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rfc:rfc2311

Network Working Group S. Dusse Request for Comments: 2311 RSA Data Security Category: Informational P. Hoffman

                                             Internet Mail Consortium
                                                          B. Ramsdell
                                                            Worldtalk
                                                         L. Lundblade
                                                             Qualcomm
                                                             L. Repka
                                                             Netscape
                                                           March 1998
               S/MIME Version 2 Message Specification

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1998).  All Rights Reserved.

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 privacy and data security
 (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.

Dusse, et. al. Informational [Page 1] RFC 2311 S/MIME Version 2 Message Specification March 1998

 Please note: The information in this document is historical material
 being published for the public record. It is not an IETF standard.
 The use of the word "standard" in this document indicates a standard
 for adopters of S/MIME version 2, not an IETF standard.

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 memo defines how to create a MIME body part that has been
 cryptographically enhanced according to PKCS #7 [PKCS-7]. This memo
 also defines the application/pkcs7-mime MIME type that can be used to
 transport those body parts. This memo also defines how to create
 certification requests that conform to PKCS #10 [PKCS-10], and the
 application/pkcs10 MIME type for transporting those requests.
 This memo also discusses how to use the multipart/signed MIME type
 defined in [MIME-SECURE] to transport S/MIME signed messages. This
 memo also defines the application/pkcs7-signature MIME type, which is
 also used to transport S/MIME signed messages. This specification is
 compatible with PKCS #7 in that it uses the data types defined by
 PKCS #7.
 In order to create S/MIME messages, an agent has to follow
 specifications in this memo, as well as some of the specifications
 listed in the following documents:
  1. "PKCS #1: RSA Encryption", [PKCS-1]
  2. "PKCS #7: Cryptographic Message Syntax", [PKCS-7]
  3. "PKCS #10: Certification Request Syntax", [PKCS-10]
 Throughout this memo, 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

Dusse, et. al. Informational [Page 2] RFC 2311 S/MIME Version 2 Message Specification March 1998

 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

 Throughout this memo, the terms MUST, MUST NOT, SHOULD, and SHOULD
 NOT are used in capital letters. This conforms to the definitions in
 [MUSTSHOULD].  [MUSTSHOULD] defines the use of these key words to
 help make the intent of standards track documents as clear as
 possible. The same key words are used in this document to help
 implementors achieve interoperability.

1.3 Definitions

 For the purposes of this memo, the following definitions apply.
 ASN.1: Abstract Syntax Notation One, as defined in CCITT X.208.
 BER: Basic Encoding Rules for ASN.1, as defined in CCITT X.209.
 Certificate: A type that binds an entity's distinguished name to a
 public key with a digital signature.
 DER: Distinguished Encoding Rules for ASN.1, as defined in CCITT
 X.509.
 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 may be sent via a channel that only transmits 7-bit
 data.

1.4 Compatibility with Prior Practice of S/MIME

 Appendix C contains important information about how S/MIME agents
 following this specification should act in order to have the greatest
 interoperability with earlier implementations of S/MIME.

Dusse, et. al. Informational [Page 3] RFC 2311 S/MIME Version 2 Message Specification March 1998

2. PKCS #7 Options

 The PKCS #7 message format 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.

2.1 DigestAlgorithmIdentifier

 Receiving agents MUST support SHA-1 [SHA1] and MD5 [MD5].
 Sending agents SHOULD use SHA-1.

2.2 DigestEncryptionAlgorithmIdentifier

 Receiving agents MUST support rsaEncryption, defined in [PKCS-1].
 Receiving agents MUST support verification of signatures using RSA
 public key sizes from 512 bits to 1024 bits.
 Sending agents MUST support rsaEncryption. Outgoing messages are
 signed with a user's private key. The size of the private key is
 determined during key generation.

2.3 KeyEncryptionAlgorithmIdentifier

 Receiving agents MUST support rsaEncryption. Incoming encrypted
 messages contain symmetric keys which are to be decrypted with a
 user's private key.  The size of the private key is determined during
 key generation.
 Sending agents MUST support rsaEncryption. Sending agents MUST
 support encryption of symmetric keys with RSA public keys at key
 sizes from 512 bits to 1024 bits.

2.4 General Syntax

 The PKCS #7 defines six distinct content types: "data", "signedData",
 "envelopedData", "signedAndEnvelopedData", "digestedData", and
 "encryptedData". Receiving agents MUST support the "data",
 "signedData" and "envelopedData" content types. Sending agents may or
 may not send out any of the content types, depending on the services
 that the agent supports.

