Movatterモバイル変換

Network Working Group                                          S. DusseRequest for Comments: 2311                            RSA Data SecurityCategory: Informational                                      P. Hoffman                                               Internet Mail Consortium                                                            B. Ramsdell                                                              Worldtalk                                                           L. Lundblade                                                               Qualcomm                                                               L. Repka                                                               Netscape                                                             March 1998S/MIME Version 2 Message SpecificationStatus of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1998).  All Rights Reserved.1. Introduction   S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a   consistent way to send and receive secure MIME data. Based on the   popular Internet MIME standard, S/MIME provides the following   cryptographic security services for electronic messaging   applications: authentication, message integrity and non-repudiation   of origin (using digital signatures) and privacy and data security   (using encryption).   S/MIME can be used by traditional mail user agents (MUAs) to add   cryptographic security services to mail that is sent, and to   interpret cryptographic security services in mail that is received.   However, S/MIME is not restricted to mail; it can be used with any   transport mechanism that transports MIME data, such as HTTP. As such,   S/MIME takes advantage of the object-based features of MIME and   allows secure messages to be exchanged in mixed-transport systems.   Further, S/MIME can be used in automated message transfer agents that   use cryptographic security services that do not require any human   intervention, such as the signing of software-generated documents and   the encryption of FAX messages sent over the Internet.Dusse, et. al.               Informational                      [Page 1]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   Please note: The information in this document is historical material   being published for the public record. It is not an IETF standard.   The use of the word "standard" in this document indicates a standard   for adopters of S/MIME version 2, not an IETF standard.1.1 Specification Overview   This document describes a protocol for adding cryptographic signature   and encryption services to MIME data. The MIME standard [MIME-SPEC]   provides a general structure for the content type of Internet   messages and allows extensions for new content type applications.   This memo defines how to create a MIME body part that has been   cryptographically enhanced according to PKCS #7 [PKCS-7]. This memo   also defines the application/pkcs7-mime MIME type that can be used to   transport those body parts. This memo also defines how to create   certification requests that conform to PKCS #10 [PKCS-10], and the   application/pkcs10 MIME type for transporting those requests.   This memo also discusses how to use the multipart/signed MIME type   defined in [MIME-SECURE] to transport S/MIME signed messages. This   memo also defines the application/pkcs7-signature MIME type, which is   also used to transport S/MIME signed messages. This specification is   compatible with PKCS #7 in that it uses the data types defined by   PKCS #7.   In order to create S/MIME messages, an agent has to follow   specifications in this memo, as well as some of the specifications   listed in the following documents:    - "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 MIMEDusse, et. al.               Informational                      [Page 2]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   data. An automated process that sends an encrypted message might not   be able to receive an encrypted message at all, for example. Thus,   the requirements and recommendations for the two types of agents are   listed separately when appropriate.1.2 Terminology   Throughout this memo, the terms MUST, MUST NOT, SHOULD, and SHOULD   NOT are used in capital letters. This conforms to the definitions in   [MUSTSHOULD].  [MUSTSHOULD] defines the use of these key words to   help make the intent of standards track documents as clear as   possible. The same key words are used in this document to help   implementors achieve interoperability.1.3 Definitions   For the purposes of this memo, the following definitions apply.   ASN.1: Abstract Syntax Notation One, as defined in CCITT X.208.   BER: Basic Encoding Rules for ASN.1, as defined in CCITT X.209.   Certificate: A type that binds an entity's distinguished name to a   public key with a digital signature.   DER: Distinguished Encoding Rules for ASN.1, as defined in CCITT   X.509.   7-bit data: Text data with lines less than 998 characters long, where   none of the characters have the 8th bit set, and there are no NULL   characters.  <CR> and <LF> occur only as part of a <CR><LF> end of   line delimiter.   8-bit data: Text data with lines less than 998 characters, and where   none of the characters are NULL characters. <CR> and <LF> occur only   as part of a <CR><LF> end of line delimiter.   Binary data: Arbitrary data.   Transfer Encoding: A reversible transformation made on data so 8-bit   or binary data may be sent via a channel that only transmits 7-bit   data.1.4 Compatibility with Prior Practice of S/MIMEAppendix C contains important information about how S/MIME agents   following this specification should act in order to have the greatest   interoperability with earlier implementations of S/MIME.Dusse, et. al.               Informational                      [Page 3]

