# RFC 2633 (RFC2633)

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### RFC 2633 - S/MIME Version 3 Message Specification

    Network Working Group                               B. Ramsdell, Editor
    Request for Comments: 2633                                    Worldtalk
    Category: Standards Track                                     June 1999

                     S/MIME Version 3 Message Specification

    Status of this Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (1999).  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.

    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 CMS [CMS], which is derived
       from 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 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.

       In order to create S/MIME messages, an S/MIME agent has to follow
       specifications in this memo, as well as the specifications listed in
       the Cryptographic Message Syntax [CMS].

       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

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
       document are to be interpreted as described in [MUSTSHOULD].

    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.

       Receiving agent: software that interprets and processes S/MIME CMS
       objects, MIME body parts that contain CMS objects, or both.

       Sending agent: software that creates S/MIME CMS objects, MIME body
       parts that contain CMS objects, or both.

       S/MIME agent: user software that is a receiving agent, a sending
       agent, or both.

    1.4 Compatibility with Prior Practice of S/MIME

       S/MIME version 3 agents should attempt to have the greatest
       interoperability possible with S/MIME version 2 agents. S/MIME
       version 2 is described in RFC 2311 through RFC 2315, inclusive. RFC
       2311 also has historical information about the development of S/MIME.

    2. CMS Options

       CMS allows for a wide variety of options in content and algorithm
       support. This section puts forth a number of support requirements and
       recommendations in order to achieve a base level of interoperability
       among all S/MIME implementations. [CMS] provides additional details
       regarding the use of the cryptographic algorithms.

    2.1 DigestAlgorithmIdentifier

       Sending and receiving agents MUST support SHA-1 [SHA1].  Receiving
       agents SHOULD support MD5 [MD5] for the purpose of providing backward
       compatibility with MD5-digested S/MIME v2 SignedData objects.

    2.2 SignatureAlgorithmIdentifier

       Sending and receiving agents MUST support id-dsa defined in [DSS].
       The algorithm parameters MUST be absent (not encoded as NULL).

       Receiving agents SHOULD support rsaEncryption, defined in [PKCS-1].

       Sending agents SHOULD support rsaEncryption. Outgoing messages are
       signed with a user's private key. The size of the private key is
       determined during key generation.

       Note that S/MIME v2 clients are only capable of verifying digital
       signatures using the rsaEncryption algorithm.

    2.3 KeyEncryptionAlgorithmIdentifier

       Sending and receiving agents MUST support Diffie-Hellman defined in
       [DH].

       Receiving agents SHOULD 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 SHOULD support rsaEncryption.

       Note that S/MIME v2 clients are only capable of decrypting content
       encryption keys using the rsaEncryption algorithm.

    2.4 General Syntax

       CMS defines multiple content types.  Of these, only the Data,
       SignedData, and EnvelopedData content types are currently used for
       S/MIME.

    2.4.1 Data Content Type

       Sending agents MUST use the id-data content type identifier to
       indicate the message content which has had security services applied
       to it. For example, when applying a digital signature to MIME data,
       the CMS signedData encapContentInfo eContentType MUST include the
       id-data object identifier and the MIME content MUST be stored in the
       SignedData encapContentInfo eContent OCTET STRING (unless the sending
       agent is using multipart/signed, in which case the eContent is
       absent, per section 3.4.3 of this document).  As another example,
       when applying encryption to MIME data, the CMS EnvelopedData

       encryptedContentInfo ContentType MUST include the id-data object
       identifier and the encrypted MIME content MUST be stored in the
       envelopedData encryptedContentInfo encryptedContent OCTET STRING.

    2.4.2 SignedData Content Type

       Sending agents MUST use the signedData content type to apply a
       digital signature to a message or, in a degenerate case where there
       is no signature information, to convey certificates.

    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 unsigned and signed
       attributes to be included along with a signature.

       Receiving agents MUST be able to handle zero or one instance of each
       of the signed attributes listed here. Sending agents SHOULD generate
       one instance of each of the following signed attributes in each
       S/MIME message:

       - signingTime (section 2.5.1 in this document)
       - sMIMECapabilities (section 2.5.2 in this document)
       - sMIMEEncryptionKeyPreference (section 2.5.3 in this document)

       Further, receiving agents SHOULD be able to handle zero or one
       instance in the signed attributes of the signingCertificate attribute
       (section 5 in [ESS]).