2.4.1 Data Content Type

 Sending agents MUST use the "data" content type as the content within
 other content types to indicate the message content which has had
 security services applied to it.

Dusse, et. al. Informational [Page 4] RFC 2311 S/MIME Version 2 Message Specification March 1998

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.

2.4.3 EnvelopedData Content Type

 This content type is used to apply privacy protection to a message. A
 sender needs to have access to a public key for each intended message
 recipient to use this service. This content type does not provide
 authentication.

2.5 Attribute SignerInfo Type

 The SignerInfo type allows the inclusion of unauthenticated and
 authenticated 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 described in this section.
 Sending agents SHOULD be able to generate one instance of each of the
 signed attributes described in this section, and SHOULD include these
 attributes in each signed message sent.
 Additional attributes and values for these attributes may be defined
 in the future. Receiving agents SHOULD handle attributes or values
 that it does not recognize in a graceful manner.

2.5.1 Signing-Time Attribute

 The signing-time attribute is used to convey the time that a message
 was signed. Until there are trusted timestamping services, 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. 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 S/MIME Capabilities Attribute

 The S/MIME capabilities attribute includes signature algorithms (such
 as "md5WithRSAEncryption"), symmetric algorithms (such as "DES-CBC"),
 and key encipherment algorithms (such as "rsaEncryption"). It also

Dusse, et. al. Informational [Page 5] RFC 2311 S/MIME Version 2 Message Specification March 1998

 includes a non-algorithm capability which is the preference for
 signedData.  SMIMECapabilities was 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.
 The semantics of the S/MIME capabilites attribute specify a partial
 list as to what the client announcing the SMIMECapabilites can
 support. A client does not have to list every capability it supports,
 and probably should not list all its capabilities so that the
 capabilities list doesn't get too long. In an SMIMECapabilities
 encoding, the OIDs are listed in order of their preference, but
 SHOULD be logically separated along the lines of their categories
 (signature algorithms, symmetric algorithms, key encipherment
 algorithms, etc.)
 The structure of  SMIMECapabilities was designed 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.
 In the case of symmetric algorithms, 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 block size for RC5 must be specified in addition to the
 key length.
 There is a list of OIDs (the registered SMIMECapability list) that is
 centrally maintained and is separate from this memo. The list of OIDs
 is maintained by the Internet Mail Consortium at
 <http://www.imc.org/ietf-smime/oids.html>.
 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 memo,
 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 S/MIME capabilities 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 S/MIME capabilities 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.

Dusse, et. al. Informational [Page 6] RFC 2311 S/MIME Version 2 Message Specification March 1998

 Additional values for SMIMECapability may be defined in the future.
 Receiving agents MUST handle a SMIMECapabilities object that has
 values that it does not recognize in a graceful manner.

2.6 ContentEncryptionAlgorithmIdentifier

 Receiving agents MUST support decryption using the RC2 [RC2] or a
 compatible algorithm at a key size of 40 bits, hereinafter called
 "RC2/40".  Receiving agents SHOULD support decryption using DES EDE3
 CBC, hereinafter called "tripleDES" [3DES] [DES].
 Sending agents SHOULD support encryption with RC2/40 and tripleDES.

2.6.1 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 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.
  1. 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.
  1. 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 capabilitie lists 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

Dusse, et. al. Informational [Page 7] RFC 2311 S/MIME Version 2 Message Specification March 1998

 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 2.6.2.1 through 2.6.2.4 describe the decisions a sending
 agent SHOULD use in deciding which type of encryption should be
 applied to a message. These rules are ordered, so the sending agent
 SHOULD make its decision in the order given.

2.6.2.1 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) for which 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.

2.6.2.2 Rule 2: Unknown Capabilities, Known Use of Encryption

 If:
  - the sending agent has no knowledge of the encryption capabilities
    of the recipient,
  - and the sending agent has received at least one message from the
    recipient,
  - and the last encrypted message received from the recipient had a
    trusted signature on it,
 then the outgoing message SHOULD use the same encryption algorithm as
 was used on the last signed and encrypted message received from the
 recipient.

2.6.2.3 Rule 3: Unknown Capabilities, Risk of Failed Decryption

 If:
  - the sending agent has no knowledge of the encryption capabilities
    of the recipient,
  - and the sending agent is willing to risk that the recipient may
    not be able to decrypt the message,
 then the sending agent SHOULD use tripleDES.

