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Obsoletes:
Obsoleted by:
Keywords: [--------|p], secure, multipurpose, internet, mail, extensions, encryption
Network Working Group B. Ramsdell, Editor
Request for Comments: 3851 Sendmail, Inc.
Obsoletes: 2633 July 2004
Category: Standards Track
Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1
Message Specification
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document defines Secure/Multipurpose Internet Mail Extensions
(S/MIME) version 3.1. S/MIME provides a consistent way to send and
receive secure MIME data. Digital signatures provide authentication,
message integrity, and non-repudiation with proof of origin.
Encryption provides data confidentiality. Compression can be used to
reduce data size. This document obsoletes RFC 2633.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Specification Overview . . . . . . . . . . . . . . . . . 3
1.2. Terminology. . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Definitions. . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Compatibility with Prior Practice of S/MIME. . . . . . . 5
1.5. Changes Since S/MIME v3. . . . . . . . . . . . . . . . . 5
2. CMS Options. . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. DigestAlgorithmIdentifier. . . . . . . . . . . . . . . . 5
2.2. SignatureAlgorithmIdentifier . . . . . . . . . . . . . . 6
2.3. KeyEncryptionAlgorithmIdentifier . . . . . . . . . . . . 6
2.4. General Syntax . . . . . . . . . . . . . . . . . . . . . 6
2.5. Attributes and the SignerInfo Type . . . . . . . . . . . 7
2.6. SignerIdentifier SignerInfo Type . . . . . . . . . . . . 11
2.7. ContentEncryptionAlgorithmIdentifier . . . . . . . . . . 12
3. Creating S/MIME Messages . . . . . . . . . . . . . . . . . . . 14
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RFC 3851 S/MIME 3.1 Message Specification July 2004
3.1. Preparing the MIME Entity for Signing, Enveloping
or Compressing . . . . . . . . . . . . . . . . . . . . . 14
3.2. The application/pkcs7-mime Type. . . . . . . . . . . . . 19
3.3. Creating an Enveloped-only Message . . . . . . . . . . . 21
3.4. Creating a Signed-only Message . . . . . . . . . . . . . 22
3.5. Creating an Compressed-only Message. . . . . . . . . . . 26
3.6. Multiple Operations. . . . . . . . . . . . . . . . . . . 27
3.7. Creating a Certificate Management Messagetoc . . . . . . 27
3.8. Registration Requests. . . . . . . . . . . . . . . . . . 28
3.9. Identifying an S/MIME Message. . . . . . . . . . . . . . 28
4. Certificate Processing . . . . . . . . . . . . . . . . . . . . 29
4.1. Key Pair Generation. . . . . . . . . . . . . . . . . . . 29
5. Security Considerations. . . . . . . . . . . . . . . . . . . . 29
A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . . . . . . 31
B. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
B.1. Normative References . . . . . . . . . . . . . . . . . . 32
B.2. Informative References . . . . . . . . . . . . . . . . . 34
C. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
D. Editor's Address . . . . . . . . . . . . . . . . . . . . . . . 35
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 36
1. Introduction
S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a
consistent way to send and receive secure MIME data. Based on the
popular Internet MIME standard, S/MIME provides the following
cryptographic security services for electronic messaging
applications: authentication, message integrity and non-repudiation
of origin (using digital signatures), and data confidentiality (using
encryption).
S/MIME can be used by traditional mail user agents (MUAs) to add
cryptographic security services to mail that is sent, and to
interpret cryptographic security services in mail that is received.
However, S/MIME is not restricted to mail; it can be used with any
transport mechanism that transports MIME data, such as HTTP. As
such, S/MIME takes advantage of the object-based features of MIME and
allows secure messages to be exchanged in mixed-transport systems.
Further, S/MIME can be used in automated message transfer agents that
use cryptographic security services that do not require any human
intervention, such as the signing of software-generated documents and
the encryption of FAX messages sent over the Internet.
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RFC 3851 S/MIME 3.1 Message Specification July 2004
1.1. Specification Overview
This document describes a protocol for adding cryptographic signature
and encryption services to MIME data. The MIME standard [MIME-SPEC]
provides a general structure for the content type of Internet
messages and allows extensions for new content type applications.
This specification defines how to create a MIME body part that has
been cryptographically enhanced according to CMS [CMS], which is
derived from PKCS #7 [PKCS-7]. This specification also defines the
application/pkcs7-mime MIME type that can be used to transport those
body parts.
This document also discusses how to use the multipart/signed MIME
type defined in [MIME-SECURE] to transport S/MIME signed messages.
multipart/signed is used in conjunction with the application/pkcs7-
signature MIME type, which is used to transport a detached S/MIME
signature.
In order to create S/MIME messages, an S/MIME agent MUST follow the
specifications in this document, as well as the specifications listed
in the Cryptographic Message Syntax document [CMS] [CMSALG].
Throughout this specification, there are requirements and
recommendations made for how receiving agents handle incoming
messages. There are separate requirements and recommendations for
how sending agents create outgoing messages. In general, the best
strategy is to "be liberal in what you receive and conservative in
what you send". Most of the requirements are placed on the handling
of incoming messages while the recommendations are mostly on the
creation of outgoing messages.
The separation for requirements on receiving agents and sending
agents also derives from the likelihood that there will be S/MIME
systems that involve software other than traditional Internet mail
clients. S/MIME can be used with any system that transports MIME
data. An automated process that sends an encrypted message might not
be able to receive an encrypted message at all, for example. Thus,
the requirements and recommendations for the two types of agents are
listed separately when appropriate.
1.2. Terminology
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 [MUSTSHOULD].
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1.3. Definitions
For the purposes of this specification, the following definitions
apply.
ASN.1: Abstract Syntax Notation One, as defined in CCITT X.208
[X.208-88].
BER: Basic Encoding Rules for ASN.1, as defined in CCITT X.209
[X.209-88].
Certificate: A type that binds an entity's name to a public key with
a digital signature.
DER: Distinguished Encoding Rules for ASN.1, as defined in CCITT
X.509 [X.509-88].
7-bit data: Text data with lines less than 998 characters long, where
none of the characters have the 8th bit set, and there are no NULL
characters. <CR> and <LF> occur only as part of a <CR><LF> end of
line delimiter.
8-bit data: Text data with lines less than 998 characters, and where
none of the characters are NULL characters. <CR> and <LF> occur only
as part of a <CR><LF> end of line delimiter.
Binary data: Arbitrary data.
Transfer Encoding: A reversible transformation made on data so 8-bit
or binary data can be sent via a channel that only transmits 7-bit
data.
Receiving agent: Software that interprets and processes S/MIME CMS
objects, MIME body parts that contain CMS content types, or both.
Sending agent: Software that creates S/MIME CMS content types, MIME
body parts that contain CMS content types, or both.
S/MIME agent: User software that is a receiving agent, a sending
agent, or both.
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1.4. Compatibility with Prior Practice of S/MIME
S/MIME version 3.1 agents SHOULD attempt to have the greatest
interoperability possible with agents for prior versions of S/MIME.
S/MIME version 2 is described in RFC 2311 through RFC 2315, inclusive
and S/MIME version 3 is described in RFC 2630 through RFC 2634
inclusive. RFC 2311 also has historical information about the
development of S/MIME.
