.3.3 if the
optional certificate-less authentication is used. The server MUST
verify client authorization as specified in Section 3.7. The EST
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server MUST check the tls-unique value, as described in Section 3.5,
if one is submitted by the client.
The server MAY accept a certificate request for manual authorization
checking by an administrator. (Section 4.2.3 describes the use of an
HTTP 202 response to the EST client if this occurs.)
4.2.1. Simple Enrollment of Clients
When HTTPS POSTing to /simpleenroll, the client MUST include a Simple
PKI Request as specified in CMC [RFC5272], Section 3.1 (i.e., a PKCS
#10 Certification Request [RFC2986]).
The Certification Signing Request (CSR) signature provides
proof-of-possession of the client-possessed private key to the EST
server. If the CSR KeyUsage extension indicates that the private key
can be used to generate digital signatures, then the client MUST
generate the CSR signature using the private key. If the key can be
used to generate digital signatures but the requested CSR KeyUsage
extension prohibits generation of digital signatures, then the CSR
signature MAY still be generated using the private key, but the key
MUST NOT be used for any other signature operations (this is
consistent with the recommendations concerning submission of
proof-of-possession to an RA or CA as described in
[SP-800-57-Part-1]). The use of /fullcmc operations provides access
to more advanced proof-of-possession methods that are used when the
key pair cannot be used for digital signature generation (see
Section 4.3).
The HTTP content-type of "application/pkcs10" is used here. The
format of the message is as specified in [RFC5967] with a Content-
Transfer-Encoding of "base64" [RFC2045].
If the EST client authenticated using a previously installed
certificate issued by a third-party CA (see Section 2.2.1), the
client MAY include the ChangeSubjectName attribute, as defined in
[RFC6402], in the CSR to request that the subjectName and
SubjectAltName be changed in the new certificate.
The EST client MAY request additional certificates even when using an
existing certificate in the TLS client authentication. For example,
the client can use an existing certificate for TLS client
authentication when requesting a certificate that cannot be used for
TLS client authentication.
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4.2.2. Simple Re-enrollment of Clients
EST clients renew/rekey certificates with an HTTPS POST using the
operation path value of "/simplereenroll".
A certificate request employs the same format as the "simpleenroll"
request, using the same HTTP content-type. The request Subject field
and SubjectAltName extension MUST be identical to the corresponding
fields in the certificate being renewed/rekeyed. The
ChangeSubjectName attribute, as defined in [RFC6402], MAY be included
in the CSR to request that these fields be changed in the new
certificate.
If the Subject Public Key Info in the certification request is the
same as the current client certificate, then the EST server renews
the client certificate. If the public key information in the
certification request is different than the current client
certificate, then the EST server rekeys the client certificate.
4.2.3. Simple Enroll and Re-enroll Response
If the enrollment is successful, the server response MUST contain an
HTTP 200 response code with a content-type of
"application/pkcs7-mime".
A successful response MUST be a certs-only CMC Simple PKI Response,
as defined in [RFC5272], containing only the certificate that was
issued. The HTTP content-type of "application/pkcs7-mime" with an
smime-type parameter "certs-only" is used, as specified in [RFC5273].
The server MUST answer with a suitable 4xx or 5xx HTTP [RFC2616]
error code when a problem occurs. A Simple PKI Response with an HTTP
content-type of "application/pkcs7-mime" (see Section 4.3.2) MAY be
included in the response data to convey an error response. If the
content-type is not set, the response data MUST be a plaintext human-
readable error message containing explanatory information describing
why the request was rejected (for example, indicating that CSR
attributes are incomplete).
If the server responds with an HTTP [RFC2616] 202, this indicates
that the request has been accepted for processing but that a response
is not yet available. The server MUST include a Retry-After header
as defined for HTTP 503 responses. The server also MAY include
informative human-readable content. The client MUST wait at least
the specified "retry-after" time before repeating the same request.
The client repeats the initial enrollment request after the
appropriate "retry-after" interval has expired. The client SHOULD
log or inform the end-user of this event. The server is responsible
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for maintaining all states necessary to recognize and handle retry
operations as the client is stateless in this regard; it simply sends
the same request repeatedly until it receives a different response
code. All other return codes are handled as specified in HTTP
[RFC2616].
If the client closes the TLS connections while waiting for the Retry-
After time to expire, then the client initiates a new TLS connection
and performs all applicable security checks. If the client has
already generated a CSR that includes linking identity and POP
information (Section 3.5), then the CSR will need to be recreated to
incorporate the tls-unique from the new, redirected session. Note:
the key pair need not be regenerated. These are processing and
interface burdens on the client. EST server administrators are
advised to take this into consideration.