Dusse, et. al. Informational [Page 8] RFC 2311 S/MIME Version 2 Message Specification March 1998

2.6.2.4 Rule 4: Unknown Capabilities, No Risk of Failed Decryption

 If:
  - the sending agent has no knowledge of the encryption capabilities
    of the recipient,
  - and the sending agent is not willing to risk that the recipient
    may not be able to decrypt the message,
 then the sending agent MUST use RC2/40.

2.6.3 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.

2.6.4 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. It should be noted that if the sending agent
 chooses to send a message encrypted with a strong algorithm, and then
 send the same message encrypted with a weak algorithm, someone
 watching the communications channel can decipher 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 PKCS
 objects. Several MIME types as well as several PKCS objects 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 PKCS processing facilities which produces a
 PKCS object. The PKCS object is then finally wrapped in MIME.
 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].

Dusse, et. al. Informational [Page 9] RFC 2311 S/MIME Version 2 Message Specification March 1998

3.1 Preparing the MIME Entity for Signing or Enveloping

 S/MIME is used to secure MIME entities. A MIME entity may 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.
 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 PKCS #7 objects 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
 when signing.  Description of the procedures from [MIME-SPEC] are
 repeated here, but the reader should 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 be
 signed, enveloped, or both signed and enveloped. 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 in order that the message can be
 forwarded to any environment without modification.
 These steps are descriptive rather than prescriptive. The implementor
 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 removed, 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.

Dusse, et. al. Informational [Page 10] RFC 2311 S/MIME Version 2 Message Specification March 1998

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 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 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 at
 all is required.  S/MIME implementations MUST be able to deal with
 binary MIME objects. If no Content-Transfer-Encoding header is
 present, the transfer encoding should be considered 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

Dusse, et. al. Informational [Page 11] RFC 2311 S/MIME Version 2 Message Specification March 1998

 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.

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 clear. 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.
 [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 must 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.

Dusse, et. al. Informational [Page 12] RFC 2311 S/MIME Version 2 Message Specification March 1998

     Content-Type: multipart/mixed; boundary=bar
  1. -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?
     I agree. 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
  1. -bar

Content-Type: image/jpeg

     Content-Transfer-Encoding: base64
     iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
     jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
     uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
     HOxEa44b+EI=
  1. -bar–

3.2 The application/pkcs7-mime Type

 The application/pkcs7-mime type is used to carry PKCS #7 objects of
 several types including envelopedData and signedData. The details of
 constructing these entities is described in subsequent sections. This
 section describes the general characteristics of the
 application/pkcs7-mime type.
 This MIME type always carries a single PKCS #7 object. The PKCS #7
 object must always be BER encoding of the ASN.1 syntax describing the
 object. The contentInfo field of the carried PKCS #7 object always
 contains a MIME entity that is prepared as described in section 3.1.
 The contentInfo field must never be empty.
 Since PKCS #7 objects 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.

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 Note that this discussion refers to the transfer encoding of the PKCS
 #7 object or "outside" MIME entity. It is completely distinct from,
 and unrelated to, the transfer encoding of the MIME entity secured by
 the PKCS #7 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:
 MIME Type                      File Extension
 application/pkcs7-mime              .p7m
 (signedData, envelopedData)
 application/pkcs7-mime              .p7c
 (degenerate signedData
 "certs-only" message)
 application/pkcs7-signature         .p7s
 application/pkcs10                  .p10
 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

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 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.

3.3 Creating an Enveloped-only Message

 This section describes the format for enveloping a MIME entity
 without signing it.
   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
           PKCS #7 object of type envelopedData.
   Step 3. The PKCS #7 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 and SignedData, and multipart/signed. In
 general, the multipart/signed form is preferred for sending, and
 receiving agents SHOULD 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
 should be chosen because it depends on the capabilities of all the

Dusse, et. al. Informational [Page 15] RFC 2311 S/MIME Version 2 Message Specification March 1998

 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.
 Messages signed using the signedData format cannot be viewed by a
 recipient unless they have S/MIME facilities. However, if they have
 S/MIME facilities, these messages can always be verified if they were
 not changed in transit.

3.4.2 Signing Using application/pkcs7-mime and 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
           PKCS #7 object of type signedData
   Step 3. The PKCS #7 object is inserted into an
           application/pkcs7-mime MIME entity
 The smime-type parameter for messages using application/pkcs7-mime
 and 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

Dusse, et. al. Informational [Page 16] RFC 2311 S/MIME Version 2 Message Specification March 1998

3.4.3 Signing Using the multipart/signed Format

 This format is a clear-signing format. Recipients without any S/MIME
 or PKCS 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 to be signed; the second part contains the
 signature, which is a PKCS #7 detached signature.