1.5. Changes Since S/MIME v3
The RSA public key algorithm was changed to a MUST implement key
wrapping algorithm, and the Diffie-Hellman algorithm changed to a
SHOULD implement.
The AES symmetric encryption algorithm has been included as a SHOULD
implement.
The RSA public key algorithm was changed to a MUST implement
signature algorithm.
Ambiguous language about the use of "empty" SignedData messages to
transmit certificates was clarified to reflect that transmission of
certificate revocation lists is also allowed.
The use of binary encoding for some MIME entities is now explicitly
discussed.
Header protection through the use of the message/rfc822 MIME type has
been added.
Use of the CompressedData CMS type is allowed, along with required
MIME type and file extension additions.
2. CMS Options
CMS allows for a wide variety of options in content and algorithm
support. This section puts forth a number of support requirements
and recommendations in order to achieve a base level of
interoperability among all S/MIME implementations. [CMSALG] provides
additional details regarding the use of the cryptographic algorithms.
2.1. DigestAlgorithmIdentifier
Sending and receiving agents MUST support SHA-1 [CMSALG]. Receiving
agents SHOULD support MD5 [CMSALG] for the purpose of providing
backward compatibility with MD5-digested S/MIME v2 SignedData
objects.
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2.2. SignatureAlgorithmIdentifier
Receiving agents MUST support id-dsa-with-sha1 defined in [CMSALG].
The algorithm parameters MUST be absent (not encoded as NULL).
Receiving agents MUST support rsaEncryption, defined in [CMSALG].
Sending agents MUST support either id-dsa-with-sha1 or rsaEncryption.
If using rsaEncryption, sending and receiving agents MUST support the
digest algorithms in section 2.1 as specified.
Note that S/MIME v3 clients might only implement signing or signature
verification using id-dsa-with-sha1, and might also use id-dsa as an
AlgorithmIdentifier in this field. Receiving clients SHOULD
recognize id-dsa as equivalent to id-dsa-with-sha1, and sending
clients MUST use id-dsa-with-sha1 if using that algorithm. Also note
that S/MIME v2 clients are only required to verify digital signatures
using the rsaEncryption algorithm with SHA-1 or MD5, and might not
implement id-dsa-with-sha1 or id-dsa at all.
2.3. KeyEncryptionAlgorithmIdentifier
Sending and receiving agents MUST support rsaEncryption, defined in
[CMSALG].
Sending and receiving agents SHOULD support Diffie-Hellman defined in
[CMSALG], using the ephemeral-static mode.
Note that S/MIME v3 clients might only implement key encryption and
decryption using the Diffie-Hellman algorithm. Also note that S/MIME
v2 clients are only capable of decrypting content-encryption keys
using the rsaEncryption algorithm.
2.4. General Syntax
There are several CMS content types. Of these, only the Data,
SignedData, EnvelopedData, and CompressedData content types are
currently used for S/MIME.
2.4.1. Data Content Type
Sending agents MUST use the id-data content type identifier to
identify the "inner" MIME message content. For example, when
applying a digital signature to MIME data, the CMS SignedData
encapContentInfo eContentType MUST include the id-data object
identifier and the MIME content MUST be stored in the SignedData
encapContentInfo eContent OCTET STRING (unless the sending agent is
using multipart/signed, in which case the eContent is absent, per
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section 3.4.3 of this document). As another example, when applying
encryption to MIME data, the CMS EnvelopedData encryptedContentInfo
contentType MUST include the id-data object identifier and the
encrypted MIME content MUST be stored in the EnvelopedData
encryptedContentInfo encryptedContent OCTET STRING.
2.4.2. SignedData Content Type
Sending agents MUST use the SignedData content type to apply a
digital signature to a message or, in a degenerate case where there
is no signature information, to convey certificates. Applying a
signature to a message provides authentication, message integrity,
and non-repudiation of origin.
2.4.3. EnvelopedData Content Type
This content type is used to apply data confidentiality to a message.
A sender needs to have access to a public key for each intended
message recipient to use this service.
2.4.4. CompressedData Content Type
This content type is used to apply data compression to a message.
This content type does not provide authentication, message integrity,
non-repudiation, or data confidentiality, and is only used to reduce
message size.
See section 3.6 for further guidance on the use of this type in
conjunction with other CMS types.
2.5. Attributes and the SignerInfo Type
The SignerInfo type allows the inclusion of unsigned and signed
attributes to be included along with a signature.
Receiving agents MUST be able to handle zero or one instance of each
of the signed attributes listed here. Sending agents SHOULD generate
one instance of each of the following signed attributes in each
S/MIME message:
- signingTime (section 2.5.1 in this document)
- sMIMECapabilities (section 2.5.2 in this document)
- sMIMEEncryptionKeyPreference (section 2.5.3 in this document)
- id-messageDigest (section 11.2 in [CMS])
- id-contentType (section 11.1 in [CMS])
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Further, receiving agents SHOULD be able to handle zero or one
instance in the signingCertificate signed attribute, as defined in
section 5 of [ESS].
Sending agents SHOULD generate one instance of the signingCertificate
signed attribute in each SignerInfo structure.
Additional attributes and values for these attributes might be
defined in the future. Receiving agents SHOULD handle attributes or
values that it does not recognize in a graceful manner.
Interactive sending agents that include signed attributes that are
not listed here SHOULD display those attributes to the user, so that
the user is aware of all of the data being signed.
2.5.1. Signing-Time Attribute
The signing-time attribute is used to convey the time that a message
was signed. The time of signing will most likely be created by a
message originator and therefore is only as trustworthy as the
originator.
Sending agents MUST encode signing time through the year 2049 as
UTCTime; signing times in 2050 or later MUST be encoded as
GeneralizedTime. When the UTCTime CHOICE is used, S/MIME agents MUST
interpret the year field (YY) as follows:
if YY is greater than or equal to 50, the year is interpreted as
19YY; if YY is less than 50, the year is interpreted as 20YY.
2.5.2. SMIMECapabilities Attribute
The SMIMECapabilities attribute includes signature algorithms (such
as "sha1WithRSAEncryption"), symmetric algorithms (such as "DES-
EDE3-CBC"), and key encipherment algorithms (such as
"rsaEncryption"). There are also several identifiers which indicate
support for other optional features such as binary encoding and
compression. The SMIMECapabilities were designed to be flexible and
extensible so that, in the future, a means of identifying other
capabilities and preferences such as certificates can be added in a
way that will not cause current clients to break.
If present, the SMIMECapabilities attribute MUST be a
SignedAttribute; it MUST NOT be an UnsignedAttribute. CMS defines
SignedAttributes as a SET OF Attribute. The SignedAttributes in a
signerInfo MUST NOT include multiple instances of the
SMIMECapabilities attribute. CMS defines the ASN.1 syntax for
Attribute to include attrValues SET OF AttributeValue. A
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SMIMECapabilities attribute MUST only include a single instance of
AttributeValue. There MUST NOT be zero or multiple instances of
AttributeValue present in the attrValues SET OF AttributeValue.
The semantics of the SMIMECapabilities attribute specify a partial
list as to what the client announcing the SMIMECapabilities can
support. A client does not have to list every capability it
supports, and need not list all its capabilities so that the
capabilities list doesn't get too long. In an SMIMECapabilities
attribute, the object identifiers (OIDs) are listed in order of their
preference, but SHOULD be separated logically along the lines of
their categories (signature algorithms, symmetric algorithms, key
encipherment algorithms, etc.)