       Sending agents SHOULD generate one instance of the signingCertificate
       signed attribute in each S/MIME message.

       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.

       Sending agents that include signed attributes that are not listed
       here SHOULD display those attributes to the user, so that the user is
       aware of all of the data being signed.

    2.5.1 Signing-Time Attribute

       The signing-time attribute is used to convey the time that a message
       was signed. 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. When the UTCTime CHOICE is used, S/MIME agents MUST
       interpret the year field (YY) as follows:

       if YY is greater than or equal to 50, the year is interpreted as
       19YY; if YY is less than 50, the year is interpreted as 20YY.

    2.5.2 SMIMECapabilities Attribute

       The SMIMECapabilities attribute includes signature algorithms (such
       as "sha1WithRSAEncryption"), symmetric algorithms (such as "DES-
       EDE3-CBC"), and key encipherment algorithms (such as
       "rsaEncryption"). It also includes a non-algorithm capability which
       is the preference for signedData. The SMIMECapabilities were designed
       to be flexible and extensible so that, in the future, a means of
       identifying other capabilities and preferences such as certificates
       can be added in a way that will not cause current clients to break.

       If present, the SMIMECapabilities attribute MUST be a
       SignedAttribute; it MUST NOT be an UnsignedAttribute. CMS defines
       SignedAttributes as a SET OF Attribute. The SignedAttributes in a
       signerInfo MUST NOT include multiple instances of the
       SMIMECapabilities attribute. CMS defines the ASN.1 syntax for
       Attribute to include attrValues SET OF AttributeValue. A
       SMIMECapabilities attribute MUST only include a single instance of
       AttributeValue.  There MUST NOT be zero or multiple instances of
       AttributeValue present in the attrValues SET OF AttributeValue.

       The semantics of the SMIMECapabilites 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
       attribute, 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 the SMIMECapabilities attribute is to facilitate
       simple table lookups and binary comparisons in order to determine
       matches. For instance, the DER-encoding for the SMIMECapability for
       DES EDE3 CBC MUST be identically encoded regardless of the
       implementation.

       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 (OIDs Used with S/MIME) 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>. Note that all OIDs
       associated with the MUST and SHOULD implement algorithms are included
       in section A of this document.

       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 draft,
       rsaEncryption was ambiguous because it could refer to either a
       signature algorithm or a key encipherment algorithm. In the event
       that an OID is ambiguous, it needs to be arbitrated by the maintainer
       of the registered SMIMECapabilities list as to which type of
       algorithm will use the OID, and a new OID MUST be allocated under the
       smimeCapabilities OID to satisfy the other use of the OID.

       The registered SMIMECapabilities list specifies the parameters for
       OIDs that need them, most notably key lengths in the case of
       variable-length symmetric ciphers. In the event that there are no
       differentiating parameters for a particular OID, the parameters MUST
       be omitted, and MUST NOT be encoded as NULL.

       Additional values for the SMIMECapabilities attribute 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.

    2.5.3 Encryption Key Preference Attribute

       The encryption key preference attribute allows the signer to
       unambiguously describe which of the signer's certificates has the
       signer's preferred encryption key. This attribute is designed to
       enhance behavior for interoperating with those clients which use
       separate keys for encryption and signing. This attribute is used to

       convey to anyone viewing the attribute which of the listed
       certificates should be used for encrypting a session key for future
       encrypted messages.

       If present, the SMIMEEncryptionKeyPreference attribute MUST be a
       SignedAttribute; it MUST NOT be an UnsignedAttribute. CMS defines
       SignedAttributes as a SET OF Attribute. The SignedAttributes in a
       signerInfo MUST NOT include multiple instances of the
       SMIMEEncryptionKeyPreference attribute.  CMS defines the ASN.1 syntax
       for Attribute to include attrValues SET OF AttributeValue. A
       SMIMEEncryptionKeyPreference attribute MUST only include a single
       instance of AttributeValue.  There MUST NOT be zero or multiple
       instances of AttributeValue present in the attrValues SET OF
       AttributeValue.

