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Keywords: S/MIME, secure/multipurpose, internet, mail, extensions
Network Working Group T. Dean
Request for Comments: 3183 W. Ottaway
Category: Experimental QinetiQ
October 2001
Domain Security Services using S/MIME
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document describes how the S/MIME (Secure/Multipurpose Internet
Mail Extensions) protocol can be processed and generated by a number
of components of a communication system, such as message transfer
agents, guards and gateways to deliver security services. These
services are collectively referred to as 'Domain Security Services'.
Acknowledgements
Significant comments were made by Luis Barriga, Greg Colla, Trevor
Freeman, Russ Housley, Dave Kemp, Jim Schaad and Michael Zolotarev.
1. Introduction
The S/MIME [1] series of standards define a data encapsulation format
for the provision of a number of security services including data
integrity, confidentiality, and authentication. S/MIME is designed
for use by messaging clients to deliver security services to
distributed messaging applications.
The mechanisms described in this document are designed to solve a
number of interoperability problems and technical limitations that
arise when different security domains wish to communicate securely,
for example when two domains use incompatible messaging technologies
such as the X.400 series and SMTP/MIME, or when a single domain
wishes to communicate securely with one of its members residing on an
untrusted domain. The scenarios covered by this document are
domain-to-domain, individual-to-domain and domain-to-individual
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communications. This document is also applicable to organizations
and enterprises that have internal PKIs which are not accessible by
the outside world, but wish to interoperate securely using the S/MIME
protocol.
There are many circumstances when it is not desirable or practical to
provide end-to-end (desktop-to-desktop) security services,
particularly between different security domains. An organization
that is considering providing end-to-end security services will
typically have to deal with some if not all of the following issues:
1) Heterogeneous message access methods: Users are accessing mail
using mechanisms which re-format messages, such as using Web
browsers. Message reformatting in the Message Store makes end-
to-end encryption and signature validation impossible.
2) Message screening and audit: Server-based mechanisms such as
searching for prohibited words or other content, virus scanning,
and audit, are incompatible with end-to-end encryption.
3) PKI deployment issues: There may not be any certificate paths
between two organizations. Or an organization may be sensitive
about aspects of its PKI and unwilling to expose them to outside
access. Also, full PKI deployment for all employees, may be
expensive, not necessary or impractical for large organizations.
For any of these reasons, direct end-to-end signature validation
and encryption are impossible.
4) Heterogeneous message formats: One organization using X.400 series
protocols wishes to communicate with another using SMTP. Message
reformatting at gateways makes end-to-end encryption and signature
validation impossible.
This document describes an approach to solving these problems by
providing message security services at the level of a domain or an
organization. This document specifies how these 'domain security
services' can be provided using the S/MIME protocol. Domain security
services may replace or complement mechanisms at the desktop. For
example, a domain may decide to provide desktop-to-desktop signatures
but domain-to-domain encryption services. Or it may allow desktop-
to-desktop services for intra-domain use, but enforce domain-based
services for communication with other domains.
Domain services can also be used by individual members of a
corporation who are geographically remote and who wish to exchange
encrypted and/or signed messages with their base.
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Whether or not a domain based service is inherently better or worse
than desktop based solutions is an open question. Some experts
believe that only end-to-end solutions can be truly made secure,
while others believe that the benefits offered by such things as
content checking at domain boundaries offers considerable increase in
practical security for many real systems. The additional service of
allowing signature checking at several points on a communications
path is also an extra benefit in many situations. This debate is
outside the scope of this document. What is offered here is a set of
tools that integrators can tailor in different ways to meet different
needs in different circumstances.
Message transfer agents (MTAs), guards, firewalls and protocol
translation gateways all provide domain security services. As with
desktop based solutions, these components must be resilient against a
wide variety of attacks intended to subvert the security services.
Therefore, careful consideration should be given to security of these
components, to make sure that their siting and configuration
minimises the possibility of attack.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [2].
2. Overview of Domain Security Services
This section gives an informal overview of the security services that
are provided by S/MIME between different security domains. These
services are provided by a combination of mechanisms in the sender's
and recipient's domains.
Later sections describe definitively how these services map onto
elements of the S/MIME protocol.
The following security mechanisms are specified in this document:
1. Domain signature
2. Review signature
3. Additional attributes signature
4. Domain encryption and decryption
The signature types defined in this document are referred to as
DOMSEC defined signatures.
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The term 'security domain' as used in this document is defined as a
collection of hardware and personnel operating under a single
security authority and performing a common business function.
Members of a security domain will of necessity share a high degree of
mutual trust, due to their shared aims and objectives.
A security domain is typically protected from direct outside attack
by physical measures and from indirect (electronic) attack by a
combination of firewalls and guards at network boundaries. The
interface between two security domains is termed a 'security
boundary'. One example of a security domain is an organizational
network ('Intranet').