The structure of the SMIMECapabilities attribute is to facilitate
simple table lookups and binary comparisons in order to determine
matches. For instance, the DER-encoding for the SMIMECapability for
DES EDE3 CBC MUST be identically encoded regardless of the
implementation. Because of the requirement for identical encoding,
individuals documenting algorithms to be used in the
SMIMECapabilities attribute SHOULD explicitly document the correct
byte sequence for the common cases.
For any capability, the associated parameters for the OID MUST
specify all of the parameters necessary to differentiate between two
instances of the same algorithm. For instance, the number of rounds
and the block size for RC5 needs to be specified in addition to the
key length.
The OIDs that correspond to algorithms SHOULD use the same OID as the
actual algorithm, except in the case where the algorithm usage is
ambiguous from the OID. For instance, in an earlier specification,
rsaEncryption was ambiguous because it could refer to either a
signature algorithm or a key encipherment algorithm. In the event
that an OID is ambiguous, it needs to be arbitrated by the maintainer
of the registered SMIMECapabilities list as to which type of
algorithm will use the OID, and a new OID MUST be allocated under the
smimeCapabilities OID to satisfy the other use of the OID.
The registered SMIMECapabilities list specifies the parameters for
OIDs that need them, most notably key lengths in the case of
variable-length symmetric ciphers. In the event that there are no
differentiating parameters for a particular OID, the parameters MUST
be omitted, and MUST NOT be encoded as NULL.
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Additional values for the SMIMECapabilities attribute might be
defined in the future. Receiving agents MUST handle a
SMIMECapabilities object that has values that it does not recognize
in a graceful manner.
Section 2.7.1 explains a strategy for caching capabilities.
2.5.2.1. SMIMECapability For the RC2 Algorithm
For the RC2 algorithm preference SMIMECapability, the capabilityID
MUST be set to the value rc2-cbc as defined in [CMSALG]. The
parameters field MUST contain SMIMECapabilitiesParametersForRC2CBC
(see appendix A).
Please note that the SMIMECapabilitiesParametersForRC2CBC is a single
INTEGER which contains the effective key length (NOT the
corresponding RC2 parameter version value). So, for example, for RC2
with a 128-bit effective key length, the parameter would be encoded
as the INTEGER value 128, NOT the corresponding parameter version of
58.
2.5.3. Encryption Key Preference Attribute
The encryption key preference attribute allows the signer to
unambiguously describe which of the signer's certificates has the
signer's preferred encryption key. This attribute is designed to
enhance behavior for interoperating with those clients that use
separate keys for encryption and signing. This attribute is used to
convey to anyone viewing the attribute which of the listed
certificates is appropriate for encrypting a session key for future
encrypted messages.
If present, the SMIMEEncryptionKeyPreference attribute MUST be a
SignedAttribute; it MUST NOT be an UnsignedAttribute. CMS defines
SignedAttributes as a SET OF Attribute. The SignedAttributes in a
signerInfo MUST NOT include multiple instances of the
SMIMEEncryptionKeyPreference attribute. CMS defines the ASN.1 syntax
for Attribute to include attrValues SET OF AttributeValue. A
SMIMEEncryptionKeyPreference attribute MUST only include a single
instance of AttributeValue. There MUST NOT be zero or multiple
instances of AttributeValue present in the attrValues SET OF
AttributeValue.
The sending agent SHOULD include the referenced certificate in the
set of certificates included in the signed message if this attribute
is used. The certificate MAY be omitted if it has been previously
made available to the receiving agent. Sending agents SHOULD use
this attribute if the commonly used or preferred encryption
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certificate is not the same as the certificate used to sign the
message.
Receiving agents SHOULD store the preference data if the signature on
the message is valid and the signing time is greater than the
currently stored value. (As with the SMIMECapabilities, the clock
skew SHOULD be checked and the data not used if the skew is too
great.) Receiving agents SHOULD respect the sender's encryption key
preference attribute if possible. This, however, represents only a
preference and the receiving agent can use any certificate in
replying to the sender that is valid.
Section 2.7.1 explains a strategy for caching preference data.
2.5.3.1. Selection of Recipient Key Management Certificate
In order to determine the key management certificate to be used when
sending a future CMS EnvelopedData message for a particular
recipient, the following steps SHOULD be followed:
- If an SMIMEEncryptionKeyPreference attribute is found in a
SignedData object received from the desired recipient, this
identifies the X.509 certificate that SHOULD be used as the X.509
key management certificate for the recipient.
- If an SMIMEEncryptionKeyPreference attribute is not found in a
SignedData object received from the desired recipient, the set of
X.509 certificates SHOULD be searched for a X.509 certificate with
the same subject name as the signing of a X.509 certificate which
can be used for key management.
- Or use some other method of determining the user's key management
key. If a X.509 key management certificate is not found, then
encryption cannot be done with the signer of the message. If
multiple X.509 key management certificates are found, the S/MIME
agent can make an arbitrary choice between them.
2.6. SignerIdentifier SignerInfo Type
S/MIME v3.1 implementations MUST support both issuerAndSerialNumber
as well as subjectKeyIdentifier. Messages that use the
subjectKeyIdentifier choice cannot be read by S/MIME v2 clients.
It is important to understand that some certificates use a value for
subjectKeyIdentifier that is not suitable for uniquely identifying a
certificate. Implementations MUST be prepared for multiple
certificates for potentially different entities to have the same
value for subjectKeyIdentifier, and MUST be prepared to try each
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matching certificate during signature verification before indicating
an error condition.
2.7. ContentEncryptionAlgorithmIdentifier
Sending and receiving agents MUST support encryption and decryption
with DES EDE3 CBC, hereinafter called "tripleDES" [CMSALG].
Receiving agents SHOULD support encryption and decryption using the
RC2 [CMSALG] or a compatible algorithm at a key size of 40 bits,
hereinafter called "RC2/40". Sending and receiving agents SHOULD
support encryption and decryption with AES [CMSAES] at a key size of
128, 192, and 256 bits.
2.7.1. Deciding Which Encryption Method To Use
When a sending agent creates an encrypted message, it has to decide
which type of encryption to use. The decision process involves using
information garnered from the capabilities lists included in messages
received from the recipient, as well as out-of-band information such
as private agreements, user preferences, legal restrictions, and so
on.
Section 2.5.2 defines a method by which a sending agent can
optionally announce, among other things, its decrypting capabilities
in its order of preference. The following method for processing and
remembering the encryption capabilities attribute in incoming signed
messages SHOULD be used.
- If the receiving agent has not yet created a list of capabilities
for the sender's public key, then, after verifying the signature
on the incoming message and checking the timestamp, the receiving
agent SHOULD create a new list containing at least the signing
time and the symmetric capabilities.
- If such a list already exists, the receiving agent SHOULD verify
that the signing time in the incoming message is greater than the
signing time stored in the list and that the signature is valid.
If so, the receiving agent SHOULD update both the signing time and
capabilities in the list. Values of the signing time that lie far
in the future (that is, a greater discrepancy than any reasonable
clock skew), or a capabilities list in messages whose signature
could not be verified, MUST NOT be accepted.
The list of capabilities SHOULD be stored for future use in creating
messages.