       The sending agent SHOULD include the referenced certificate in the
       set of certificates included in the signed message if this attribute
       is used.  The certificate may be omitted if it has been previously
       made available to the receiving agent.  Sending agents SHOULD use
       this attribute if the commonly used or preferred encryption
       certificate is not the same as the certificate used to sign the
       message.

       Receiving agents SHOULD store the preference data if the signature on
       the message is valid and the signing time is greater than the
       currently stored value.  (As with the SMIMECapabilities, the clock
       skew should be checked and the data not used if the skew is too
       great.)  Receiving agents SHOULD respect the sender's encryption key
       preference attribute if possible.  This however represents only a
       preference and the receiving agent may use any certificate in
       replying to the sender that is valid.

    2.5.3.1 Selection of Recipient Key Management Certificate

       In order to determine the key management certificate to be used when
       sending a future CMS envelopedData message for a particular
       recipient, the following steps SHOULD be followed:

        - If an SMIMEEncryptionKeyPreference attribute is found in a
       signedData object received from the desired recipient, this
       identifies the X.509 certificate that should be used as the X.509
       key management certificate for the recipient.

       - If an SMIMEEncryptionKeyPreference attribute is not found in a
       signedData object received from the desired recipient, the set of
       X.509 certificates should be searched for a X.509 certificate with
       the same subject name as the signing X.509 certificate which can
       be used for key management.

       - Or use some other method of determining the user's key management
       key. If a X.509 key management certificate is not found, then
       encryption cannot be done with the signer of the message. If multiple
       X.509 key management certificates are found, the S/MIME agent can
       make an arbitrary choice between them.

    2.6 SignerIdentifier SignerInfo Type

       S/MIME v3 requires the use of SignerInfo version 1, that is the
       issuerAndSerialNumber CHOICE MUST be used for SignerIdentifier.

    2.7 ContentEncryptionAlgorithmIdentifier

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

    2.7.1 Deciding Which Encryption Method To Use

       When a sending agent creates an encrypted message, it has to decide
       which type of encryption to use. The decision process involves using
       information garnered from the capabilities lists included in messages
       received from the recipient, as well as out-of-band information such
       as private agreements, user preferences, legal restrictions, and so
       on.

       Section 2.5 defines a method by which a sending agent can optionally
       announce, among other things, its decrypting capabilities in its
       order of preference. The following method for processing and
       remembering the encryption capabilities attribute in incoming signed
       messages SHOULD be used.

       -  If the receiving agent has not yet created a list of capabilities
          for the sender's public key, then, after verifying the signature
          on the incoming message and checking the timestamp, the receiving
          agent SHOULD create a new list containing at least the signing
          time and the symmetric capabilities.

        - If such a list already exists, the receiving agent SHOULD verify
          that the signing time in the incoming message is greater than
          the signing time stored in the list and that the signature is
          valid. If so, the receiving agent SHOULD update both the signing
          time and capabilities in the list. Values of the signing time that
          lie far in the future (that is, a greater discrepancy than any
          reasonable clock skew), or a capabilities list in messages whose
          signature could not be verified, MUST NOT be accepted.

       The list of capabilities SHOULD be stored for future use in creating
       messages.

       Before sending a message, the sending agent MUST decide whether it is
       willing to use weak encryption for the particular data in the
       message.  If the sending agent decides that weak encryption is
       unacceptable for this data, then the sending agent MUST NOT use a
       weak algorithm such as RC2/40.  The decision to use or not use weak
       encryption overrides any other decision in this section about which
       encryption algorithm to use.

       Sections 2.7.2.1 through 2.7.2.4 describe the decisions a sending
       agent SHOULD use in deciding which type of encryption should be
       applied to a message.  These rules are ordered, so the sending agent
       SHOULD make its decision in the order given.

    2.7.1.1 Rule 1: Known Capabilities

       If the sending agent has received a set of capabilities from the
       recipient for the message the agent is about to encrypt, then the
       sending agent SHOULD use that information by selecting the first
       capability in the list (that is, the capability most preferred by the
       intended recipient) 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.

    2.7.1.2 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.