2.1 Domain Signature
A domain signature is an S/MIME signature generated on behalf of a
set of users in a domain. A domain signature can be used to
authenticate information sent between domains or between a certain
domain and one of its individuals, for example, when two 'Intranets'
are connected using the Internet, or when an Intranet is connected to
a remote user over the Internet. It can be used when two domains
employ incompatible signature schemes internally or when there are no
certification links between their PKIs. In both cases messages from
the originator's domain are signed over the original message and
signature (if present) using an algorithm, key, and certificate which
can be processed by the recipient(s) or the recipient(s) domain. A
domain signature is sometimes referred to as an "organizational
signature".
2.2 Review Signature
A third party may review messages before they are forwarded to the
final recipient(s) who may be in the same or a different security
domain. Organizational policy and good security practice often
require that messages be reviewed before they are released to
external recipients. Having reviewed a message, an S/MIME signature
is added to it - a review signature. An agent could check the review
signature at the domain boundary, to ensure that only reviewed
messages are released.
2.3 Additional Attributes Signature
A third party can add additional attributes to a signed message. An
S/MIME signature is used for this purpose - an additional attributes
signature. An example of an additional attribute is the 'Equivalent
Label' attribute defined in ESS [3].
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2.4 Domain Encryption and Decryption
Domain encryption is S/MIME encryption performed on behalf of a
collection of users in a domain. Domain encryption can be used to
protect information between domains, for example, when two
'Intranets' are connected using the Internet. It can also be used
when end users do not have PKI/encryption capabilities at the
desktop, or when two domains employ incompatible encryption schemes
internally. In the latter case messages from the originator's domain
are encrypted (or re-encrypted) using an algorithm, key, and
certificate which can be decrypted by the recipient(s) or an entity
in their domain. This scheme also applies to protecting information
between a single domain and one of its members when both are
connected using an untrusted network, e.g., the Internet.
3. Mapping of the Signature Services to the S/MIME Protocol
This section describes the S/MIME protocol elements that are used to
provide the security services described above. ESS [3] introduces
the concept of triple-wrapped messages that are first signed, then
encrypted, then signed again. This document also uses this concept
of triple-wrapping. In addition, this document also uses the concept
of 'signature encapsulation'. 'Signature encapsulation' denotes a
signed or unsigned message that is wrapped in a signature, this
signature covering both the content and the first (inner) signature,
if present.
Signature encapsulation MAY be performed on the inner and/or the
outer signature of a triple-wrapped message.
For example, the originator signs a message which is then
encapsulated with an 'additional attributes' signature. This is then
encrypted. A reviewer then signs this encrypted data, which is then
encapsulated by a domain signature.
There is a possibility that some policies will require signatures to
be added in a specific order. By only allowing signatures to be
added by encapsulation it is possible to determine the order in which
the signatures have been added.
A DOMSEC defined signature MAY encapsulate a message in one of the
following ways:
1) An unsigned message has an empty signature layer added to it
(i.e., the message is wrapped in a signedData that has a
signerInfos which contains no elements). This is to enable
backward compatibility with S/MIME software that does not have a
DOMSEC capability. Since the signerInfos will contain no signers
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the eContentType, within the EncapsulatedContentInfo, MUST be id-
data as described in CMS [5]. However, the eContent field will
contain the unsigned message instead of being left empty as
suggested in section 5.2 in CMS [5]. This is so that when the
DOMSEC defined signature is added, as defined in method 2) below,
the signature will cover the unsigned message.
2) Signature Encapsulation is used to wrap the original signed
message with a DOMSEC defined signature. This is so that the
DOMSEC defined signature covers the message and all the previously
added signatures. Also, it is possible to determine that the
DOMSEC defined signature was added after the signatures that are
already there.
3.1 Naming Conventions and Signature Types
An entity receiving an S/MIME signed message would normally expect
the signature to be that of the originator of the message. However,
the message security services defined in this document require the
recipient to be able to accept messages signed by other entities
and/or the originator. When other entities sign the message the name
in the certificate will not match the message sender's name. An
S/MIME compliant implementation would normally flag a warning if
there were a mismatch between the name in the certificate and the
message sender's name. (This check prevents a number of types of
masquerade attack.)
In the case of domain security services, this warning condition
SHOULD be suppressed under certain circumstances. These
circumstances are defined by a naming convention that specifies the
form that the signers name SHOULD adhere to. Adherence to this
naming convention avoids the problems of uncontrolled naming and the
possible masquerade attacks that this would produce.
As an assistance to implementation, a signed attribute is defined to
be included in the S/MIME signature - the 'signature type' attribute.
On receiving a message containing this attribute, the naming
convention checks are invoked.
Implementations conforming to this standard MUST support the naming
convention for signature generation and verification.
Implementations conforming to this standard MUST recognize the
signature type attribute for signature verification. Implementations
conforming to this standard MUST support the signature type attribute
for signature generation.