Before sending a message, the sending agent MUST decide whether it is
willing to use weak encryption for the particular data in the
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message. If the sending agent decides that weak encryption is
unacceptable for this data, then the sending agent MUST NOT use a
weak algorithm such as RC2/40. The decision to use or not use weak
encryption overrides any other decision in this section about which
encryption algorithm to use.
Sections 2.7.2.1 through 2.7.2.4 describe the decisions a sending
agent SHOULD use in deciding which type of encryption will be applied
to a message. These rules are ordered, so the sending agent SHOULD
make its decision in the order given.
2.7.1.1. Rule 1: Known Capabilities
If the sending agent has received a set of capabilities from the
recipient for the message the agent is about to encrypt, then the
sending agent SHOULD use that information by selecting the first
capability in the list (that is, the capability most preferred by the
intended recipient) that the sending agent knows how to encrypt. The
sending agent SHOULD use one of the capabilities in the list if the
agent reasonably expects the recipient to be able to decrypt the
message.
2.7.1.2. Rule 2: Unknown Capabilities, Unknown Version of S/MIME
If the following two conditions are met:
- the sending agent has no knowledge of the encryption capabilities
of the recipient,
- and the sending agent has no knowledge of the version of S/MIME of
the recipient,
then the sending agent SHOULD use tripleDES because it is a stronger
algorithm and is required by S/MIME v3. If the sending agent chooses
not to use tripleDES in this step, it SHOULD use RC2/40.
2.7.2. Choosing Weak Encryption
Like all algorithms that use 40 bit keys, RC2/40 is considered by
many to be weak encryption. A sending agent that is controlled by a
human SHOULD allow a human sender to determine the risks of sending
data using RC2/40 or a similarly weak encryption algorithm before
sending the data, and possibly allow the human to use a stronger
encryption method such as tripleDES.
2.7.3. Multiple Recipients
If a sending agent is composing an encrypted message to a group of
recipients where the encryption capabilities of some of the
recipients do not overlap, the sending agent is forced to send more
than one message. Please note that if the sending agent chooses to
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send a message encrypted with a strong algorithm, and then send the
same message encrypted with a weak algorithm, someone watching the
communications channel could learn the contents of the strongly-
encrypted message simply by decrypting the weakly-encrypted message.
3. Creating S/MIME Messages
This section describes the S/MIME message formats and how they are
created. S/MIME messages are a combination of MIME bodies and CMS
content types. Several MIME types as well as several CMS content
types are used. The data to be secured is always a canonical MIME
entity. The MIME entity and other data, such as certificates and
algorithm identifiers, are given to CMS processing facilities which
produce a CMS object. Finally, the CMS object is wrapped in MIME.
The Enhanced Security Services for S/MIME [ESS] document provides
descriptions of how nested, secured S/MIME messages are formatted.
ESS provides a description of how a triple-wrapped S/MIME message is
formatted using multipart/signed and application/pkcs7-mime for the
signatures.
S/MIME provides one format for enveloped-only data, several formats
for signed-only data, and several formats for signed and enveloped
data. Several formats are required to accommodate several
environments, in particular for signed messages. The criteria for
choosing among these formats are also described.
The reader of this section is expected to understand MIME as
described in [MIME-SPEC] and [MIME-SECURE].
3.1. Preparing the MIME Entity for Signing, Enveloping or Compressing
S/MIME is used to secure MIME entities. A MIME entity can be a sub-
part, sub-parts of a message, or the whole message with all its sub-
parts. A MIME entity that is the whole message includes only the
MIME headers and MIME body, and does not include the RFC-822 headers.
Note that S/MIME can also be used to secure MIME entities used in
applications other than Internet mail. If protection of the RFC-822
headers is required, the use of the message/rfc822 MIME type is
explained later in this section.
The MIME entity that is secured and described in this section can be
thought of as the "inside" MIME entity. That is, it is the
"innermost" object in what is possibly a larger MIME message.
Processing "outside" MIME entities into CMS content types is
described in Section 3.2, 3.4, and elsewhere.
The procedure for preparing a MIME entity is given in [MIME-SPEC].
The same procedure is used here with some additional restrictions
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RFC 3851 S/MIME 3.1 Message Specification July 2004
when signing. Description of the procedures from [MIME-SPEC] are
repeated here, but it is suggested that the reader refer to that
document for the exact procedure. This section also describes
additional requirements.
A single procedure is used for creating MIME entities that are to
have any combination of signing, enveloping, and compressing applied.
Some additional steps are recommended to defend against known
corruptions that can occur during mail transport that are of
particular importance for clear-signing using the multipart/signed
format. It is recommended that these additional steps be performed
on enveloped messages, or signed and enveloped messages, so that the
message can be forwarded to any environment without modification.
These steps are descriptive rather than prescriptive. The
implementer is free to use any procedure as long as the result is the
same.
Step 1. The MIME entity is prepared according to the local
conventions.
Step 2. The leaf parts of the MIME entity are converted to canonical
form.
Step 3. Appropriate transfer encoding is applied to the leaves of
the MIME entity.
When an S/MIME message is received, the security services on the
message are processed, and the result is the MIME entity. That MIME
entity is typically passed to a MIME-capable user agent where, it is
further decoded and presented to the user or receiving application.
In order to protect outer, non-content related message headers (for
instance, the "Subject", "To", "From" and "CC" fields), the sending
client MAY wrap a full MIME message in a message/rfc822 wrapper in
order to apply S/MIME security services to these headers. It is up
to the receiving client to decide how to present these "inner"
headers along with the unprotected "outer" headers.
When an S/MIME message is received, if the top-level protected MIME
entity has a Content-Type of message/rfc822, it can be assumed that
the intent was to provide header protection. This entity SHOULD be
presented as the top-level message, taking into account header
merging issues as previously discussed.
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RFC 3851 S/MIME 3.1 Message Specification July 2004
3.1.1. Canonicalization
Each MIME entity MUST be converted to a canonical form that is
uniquely and unambiguously representable in the environment where the
signature is created and the environment where the signature will be
verified. MIME entities MUST be canonicalized for enveloping and
compressing as well as signing.
The exact details of canonicalization depend on the actual MIME type
and subtype of an entity, and are not described here. Instead, the
standard for the particular MIME type SHOULD be consulted. For
example, canonicalization of type text/plain is different from
canonicalization of audio/basic. Other than text types, most types
have only one representation regardless of computing platform or
environment which can be considered their canonical representation.
In general, canonicalization will be performed by the non-security
part of the sending agent rather than the S/MIME implementation.
The most common and important canonicalization is for text, which is
often represented differently in different environments. MIME
entities of major type "text" MUST have both their line endings and
character set canonicalized. The line ending MUST be the pair of
characters <CR><LF>, and the charset SHOULD be a registered charset
[CHARSETS]. The details of the canonicalization are specified in
[MIME-SPEC]. The chosen charset SHOULD be named in the charset
parameter so that the receiving agent can unambiguously determine the
charset used.
Note that some charsets such as ISO-2022 have multiple
representations for the same characters. When preparing such text
for signing, the canonical representation specified for the charset
MUST be used.
3.1.2. Transfer Encoding
When generating any of the secured MIME entities below, except the
signing using the multipart/signed format, no transfer encoding is
required at all. S/MIME implementations MUST be able to deal with
binary MIME objects. If no Content-Transfer-Encoding header is
present, the transfer encoding is presumed to be 7BIT.