The EST client MAY also make the certificate response, and associated
private key, available to end-entity software for use as an
end-entity certificate.
4.3. Full CMC
An EST client can request a certificate from an EST server with an
HTTPS POST using the operation path value of "/fullcmc". Support for
the /fullcmc function is OPTIONAL for both clients and servers.
4.3.1. Full CMC Request
If the HTTP POST to /fullcmc is not a valid Full PKI Request, the
server MUST reject the message. The HTTP content-type used is
"application/pkcs7-mime" with an smime-type parameter "CMC-request",
as specified in [RFC5273]. The body of the message is the binary
value of the encoding of the PKI Request with a
Content-Transfer-Encoding of "base64" [RFC2045].
4.3.2. Full CMC Response
If the enrollment is successful, the server response MUST include an
HTTP 200 response code with a content-type of
"application/pkcs7-mime" as specified in [RFC5273]. The response
data includes either the Simple PKI Response with an smime-type
parameter of "certs-only" or the Full PKI Response with an smime-type
parameter "CMC-response", as specified in Section 3.2.1 of [RFC5751].
The body of the message is the binary value of the encoding of the
PKI Response with a Content-Transfer-Encoding of "base64" [RFC2045].
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When rejecting a request, the server MUST specify either an HTTP 4xx
error or an HTTP 5xx error. A CMC response with the content-type of
"application/pkcs7-mime" MUST be included in the response data for
any CMC error response.
All other return codes are handled as specified in Section 4.2.3 or
HTTP [RFC2616]. For example, a client interprets an HTTP 404 or 501
response to indicate that this service is not implemented.
4.4. Server-Side Key Generation
An EST client may request a private key and associated certificate
from an EST server using an HTTPS POST with an operation path value
of "/serverkeygen". Support for the /serverkeygen function is
OPTIONAL.
A client MUST authenticate an EST server, as specified in
Section 3.3.1 if certificate-based authentication is used or
Section 3.3.3 if the optional certificate-less authentication is
used, and check the server's authorization as given in Section 3.6.
The EST server MUST authenticate the client, as specified in
Section 3.3.2 if certificate-based authenticated is used or
Section 3.3.3 if the optional certificate-less authentication is
used, and check the client's authorization as given in Section 3.7.
The EST server applies whatever authorization or logic it chooses to
determine if the private key and certificate should be provided.
Cipher suites that have a NULL confidentiality algorithm MUST NOT be
used as they will disclose the contents of an unprotected private
key.
Proper random number and key generation [RFC4086] is a server
implementation responsibility, and server archiving of generated keys
is determined by CA policy. The key pair and certificate are
transferred over the TLS session. The cipher suite used to return
the private key and certificate MUST offer confidentiality
commensurate with the private key being delivered to the client.
The EST client MAY request additional certificates even when using an
existing certificate in the TLS client authentication. For example,
the client can use an existing certificate for TLS client
authentication when requesting a certificate that cannot be used for
TLS client authentication.
Pritikin, et al. Standards Track [Page 31]
RFC 7030 EST October 2013
4.4.1. Server-Side Key Generation Request
The certificate request is HTTPS POSTed and is the same format as for
the "/simpleenroll" and "/simplereenroll" path extensions with the
same content-type and transfer encoding.
In all respects, the server SHOULD treat the CSR as it would any
enroll or re-enroll CSR; the only distinction here is that the server
MUST ignore the public key values and signature in the CSR. These
are included in the request only to allow re-use of existing
codebases for generating and parsing such requests.
If the client desires to receive the private key with encryption that
exists outside of and in addition to that of the TLS transport used
by EST or if server policy requires that the key be delivered in such
a form, the client MUST include an attribute in the CSR indicating
the encryption key to be used. Both symmetric and asymmetric
encryption are supported as described in the following subsections.
The client MUST also include an SMIMECapabilities attribute
([RFC2633], Section 2.5) in the CSR to indicate the key encipherment
algorithms the client is willing to use.
It is strongly RECOMMENDED that the clients request that the returned
private key be afforded the additional security of the Cryptographic
Message Syntax (CMS) EnvelopedData in addition to the TLS-provided
security to protect against unauthorized disclosure.