    2.7.1.3 Rule 3: Unknown Capabilities, Unknown Version of S/MIME

       If:

        - the sending agent has no knowledge of the encryption capabilities
          of the recipient,
        - and the sending agent has no knowledge of the version of S/MIME
          of the recipient,

       then the sending agent SHOULD use tripleDES because it is a stronger
       algorithm and is required by S/MIME v3. If the sending agent chooses
       not to use tripleDES in this step, it SHOULD use RC2/40.

    2.7.2 Choosing Weak Encryption

       Like all algorithms that use 40 bit keys, RC2/40 is considered by
       many to be weak encryption. A sending agent that is controlled by a
       human SHOULD allow a human sender to determine the risks of sending
       data using RC2/40 or a similarly weak encryption algorithm before
       sending the data, and possibly allow the human to use a stronger
       encryption method such as tripleDES.

    2.7.3 Multiple Recipients

       If a sending agent is composing an encrypted message to a group of
       recipients where the encryption capabilities of some of the
       recipients do not overlap, the sending agent is forced to send more
       than one message. 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 may be able to learn the contents
       of the strongly-encrypted message simply by decrypting the weakly-
       encrypted message.

    3. Creating S/MIME Messages

       This section describes the S/MIME message formats and how they are
       created. S/MIME messages are a combination of MIME bodies and CMS
       objects. Several MIME types as well as several CMS 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 CMS processing facilities which produces a
       CMS object.  The CMS object is then finally wrapped in MIME. The
       Enhanced Security Services for S/MIME [ESS] document provides
       examples of how nested, secured S/MIME messages are formatted.  ESS
       provides an example of how a triple-wrapped S/MIME message is
       formatted using multipart/signed and application/pkcs7-mime for the
       signatures.

       S/MIME provides one format for enveloped-only data, several formats
       for signed-only data, and several formats for signed and enveloped
       data. Several formats are required to accommodate several
       environments, in particular for signed messages. The criteria for
       choosing among these formats are also described.

       The reader of this section is expected to understand MIME as
       described in [MIME-SPEC] and [MIME-SECURE].

    3.1 Preparing the MIME Entity for Signing 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 CMS 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 processed, and the result is the MIME entity. That MIME
       entity is typically passed to a MIME-capable user agent where, it is
       further decoded and presented to the user or receiving application.

    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 non-security
       part of the sending agent rather than the S/MIME implementation.

       The most common and important canonicalization is for text, which is
       often represented differently in different environments. MIME
       entities of major type "text" must have both their line endings and
       character set canonicalized. The line ending must be the pair of
       characters <CR><LF>, and the charset should be a registered charset
       [CHARSETS].  The details of the canonicalization are specified in
       [MIME-SPEC]. The chosen charset SHOULD be named in the charset
       parameter so that the receiving agent can unambiguously determine the
       charset used.

       Note that some charsets such as ISO-2022 have multiple
       representations for the same characters. When preparing such text for
       signing, the canonical representation specified for the charset MUST
       be used.

    3.1.2 Transfer Encoding

       When generating any of the secured MIME entities below, except the
       signing using the multipart/signed format, no transfer encoding 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
       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.

         Content-Type: multipart/mixed; boundary=bar

         --bar
         Content-Type: text/plain; charset=iso-8859-1
         Content-Transfer-Encoding: quoted-printable

         =A1Hola Michael!

         How do you like the new S/MIME specification?

         I agree. It's generally a good idea to encode lines that begin with
         From=20 because some mail transport agents will insert a
         greater-than (>) sign, thus invalidating the signature.

         Also, in some cases it might be desirable to encode any  =20
         trailing whitespace that occurs on lines in order to ensure  =20
         that the message signature is not invalidated when passing  =20
         a gateway that modifies such whitespace (like BITNET).  =20

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

         iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
         jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
         uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
         HOxEa44b+EI=

         --bar--

    3.2 The application/pkcs7-mime Type

       The application/pkcs7-mime type is used to carry CMS 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.

       The carried CMS object always contains a MIME entity that is prepared
       as described in section 3.1 if the eContentType is id-data. Other
       contents may be carried when the eContentType contains different
       values. See [ESS] for an example of this with signed receipts.

       Since CMS 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 CMS
       object or "outside" MIME entity. It is completely distinct from, and
       unrelated to, the transfer encoding of the MIME entity secured by the
       CMS object, the "inside" object, which is described in section 3.1.