S/MIME implementations SHOULD however use transfer encoding described
in section 3.1.3 for all MIME entities they secure. The reason for
securing only 7-bit MIME entities, even for enveloped data that are
not exposed to the transport, is that it allows the MIME entity to be
handled in any environment without changing it. For example, a
trusted gateway might remove the envelope, but not the signature, of
a message, and then forward the signed message on to the end
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RFC 3851 S/MIME 3.1 Message Specification July 2004
recipient so that they can verify the signatures directly. If the
transport internal to the site is not 8-bit clean, such as on a
wide-area network with a single mail gateway, verifying the signature
will not be possible unless the original MIME entity was only 7-bit
data.
S/MIME implementations which "know" that all intended recipient(s)
are capable of handling inner (all but the outermost) binary MIME
objects SHOULD use binary encoding as opposed to a 7-bit-safe
transfer encoding for the inner entities. The use of a 7-bit-safe
encoding (such as base64) would unnecessarily expand the message
size. Implementations MAY "know" that recipient implementations are
capable of handling inner binary MIME entities either by interpreting
the id-cap-preferBinaryInside sMIMECapabilities attribute, by prior
agreement, or by other means.
If one or more intended recipients are unable to handle inner binary
MIME objects, or if this capability is unknown for any of the
intended recipients, S/MIME implementations SHOULD use transfer
encoding described in section 3.1.3 for all MIME entities they
secure.
3.1.3. Transfer Encoding for Signing Using multipart/signed
If a multipart/signed entity is ever to be transmitted over the
standard Internet SMTP infrastructure or other transport that is
constrained to 7-bit text, it MUST have transfer encoding applied so
that it is represented as 7-bit text. MIME entities that are 7-bit
data already need no transfer encoding. Entities such as 8-bit text
and binary data can be encoded with quoted-printable or base-64
transfer encoding.
The primary reason for the 7-bit requirement is that the Internet
mail transport infrastructure cannot guarantee transport of 8-bit or
binary data. Even though many segments of the transport
infrastructure now handle 8-bit and even binary data, it is sometimes
not possible to know whether the transport path is 8-bit clean. If a
mail message with 8-bit data were to encounter a message transfer
agent that can not transmit 8-bit or binary data, the agent has three
options, none of which are acceptable for a clear-signed message:
- The agent could change the transfer encoding; this would
invalidate the signature.
- The agent could transmit the data anyway, which would most likely
result in the 8th bit being corrupted; this too would invalidate
the signature.
- The agent could return the message to the sender.
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RFC 3851 S/MIME 3.1 Message Specification July 2004
[MIME-SECURE] prohibits an agent from changing the transfer encoding
of the first part of a multipart/signed message. If a compliant
agent that can not transmit 8-bit or binary data encounters a
multipart/signed message with 8-bit or binary data in the first part,
it would have to return the message to the sender as undeliverable.
3.1.4. Sample Canonical MIME Entity
This example shows a multipart/mixed message with full transfer
encoding. This message contains a text part and an attachment. The
sample message text includes characters that are not US-ASCII and
thus need to be transfer encoded. Though not shown here, the end of
each line is <CR><LF>. The line ending of the MIME headers, the
text, and transfer encoded parts, all MUST be <CR><LF>.
Note that this example is not of an S/MIME message.
Content-Type: multipart/mixed; boundary=bar
--bar
Content-Type: text/plain; charset=iso-8859-1
Content-Transfer-Encoding: quoted-printable
=A1Hola Michael!
How do you like the new S/MIME specification?
It's generally a good idea to encode lines that begin with
From=20because some mail transport agents will insert a greater-
than (>) sign, thus invalidating the signature.
Also, in some cases it might be desirable to encode any =20
trailing whitespace that occurs on lines in order to ensure =20
that the message signature is not invalidated when passing =20
a gateway that modifies such whitespace (like BITNET). =20
--bar
Content-Type: image/jpeg
Content-Transfer-Encoding: base64
iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
HOxEa44b+EI=
--bar--
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RFC 3851 S/MIME 3.1 Message Specification July 2004
3.2. The application/pkcs7-mime Type
The application/pkcs7-mime type is used to carry CMS content types
including EnvelopedData, SignedData, and CompressedData. The details
of constructing these entities is described in subsequent sections.
This section describes the general characteristics of the
application/pkcs7-mime type.
The carried CMS object always contains a MIME entity that is prepared
as described in section 3.1 if the eContentType is id-data. Other
contents MAY be carried when the eContentType contains different
values. See [ESS] for an example of this with signed receipts.
Since CMS content types are binary data, in most cases base-64
transfer encoding is appropriate, in particular, when used with SMTP
transport. The transfer encoding used depends on the transport
through which the object is to be sent, and is not a characteristic
of the MIME type.
Note that this discussion refers to the transfer encoding of the CMS
object or "outside" MIME entity. It is completely distinct from, and
unrelated to, the transfer encoding of the MIME entity secured by the
CMS object, the "inside" object, which is described in section 3.1.
Because there are several types of application/pkcs7-mime objects, a
sending agent SHOULD do as much as possible to help a receiving agent
know about the contents of the object without forcing the receiving
agent to decode the ASN.1 for the object. The MIME headers of all
application/pkcs7-mime objects SHOULD include the optional "smime-
type" parameter, as described in the following sections.
3.2.1. The name and filename Parameters
For the application/pkcs7-mime, sending agents SHOULD emit the
optional "name" parameter to the Content-Type field for compatibility
with older systems. Sending agents SHOULD also emit the optional
Content-Disposition field [CONTDISP] with the "filename" parameter.
If a sending agent emits the above parameters, the value of the
parameters SHOULD be a file name with the appropriate extension:
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RFC 3851 S/MIME 3.1 Message Specification July 2004
MIME Type File Extension
application/pkcs7-mime (SignedData, EnvelopedData) .p7m
application/pkcs7-mime (degenerate SignedData .p7c
certificate management message)
application/pkcs7-mime (CompressedData) .p7z
application/pkcs7-signature (SignedData) .p7s
In addition, the file name SHOULD be limited to eight characters
followed by a three letter extension. The eight character filename
base can be any distinct name; the use of the filename base "smime"
SHOULD be used to indicate that the MIME entity is associated with
S/MIME.
Including a file name serves two purposes. It facilitates easier use
of S/MIME objects as files on disk. It also can convey type
information across gateways. When a MIME entity of type
application/pkcs7-mime (for example) arrives at a gateway that has no
special knowledge of S/MIME, it will default the entity's MIME type
to application/octet-stream and treat it as a generic attachment,
thus losing the type information. However, the suggested filename
for an attachment is often carried across a gateway. This often
allows the receiving systems to determine the appropriate application
to hand the attachment off to, in this case, a stand-alone S/MIME
processing application. Note that this mechanism is provided as a
convenience for implementations in certain environments. A proper
S/MIME implementation MUST use the MIME types and MUST NOT rely on
the file extensions.
3.2.2. The smime-type parameter
The application/pkcs7-mime content type defines the optional "smime-
type" parameter. The intent of this parameter is to convey details
about the security applied (signed or enveloped) along with
information about the contained content. This specification defines
the following smime-types.
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RFC 3851 S/MIME 3.1 Message Specification July 2004
Name CMS type Inner Content
enveloped-data EnvelopedData id-data
signed-data SignedData id-data
certs-only SignedData none
compressed-data CompressedData id-data
In order for consistency to be obtained with future specifications,
the following guidelines SHOULD be followed when assigning a new
smime-type parameter.