4.4.1.1. Requests for Symmetric Key Encryption of the Private Key
To specify a symmetric encryption key to be used to encrypt the
server-generated private key, the client MUST include a
DecryptKeyIdentifier attribute (as defined in Section 2.2.5 of
[RFC4108]) specifying the identifier of the secret key to be used by
the server to encrypt the private key. While that attribute was
originally designated for specifying a firmware encryption key, it
exactly mirrors the requirements for specifying a secret key to
encrypt a private key. If the server does not have a secret key
matching the identifier specified by the client, the request MUST be
terminated and an error returned to the client. Distribution of the
key specified by the DecryptKeyIdentifier to the key generator and
the client is outside the scope of this document.
Pritikin, et al. Standards Track [Page 32]
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4.4.1.2. Requests for Asymmetric Encryption of the Private Key
To specify an asymmetric encryption key to be used to encrypt the
server-generated private key, the client MUST include an
AsymmetricDecryptKeyIdentifier attribute. The
AsymmetricDecryptKeyIdentifier attribute is defined as:
id-aa-asymmDecryptKeyID OBJECT IDENTIFIER ::= {
id-aa 54 }
The asymmetric-decrypt-key-identifier attribute values have ASN.1
type AsymmetricDecryptKeyIdentifier (where ASN.1 is defined in
[X.680])::
AsymmetricDecryptKeyIdentifier ::= OCTET STRING
If the server does not have a public key matching the identifier
specified by the client, the request MUST be terminated and an error
returned to the client. Distribution of the key specified by the
AsymmetricDecryptKeyIdentifier to the key generator and the client is
outside the scope of this document. If the key identified is bound
to an X.509 certificate, then the key MUST either explicitly support
keyTransport or keyAgreement or its use MUST be unrestricted.
4.4.2. Server-Side Key Generation Response
If the request is successful, the server response MUST have an HTTP
200 response code with a content-type of "multipart/mixed" consisting
of two parts: one part is the private key data and the other part is
the certificate data.
The format in which the private key data part is returned is
dependent on whether the private key is being returned with
additional encryption on top of that provided by TLS.
If additional encryption is not being employed, the private key data
MUST be placed in an "application/pkcs8". An "application/pkcs8"
part consists of the base64-encoded DER-encoded [X.690]
PrivateKeyInfo with a Content-Transfer-Encoding of "base64"
[RFC2045].
If additional encryption is being employed, the private key is placed
inside of a CMS SignedData. The SignedData is signed by the party
that generated the private key, which may or may not be the EST
server or the EST CA. The SignedData is further protected by placing
it inside of a CMS EnvelopedData, as described in Section 4 of
[RFC5958]. The following list shows how the EncryptedData is used,
depending on the type of protection key specified by the client.
Pritikin, et al. Standards Track [Page 33]
RFC 7030 EST October 2013
o If the client specified a symmetric encryption key to protect the
server-generated private key, the EnvelopedData content is
encrypted using the secret key identified in the request. The
EnvelopedData RecipientInfo field MUST indicate the key-encryption
kekri key management technique. The values are as follows:
version is set to 4, key-encryption key identifier (kekid) is set
to the value of the DecryptKeyIdentifier from Section 4.4.1.1;
keyEncryptionAlgorithm is set to one of the key wrap algorithms
that the client included in the SMIMECapabilities accompanying the
request; and encryptedKey is the encrypted key.
o If the client specified an asymmetric encryption key suitable for
key transport operations to protect the server-generated private
key, the EnvelopedData content is encrypted using a randomly
generated symmetric encryption key. The cryptographic strength of
the symmetric encryption key SHOULD be equivalent to the client-
specified asymmetric key. The EnvelopedData RecipientInfo field
MUST indicate the KeyTransRecipientInfo (ktri) key management
technique. In KeyTransRecipientInfo, the RecipientIdentifier
(rid) is either the subjectKeyIdentifier copied from the attribute
defined in Section 4.4.1.2 or the server determines an associated
issuerAndSerialNumber from the attribute; version is derived from
the choice of rid [RFC5652], keyEncryptionAlgorithm is set to one
of the key wrap algorithms that the client included in the
SMIMECapabilities accompanying the request, and encryptedKey is
the encrypted key.
o If the client specified an asymmetric encryption key suitable for
key agreement operations to protect the server-generated private
key, the EnvelopedData content is encrypted using a randomly
generated symmetric encryption key. The cryptographic strength of
the symmetric encryption key SHOULD be equivalent to the client-
specified asymmetric key. The EnvelopedData RecipientInfo field
MUST indicate the KeyAgreeRecipientInfo (kari) key management
technique. In the KeyAgreeRecipientInfo type, version,
originator, and user keying material (ukm) are as in [RFC5652],
and keyEncryptionAlgorithm is set to one of the key wrap
algorithms that the client included in the SMIMECapabilities
accompanying the request. The recipient's key identifier is
either copied from the attribute defined in Section 4.4.1.2 to
subjectKeyIdentifier or the server determines an
IssuerAndSerialNumber that corresponds to the value provided in
the attribute.