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3.1.1 Naming Conventions
The following naming conventions are specified for agents generating
signatures specified in this document:
* For a domain signature, an agent generating this signature MUST be
named 'domain-signing-authority'
* For a review signature, an agent generating this signature MUST be
named 'review-authority'.
* For an additional attributes signature, an agent generating this
signature MUST be named 'attribute-authority'.
This name shall appear as the 'common name (CN)' component of the
subject field in the X.509 certificate. There MUST be only one CN
component present. Additionally, if the certificate contains an RFC
822 address, this name shall appear in the end entity component of
the address - on the left-hand side of the '@' symbol.
In the case of a domain signature, an additional naming rule is
defined: the 'name mapping rule'. The name mapping rule states that
for a domain signing authority, the domain part of its name MUST be
the same as, or an ascendant of, the domain name of the message
originator(s) that it is representing. The domain part is defined as
follows:
* In the case of an X.500 distinguished subject name of an X.509
certificate, the domain part is the country, organization,
organizational unit, state, and locality components of the
distinguished name.
* In the case of an RFC 2247 distinguished name, the domain part is
the domain components of the distinguished name.
* If the certificate contains an RFC 822 address, the domain part is
defined to be the RFC 822 address component on the right-hand side
of the '@' symbol.
For example, a domain signing authority acting on behalf of John Doe
of the Acme corporation, whose distinguished name is 'cn=John Doe,
ou=marketing,o=acme,c=us' and whose e-mail address is
John.Doe@marketing.acme.com, could have a certificate containing a
distinguished name of
'cn=domain-signing-authority,o=acme,c=us' and an RFC 822 address of
'domain-signing-authority@acme.com'. If John Doe has an RFC 2247
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defined address of 'cn=John Doe,dc=marketing,dc=acme,dc=us' then an
address of 'cn=domain-signing-authority,dc=acme,dc=us' could be used
to represent the domain signing authority.
When the X.500 distinguished subject name has consecutive
organizational units and/or localities it is important to understand
the ordering of these values in order to determine if the domain part
of the domain signature is an ascendant. In this case, when parsing
the distinguished subject name from the most significant component
(i.e., country, locality or organization) the parsed organizational
unit or locality is deemed to be the ascendant of consecutive
(unparsed) organizational units or localities.
When parsing an RFC 2247 subject name from the most significant
component (i.e., the 'dc' entry that represents the country, locality
or organization) the parsed 'dc' entry is deemed to be the ascendant
of consecutive (unparsed) 'dc' entries.
For example, a domain signing authority acting on behalf of John Doe
of the Acme corporation, whose distinguished name is 'cn=John Doe,
ou=marketing,ou=defence,o=acme,c=us' and whose e-mail address is
John.Doe@marketing.defence.acme.com, could have a certificate
containing a distinguished name of 'cn=domain-signing-
authority,ou=defence,o=acme,c=us' and an RFC 822 address of 'domain-
signing-authority@defence.acme.com'. If John Doe has an RFC 2247
defined address of 'cn=John
Doe,dc=marketing,dc=defense,dc=acme,dc=us' then the domain signing
authority could have a distinguished name of 'cn=domain-signing-
authority,dc=defence,dc=acme,dc=us'.
Any message received where the domain part of the domain signing
agent's name does not match, or is not an ascendant of, the
originator's domain name MUST be flagged.
This naming rule prevents agents from one organization masquerading
as domain signing authorities on behalf of another. For the other
types of signature defined in this document, no such named mapping
rule is defined.
Implementations conforming to this standard MUST support this name
mapping convention as a minimum. Implementations MAY choose to
supplement this convention with other locally defined conventions.
However, these MUST be agreed between sender and recipient domains
prior to secure exchange of messages.
On verifying the signature, a receiving agent MUST ensure that the
naming convention has been adhered to. Any message that violates the
convention MUST be flagged.
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3.1.2 Signature Type Attribute
An S/MIME signed attribute is used to indicate the type of signature.
This should be used in conjunction with the naming conventions
specified in the previous section. When an S/MIME signed message
containing the signature type attribute is received it triggers the
software to verify that the correct naming convention has been used.
The ASN.1 [4] notation of this attribute is: -
SignatureType ::= SEQUENCE OF OBJECT IDENTIFIER
id-sti OBJECT IDENTIFIER ::= {iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) 9 }
-- signature type identifier
If present, the SignatureType attribute MUST be a signed attribute,
as defined in [5]. If the SignatureType attribute is absent and
there are no further encapsulated signatures the recipient SHOULD
assume that the signature is that of the message originator.
All of the signatures defined here are generated and processed as
described in [5]. They are distinguished by the presence of the
following values in the SignatureType signed attribute:
id-sti-domainSig OBJECT IDENTIFIER ::= { id-sti 2 }
-- domain signature.
id-sti-addAttribSig OBJECT IDENTIFIER ::= { id-sti 3 }
-- additional attributes signature.
id-sti-reviewSig OBJECT IDENTIFIER ::= { id-sti 4 }
-- review signature.