3.4.3.1 The application/pkcs7-signature MIME Type

 This MIME type always contains a single PKCS #7 object of type
 signedData.  The contentInfo field of the PKCS #7 object must be
 empty. The signerInfos field contains the signatures for the MIME
 entity. The details of the registered type are given in Appendix D.
 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 PKCS #7 processing in order
           to obtain an object of type signedData with an empty
           contentInfo field.
   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 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.

Dusse, et. al. Informational [Page 17] RFC 2311 S/MIME Version 2 Message Specification March 1998

 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 used in the calculation of
 the Message Integrity Check. The value of the micalg parameter SHOULD
 be one of the following:
 Algorithm used     Value
 --------------     ---------
 MD5                md5
 SHA-1              sha1
 any other          unknown
 (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.

3.4.3.3 Sample multipart/signed Message

     Content-Type: multipart/signed;
        protocol="application/pkcs7-signature";
        micalg=sha1; boundary=boundary42
  1. -boundary42

Content-Type: text/plain

     This is a clear-signed message.
  1. -boundary42

Content-Type: application/pkcs7-signature; name=smime.p7s

     Content-Transfer-Encoding: base64
     Content-Disposition: attachment; filename=smime.p7s
     ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
     4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
     n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
     7GhIGfHfYT64VQbnj756
  1. -boundary42–

3.5 Signing and Encrypting

 To achieve signing and enveloping, any of the signed-only and
 encrypted-only formats may be nested. This is allowed because the
 above formats are all MIME entities, and because they all secure MIME
 entities.

Dusse, et. al. Informational [Page 18] RFC 2311 S/MIME Version 2 Message Specification March 1998

 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 either sign a message first, or to envelope the
 message first. It is up to the implementor 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 may be useful in an environment were automatic signature
 verification is desired, as no private key material is required to
 verify a signature.

3.6 Creating a Certificates-only Message

 The certificates only message or MIME entity is used to transport
 certificates, such as in response to a registration request. This
 format can also be used to convey CRLs.
   Step 1. The certificates are made available to the PKCS #7
           generating process which creates a PKCS #7 object of type
           signedData.  The contentInfo and signerInfos fields must be
           empty.
   Step 2. The PKCS #7 signedData object is enclosed in an
           application/pkcs7-mime MIME entity
 The smime-type parameter for a certs-only message is "certs-only".
 The file extension for this type of message is ".p7c".

3.7 Creating a Registration Request

 A typical application which allows a user to generate cryptographic
 information has to submit that information to a certification
 authority, who transforms it into a certificate. PKCS #10 describes a
 syntax for certification requests. The application/pkcs10 body type
 MUST be used to transfer a PKCS #10 certification request.
 The details of certification requests and the process of obtaining a
 certificate are beyond the scope of this memo. Instead, only the
 format of data used in application/pkcs10 is defined.

3.7.1 Format of the application/pkcs10 Body

 PKCS #10 defines the ASN.1 type CertificationRequest for use in
 submitting a certification request. Therefore, when the MIME content
 type application/pkcs10 is used, the body MUST be a
 CertificationRequest, encoded using the Basic Encoding Rules (BER).

Dusse, et. al. Informational [Page 19] RFC 2311 S/MIME Version 2 Message Specification March 1998

 Although BER is specified, instead of the more restrictive DER, a
 typical application will use DER since the CertificationRequest's
 CertificationRequestInfo has to be DER-encoded in order to be signed.
 A robust application SHOULD output DER, but allow BER or DER on
 input.
 Data produced by BER or DER is 8-bit, but many transports are limited
 to 7-bit data. Therefore, a suitable 7-bit Content-Transfer-Encoding
 SHOULD be applied. The base64 Content-Transfer-Encoding SHOULD be
 used with application/pkcs10, although any 7-bit transfer encoding
 may work.