In all three additional encryption cases, the EnvelopedData is
returned in the response as an "application/pkcs7-mime" part with an
smime-type parameter of "server-generated-key" and a Content-
Transfer-Encoding of "base64".
Pritikin, et al. Standards Track [Page 34]
RFC 7030 EST October 2013
The certificate data part is an "application/pkcs7-mime" and exactly
matches the certificate response to /simpleenroll.
When rejecting a request, the server MUST specify either an HTTP 4xx
error or an HTTP 5xx error. If the content-type is not set, the
response data MUST be a plaintext human-readable error message.
4.5. CSR Attributes
CA policy may allow inclusion of client-provided attributes in
certificates that it issues, and some of these attributes may
describe information that is not available to the CA. In addition, a
CA may desire to certify a certain type of public key and a client
may not have a priori knowledge of that fact. Therefore, clients
SHOULD request a list of expected attributes that are required, or
desired, by the CA in an enrollment request or if dictated by local
policy.
The EST server SHOULD NOT require client authentication or
authorization to reply to this request.
Requesting CSR attributes is optional, but clients are advised that
CAs may refuse enrollment requests that are not encoded according to
the CA's policy.
4.5.1. CSR Attributes Request
The EST client requests a list of CA-desired CSR attributes from the
CA by sending an HTTPS GET message to the EST server with an
operations path of "/csrattrs".
4.5.2. CSR Attributes Response
If locally configured policy for an authenticated EST client
indicates a CSR Attributes Response is to be provided, the server
response MUST include an HTTP 200 response code. An HTTP response
code of 204 or 404 indicates that a CSR Attributes Response is not
available. Regardless of the response code, the EST server and CA
MAY reject any subsequent enrollment requests for any reason, e.g.,
incomplete CSR attributes in the request.
Pritikin, et al. Standards Track [Page 35]
RFC 7030 EST October 2013
Responses to attribute request messages MUST be encoded as the
content-type of "application/csrattrs" with a
Content-Transfer-Encoding of "base64" [RFC2045]. The syntax for
application/csrattrs body is as follows:
CsrAttrs ::= SEQUENCE SIZE (0..MAX) OF AttrOrOID
AttrOrOID ::= CHOICE (oid OBJECT IDENTIFIER, attribute Attribute }
Attribute { ATTRIBUTE:IOSet } ::= SEQUENCE {
type ATTRIBUTE.&id({IOSet}),
values SET SIZE(1..MAX) OF ATTRIBUTE.&Type({IOSet}{@type}) }
An EST server includes zero or more OIDs or attributes [RFC2986] that
it requests the client to use in the certification request. The
client MUST ignore any OID or attribute it does not recognize. When
the server encodes CSR Attributes as an empty SEQUENCE, it means that
the server has no specific additional information it desires in a
client certification request (this is functionally equivalent to an
HTTP response code of 204 or 404).
If the CA requires a particular crypto system or use of a particular
signature scheme (e.g., certification of a public key based on a
certain elliptic curve, or signing using a certain hash algorithm) it
MUST provide that information in the CSR Attribute Response. If an
EST server requires the linking of identity and POP information (see
Section 3.5), it MUST include the challengePassword OID in the CSR
Attributes Response.
The structure of the CSR Attributes Response SHOULD, to the greatest
extent possible, reflect the structure of the CSR it is requesting.
Requests to use a particular signature scheme (e.g. using a
particular hash function) are represented as an OID to be reflected
in the SignatureAlgorithm of the CSR. Requests to use a particular
crypto system (e.g., certification of a public key based on a certain
elliptic curve) are represented as an attribute, to be reflected as
the AlgorithmIdentifier of the SubjectPublicKeyInfo, with a type
indicating the algorithm and the values indicating the particular
parameters specific to the algorithm. Requests for descriptive
information from the client are made by an attribute, to be
represented as Attributes of the CSR, with a type indicating the
[RFC2985] extensionRequest and the values indicating the particular
attributes desired to be included in the resulting certificate's
extensions.
The sequence is Distinguished Encoding Rules (DER) encoded [X.690]
and then base64 encoded (Section 4 of [RFC4648]). The resulting text
forms the application/csrattr body, without headers.