For completeness, an attribute type is also specified for an
originator signature. However, this signature type is optional. It
is defined as follows:
id-sti-originatorSig OBJECT IDENTIFIER ::= { id-sti 1 }
-- originator's signature.
All signature types, except the originator type, MUST encapsulate
other signatures. Note a DOMSEC defined signature could be
encapsulating an empty signature as defined in section 3.
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A SignerInfo MUST NOT include multiple instances of SignatureType. A
signed attribute representing a SignatureType MAY include multiple
instances of different SignatureType values as an AttributeValue of
attrValues [5], as long as the SignatureType 'additional attributes'
is not present.
If there is more than one SignerInfo in a signerInfos (i.e., when
different algorithms are used) then the SignatureType attribute in
all the SignerInfos MUST contain the same content.
The following sections describe the conditions under which each of
these types of signature may be generated, and how they are
processed.
3.2 Domain Signature Generation and Verification
A 'domain signature' is a proxy signature generated on a user's
behalf in the user's domain. The signature MUST adhere to the naming
conventions in 3.1.1, including the name mapping convention. A
'domain signature' on a message authenticates the fact that the
message has been released from that domain. Before signing, a
process generating a 'domain signature' MUST first satisfy itself of
the authenticity of the message originator. This is achieved by one
of two methods. Either the 'originator's signature' is checked, if
S/MIME signatures are used inside a domain. Or if not, some
mechanism external to S/MIME is used, such as the physical address of
the originating client or an authenticated IP link.
If the originator's authenticity is successfully verified by one of
the above methods and all other signatures present are valid,
including those that have been encrypted, a 'domain signature' can be
added to a message.
If a 'domain signature' is added and the message is received by a
Mail List Agent (MLA) there is a possibility that the 'domain
signature' will be removed. To stop the 'domain signature' from
being removed the steps in section 5 MUST be followed.
An entity generating a domain signature MUST do so using a
certificate containing a subject name that follows the naming
convention specified in 3.1.1.
If the originator's authenticity is not successfully verified or all
the signatures present are not valid, a 'domain signature' MUST NOT
be generated.
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On reception, the 'domain signature' SHOULD be used to verify the
authenticity of a message. A check MUST be made to ensure that both
the naming convention and the name mapping convention have been used
as specified in this standard.
A recipient can assume that successful verification of the domain
signature also authenticates the message originator.
If there is an originator signature present, the name in that
certificate SHOULD be used to identify the originator. This
information can then be displayed to the recipient.
If there is no originator signature present, the only assumption that
can be made is the domain the message originated from.
A domain signer can be assumed to have verified any signatures that
it encapsulates. Therefore, it is not necessary to verify these
signatures before treating the message as authentic. However, this
standard does not preclude a recipient from attempting to verify any
other signatures that are present.
The 'domain signature' is indicated by the presence of the value id-
sti-domainSig in a 'signature type' signed attribute.
There MAY be one or more 'domain signature' signatures in an S/MIME
encoding.
3.3 Additional Attributes Signature Generation and Verification
The 'additional attributes' signature type indicates that the
SignerInfo contains additional attributes that are associated with
the message.
All attributes in the applicable SignerInfo MUST be treated as
additional attributes. Successful verification of an 'additional
attributes' signature means only that the attributes are
authentically bound to the message. A recipient MUST NOT assume that
its successful verification also authenticates the message
originator.
An entity generating an 'additional attributes' signature MUST do so
using a certificate containing a subject name that follows the naming
convention specified in 3.1.1. On reception, a check MUST be made to
ensure that the naming convention has been used.
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A signer MAY include any of the attributes listed in [3] or in this
document when generating an 'additional attributes' signature. The
following attributes have a special meaning, when present in an
'additional attributes' signature:
1) Equivalent Label: label values in this attribute are to be treated
as equivalent to the security label contained in an encapsulated
SignerInfo, if present.
2) Security Label: the label value indicates the aggregate
sensitivity of the inner message content plus any encapsulated
signedData and envelopedData containers. The label on the
original data is indicated by the value in the originator's
signature, if present.
An 'additional attributes' signature is indicated by the presence of
the value id-sti-addAttribSig in a 'signature type' signed attribute.
Other Object Identifiers MUST NOT be included in the sequence of OIDs
if this value is present.
There MAY be multiple 'additional attributes' signatures in an S/MIME
encoding.
3.4 Review Signature Generation and Verification
The review signature indicates that the signer has reviewed the
message. Successful verification of a review signature means only
that the signer has approved the message for onward transmission to
the recipient(s). When the recipient is in another domain, a device
on a domain boundary such as a Mail Guard or firewall may be
configured to check review signatures. A recipient MUST NOT assume
that its successful verification also authenticates the message
originator.
An entity generating a signed review signature MUST do so using a
certificate containing a subject name that follows the naming
convention specified in 3.1.1. On reception, a check MUST be made to
ensure that the naming convention has been used.