1. If both signing and encryption can be applied to the content, then
two values for smime-type SHOULD be assigned "signed-*" and
"encrypted-*". If one operation can be assigned then this can be
omitted. Thus since "certs-only" can only be signed, "signed-" is
omitted.
2. A common string for a content OID SHOULD be assigned. We use
"data" for the id-data content OID when MIME is the inner content.
3. If no common string is assigned. Then the common string of
"OID.<oid>" is recommended (for example, "OID.1.3.6.1.5.5.7.6.1"
would be DES40).
It is explicitly intended that this field be a suitable hint for mail
client applications to indicate whether a message is "signed" or
"encrypted" without having to tunnel into the CMS payload.
3.3. Creating an Enveloped-only Message
This section describes the format for enveloping a MIME entity
without signing it. It is important to note that sending enveloped
but not signed messages does not provide for data integrity. It is
possible to replace ciphertext in such a way that the processed
message will still be valid, but the meaning can be altered.
Step 1. The MIME entity to be enveloped is prepared according to
section 3.1.
Step 2. The MIME entity and other required data is processed into a
CMS object of type EnvelopedData. In addition to encrypting a copy
of the content-encryption key for each recipient, a copy of the
content-encryption key SHOULD be encrypted for the originator and
included in the EnvelopedData (see [CMS] Section 6).
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RFC 3851 S/MIME 3.1 Message Specification July 2004
Step 3. The EnvelopedData object is wrapped in a CMS ContentInfo
object.
Step 4. The ContentInfo object is inserted into an
application/pkcs7-mime MIME entity.
The smime-type parameter for enveloped-only messages is "enveloped-
data". The file extension for this type of message is ".p7m".
A sample message would be:
Content-Type: application/pkcs7-mime; smime-type=enveloped-data;
name=smime.p7m
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7m
rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6
7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H
f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
0GhIGfHfQbnj756YT64V
3.4. Creating a Signed-only Message
There are two formats for signed messages defined for S/MIME:
application/pkcs7-mime with SignedData, and multipart/signed. In
general, the multipart/signed form is preferred for sending, and
receiving agents MUST be able to handle both.
3.4.1. Choosing a Format for Signed-only Messages
There are no hard-and-fast rules when a particular signed-only format
is chosen because it depends on the capabilities of all the receivers
and the relative importance of receivers with S/MIME facilities being
able to verify the signature versus the importance of receivers
without S/MIME software being able to view the message.
Messages signed using the multipart/signed format can always be
viewed by the receiver whether they have S/MIME software or not.
They can also be viewed whether they are using a MIME-native user
agent or they have messages translated by a gateway. In this
context, "be viewed" means the ability to process the message
essentially as if it were not a signed message, including any other
MIME structure the message might have.
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RFC 3851 S/MIME 3.1 Message Specification July 2004
Messages signed using the SignedData format cannot be viewed by a
recipient unless they have S/MIME facilities. However, the
SignedData format protects the message content from being changed by
benign intermediate agents. Such agents might do line wrapping or
content-transfer encoding changes which would break the signature.
3.4.2. Signing Using application/pkcs7-mime with SignedData
This signing format uses the application/pkcs7-mime MIME type. The
steps to create this format are:
Step 1. The MIME entity is prepared according to section 3.1.
Step 2. The MIME entity and other required data is processed into a
CMS object of type SignedData.
Step 3. The SignedData object is wrapped in a CMS ContentInfo
object.
Step 4. The ContentInfo object is inserted into an
application/pkcs7-mime MIME entity.
The smime-type parameter for messages using application/pkcs7-mime
with SignedData is "signed-data". The file extension for this type
of message is ".p7m".
A sample message would be:
Content-Type: application/pkcs7-mime; smime-type=signed-data;
name=smime.p7m
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7m
567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7
77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH
HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh
6YT64V0GhIGfHfQbnj75
3.4.3. Signing Using the multipart/signed Format
This format is a clear-signing format. Recipients without any S/MIME
or CMS processing facilities are able to view the message. It makes
use of the multipart/signed MIME type described in [MIME-SECURE].
The multipart/signed MIME type has two parts. The first part
contains the MIME entity that is signed; the second part contains the
"detached signature" CMS SignedData object in which the
encapContentInfo eContent field is absent.
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RFC 3851 S/MIME 3.1 Message Specification July 2004
3.4.3.1. The application/pkcs7-signature MIME Type
This MIME type always contains a CMS ContentInfo containing a single
CMS object of type SignedData. The SignedData encapContentInfo
eContent field MUST be absent. The signerInfos field contains the
signatures for the MIME entity.
The file extension for signed-only messages using application/pkcs7-
signature is ".p7s".
3.4.3.2. Creating a multipart/signed Message
Step 1. The MIME entity to be signed is prepared according to
section 3.1, taking special care for clear-signing.
Step 2. The MIME entity is presented to CMS processing in order to
obtain an object of type SignedData in which the encapContentInfo
eContent field is absent.
Step 3. The MIME entity is inserted into the first part of a
multipart/signed message with no processing other than that described
in section 3.1.
Step 4. Transfer encoding is applied to the "detached signature" CMS
SignedData object and it is inserted into a MIME entity of type
application/pkcs7-signature.
Step 5. The MIME entity of the application/pkcs7-signature is
inserted into the second part of the multipart/signed entity.
The multipart/signed Content type has two required parameters: the
protocol parameter and the micalg parameter.
The protocol parameter MUST be "application/pkcs7-signature". Note
that quotation marks are required around the protocol parameter
because MIME requires that the "/" character in the parameter value
MUST be quoted.
The micalg parameter allows for one-pass processing when the
signature is being verified. The value of the micalg parameter is
dependent on the message digest algorithm(s) used in the calculation
of the Message Integrity Check. If multiple message digest
algorithms are used they MUST be separated by commas per [MIME-
SECURE]. The values to be placed in the micalg parameter SHOULD be
from the following:
Ramsdell Standards Track [Page 24]
RFC 3851 S/MIME 3.1 Message Specification July 2004
Algorithm Value
used
MD5 md5
SHA-1 sha1
SHA-256 sha256
SHA-384 sha384
SHA-512 sha512
Any other (defined separately in algorithm profile or "unknown"
if not defined)
(Historical note: some early implementations of S/MIME emitted and
expected "rsa-md5" and "rsa-sha1" for the micalg parameter.)
Receiving agents SHOULD be able to recover gracefully from a micalg
parameter value that they do not recognize.
The SHA-256, SHA-384, and SHA-512 algorithms [FIPS180-2] are not
currently recommended in S/MIME, and are included here for
completeness.
3.4.3.3. Sample multipart/signed Message
Content-Type: multipart/signed;
protocol="application/pkcs7-signature";
micalg=sha1; boundary=boundary42
--boundary42
Content-Type: text/plain
This is a clear-signed message.
--boundary42
Content-Type: application/pkcs7-signature; name=smime.p7s
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7s
ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
7GhIGfHfYT64VQbnj756
--boundary42--
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RFC 3851 S/MIME 3.1 Message Specification July 2004
The content that is digested (the first part of the multipart/signed)
are the bytes:
43 6f 6e 74 65 6e 74 2d 54 79 70 65 3a 20 74 65 78 74 2f 70 6c 61 69
6e 0d 0a 0d 0a 54 68 69 73 20 69 73 20 61 20 63 6c 65 61 72 2d 73 69
67 6e 65 64 20 6d 65 73 73 61 67 65 2e 0d 0a
3.5. Creating an Compressed-only Message
This section describes the format for compressing a MIME entity.