       Because there are several types of application/pkcs7-mime objects, a
       sending agent SHOULD do as much as possible to help a receiving agent
       know about the contents of the object without forcing the receiving
       agent to decode the ASN.1 for the object. The MIME headers of all
       application/pkcs7-mime objects SHOULD include the optional "smime-
       type" parameter, as described in the following sections.

    3.2.1 The name and filename Parameters

       For the application/pkcs7-mime, sending agents SHOULD emit the
       optional "name" parameter to the Content-Type field for compatibility
       with older systems. Sending agents SHOULD also emit the optional
       Content-Disposition field [CONTDISP] with the "filename" parameter.
       If a sending agent emits the above parameters, the value of the
       parameters SHOULD be a file name with the appropriate extension:

       MIME Type                                File Extension

       Application/pkcs7-mime (signedData,      .p7m
       envelopedData)

       Application/pkcs7-mime (degenerate       .p7c
       signedData "certs-only" message)

       Application/pkcs7-signature              .p7s

       In addition, the file name SHOULD be limited to eight characters
       followed by a three letter extension. The eight character filename
       base can be any distinct name; the use of the filename base "smime"
       SHOULD be used to indicate that the MIME entity is associated with
       S/MIME.

       Including a file name serves two purposes. It facilitates easier use
       of S/MIME objects as files on disk. It also can convey type
       information across gateways. When a MIME entity of type
       application/pkcs7-mime (for example) arrives at a gateway that has no
       special knowledge of S/MIME, it will default the entity's MIME type
       to application/octet-stream and treat it as a generic attachment,
       thus losing the type information. However, the suggested filename for
       an attachment is often carried across a gateway. This often allows
       the receiving systems to determine the appropriate application to
       hand the attachment off to, in this case a stand-alone S/MIME
       processing application. Note that this mechanism is provided as a
       convenience for implementations in certain environments. A proper
       S/MIME implementation MUST use the MIME types and MUST NOT rely on
       the file extensions.

    3.2.2 The smime-type parameter

       The application/pkcs7-mime content type defines the optional "smime-
       type" parameter. The intent of this parameter is to convey details
       about the security applied (signed or enveloped) along with
       infomation about the contained content. This memo defines the
       following smime-types.

       Name                   Security                Inner Content

       enveloped-data         EnvelopedData           id-data

       signed-data            SignedData              id-data

       certs-only             SignedData              none

       In order that consistency can be obtained with future, the following
       guidelines should be followed when assigning a new smime-type
       parameter.

       1. If both signing and encryption can be applied to the content, then
       two values for smime-type SHOULD be assigned "signed-*" and
       "encrypted-*".  If one operation can be assigned then this may be
       omitted. Thus since "certs-only" can only be signed, "signed-" is
       omitted.

       2. A common string for a content oid should be assigned. We use
       "data" for the id-data content OID when MIME is the inner content.

       3. If no common string is assigned.  Then the common string of
       "OID.<oid>" is recommended (for example, "OID.1.3.6.1.5.5.7.6.1"
       would be DES40).

    3.3 Creating an Enveloped-only Message

       This section describes the format for enveloping a MIME entity
       without signing it. It is important to note that sending enveloped
       but not signed messages does not provide for data integrity. It is
       possible to replace ciphertext in such a way that the processed
       message will still be valid, but the meaning may be altered.

       Step 1. The MIME entity to be enveloped is prepared according to
       section 3.1.

       Step 2. The MIME entity and other required data is processed into a
       CMS object of type envelopedData. In addition to encrypting a copy of
       the content-encryption key for each recipient, a copy of the content
       encryption key SHOULD be encrypted for the originator and included in
       the envelopedData (see CMS Section 6).

       Step 3. The CMS object is inserted into an application/pkcs7-mime
       MIME entity.

       The smime-type parameter for enveloped-only messages is "enveloped-
       data". The file extension for this type of message is ".p7m".