Pritikin, et al. Standards Track [Page 36]
RFC 7030 EST October 2013
For example, if a CA requests a client to submit a certification
request containing the challengePassword (indicating that linking of
identity and POP information is requested; see Section 3.5), an
extensionRequest with the Media Access Control (MAC) address
([RFC2307]) of the client, and to use the secp384r1 elliptic curve
and to sign with the SHA384 hash function. Then, it takes the
following:
OID: challengePassword (1.2.840.113549.1.9.7)
Attribute: type = extensionRequest (1.2.840.113549.1.9.14)
value = macAddress (1.3.6.1.1.1.1.22)
Attribute: type = id-ecPublicKey (1.2.840.10045.2.1)
value = secp384r1 (1.3.132.0.34)
OID: ecdsaWithSHA384 (1.2.840.10045.4.3.3)
and encodes them into an ASN.1 SEQUENCE to produce:
30 41 06 09 2a 86 48 86 f7 0d 01 09 07 30 12 06 07 2a 86 48 ce 3d
02 01 31 07 06 05 2b 81 04 00 22 30 16 06 09 2a 86 48 86 f7 0d 01
09 0e 31 09 06 07 2b 06 01 01 01 01 16 06 08 2a 86 48 ce 3d 04 03
03
and then base64 encodes the resulting ASN.1 SEQUENCE to produce:
MEEGCSqGSIb3DQEJBzASBgcqhkjOPQIBMQcGBSuBBAAiMBYGCSqGSIb3DQEJDjEJ
BgcrBgEBAQEWBggqhkjOPQQDAw==
5. IANA Considerations
Section 4.4.1.2 defines an OID that has been registered in an arc
delegated by the IANA to the PKIX working group.
IANA has registered the following:
IANA updated the well-known URI registry with the following filled-in
template from [RFC5785].
URI suffix: est
Change controller: IETF
Pritikin, et al. Standards Track [Page 37]
RFC 7030 EST October 2013
IANA has updated the "Application Media Types" registry with the
following filled-in templates from [RFC6838].
The media subtype for CSR attributes in a CSR Attributes Response is
application/csrattrs.
Type name: application
Subtype name: csrattrs
Required parameters: None
Optional parameters: None
Encoding considerations: binary;
Security Considerations:
Clients request a list of attributes that servers wish to be in
certification requests. The request/response is normally done
in a TLS-protected tunnel.
Interoperability considerations: None
Published specification: This memo.
Applications which use this media type: Enrollment over Secure
Transport (EST)
Additional information:
Magic number(s): None
File extension: .csrattrs
Person & email address to contact for further information:
Dan Harkins <dharkins@arubanetworks.com>
Restrictions on usage: None
Author: Dan Harkins <dharkins@arubanetworks.com>
Intended usage: COMMON
Change controller: The IESG <iesg@ietf.org>
Pritikin, et al. Standards Track [Page 38]
RFC 7030 EST October 2013
The application/pkcs7-mime content-type defines the optional
"smime-type" parameter [RFC5751] with a set of specific values. This
document adds another value, "server-generated-key", as the parameter
value for Server-side Key Generation Response.
6. Security Considerations
Support for Basic authentication, as specified in HTTP [RFC2617],
allows the server access to a client's cleartext password. This
provides support for legacy username/password databases but requires
exposing the plaintext password to the EST server. Use of a PIN or
one-time password can help mitigate such exposure, but it is
RECOMMENDED that EST clients use such credentials only once to obtain
a client certificate (that will be used during future interactions
with the EST server).
When a client uses the Implicit TA database for certificate
validation (see Section 3), then authorization proceeds as specified
in Section 3.6.2. In this situation, the client has validated the
server as being a responder that is certified by a third party for
the URI configured, but it cannot verify that the responder is
authorized to act as an RA for the PKI in which the client is trying
to enroll. Clients using an Implicit Trust Anchor database are
RECOMMENDED to use only TLS-based client authentication (to prevent
exposing HTTP-based client authentication information). It is
RECOMMENDED that such clients include "Linking Identity and POP
Information" (Section 3.5) in requests (to prevent such requests from
being forwarded to a real EST server by a man in the middle). It is
RECOMMENDED that the Implicit Trust Anchor database used for EST
server authentication be carefully managed to reduce the chance of a
third-party CA with poor certification practices from being trusted.
Disabling the Implicit Trust Anchor database after successfully
receiving the Distribution of CA certificates response
(Section 4.1.3) limits any vulnerability to the first TLS exchange.
Certificate-less TLS cipher suites that maintain security and perform
the mutual authentication necessary for enrollment have the following
properties:
o the only information leaked by an active attack is whether or not
a single guess of the secret is correct.
o any advantage an adversary gains is through interaction and not
computation.
o it is possible to perform countermeasures, such as exponential
backoff after a certain number of failed attempts, to frustrate
repeated active attacks.