A review signature is indicated by the presence of the value id-sti-
reviewSig in a 'signature type' signed attribute.
There MAY be multiple review signatures in an S/MIME encoding.
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3.5 Originator Signature
The 'originator signature' is used to indicate that the signer is the
originator of the message and its contents. It is included in this
document for completeness only. An originator signature is indicated
either by the absence of the signature type attribute, or by the
presence of the value id-sti-originatorSig in a 'signature type'
signed attribute.
4. Encryption and Decryption
Message encryption may be performed by a third party on behalf of a
set of originators in a domain. This is referred to as domain
encryption. Message decryption may be performed by a third party on
behalf of a set of recipients in a domain. This is referred to as
domain decryption. The third party that performs these processes is
referred to in this section as a "Domain Confidentiality Authority"
(DCA). Both of these processes are described in this section.
Messages may be encrypted for decryption by the final recipient
and/or by a DCA in the recipient's domain. The message may also be
encrypted for decryption by a DCA in the originator's domain (e.g.,
for content analysis, audit, key word scanning, etc.). The choice of
which of these is actually performed is a system specific issue that
depends on system security policy. It is therefore outside the scope
of this document. These processes of encryption and decryption
processes are shown in the following table.
--------------------------------------------------------------------
| | Recipient Decryption | Domain Decryption |
|------------------------|----------------------|--------------------|
| Originator Encryption | Case(a) | Case(b) |
| Domain Encryption | Case(c) | Case(d) |
--------------------------------------------------------------------
Case (a), encryption of messages by the originator for decryption by
the final recipient(s), is described in CMS [5]. In cases (c) and
(d), encryption is performed not by the originator but by the DCA in
the originator's domain. In cases (b) and (d), decryption is
performed not by the recipient(s) but by the DCA in the recipient's
domain.
A client implementation that conforms to this standard MUST support
case (b) for transmission, case (c) for reception and case (a) for
transmission and reception.
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A DCA implementation that conforms to this standard MUST support
cases (c) and (d), for transmission, and cases (b) and (d) for
reception. In cases (c) and (d) the 'domain signature' SHOULD be
applied before the encryption. In cases (b) and (d) the message
SHOULD be decrypted before the originators 'domain signature' is
obtained and verified.
The process of encryption and decryption is documented in CMS [5].
The only additional requirement introduced by domain encryption and
decryption is for greater flexibility in the management of keys, as
described in the following subsections. As with signatures, a naming
convention and name mapping convention are used to locate the correct
public key.
The mechanisms described below are applicable both to key agreement
and key transport systems, as documented in CMS [5]. The phrase
'encryption key' is used as a collective term to cover the key
management keys used by both techniques.
The mechanisms below are also applicable to individual roving users
who wish to encrypt messages that are sent back to base.
4.1 Domain Confidentiality Naming Conventions
A DCA MUST be named 'domain-confidentiality-authority'. This name
MUST appear in the 'common name(CN)' component of the subject field
in the X.509 certificate. Additionally, if the certificate contains
an RFC 822 address, this name MUST appear in the end entity part of
the address, i.e., on the left-hand side of the '@' symbol.
Along with this naming convention, an additional naming rule is
defined: the 'name mapping rule'. The name mapping rule states that
for a DCA, the domain part of its name MUST be the same as, or an
ascendant of (as defined in section 3.1.1), the domain name of the
set of entities that it represents. The domain part is defined as
follows:
* In the case of an X.500 distinguished name of an X.509
certificate, the domain part is the country, organization,
organizational unit, state, and locality components of the
distinguished name.
* In the case of an RFC 2247 distinguished name, the domain part is
the domain components of the distinguished name.
* If the certificate contains an RFC 822 address, the domain part is
defined to be the RFC 822 address part on the right-hand side of
the '@' symbol.
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For example, a DCA acting on behalf of John Doe of the Acme
corporation, whose distinguished name is 'cn=John Doe,ou=marketing,
o=acme,c=us' and whose e-mail address is John.Doe@marketing.acme.com,
could have a certificate containing a distinguished name of
'cn=domain-confidentiality-authority,o=acme,c=us' and an e-mail
address of 'domain-confidentiality-authority@acme.com'. If John Doe
has an RFC 2247 defined address of 'cn=John Doe,dc=marketing,
dc=defense,dc=acme,dc=us' then the domain signing authority could
have a distinguished name of
'cn=domain-signing-authority,dc=defence,dc=acme,dc=us'. The key
associated with this certificate would be used for encrypting
messages for John Doe.
Any message received where the domain part of the domain encrypting
agents name does not match, or is not an ascendant of, the domain
name of the entities it represents MUST be flagged.
This naming rule prevents messages being encrypted for the wrong
domain decryption agent.
Implementations conforming to this standard MUST support this name
mapping convention as a minimum. Implementations may choose to
supplement this convention with other locally defined conventions.
However, these MUST be agreed between sender and recipient domains
prior to sending any messages.