Please note that versions of S/MIME prior to 3.1 did not specify any
use of CompressedData, and will not recognize it. The use of a
capability to indicate the ability to receive CompressedData is
described in [CMSCOMPR] and is the preferred method for
compatibility.
Step 1. The MIME entity to be compressed is prepared according to
section 3.1.
Step 2. The MIME entity and other required data is processed into a
CMS object of type CompressedData.
Step 3. The CompressedData object is wrapped in a CMS ContentInfo
object.
Step 4. The ContentInfo object is inserted into an
application/pkcs7-mime MIME entity.
The smime-type parameter for compressed-only messages is
"compressed-data". The file extension for this type of message is
".p7z".
A sample message would be:
Content-Type: application/pkcs7-mime; smime-type=compressed-data;
name=smime.p7z
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7z
rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6
7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H
f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
0GhIGfHfQbnj756YT64V
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RFC 3851 S/MIME 3.1 Message Specification July 2004
3.6. Multiple Operations
The signed-only, encrypted-only, and compressed-only MIME formats can
be nested. This works because these formats are all MIME entities
that encapsulate other MIME entities.
An S/MIME implementation MUST be able to receive and process
arbitrarily nested S/MIME within reasonable resource limits of the
recipient computer.
It is possible to apply any of the signing, encrypting, and
compressing operations in any order. It is up to the implementer and
the user to choose. When signing first, the signatories are then
securely obscured by the enveloping. When enveloping first the
signatories are exposed, but it is possible to verify signatures
without removing the enveloping. This can be useful in an
environment were automatic signature verification is desired, as no
private key material is required to verify a signature.
There are security ramifications to choosing whether to sign first or
encrypt first. A recipient of a message that is encrypted and then
signed can validate that the encrypted block was unaltered, but
cannot determine any relationship between the signer and the
unencrypted contents of the message. A recipient of a message that
is signed-then-encrypted can assume that the signed message itself
has not been altered, but that a careful attacker could have changed
the unauthenticated portions of the encrypted message.
When using compression, keep the following guidelines in mind:
- Compression of binary encoded encrypted data is discouraged, since
it will not yield significant compression. Base64 encrypted data
could very well benefit, however.
- If a lossy compression algorithm is used with signing, you will
need to compress first, then sign.
3.7. Creating a Certificate Management Message
The certificate management message or MIME entity is used to
transport certificates and/or certificate revocation lists, such as
in response to a registration request.
Step 1. The certificates and/or certificate revocation lists are
made available to the CMS generating process which creates a CMS
object of type SignedData. The SignedData encapContentInfo eContent
field MUST be absent and signerInfos field MUST be empty.
Ramsdell Standards Track [Page 27]
RFC 3851 S/MIME 3.1 Message Specification July 2004
Step 2. The SignedData object is wrapped in a CMS ContentInfo
object.
Step 3. The ContentInfo object is enclosed in an application/pkcs7-
mime MIME entity.
The smime-type parameter for a certificate management message is
"certs-only". The file extension for this type of message is ".p7c".
3.8. Registration Requests
A sending agent that signs messages MUST have a certificate for the
signature so that a receiving agent can verify the signature. There
are many ways of getting certificates, such as through an exchange
with a certificate authority, through a hardware token or diskette,
and so on.
S/MIME v2 [SMIMEV2] specified a method for "registering" public keys
with certificate authorities using an application/pkcs10 body part.
Since that time, the IETF PKIX Working Group has developed other
methods for requesting certificates. However, S/MIME v3.1 does not
require a particular certificate request mechanism.
3.9. Identifying an S/MIME Message
Because S/MIME takes into account interoperation in non-MIME
environments, several different mechanisms are employed to carry the
type information, and it becomes a bit difficult to identify S/MIME
messages. The following table lists criteria for determining whether
or not a message is an S/MIME message. A message is considered an
S/MIME message if it matches any of the criteria listed below.
The file suffix in the table below comes from the "name" parameter in
the content-type header, or the "filename" parameter on the content-
disposition header. These parameters that give the file suffix are
not listed below as part of the parameter section.
MIME type: application/pkcs7-mime
parameters: any
file suffix: any
MIME type: multipart/signed
parameters: protocol="application/pkcs7-signature"
file suffix: any
MIME type: application/octet-stream
parameters: any
file suffix: p7m, p7s, p7c, p7z
Ramsdell Standards Track [Page 28]
RFC 3851 S/MIME 3.1 Message Specification July 2004
4. Certificate Processing
A receiving agent MUST provide some certificate retrieval mechanism
in order to gain access to certificates for recipients of digital
envelopes. This specification does not cover how S/MIME agents
handle certificates, only what they do after a certificate has been
validated or rejected. S/MIME certificate issues are covered in
[CERT31].
At a minimum, for initial S/MIME deployment, a user agent could
automatically generate a message to an intended recipient requesting
that recipient's certificate in a signed return message. Receiving
and sending agents SHOULD also provide a mechanism to allow a user to
"store and protect" certificates for correspondents in such a way so
as to guarantee their later retrieval.
4.1. Key Pair Generation
All generated key pairs MUST be generated from a good source of non-
deterministic random input [RANDOM] and the private key MUST be
protected in a secure fashion.
If an S/MIME agent needs to generate an RSA key pair, then the S/MIME
agent or some related administrative utility or function SHOULD
generate RSA key pairs using the following guidelines. A user agent
SHOULD generate RSA key pairs at a minimum key size of 768 bits. A
user agent MUST NOT generate RSA key pairs less than 512 bits long.
Creating keys longer than 1024 bits can cause some older S/MIME
receiving agents to not be able to verify signatures, but gives
better security and is therefore valuable. A receiving agent SHOULD
be able to verify signatures with keys of any size over 512 bits.
Some agents created in the United States have chosen to create 512
bit keys in order to get more advantageous export licenses. However,
512 bit keys are considered by many to be cryptographically insecure.
Implementers SHOULD be aware that multiple (active) key pairs can be
associated with a single individual. For example, one key pair can
be used to support confidentiality, while a different key pair can be
used for authentication.
5. Security Considerations
40-bit encryption is considered weak by most cryptographers. Using
weak cryptography in S/MIME offers little actual security over
sending plaintext. However, other features of S/MIME, such as the
specification of tripleDES and the ability to announce stronger
cryptographic capabilities to parties with whom you communicate,
allow senders to create messages that use strong encryption. Using
weak cryptography is never recommended unless the only alternative is
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RFC 3851 S/MIME 3.1 Message Specification July 2004
no cryptography. When feasible, sending and receiving agents SHOULD
inform senders and recipients of the relative cryptographic strength
of messages.
It is impossible for most software or people to estimate the value of
a message. Further, it is impossible for most software or people to
estimate the actual cost of decrypting a message that is encrypted
with a key of a particular size. Further, it is quite difficult to
determine the cost of a failed decryption if a recipient cannot
decode a message. Thus, choosing between different key sizes (or
choosing whether to just use plaintext) is also impossible. However,
decisions based on these criteria are made all the time, and
therefore this specification gives a framework for using those
estimates in choosing algorithms.