3.7.2 Sending and Receiving an application/pkcs10 Body Part

 For sending a certificate-signing request, the application/pkcs10
 message format MUST be used to convey a PKCS #10 certificate-signing
 request. Note that for sending certificates and CRLs messages without
 any signed content, the application/pkcs7-mime message format MUST be
 used to convey a degenerate PKCS #7 signedData "certs-only" message.
 To send an application/pkcs10 body, the application generates the
 cryptographic information for the user. The details of the
 cryptographic information are beyond the scope of this memo.
   Step 1. The cryptographic information is placed within a PKCS #10
           CertificationRequest.
   Step 2. The CertificationRequest is encoded according to BER or DER
           (typically, DER).
   Step 3. As a typical step, the DER-encoded CertificationRequest is
           also base64 encoded so that it is 7-bit data suitable for
           transfer in SMTP. This then becomes the body of an
           application/pkcs10 body part.
 The result might look like this:
     Content-Type: application/pkcs10; name=smime.p10
     Content-Transfer-Encoding: base64
     Content-Disposition: attachment; filename=smime.p10
     rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6
     7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H
     f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
     0GhIGfHfQbnj756YT64V
 A typical application only needs to send a certification request. It
 is a certification authority that has to receive and process the

Dusse, et. al. Informational [Page 20] RFC 2311 S/MIME Version 2 Message Specification March 1998

 request. The steps for recovering the CertificationRequest from the
 message are straightforward but are not presented here. The
 procedures for processing the certification request are beyond the
 scope of this document.

3.8 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 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:   application/pkcs10
 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, aps, p7c, p10

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 memo does not cover how S/MIME agents handle
 certificates, only what they do after a certificate has been
 validated or rejected. S/MIME certification issues are covered in a
 different document.
 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

Dusse, et. al. Informational [Page 21] RFC 2311 S/MIME Version 2 Message Specification March 1998

 "store and protect" certificates for correspondents in such a way so
 as to guarantee their later retrieval.

4.1 Key Pair Generation

 An S/MIME agent or some related administrative utility or function
 MUST be capable of generating RSA key pairs on behalf of the user.
 Each key pair MUST be generated from a good source of non-
 deterministic random input and protected in a secure fashion.
 A user agent SHOULD generate RSA key pairs at a minimum key size of
 768 bits and a maximum key size of 1024 bits. A user agent MUST NOT
 generate RSA key pairs less than 512 bits long. 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.
 Implementors should be aware that multiple (active) key pairs may be
 associated with a single individual. For example, one key pair may be
 used to support confidentiality, while a different key pair may be
 used for authentication.

5. Security Considerations

 This entire memo discusses security. Security issues not covered in
 other parts of the memo include:
 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
 no cryptography. When feasible, sending and receiving agents should
 inform senders and recipients 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 memo gives a framework for using those estimates in
 choosing algorithms.

Dusse, et. al. Informational [Page 22] RFC 2311 S/MIME Version 2 Message Specification March 1998

 If a sending agent is sending the same message using different
 strengths of cryptography, an attacker watching the communications
 channel can 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.

Dusse, et. al. Informational [Page 23] RFC 2311 S/MIME Version 2 Message Specification March 1998

A. Object Identifiers and Syntax

 The syntax for SMIMECapability is:
 SMIMECapability ::= SEQUENCE {
     capabilityID OBJECT IDENTIFIER,
     parameters OPTIONAL ANY DEFINED BY capabilityID }
 SMIMECapabilities ::= SEQUENCE OF SMIMECapability

A.1 Content Encryption Algorithms

RC2-CBC OBJECT IDENTIFIER ::= {iso(1) member-body(2) us(840) rsadsi(113549) encryptionAlgorithm(3) 2}

For the effective-key-bits (key size) greater than 32 and less than 256, the RC2-CBC algorithm parameters are encoded as:

RC2-CBC parameter ::= SEQUENCE { rc2ParameterVersion INTEGER, iv OCTET STRING (8)}

For the effective-key-bits of 40, 64, and 128, the rc2ParameterVersion values are 160, 120, 58 respectively.

DES-EDE3-CBC OBJECT IDENTIFIER ::= {iso(1) member-body(2) us(840) rsadsi(113549) encryptionAlgorithm(3) 7}

For DES-CBC and DES-EDE3-CBC, the parameter should be encoded as:

CBCParameter :: IV

where IV ::= OCTET STRING – 8 octets.