4.2 Key Management for DCA Encryption
At the sender's domain, DCA encryption is achieved using the
recipient DCA's certificate or the end recipient's certificate. For
this, the encrypting process must be able to correctly locate the
certificate for the corresponding DCA in the recipient's domain or
the one corresponding to the end recipient. Having located the
correct certificate, the encryption process is then performed and
additional information required for decryption is conveyed to the
recipient in the recipientInfo field as specified in CMS [5]. A DCA
encryption agent MUST be named according to the naming convention
specified in section 4.1. This is so that the corresponding
certificate can be found.
No specific method for locating the certificate to the corresponding
DCA in the recipient's domain or the one corresponding to the end
recipient is mandated in this document. An implementation may choose
to access a local certificate store to locate the correct
certificate. Alternatively, a X.500 or LDAP directory may be used in
one of the following ways:
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1. The directory may store the DCA certificate in the recipient's
directory entry. When the user certificate attribute is
requested, this certificate is returned.
2. The encrypting agent maps the recipient's name to the DCA name in
the manner specified in 4.1. The user certificate attribute
associated with this directory entry is then obtained.
This document does not mandate either of these processes. Whichever
one is used, the name mapping conventions must be adhered to, in
order to maintain confidentiality.
Having located the correct certificate, the encryption process is
then performed. A recipientInfo for the DCA or end recipient is then
generated, as described in CMS [5].
DCA encryption may be performed for decryption by the end recipient
and/or by a DCA. End recipient decryption is described in CMS [5].
DCA decryption is described in section 4.3.
4.3 Key Management for DCA Decryption
DCA decryption uses a private-key belonging to the DCA and the
necessary information conveyed in the DCA's recipientInfo field.
It should be noted that domain decryption can be performed on
messages encrypted by the originator and/or by a DCA in the
originator's domain. In the first case, the encryption process is
described in CMS [5]; in the second case, the encryption process is
described in 4.2.
5. Applying a Domain Signature when Mail List Agents are Present.
It is possible that a message leaving a DOMSEC domain may encounter a
Mail List Agent (MLA) before it reaches the final recipient. There
is a possibility that this would result in the 'domain signature'
being stripped off the message. We do not want a MLA to remove the
'domain signature'. Therefore, the 'domain signature' must be
applied to the message in such a way that will prevent a MLA from
removing it.
A MLA will search a message for the "outer" signedData layer, as
defined in ESS [3] section 4.2, and strip off all signedData layers
that encapsulate this "outer" signedData layer. Where this "outer"
signedData layer is found will depend on whether the message contains
a mlExpansionHistory attribute or an envelopedData layer.
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There is a possibility that a message leaving a DOMSEC domain has
already been processed by a MLA, in which case a 'mlExpansionHistory'
attribute will be present within the message.
There is a possibility that the message will contain an envelopedData
layer. This will be the case when the message has been encrypted
within the domain for the domain's "Domain Confidentiality
Authority", see section 4.0, and, possibly, the final recipient.
How the 'domain signature' is applied will depend on what is already
present within the message. Before the 'domain signature' can be
applied the message MUST be searched for the "outer" signedData
layer, this search is complete when one of the following is found: -
- The "outer" signedData layer that includes an
mlExpansionHistory attribute or encapsulates an envelopedData
object.
- An envelopedData layer.
- The original content (that is, a layer that is neither
envelopedData nor signedData).
If a signedData layer containing a mlExpansionHistory attribute has
been found then: -
1) Strip off the signedData layer (after remembering the included
signedAttributes).
2) Search the rest of the message until an envelopedData layer or
the original content is found.
3) a) If an envelopedData layer has been found then: -
- Strip off all the signedData layers down to the
envelopedData layer.
- Locate the RecipientInfo for the local DCA and use the
information it contains to obtain the message key.
- Decrypt the encryptedContent using the message key.
- Encapsulate the decrypted message with a 'domain
signature'
- If local policy requires the message to be encrypted
using S/MIME encryption before leaving the domain then
encapsulate the 'domain signature' with an envelopedData
layer containing RecipientInfo structures for each of the
recipients and an originatorInfo value built from
information describing this DCA.
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If local policy does not require the message to be
encrypted using S/MIME encryption but there is an
envelopedData at a lower level within the message then
the 'domain signature' MUST be encapsulated by an
envelopedData as described above.
An example when it may not be local policy to require
S/MIME encryption is when there is a link crypto present.
b) If an envelopedData layer has not been found then: -
- Encapsulate the new message with a 'domain signature'.
4) Encapsulate the new message in a signedData layer, adding the
signedAttributes from the signedData layer that contained the
mlExpansionHistory attribute.
If no signedData layer containing a mlExpansionHistory attribute has
been found but an envelopedData has been found then: -
1) Strip off all the signedData layers down to the envelopedData
layer.
2) Locate the RecipientInfo for the local DCA and use the
information it contains to obtain the message key.