       A sample message would be:

           Content-Type: application/pkcs7-mime; smime-type=enveloped-data;
                name=smime.p7m
           Content-Transfer-Encoding: base64
           Content-Disposition: attachment; filename=smime.p7m

           rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6
           7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H
           f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
           0GhIGfHfQbnj756YT64V

    3.4 Creating a Signed-only Message

       There are two formats for signed messages defined for S/MIME:
       application/pkcs7-mime with SignedData, and multipart/signed. In
       general, the multipart/signed form is preferred for sending, and
       receiving agents 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 with SignedData

       This signing format uses the application/pkcs7-mime MIME type. The
       steps to create this format are:

       Step 1. The MIME entity is prepared according to section 3.1

       Step 2. The MIME entity and other required data is processed into a
       CMS object of type signedData

       Step 3. The CMS object is inserted into an application/pkcs7-mime
       MIME entity

       The smime-type parameter for messages using application/pkcs7-mime
       with SignedData is "signed-data". The file extension for this type of
       message is ".p7m".

       A sample message would be:

           Content-Type: application/pkcs7-mime; smime-type=signed-data;
                name=smime.p7m
           Content-Transfer-Encoding: base64
           Content-Disposition: attachment; filename=smime.p7m

           567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7
           77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH
           HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh
           6YT64V0GhIGfHfQbnj75

    3.4.3 Signing Using the multipart/signed Format

       This format is a clear-signing format. Recipients without any S/MIME
       or CMS processing facilities are able to view the message. It makes
       use of the multipart/signed MIME type described in [MIME-SECURE]. The
       multipart/signed MIME type has two parts. The first part contains the
       MIME entity that is signed; the second part contains the "detached
       signature" CMS SignedData object in which the encapContentInfo
       eContent field is absent.

    3.4.3.1 The application/pkcs7-signature MIME Type

       This MIME type always contains a single CMS object of type
       signedData.  The signedData encapContentInfo eContent field MUST be
       absent. The signerInfos field contains the signatures for the MIME
       entity.

       The file extension for signed-only messages using application/pkcs7-
       signature is ".p7s".

    3.4.3.2 Creating a multipart/signed Message

       Step 1. The MIME entity to be signed is prepared according to section
       3.1, taking special care for clear-signing.

       Step 2. The MIME entity is presented to CMS processing in order to
       obtain an object of type signedData in which the encapContentInfo
       eContent field is absent.

       Step 3. The MIME entity is inserted into the first part of a
       multipart/signed message with no processing other than that described
       in section 3.1.

       Step 4. Transfer encoding is applied to the "detached signature" CMS
       SignedData object and it is inserted into a MIME entity of type
       application/pkcs7-signature.

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

       The multipart/signed Content type has two required parameters: the
       protocol parameter and the micalg parameter.

       The protocol parameter MUST be "application/pkcs7-signature". Note
       that quotation marks are required around the protocol parameter
       because MIME requires that the "/" character in the parameter value
       MUST be quoted.

       The micalg parameter allows for one-pass processing when the
       signature is being verified. The value of the micalg parameter is
       dependent on the message digest algorithm(s) used in the calculation
       of the Message Integrity Check. If multiple message digest algorithms
       are used they MUST be separated by commas per [MIME-SECURE]. The
       values to be placed in the micalg parameter SHOULD be from the
       following:

       Algorithm   Value
       used

       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

           --boundary42
           Content-Type: text/plain

           This is a clear-signed message.

           --boundary42
           Content-Type: application/pkcs7-signature; name=smime.p7s
           Content-Transfer-Encoding: base64
           Content-Disposition: attachment; filename=smime.p7s

           ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
           4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
           n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
           7GhIGfHfYT64VQbnj756

           --boundary42--

    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.

       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.

       There are security ramifications to choosing whether to sign first or
       encrypt first. A recipient of a message that is encrypted and then
       signed can validate that the encrypted block was unaltered, but
       cannot determine any relationship between the signer and the
       unencrypted contents of the message. A recipient of a message that is
       signed-then-encrypted can assume that the signed message itself has
       not been altered, but that a careful attacker may have changed the
       unauthenticated portions of the encrypted message.

    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 CMS generating
       process which creates a CMS object of type signedData. The signedData
       encapContentInfo eContent field MUST be absent and signerInfos field
       MUST be empty.

       Step 2. The CMS 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 Registration Requests

       A sending agent that signs messages MUST have a certificate for the
       signature so that a receiving agent can verify the signature. There
       are many ways of getting certificates, such as through an exchange
       with a certificate authority, through a hardware token or diskette,
       and so on.