Pritikin, et al. Standards Track [Page 39]
RFC 7030 EST October 2013
Using a certificate-less cipher suite that does not have the
properties listed above would render the results of enrollment void
and potentially result in certificates being issued to
unauthenticated and/or unauthorized entities.
When using a certificate-less TLS cipher suite, the shared secret
used for authentication and authorization cannot be shared with an
entity that is not a party to the exchange: someone other than the
client and the server. Any additional sharing of secrets voids the
security afforded by a certificate-less cipher suite. Exposure of a
shared secret used by a certificate-less cipher suite to a third
party enables client impersonation that can result in corruption of a
client's trust anchor database.
TLS cipher suites that include "_EXPORT_" and "_DES_" in their names
MUST NOT be used. These ciphers do not offer a sufficient level of
protection; 40-bit crypto in 2013 doesn't offer acceptable
protection, and the use of DES is deprecated.
As described in CMC, Section 6.7 of [RFC5272], "For keys that can be
used as signature keys, signing the certification request with the
private key serves as a POP on that key pair". The inclusion of tls-
unique within the certification request links the proof-of-possession
to the TLS proof-of-identity by enforcing that the POP operation
occurred while the TLS session was active. This implies to the
server that the authenticated client currently has access to the
private key. If the authenticated client is known to have specific
capabilities, such as hardware protection for authentication
credentials and key storage, this implication is strengthened but not
proven.
The server-side key generation method allows keys to be transported
over the TLS connection to the client without any application-layer
protection. The distribution of private key material is inherently
risky. Private key distribution uses the encryption mode of the
negotiated TLS cipher suite. Keys are not protected by preferred key
wrapping methods such as AES Key Wrap [RFC3394] or as specified in
[RFC5958] as encryption of the private key beyond that provided by
TLS is optional. It is RECOMMENDED that EST servers not support this
operation by default. It is RECOMMENDED that clients not request
this service unless there is a compelling operational benefit. Use
of an Implicit Trust Anchor database is NOT RECOMMENDED when
server-side key generation is employed. The use of an encrypted CMS
Server-Side Key Generation Response is RECOMMENDED.
Regarding the CSR attributes that the CA may list for inclusion in an
enrollment request, there are no real inherent security issues with
the content being conveyed, but an adversary who is able to interpose
Pritikin, et al. Standards Track [Page 40]
RFC 7030 EST October 2013
herself into the conversation could exclude attributes that a server
may want, include attributes that a server may not want, and render
meaningless other attributes that a server may want.
ASN.1 encoding rules (e.g., DER and BER) have a type-length-value
structure, and it is easy to construct malicious content with invalid
length fields that can cause buffer overrun conditions. ASN.1
encoding rules allow for arbitrary levels of nesting, which may make
it possible to construct malicious content that will cause a stack
overflow. Interpreters of ASN.1 structures should be aware of these
issues and should take appropriate measures to guard against buffer
overflows and stack overruns in particular, and malicious content in
general.
7. References
7.1. Normative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP", RFC
2585, May 1999.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC2633] Ramsdell, B., "S/MIME Version 3 Message Specification",
RFC 2633, June 1999.
[RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification
Request Syntax Specification Version 1.7", RFC 2986,
November 2000.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
Pritikin, et al. Standards Track [Page 41]
RFC 7030 EST October 2013
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4108] Housley, R., "Using Cryptographic Message Syntax (CMS) to
Protect Firmware Packages", RFC 4108, August 2005.
[RFC4210] Adams, C., Farrell, S., Kause, T., and T. Mononen,
"Internet X.509 Public Key Infrastructure Certificate
Management Protocol (CMP)", RFC 4210, September 2005.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5272] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC)", RFC 5272, June 2008.
[RFC5273] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC): Transport Protocols", RFC 5273, June 2008.
[RFC5274] Schaad, J. and M. Myers, "Certificate Management Messages
over CMS (CMC): Compliance Requirements", RFC 5274, June
2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, February 2010.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
Pritikin, et al. Standards Track [Page 42]
RFC 7030 EST October 2013
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785, April
2010.
[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, July 2010.
[RFC5958] Turner, S., "Asymmetric Key Packages", RFC 5958, August
2010.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011.
[RFC6402] Schaad, J., "Certificate Management over CMS (CMC)
Updates", RFC 6402, November 2011.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, December
2011.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13, RFC
6838, January 2013.