A.2 Digest Algorithms

md5 OBJECT IDENTIFIER ::= {iso(1) member-body(2) us(840) rsadsi(113549) digestAlgorithm(2) 5}

sha-1 OBJECT IDENTIFIER ::= {iso(1) identified-organization(3) oiw(14) secsig(3) algorithm(2) 26}

A.3 Asymmetric Encryption Algorithms

rsaEncryption OBJECT IDENTIFIER ::= {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1}

Dusse, et. al. Informational [Page 24] RFC 2311 S/MIME Version 2 Message Specification March 1998

rsa OBJECT IDENTIFIER ::=

   {joint-iso-ccitt(2) ds(5) algorithm(8) encryptionAlgorithm(1) 1}

A.4 Signature Algorithms

md2WithRSAEncryption OBJECT IDENTIFIER ::=

   {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 2}

md5WithRSAEncryption OBJECT IDENTIFIER ::=

   {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 4}

sha-1WithRSAEncryption OBJECT IDENTIFIER ::=

   {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 5}

A.5 Signed Attributes

signingTime OBJECT IDENTIFIER ::=

   {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 5}

smimeCapabilities OBJECT IDENTIFIER ::=

  {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15}

Dusse, et. al. Informational [Page 25] RFC 2311 S/MIME Version 2 Message Specification March 1998

B. References

 [3DES] W. Tuchman, "Hellman Presents No Shortcut Solutions To DES,"
 IEEE Spectrum, v. 16, n. 7, July 1979, pp40-41.
 [CHARSETS] Character sets assigned by IANA. See
 <ftp://ftp.isi.edu/in-notes/iana/assignments/character-sets>.
 [CONTDISP] Troost, R., Dorner, S and K. Moore, "Communicating
 Presentation Information in Internet Messages:  The Content-
 Disposition Header Field", RFC 2183, August 1997.
 [DES] ANSI X3.106, "American National Standard for Information
 Systems-Data Link Encryption," American National Standards Institute,
 1983.
 [MD5] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, April
 1992.
 [MIME-SPEC] The primary definition of MIME.
 Freed, N., and N. Borenstein, "MIME Part 1: Format of Internet
 Message Bodies", RFC 2045, November 1996.
 Freed, N., and N. Borenstein, "MIME Part 2: Media Types", RFC 2046,
 November 1996.
 Moore, K., "MIME Part 3: Message Header Extensions for Non-ASCII
 Text", RFC 2047, November 1996.
 Freed, N., Klensin, J., and J. Postel, "MIME Part 4: Registration
 Procedures", RFC 2048, November 1996.
 Freed, N., and N. Borenstein, "MIME Part 5: 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.
 [PKCS-1] Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", RFC
 2313, March 1998.
 [PKCS-7] Kaliski, B., "PKCS #7: Cryptographic Message Syntax Version
 1.5", RFC 2315, March 1998.

Dusse, et. al. Informational [Page 26] RFC 2311 S/MIME Version 2 Message Specification March 1998

 [PKCS-10] Kaliski, B., "PKCS #10: Certification Request Syntax
 Version 1.5", RFC 2314, March 1998.
 [RC2] Rivest, R., "Description of the RC2(r) Encryption Algorithm",
 RFC 2268, January 1998.
 [SHA1] NIST FIPS PUB 180-1, "Secure Hash Standard," National
 Institute of Standards and Technology, U.S. Department of Commerce,
 DRAFT, 31 May 1994.

Dusse, et. al. Informational [Page 27] RFC 2311 S/MIME Version 2 Message Specification March 1998

C. Compatibility with Prior Practice in S/MIME

 S/MIME was originally developed by RSA Data Security, Inc. Many
 developers implemented S/MIME agents before this document was
 published. All S/MIME receiving agents SHOULD make every attempt to
 interoperate with these earlier implementations of S/MIME.

C.1 Early MIME Types

 Some early implementations of S/MIME agents used the following MIME
 types:
 application/x-pkcs7-mime
 application/x-pkcs7-signature
 application/x-pkcs10
 In each case, the "x-" subtypes correspond to the subtypes described
 in this document without the "x-".

C.2 Profiles

 Early S/MIME documentation had two profiles for encryption:
 "restricted" and "unrestricted". The difference between these
 profiles historically came about due to US Government export
 regulations, as described at the end of this section. It is expected
 that in the future, there will be few agents that only use the
 restricted profile.
 Briefly, the restricted profile required the ability to encrypt and
 decrypt using RSA's trade-secret RC2 algorithm in CBC mode with 40-
 bit keys. The unrestricted profile required the ability to encrypt
 and decrypt using RSA's trade-secret RC2 algorithm in CBC mode with
 40-bit keys, and to encrypt and decrypt using tripleDES. The
 restricted profile also had non-mandatory suggestions for other
 algorithms, but these were not widely implemented.
 It is important to note that many current implementations of S/MIME
 use the restricted profile.