       S/MIME v2 [SMIMEV2] specified a method for "registering" public keys
       with certificate authorities using an application/pkcs10 body part.
       The IETF's PKIX Working Group is preparing another method for
       requesting certificates; however, that work was not finished at the
       time of this memo. S/MIME v3 does not specify how to request a

       certificate, but instead mandates that every sending agent already
       has a certificate. Standardization of certificate management is being
       pursued separately in the IETF.

    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:   multipart/signed
       parameters:  protocol="application/pkcs7-signature"
       file suffix: any

       MIME type:   application/octet-stream
       parameters:  any
       file suffix: p7m, p7s, p7c

    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
       [CERT3].

       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

       If an S/MIME agent needs to generate a key pair, then the S/MIME
       agent or some related administrative utility or function MUST be
       capable of generating separate DH and DSS public/private key pairs on
       behalf of the user. Each key pair MUST be generated from a good
       source of non-deterministic random input [RANDOM] and the private key
       MUST be protected in a secure fashion.

       If an S/MIME agent needs to generate a key pair, then the S/MIME
       agent or some related administrative utility or function SHOULD
       generate RSA key pairs.

       A user agent SHOULD generate RSA key pairs at a minimum key size of
       768 bits. A user agent MUST NOT generate RSA key pairs less than 512
       bits long. Creating keys longer than 1024 bits may cause some older
       S/MIME receiving agents to not be able to verify signatures, but
       gives better security and is therefore valuable. A receiving agent
       SHOULD be able to verify signatures with keys of any size over 512
       bits. Some agents created in the United States have chosen to create
       512 bit keys in order to get more advantageous export licenses.
       However, 512 bit keys are considered by many to be cryptographically
       insecure.  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

       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.

       If a sending agent is sending the same message using different
       strengths of cryptography, an attacker watching the communications
       channel may be able to determine the contents of the strongly-
       encrypted message by decrypting the weakly-encrypted version.  In
       other words, a sender should not send a copy of a message using
       weaker cryptography than they would use for the original of the
       message.

       Modification of the ciphertext can go undetected if authentication is
       not also used, which is the case when sending EnvelopedData without
       wrapping it in SignedData or enclosing SignedData within it.

    A. ASN.1 Module

    SecureMimeMessageV3
      { iso(1) member-body(2) us(840) rsadsi(113549)
             pkcs(1) pkcs-9(9) smime(16) modules(0) smime(4) }

    DEFINITIONS IMPLICIT TAGS ::=
    BEGIN

    IMPORTS
    -- Cryptographic Message Syntax
        SubjectKeyIdentifier, IssuerAndSerialNumber,
    RecipientKeyIdentifier
            FROM    CryptographicMessageSyntax
                   { iso(1) member-body(2) us(840) rsadsi(113549)
                     pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1) };

    --  id-aa is the arc with all new authenticated and unauthenticated
    --  attributes produced the by S/MIME Working Group

    id-aa OBJECT IDENTIFIER ::= {iso(1) member-body(2) usa(840)
    rsadsi(113549)
            pkcs(1) pkcs-9(9) smime(16) attributes(2)}

    -- S/MIME Capabilities provides a method of broadcasting the symetric
    -- capabilities understood.  Algorithms should be ordered by preference
    -- and grouped by type

    smimeCapabilities OBJECT IDENTIFIER ::=
       {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15}

    SMIMECapability ::= SEQUENCE {
       capabilityID OBJECT IDENTIFIER,
       parameters ANY DEFINED BY capabilityID OPTIONAL }

    SMIMECapabilities ::= SEQUENCE OF SMIMECapability

    -- Encryption Key Preference provides a method of broadcasting the
    -- preferred encryption certificate.

    id-aa-encrypKeyPref OBJECT IDENTIFIER ::= {id-aa 11}

    SMIMEEncryptionKeyPreference ::= CHOICE {
       issuerAndSerialNumber   [0] IssuerAndSerialNumber,
       receipentKeyId          [1] RecipientKeyIdentifier,
       subjectAltKeyIdentifier [2] SubjectKeyIdentifier
    }

    -- The Content Encryption Algorithms defined for SMIME are:

    -- Triple-DES is the manditory algorithm with CBCParameter being the
    -- parameters

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

    CBCParameter ::= IV

    IV ::= OCTET STRING (SIZE (8..8))

    --  RC2 (or compatable) is an optional algorithm w/ RC2-CBC-paramter
    --  as the parameter

    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                   IV}

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

    -- The following list the OIDs to be used with S/MIME V3

    -- 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}

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

    -- rsa OBJECT IDENTIFIER ::=
    --    {joint-iso-ccitt(2) ds(5) algorithm(8) encryptionAlgorithm(1) 1}
    --
    -- id-dsa OBJECT IDENTIFIER ::=
    --    {iso(1) member-body(2) us(840) x9-57(10040) x9cm(4) 1 }

    -- 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}
    --
    -- id-dsa-with-sha1 OBJECT IDENTIFIER ::=
    --    {iso(1) member-body(2) us(840) x9-57(10040) x9cm(4) 3}

    -- Other Signed Attributes
    --
    -- signingTime OBJECT IDENTIFIER ::=
    --    {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
    -- 5}
    --    See [CMS] for a description of how to encode the attribute
    --    value.

    END

    B. References

       [3DES]         ANSI X9.52-1998, "Triple Data Encryption Algorithm
                      Modes of Operation", American National Standards
                      Institute, 1998.

       [CERT3]        Ramsdell, B., Editor, "S/MIME Version 3 Certificate
                      Handling", RFC 2632, June 1999.

       [CHARSETS]     Character sets assigned by IANA. See
                      <ftp://ftp.isi.edu/in-
                      notes/iana/assignments/character-sets>.

       [CMS]          Housley, R., "Cryptographic Message Syntax", RFC 2630,
                      June 1999.

       [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.

       [DH]           Rescorla, E., "Diffie-Hellman Key Agreement Method",
                      RFC 2631, June 1999.

       [DSS]          NIST FIPS PUB 186, "Digital Signature Standard", 18
                      May 1994.

       [ESS]          Hoffman, P., Editor "Enhanced Security Services for
                      S/MIME", RFC 2634, June 1999.

       [MD5]          Rivest, R., "The MD5 Message Digest Algorithm", RFC
                      1321, April 1992.

       [MIME-SPEC]    The primary definition of MIME. "MIME Part 1: Format
                      of Internet Message Bodies", RFC 2045; "MIME Part 2:
                      Media Types", RFC 2046; "MIME Part 3: Message Header
                      Extensions for Non-ASCII Text", RFC 2047; "MIME Part
                      4: Registration Procedures", RFC 2048; "MIME Part 5:
                      Conformance Criteria and Examples", RFC 2049,
                      September 1993.

       [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", BCP14, RFC 2119, March 1997.

       [PKCS-1]       Kaliski, B., "PKCS #1: RSA Encryption Version 2.0",
                      RFC 2437, October 1998.

       [PKCS-7]       Kaliski, B., "PKCS #7: Cryptographic Message Syntax
                      Version 1.5", RFC 2315, March 1998.

       [RANDOM]       Eastlake, 3rd, D., Crocker, S. and J. Schiller,
                      "Randomness Recommendations for Security", RFC 1750,
                      December 1994.

       [RC2]          Rivest, R., "A 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, 31May 1994.

       [SMIMEV2]      Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L.
                      and L.  Repka, "S/MIME Version 2 Message
                      Specification", RFC 2311, March 1998.

    C. Acknowledgements

       Many thanks go out to the other authors of the S/MIME Version 2
       Message Specification RFC:  Steve Dusse, Paul Hoffman, Laurence
       Lundblade and Lisa Repka. Without v2, there wouldn't be a v3.

       A number of the members of the S/MIME Working Group have also worked
       very hard and contributed to this document. Any list of people is
       doomed to omission, and for that I apologize. In alphabetical order,
       the following people stand out in my mind due to the fact that they
       made direct contributions to this document.

       Dave Crocker
       Bill Flanigan
       Paul Hoffman
       Russ Housley
       John Pawling
       Jim Schaad

    Editor's Address

       Blake Ramsdell
       Worldtalk
       17720 NE 65th St Ste 201
       Redmond, WA 98052

       Phone: +1 425 376 0225
       EMail: blaker@deming.com

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