3) Decrypt the encryptedContent using the message key.
4) Encapsulate the decrypted message with a 'domain signature'
5) If local policy requires the message to be encrypted before
leaving the domain then encapsulate the 'domain signature' with
an envelopedData layer containing RecipientInfo structures for
each of the recipients and an originatorInfo value built from
information describing this DCA.
If local policy does not require the message to be encrypted
using S/MIME encryption but there is an envelopedData at a
lower level within the message then the 'domain signature' MUST
be encapsulated by an envelopedData as described above.
If no signedData layer containing a mlExpansionHistory attribute has
been found and no envelopedData has been found then: -
1) Encapsulate the message in a 'domain signature'.
5.1 Examples of Rule Processing
The following examples help explain the above rules. All of the
signedData objects are valid and none of them are a domain signature.
If a signedData object was a domain signature then it would not be
necessary to validate any further signedData objects.
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1) A message (S1 (Original Content)) (where S = signedData) in which
the signedData does not include an mlExpansionHistory attribute is
to have a 'domain signature' applied. The signedData, S1, is
verified. No "outer" signedData is found, after searching for one
as defined above, since the original content is found, nor is an
envelopedData or a mlExpansionHistory attribute found. A new
signedData layer, S2, is created that contains a 'domain
signature', resulting in the following message sent out of the
domain (S2 (S1 (Original Content))).
2) A message (S3 (S2 (S1 (Original Content))) in which none of the
signedData layers includes an mlExpansionHistory attribute is to
have a 'domain signature' applied. The signedData objects S1, S2
and S3 are verified. There is not an original, "outer" signedData
layer since the original content is found, nor is an envelopedData
or a mlExpansionHistory attribute found. A new signedData layer,
S4, is created that contains a 'domain signature', resulting in
the following message sent out of the domain (S4 (S3 (S2 (S1
(Original Content))).
3) A message (E1 (S1 (Original Content))) (where E = envelopedData)
in which S1 does not include a mlExpansionHistory attribute is to
have a 'domain signature' applied. There is not an original,
received "outer" signedData layer since the envelopedData, E1, is
found at the outer layer. The encryptedContent is decrypted. The
signedData, S1, is verified. The decrypted content is wrapped in
a new signedData layer, S2, which contains a 'domain signature'.
If local policy requires the message to be encrypted, using S/MIME
encryption, before it leaves the domain then this new message is
wrapped in an envelopedData layer, E2, resulting in the following
message sent out of the domain (E2 (S2 (S1 (Original Content)))),
else the message is not wrapped in an envelopedData layer
resulting in the following message (S2 (S1 (Original Content)))
being sent.
4) A message (S2 (E1 (S1 (Original Content)))) in which S2 includes a
mlExpansionHistory attribute is to have a 'domain signature'
applied. The signedData object S2 is verified. The
mlExpansionHistory attribute is found in S2, so S2 is the "outer"
signedData. The signed attributes in S2 are remembered for later
inclusion in the new outer signedData that is applied to the
message. S2 is stripped off and the message is decrypted. The
signedData object S1 is verified. The decrypted message is
wrapped in a signedData layer, S3, which contains a 'domain
signature'. If local policy requires the message to be encrypted,
using S/MIME encryption, before it leaves the domain then this new
message is wrapped in an envelopedData layer, E2. A new
signedData layer, S4, is then wrapped around the envelopedData,
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E2, resulting in the following message sent out of the domain (S4
(E2 (S3 (S1 (Original Content))))). If local policy does not
require the message to be encrypted, using S/MIME encryption,
before it leaves the domain then the message is not wrapped in an
envelopedData layer but is wrapped in a new signedData layer, S4,
resulting in the following message sent out of the domain (S4 (S3
(S1 (Original Content). The signedData S4, in both cases,
contains the signed attributes from S2.
5) A message (S3 (S2 (E1 (S1 (Original Content))))) in which none of
the signedData layers include a mlExpansionHistory attribute is to
have a 'domain signature' applied. The signedData objects S3 and
S2 are verified. When the envelopedData E1 is found the
signedData objects S3 and S2 are stripped off. The
encryptedContent is decrypted. The signedData object S1 is
verified. The decrypted content is wrapped in a new signedData
layer, S4, which contains a 'domain signature'. If local policy
requires the message to be encrypted, using S/MIME encryption,
before it leaves the domain then this new message is wrapped in an
envelopedData layer, E2, resulting in the following message sent
out of the domain (E2 (S4 (S1 (Original Content)))), else the
message is not wrapped in an envelopedData layer resulting in the
following message (S4 (S1 (Original Content))) being sent.
6) A message (S3 (S2 (E1 (S1 (Original Content))))) in which S3
includes a mlExpansionHistory attribute is to have a 'domain
signature' applied. The signedData objects S3 and S2 are
verified. The mlExpansionHistory attribute is found in S3, so S3
is the "outer" signedData. The signed attributes in S3 are
remembered for later inclusion in the new outer signedData that
is applied to the message. The signedData object S3 is stripped
off. When the envelopedData layer, E1, is found the signedData
object S2 is stripped off. The encryptedContent is decrypted.