[X.680] ITU-T Recommendation X.680 (2008) | ISO/IEC 8824-1:2008,
"Abstract Syntax Notation One (ASN.1): Specification of
basic notation", November 2008,
<http://www.itu.int/rec/T-REC-X.680-200811-I/en>.
[X.690] ITU-T Recommendation X.690 (2008) | ISO/IEC 8825-1:2008,
"ASN.1 encoding rules: Specification of Basic Encoding
Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER)", November 2008,
<http://www.itu.int/rec/T-REC-X.690-200811-I/en>.
7.2. Informative References
[IDevID] IEEE Standards Association, "IEEE 802.1AR Secure Device
Identifier", December 2009, <http://standards.ieee.org/
findstds/standard/802.1AR-2009.html>.
[RFC2307] Howard, L., "An Approach for Using LDAP as a Network
Information Service", RFC 2307, March 1998.
Pritikin, et al. Standards Track [Page 43]
RFC 7030 EST October 2013
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC2985] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
Classes and Attribute Types Version 2.0", RFC 2985,
November 2000.
[RFC3394] Schaad, J. and R. Housley, "Advanced Encryption Standard
(AES) Key Wrap Algorithm", RFC 3394, September 2002.
[RFC5054] Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
"Using the Secure Remote Password (SRP) Protocol for TLS
Authentication", RFC 5054, November 2007.
[RFC5967] Turner, S., "The application/pkcs10 Media Type", RFC 5967,
August 2010.
[RFC6403] Zieglar, L., Turner, S., and M. Peck, "Suite B Profile of
Certificate Management over CMS", RFC 6403, November 2011.
[SHS] National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", Federal Information Processing
Standard Publication 180-4, March 2012,
<http://csrc.nist.gov/publications/fips/
fips180-4/fips-180-4.pdf>.
[SP-800-57-Part-1]
National Institute of Standards and Technology,
"Recommendation for Key Management - Part 1: General
(Revision 3)", July 2012,
<http://csrc.nist.gov/publications/nistpubs/800-57/
sp800-57_part1_rev3_general.pdf>.
Pritikin, et al. Standards Track [Page 44]
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Appendix A. Operational Scenario Example Messages
(Informative)
This section expands on the Operational Scenario Overviews by
providing detailed examples of the messages at each TLS layer.
A.1. Obtaining CA Certificates
The following is an example of a valid /cacerts exchange.
During the initial TLS handshake, the client can ignore the optional
server-generated "certificate request" and can instead proceed with
the HTTP GET request:
GET /.well-known/est/cacerts HTTP/1.1
User-Agent: curl/7.22.0 (i686-pc-linux-gnu) libcurl/7.22.0 OpenS
SL/1.0.1 zlib/1.2.3.4 libidn/1.23 librtmp/2.3
Host: 192.0.2.1:8085
Accept: */*
In response, the server provides the current CA certificates:
HTTP/1.1 200 OK
Status: 200 OK
Content-Type: application/pkcs7-mime
Content-Transfer-Encoding: base64
Content-Length: 4246
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Pritikin, et al. Standards Track [Page 45]
RFC 7030 EST October 2013
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Pritikin, et al. Standards Track [Page 46]
RFC 7030 EST October 2013
A.2. CSR Attributes
The following is an example of a valid /csrattrs exchange. During
this exchange, the EST client authenticates itself using an existing
certificate issued by the CA for which the EST server provides
services.
The initial TLS handshake is identical to the enrollment example
handshake. The HTTP GET request:
GET /.well-known/est/csrattrs HTTP/1.1
User-Agent: curl/7.22.0 (i686-pc-linux-gnu) libcurl/7.22.0 OpenS
SL/1.0.1 zlib/1.2.3.4 libidn/1.23 librtmp/2.3
Host: 192.0.2.1:8085
Accept: */*
In response, the server provides suggested attributes that are
appropriate for the authenticated client. In this example, the EST
server also includes two example attributes that the client would
ignore unless the attribute type is known to the client:
HTTP/1.1 200 OK
Status: 200 OK
Content-Type: application/csrattrs
Content-Transfer-Encoding: base64
Content-Length: 171
MHwGBysGAQEBARYwIgYDiDcBMRsTGVBhcnNlIFNFVCBhcyAyLjk5OS4xIGRhdGEG
CSqGSIb3DQEJBzAsBgOINwIxJQYDiDcDBgOINwQTGVBhcnNlIFNFVCBhcyAyLjk5
OS4yIGRhdGEGCSskAwMCCAEBCwYJYIZIAWUDBAIC
A.3. Enroll/Re-enroll
The following is an example of a valid /simpleenroll exchange. The
data messages for /simplereenroll are similar.