If a sending agent is sending the same message using different
strengths of cryptography, an attacker watching the communications
channel might be able to determine the contents of the strongly-
encrypted message by decrypting the weakly-encrypted version. In
other words, a sender SHOULD NOT send a copy of a message using
weaker cryptography than they would use for the original of the
message.
Modification of the ciphertext can go undetected if authentication is
not also used, which is the case when sending EnvelopedData without
wrapping it in SignedData or enclosing SignedData within it.
See RFC 3218 [MMA] for more information about thwarting the adaptive
chosen ciphertext vulnerability in PKCS #1 Version 1.5
implementations.
In some circumstances the use of the Diffie-Hellman key agreement
scheme in a prime order subgroup of a large prime p is vulnerable to
certain attacks known as "small-subgroup" attacks. Methods exist,
however, to prevent these attacks. These methods are described in
RFC 2785 [DHSUB].
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RFC 3851 S/MIME 3.1 Message Specification July 2004
A. ASN.1 Module
SecureMimeMessageV3dot1
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) msg-v3dot1(21) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
-- Cryptographic Message Syntax
SubjectKeyIdentifier, IssuerAndSerialNumber,
RecipientKeyIdentifier
FROM CryptographicMessageSyntax
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2001(14) };
-- id-aa is the arc with all new authenticated and unauthenticated
-- attributes produced the by S/MIME Working Group
id-aa OBJECT IDENTIFIER ::= {iso(1) member-body(2) usa(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) attributes(2)}
-- S/MIME Capabilities provides a method of broadcasting the symmetric
-- capabilities understood. Algorithms SHOULD be ordered by
-- preference and grouped by type
smimeCapabilities OBJECT IDENTIFIER ::=
{iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15}
SMIMECapability ::= SEQUENCE {
capabilityID OBJECT IDENTIFIER,
parameters ANY DEFINED BY capabilityID OPTIONAL }
SMIMECapabilities ::= SEQUENCE OF SMIMECapability
-- Encryption Key Preference provides a method of broadcasting the
-- preferred encryption certificate.
id-aa-encrypKeyPref OBJECT IDENTIFIER ::= {id-aa 11}
SMIMEEncryptionKeyPreference ::= CHOICE {
issuerAndSerialNumber [0] IssuerAndSerialNumber,
receipentKeyId [1] RecipientKeyIdentifier,
subjectAltKeyIdentifier [2] SubjectKeyIdentifier
}
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RFC 3851 S/MIME 3.1 Message Specification July 2004
id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 16 }
id-cap OBJECT IDENTIFIER ::= { id-smime 11 }
-- The preferBinaryInside indicates an ability to receive messages
-- with binary encoding inside the CMS wrapper
id-cap-preferBinaryInside OBJECT IDENTIFIER ::= { id-cap 1 }
-- The following list the OIDs to be used with S/MIME V3
-- Signature Algorithms Not Found in [CMSALG]
--
-- md2WithRSAEncryption OBJECT IDENTIFIER ::=
-- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
-- 2}
--
-- Other Signed Attributes
--
-- signingTime OBJECT IDENTIFIER ::=
-- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
-- 5}
-- See [CMS] for a description of how to encode the attribute
-- value.
SMIMECapabilitiesParametersForRC2CBC ::= INTEGER
-- (RC2 Key Length (number of bits))
END
B. References
B.1. Normative References
[CERT31] Ramsdell, B., Ed., "S/MIME Version 3.1 Certificate
Handling", RFC 3850, July 2004.
[CHARSETS] Character sets assigned by IANA. See
http://www.iana.org/assignments/character-sets
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
3852, July 2004.
[CMSAES] Schaad, J., "Use of the Advanced Encryption Standard
(AES) Encryption Algorithm in Cryptographic Message
Syntax (CMS)", RFC 3565, July 2003.
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RFC 3851 S/MIME 3.1 Message Specification July 2004
[CMSALG] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
[CMSCOMPR] Gutmann, P., "Compressed Data Content Type for
Cryptographic Message Syntax (CMS)", RFC 3274, June
2002.
[CONTDISP] Troost, R., Dorner, S., and K. Moore, "Communicating
Presentation Information in Internet Messages: The
Content-Disposition Header Field", RFC 2183, August
1997.
[ESS] Hoffman, P., "Enhanced Security Services for S/MIME",
RFC 2634, June 1999.
[FIPS180-2] "Secure Hash Signature Standard (SHS)", National
Institute of Standards and Technology (NIST). FIPS
Publication 180-2.
[MIME-SPEC] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", RFC 2045, November 1996.
Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part Two: Media Types", RFC
2046, November 1996.
Moore, K., "MIME (Multipurpose Internet Mail
Extensions) Part Three: Message Header Extensions for
Non-ASCII Text", RFC 2047, November 1996.
Freed, N., Klensin, J., and J. Postel, "Multipurpose
Internet Mail Extensions (MIME) Part Four: Registration
Procedures", BCP 13, RFC 2048, November 1996.
Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part Five: Conformance Criteria
and Examples", RFC 2049, November 1996.
[MIME-SECURE] Galvin, J., Murphy, S., Crocker, S., and N. Freed,
"Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC 1847, October 1995.
[MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[X.208-88] CCITT. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988.
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RFC 3851 S/MIME 3.1 Message Specification July 2004
[X.209-88] CCITT. Recommendation X.209: Specification of Basic
Encoding Rules for Abstract Syntax Notation One
(ASN.1). 1988.
[X.509-88] CCITT. Recommendation X.509: The Directory -
Authentication Framework. 1988.
B.2. Informative References
[DHSUB] Zuccherato, R., "Methods for Avoiding the "Small-
Subgroup" Attacks on the Diffie-Hellman Key Agreement
Method for S/MIME", RFC 2785, March 2000.
[MMA] Rescorla, E., "Preventing the Million Message Attack on
Cryptographic Message Syntax", RFC 3218, January 2002.
[PKCS-7] Kaliski, B., "PKCS #7: Cryptographic Message Syntax
Version 1.5", RFC 2315, March 1998.
[RANDOM] Eastlake 3rd, D., Crocker, S., and J. Schiller,
"Randomness Recommendations for Security", RFC 1750,
December 1994.
[SMIMEV2] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L.,
and L. Repka, "S/MIME Version 2 Message Specification",
RFC 2311, March 1998.
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RFC 3851 S/MIME 3.1 Message Specification July 2004
C. Acknowledgements
Many thanks go out to the other authors of the S/MIME Version 2
Message Specification RFC: Steve Dusse, Paul Hoffman, Laurence
Lundblade and Lisa Repka.
A number of the members of the S/MIME Working Group have also worked
very hard and contributed to this document. Any list of people is
doomed to omission, and for that I apologize. In alphabetical order,
the following people stand out in my mind due to the fact that they
made direct contributions to this document.
Tony Capel
Piers Chivers
Dave Crocker
Bill Flanigan
Peter Gutmann
Paul Hoffman
Russ Housley
William Ottaway
John Pawling
Jim Schaad
D. Editor's Address
Blake Ramsdell
Sendmail, Inc.
704 228th Ave NE #775
Sammamish, WA 98074
EMail: blake@sendmail.com
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RFC 3851 S/MIME 3.1 Message Specification July 2004
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