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RFC 2311 - S/MIME Version 2 Message Specification

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Network Working Group                                          S. Dusse
Request for Comments: 2311                            RSA Data Security
Category: Informational                                      P. Hoffman
                                               Internet Mail Consortium
                                                            B. Ramsdell
                                                           L. Lundblade
                                                               L. Repka
                                                             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.

   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:

    - "PKCS #1: RSA Encryption", [PKCS-1]
    - "PKCS #7: Cryptographic Message Syntax", [PKCS-7]
    - "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

   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

   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

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.

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.

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

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

   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

   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.

   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

   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.

    - 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 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

   Before sending a message, the sending agent MUST decide whether it is
   willing to use weak encryption for the particular data in the

   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 through 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. 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. Rule 2: Unknown Capabilities, Known Use of Encryption

    - 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
    - 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. Rule 3: Unknown Capabilities, Risk of Failed Decryption

    - 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. Rule 4: Unknown Capabilities, No Risk of Failed Decryption

    - 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

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].

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

     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.

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

   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

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.

       Content-Type: multipart/mixed; boundary=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

       Content-Type: image/jpeg
       Content-Transfer-Encoding: base64



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.

   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

   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.

3.3 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;
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7m


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

   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;
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7m


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. 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". 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

     Step 5. The MIME entity of the application/pkcs7-signature is
             inserted into the second part of the multipart/signed

   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 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. Sample multipart/signed Message

       Content-Type: multipart/signed;
          micalg=sha1; boundary=boundary42

       Content-Type: text/plain

       This is a clear-signed message.

       Content-Type: application/pkcs7-signature; name=smime.p7s
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7s



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

   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

     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).

   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

   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

     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


   A typical application only needs to send a certification request. It
   is a certification authority that has to receive and process the

   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

   "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

   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.

   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.

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

 {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.

 {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

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

 {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}

     {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

     {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}

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

   [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,

   [MD5] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, April

   [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.

   [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.

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


   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

   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.

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

   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

   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

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

   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

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

   similarly treats the application/pkcs7-mime MIME entity as a simple
   leaf node of the MIME structure and makes it available to viewer

   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

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.

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
   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

   Lisa Repka
   Netscape Communications Corporation
   501 East Middlefield Road
   Mountain View, CA  94043

   Phone: (415) 254-1900
   EMail: repka@netscape.com

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

   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


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