During this exchange, the EST client uses an out-of-band distributed
username/password to authenticate itself to the EST server. This is
the normal HTTP WWW-Authenticate behavior and is included here for
informative purposes. When an existing TLS client certificate is
used, the server might skip requesting the HTTP WWW-Authenticate
header, such as during a /simplereenroll operation.
During the initial TLS handshake, the client can ignore the optional
server-generated "certificate request" and can instead proceed with
the HTTP POST request. In response to the initial HTTP POST attempt,
Pritikin, et al. Standards Track [Page 47]
RFC 7030 EST October 2013
the server requests WWW-Authenticate from the client (this might
occur even if the client used a client certificate, as detailed in
the normative text above):
HTTP/1.1 401 Unauthorized
Content-Length: 0
WWW-Authenticate: Digest qop="auth", realm="estrealm",
nonce="1368141352"
In the subsequent HTTP POST, the username/password is included, along
with the complete application/pkcs10 content:
POST /.well-known/est/simpleenroll HTTP/1.1
Authorization: Digest username="estuser", realm="estrealm", nonc
e="1368141352", uri="/.well-known/est/simpleenroll", cnonce="M
TYwMzg3", nc=00000001, qop="auth", response="144cc27f96046f1d70e
b16db20f75f22"
Host: 192.0.2.1:8085
Accept: */*
Content-Type: application/pkcs10
Content-Transfer-Encoding: base64
Content-Length: 882
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PyMFcM15E9gtUVee5C62bVwuk/tbnGsbwQ==
Pritikin, et al. Standards Track [Page 48]
RFC 7030 EST October 2013
The EST server uses the username/password to perform
authentication/authorization and responds with the issued
certificate:
HTTP/1.1 200 OK
Status: 200 OK
Content-Type: application/pkcs7-mime; smime-type=certs-only
Content-Transfer-Encoding: base64
Content-Length: 1122
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R4POrT2xz8ChADEA
Pritikin, et al. Standards Track [Page 49]
RFC 7030 EST October 2013
A.4. Server Key Generation
The following is an example of a valid /serverkeygen exchange.
During this exchange, the EST client authenticates itself using an
existing certificate issued by the CA the EST server provides
services for.
The initial TLS handshake is identical to the enrollment example
handshake. An example HTTP POSTed message is:
POST /.well-known/est/serverkeygen HTTP/1.1
Host: 192.0.2.1:8085
Accept: */*
Expect: 100-continue
Content-Type: application/pkcs10
Content-Transfer-Encoding: base64
Content-Length: 963
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M/965hWIw/5dshY/wQjIfYs07bbq2ERbpJiw9bAQY34gyoVmEQ==
Pritikin, et al. Standards Track [Page 50]
RFC 7030 EST October 2013
Because the DecryptKeyIdentifier attribute is not included in this
request, the response does not include additional encryption beyond
the TLS session. The EST server response is:
HTTP/1.1 200 OK
Status: 200 OK
Content-Type: multipart/mixed ; boundary=estServerExampleBoundary
Content-Length: 3219
This is the preamble. It is to be ignored, though it
is a handy place for estServer to include an explanatory note,
including contact or support information.
--estServerExampleBoundary
Content-Type: application/pkcs8
Content-Transfer-Encoding: base64
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pM7PYH/x4BiBmgQ3bhOqTp4H
--estServerExampleBoundary
Content-Type: application/pkcs7-mime; smime-type=certs-only
Content-Transfer-Encoding: base64
Pritikin, et al. Standards Track [Page 51]
RFC 7030 EST October 2013
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--estServerExampleBoundary--
This is the epilogue. It is also to be ignored.
Appendix B. Contributors and Acknowledgements
The editors would like to thank Stephen Kent, Vinod Arjun, Jan
Vilhuber, Sean Turner, Russ Housley, and others for their feedback
and prototypes of early versions of this document. Our thanks also
go the authors of [RFC6403], around whose document we structured part
of this specification.
Pritikin, et al. Standards Track [Page 52]
RFC 7030 EST October 2013
Authors' Addresses
Max Pritikin (editor)
Cisco Systems, Inc.
510 McCarthy Drive
Milpitas, CA 95035
USA
EMail: pritikin@cisco.com
Peter E. Yee (editor)
AKAYLA, Inc.
7150 Moorland Drive
Clarksville, MD 21029
USA
EMail: peter@akayla.com
Dan Harkins (editor)
Aruba Networks
1322 Crossman Avenue
Sunnyvale, CA 94089-1113
USA
EMail: dharkins@arubanetworks.com
Pritikin, et al. Standards Track [Page 53]