C.2.1 Historical Reasons for the Existence of Two Encryption Profiles

 Due to US Government export regulations, an S/MIME agent which
 supports a strong content encryption algorithm such as DES would not
 be freely exportable outside of North America. US software
 manufacturers have been compelled to incorporate an exportable or
 "restricted" content encryption algorithm in order to create a widely
 exportable version of their product.  S/MIME agents created in the US
 and intended for US domestic use (or use under special State

Dusse, et. al. Informational [Page 28] RFC 2311 S/MIME Version 2 Message Specification March 1998

 Department export licenses) can utilize stronger, "unrestricted"
 content encryption. However, in order to achieve interoperability,
 such agents need to support whatever exportable algorithm is
 incorporated in restricted S/MIME agents.
 The RC2 symmetric encryption algorithm has been approved by the US
 Government for "expedited" export licensing at certain key sizes.
 Consequently, support for the RC2 algorithm in CBC mode is required
 for baseline interoperability in all S/MIME implementations. Support
 for other strong symmetric encryption algorithms such as RC5 CBC, DES
 CBC and DES EDE3-CBC for content encryption is strongly encouraged
 where possible.

Dusse, et. al. Informational [Page 29] RFC 2311 S/MIME Version 2 Message Specification March 1998

D. Request for New MIME Subtypes

D.1 application/pkcs7-mime

 To: ietf-types@iana.org
 Subject: Registration of MIME media type application/pkcs7-mime
 MIME media type name: application
 MIME subtype name: pkcs7-mime
 Required parameters: none
 Optional parameters: name, filename, smime-type
 Encoding considerations: Will be binary data, therefore should use
 base64 encoding
 Security considerations: Described in [PKCS-7]
 Interoperability considerations: Designed to carry data formatted
 with PKCS-7, as described in [PKCS-7]
 Published specification: RFC 2311
 Applications which use this media type: Secure Internet mail and
 other secure data transports.
 Additional information:
 File extension(s): .p7m and .p7c
 Macintosh File Type Code(s):
 Person & email address to contact for further information:
 Steve Dusse, spock@rsa.com
 Intended usage: COMMON

D.2 application/pkcs7-signature

 To: ietf-types@iana.org
 Subject: Registration of MIME media type application/pkcs7-signature
 MIME media type name: application
 MIME subtype name: pkcs7-signature
 Required parameters: none

Dusse, et. al. Informational [Page 30] RFC 2311 S/MIME Version 2 Message Specification March 1998

 Optional parameters: name, filename
 Encoding considerations: Will be binary data, therefore should use
 base64 encoding
 Security considerations: Described in [PKCS-7]
 Interoperability considerations: Designed to carry digital
 signatures with PKCS-7, as described in [PKCS-7]
 Published specification: RFC 2311
 Applications which use this media type: Secure Internet mail and
 other secure data transports.
 Additional information:
 File extension(s): .p7s
 Macintosh File Type Code(s):
 Person & email address to contact for further information:
 Steve Dusse, spock@rsa.com
 Intended usage: COMMON

D.3 application/pkcs10

 To: ietf-types@iana.org
 Subject: Registration of MIME media type application/pkcs10
 MIME media type name: application
 MIME subtype name: pkcs10
 Required parameters: none
 Optional parameters: name, filename
 Encoding considerations: Will be binary data, therefore should use
 base64 encoding
 Security considerations: Described in [PKCS-10]
 Interoperability considerations: Designed to carry digital
 certificates formatted with PKCS-10, as described in [PKCS-10]
 Published specification: RFC 2311
 Applications which use this media type: Secure Internet mail and

Dusse, et. al. Informational [Page 31] RFC 2311 S/MIME Version 2 Message Specification March 1998

 other transports where certificates are required.
 Additional information:
 File extension(s): .p10
 Macintosh File Type Code(s):
 Person & email address to contact for further information:
 Steve Dusse, spock@rsa.com
 Intended usage: COMMON

Dusse, et. al. Informational [Page 32] RFC 2311 S/MIME Version 2 Message Specification March 1998