The signedData object S1 is verified. The decrypted content is
wrapped in a new signedData layer, S4, which contains a 'domain
signature'. If local policy requires the message to be encrypted,
using S/MIME encryption, before it leaves the domain then this new
message is wrapped in an envelopedData layer, E2. A new
signedData layer, S5, is then wrapped around the envelopedData,
E2, resulting in the following message sent out of the domain (S5
(E2 (S4 (S1 (Original Content))))). If local policy does not
require the message to be encrypted, using S/MIME encryption,
before it leaves the domain then the message is not wrapped in an
envelopedData layer but is wrapped in a new signedData layer, S5,
resulting in the following message sent out of the domain (S5 (S4
(S1 (Original Content). The signedData S5, in both cases,
contains the signed attributes from S3.
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7) A message (S3 (E2 (S2 (E1 (S1 (Original Content)))))) in which S3
does not include a mlExpansionHistory attribute is to have a
'domain signature' applied. The signedData object S3 is verified.
When the envelopedData E2 is found the signedData object S3 is
stripped off. The encryptedContent is decrypted. The signedData
object S2 is verified, the envelopedData E1 is decrypted and the
signedData object S1 is verified. The signedData object S2 is
wrapped in a new signedData layer S4, which contains a 'domain
signature'. Since there is an envelopedData E1 lower down in the
message, the new message is wrapped in an envelopedData layer, E3,
resulting in the following message sent out of the domain (E3 (S4
(S2 (E1 (S1 (Original Content)))))).
6. Security Considerations
This specification relies on the existence of several well known
names, such as domain-confidentiality-authority. Organizations must
take care with these names, even if they do not support DOMSEC, so
that certificates issued in these names are only issued to legitimate
entities. If this is not true then an individual could get a
certificate associated with domain-confidentiality-authority@acme.com
and as a result might be able to read messages the a DOMSEC client
intended for others.
Implementations MUST protect all private keys. Compromise of the
signer's private key permits masquerade.
Similarly, compromise of the content-encryption key may result in
disclosure of the encrypted content.
Compromise of key material is regarded as an even more serious issue
for domain security services than for an S/MIME client. This is
because compromise of the private key may in turn compromise the
security of a whole domain. Therefore, great care should be used
when considering its protection.
Domain encryption alone is not secure and should be used in
conjunction with a domain signature to avoid a masquerade attack,
where an attacker that has obtained a DCA certificate can fake a
message to that domain pretending to be another domain.
When an encrypted DOMSEC message is sent to an end user in such a way
that the message is decrypted by the end users DCA the message will
be in plain text and therefore confidentiality could be compromised.
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If the recipient's DCA is compromised then the recipient can not
guarantee the integrity of the message. Furthermore, even if the
recipient's DCA correctly verifies a message's signatures, then a
message could be undetectably modified, when there are no signatures
on a message that the recipient can verify.
7. DOMSEC ASN.1 Module
DOMSECSyntax
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) domsec(10) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORTS All
-- The types and values defined in this module are exported for
-- use in the other ASN.1 modules. Other applications may use
-- them for their own purposes.
SignatureType ::= SEQUENCE OF OBJECT IDENTIFIER
id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 16 }
id-sti OBJECT IDENTIFIER ::= { id-smime 9 } -- signature type
identifier
-- Signature Type Identifiers
id-sti-originatorSig OBJECT IDENTIFIER ::= { id-sti 1 }
id-sti-domainSig OBJECT IDENTIFIER ::= { id-sti 2 }
id-sti-addAttribSig OBJECT IDENTIFIER ::= { id-sti 3 }
id-sti-reviewSig OBJECT IDENTIFIER ::= { id-sti 4 }
END -- of DOMSECSyntax
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8. References
[1] Ramsdell, B., "S/MIME Version 3 Message Specification", RFC 2633,
June 1999.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Hoffman, P., "Enhanced Security Services for S/MIME", RFC 2634,
June 1999.
[4] International Telecommunications Union, Recommendation X.208,
"Open systems interconnection: specification of Abstract Syntax
Notation (ASN.1)", CCITT Blue Book, 1989.
[5] Housley, R., "Cryptographic Message Syntax", RFC 2630, June 1999.
9. Authors' Addresses
Tim Dean
QinetiQ
St. Andrews Road
Malvern
Worcs
WR14 3PS
Phone: +44 (0) 1684 894239
Fax: +44 (0) 1684 896660
EMail: tbdean@QinetiQ.com
William Ottaway
QinetiQ
St. Andrews Road
Malvern
Worcs
WR14 3PS
Phone: +44 (0) 1684 894079
Fax: +44 (0) 1684 896660
EMail: wjottaway@QinetiQ.com
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10. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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