E. Encapsulating Signed Messages for Internet Transport

 The rationale behind the multiple formats for signing has to do with
 the MIME subtype defaulting rules of the application and multipart
 top-level types, and the behavior of currently deployed gateways and
 mail user agents.
 Ideally, the multipart/signed format would be the only format used
 because it provides a truly backwards compatible way to sign MIME
 entities. In a pure MIME environment with very capable user agents,
 this would be possible. The world, however, is more complex than
 this.
 One problem with the multipart/signed format occurs with gateways to
 non-MIME environments. In these environments, the gateway will
 generally not be S/MIME aware, will not recognize the
 multipart/signed type, and will default its treatment to
 multipart/mixed as is prescribed by the MIME standard. The real
 problem occurs when the gateway also applies conversions to the MIME
 structure of the original message that is being signed and is
 contained in the first part of the multipart/signed structure, such
 as the gateway converting text and attachments to the local format.
 Because the signature is over the MIME structure of the original
 message, but the original message is now decomposed and transformed,
 the signature cannot be verified. Because MIME encoding of a
 particular set of body parts can be done in many different ways,
 there is no way to reconstruct the original MIME entity over which
 the signature was computed.
 A similar problem occurs when an attempt is made to combine an
 existing user agent with a stand-alone S/MIME facility. Typical user
 agents do not have the ability to make a multipart sub-entity
 available to a stand-alone application in the same way they make leaf
 MIME entities available to "viewer" applications. This user agent
 behavior is not required by the MIME standard and thus not widely
 implemented. The result is that it is impossible for most user agents
 to hand off the entire multipart/signed entity to a stand-alone
 application.

E.1 Solutions to the Problem

 To work around these two problems, the application/pkcs7-mime type
 can be used. When going through a gateway, it will be defaulted to
 the MIME type of application/octet-stream and treated as a single
 opaque entity. That is, the message will be treated as an attachment
 of unknown type, converted into the local representation for an
 attachment and thus can be made available to an S/MIME facility
 completely intact. A similar result is achieved when a user agent

Dusse, et. al. Informational [Page 33] RFC 2311 S/MIME Version 2 Message Specification March 1998

 similarly treats the application/pkcs7-mime MIME entity as a simple
 leaf node of the MIME structure and makes it available to viewer
 applications.
 Another way to work around these problems is to encapsulate the
 multipart/signed MIME entity in a MIME entity that will not be
 damaged by the gateway. At the time that this memo is being written,
 there is a proposal for a MIME entity "application/mime" for this
 purpose. However, no implementations of S/MIME use this type of
 wrapping.

E.2 Encapsulation in an Non-MIME Environment

 While this document primarily addresses the Internet, it is useful to
 compose and receive S/MIME secured messages in non-MIME environments.
 This is particularly the case when it is desired that security be
 implemented end-to-end. Other discussion here addresses the receipt
 of S/MIME messages in non-MIME environments. Here the composition of
 multipart/signed entities is addressed.
 When a message is to be sent in such an environment, the
 multipart/signed entity is created as described above. That entity is
 then treated as an opaque stream of bits and added to the message as
 an attachment. It must have a file name that ends with ".aps", as
 this is the sole mechanism for recognizing it as an S/MIME message by
 the receiving agent.
 When this message arrives in a MIME environment, it is likely to have
 a MIME type of application/octet-stream, with MIME parameters giving
 the filename for the attachment. If the intervening gateway has
 carried the file type, it will end in ".aps" and be recognized as an
 S/MIME message.

Dusse, et. al. Informational [Page 34] RFC 2311 S/MIME Version 2 Message Specification March 1998

F. Acknowledgements

 Significant contributions to the content of this memo were made by
 many people, including Jim Schaad, Jeff Thompson, and Jeff Weinstein.

G. Authors' Addresses

 Steve Dusse
 RSA Data Security, Inc.
 100 Marine Parkway, #500
 Redwood City, CA  94065  USA
 Phone: (415) 595-8782
 EMail: spock@rsa.com
 Paul Hoffman
 Internet Mail Consortium
 127 Segre Place
 Santa Cruz, CA  95060
 Phone: (408) 426-9827
 EMail: phoffman@imc.org
 Blake Ramsdell
 Worldtalk
 13122 NE 20th St., Suite C
 Bellevue, WA 98005
 Phone: (425) 882-8861
 EMail: blaker@deming.com
 Laurence Lundblade
 QUALCOMM Incorporated
 Eudora Division
 6455 Lusk Boulevard
 San Diego, California 92121-2779
 Phone: (800) 238-3672
 EMail: lgl@qualcomm.com

Dusse, et. al. Informational [Page 35] RFC 2311 S/MIME Version 2 Message Specification March 1998

 Lisa Repka
 Netscape Communications Corporation
 501 East Middlefield Road
 Mountain View, CA  94043
 Phone: (415) 254-1900
 EMail: repka@netscape.com

Dusse, et. al. Informational [Page 36] RFC 2311 S/MIME Version 2 Message Specification March 1998

H. Full Copyright Statement

 Copyright (C) The Internet Society (1998).  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.

Dusse, et. al. Informational [Page 37]

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