RFC 2311         S/MIME Version 2 Message Specification       March 19982. PKCS #7 Options   The PKCS #7 message format allows for a wide variety of options in   content and algorithm support. This section puts forth a number of   support requirements and recommendations in order to achieve a base   level of interoperability among all S/MIME implementations.2.1 DigestAlgorithmIdentifier   Receiving agents MUST support SHA-1 [SHA1] and MD5 [MD5].   Sending agents SHOULD use SHA-1.2.2 DigestEncryptionAlgorithmIdentifier   Receiving agents MUST support rsaEncryption, defined in [PKCS-1].   Receiving agents MUST support verification of signatures using RSA   public key sizes from 512 bits to 1024 bits.   Sending agents MUST support rsaEncryption. Outgoing messages are   signed with a user's private key. The size of the private key is   determined during key generation.2.3 KeyEncryptionAlgorithmIdentifier   Receiving agents MUST support rsaEncryption. Incoming encrypted   messages contain symmetric keys which are to be decrypted with a   user's private key.  The size of the private key is determined during   key generation.   Sending agents MUST support rsaEncryption. Sending agents MUST   support encryption of symmetric keys with RSA public keys at key   sizes from 512 bits to 1024 bits.2.4 General Syntax   The PKCS #7 defines six distinct content types: "data", "signedData",   "envelopedData", "signedAndEnvelopedData", "digestedData", and   "encryptedData". Receiving agents MUST support the "data",   "signedData" and "envelopedData" content types. Sending agents may or   may not send out any of the content types, depending on the services   that the agent supports.2.4.1 Data Content Type   Sending agents MUST use the "data" content type as the content within   other content types to indicate the message content which has had   security services applied to it.Dusse, et. al.               Informational                      [Page 4]

RFC 2311         S/MIME Version 2 Message Specification       March 19982.4.2 SignedData Content Type   Sending agents MUST use the signedData content type to apply a   digital signature to a message or, in a degenerate case where there   is no signature information, to convey certificates.2.4.3 EnvelopedData Content Type   This content type is used to apply privacy protection to a message. A   sender needs to have access to a public key for each intended message   recipient to use this service. This content type does not provide   authentication.2.5 Attribute SignerInfo Type   The SignerInfo type allows the inclusion of unauthenticated and   authenticated attributes to be included along with a signature.   Receiving agents MUST be able to handle zero or one instance of each   of the signed attributes described in this section.   Sending agents SHOULD be able to generate one instance of each of the   signed attributes described in this section, and SHOULD include these   attributes in each signed message sent.   Additional attributes and values for these attributes may be defined   in the future. Receiving agents SHOULD handle attributes or values   that it does not recognize in a graceful manner.2.5.1 Signing-Time Attribute   The signing-time attribute is used to convey the time that a message   was signed. Until there are trusted timestamping services, the time   of signing will most likely be created by a message originator and   therefore is only as trustworthy as the originator.   Sending agents MUST encode signing time through the year 2049 as   UTCTime; signing times in 2050 or later MUST be encoded as   GeneralizedTime. Agents MUST interpret the year field (YY) as   follows: if YY is greater than or equal to 50, the year is   interpreted as 19YY; if YY is less than 50, the year is interpreted   as 20YY.2.5.2 S/MIME Capabilities Attribute   The S/MIME capabilities attribute includes signature algorithms (such   as "md5WithRSAEncryption"), symmetric algorithms (such as "DES-CBC"),   and key encipherment algorithms (such as "rsaEncryption"). It alsoDusse, et. al.               Informational                      [Page 5]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   includes a non-algorithm capability which is the preference for   signedData.  SMIMECapabilities was designed to be flexible and   extensible so that, in the future, a means of identifying other   capabilities and preferences such as certificates can be added in a   way that will not cause current clients to break.   The semantics of the S/MIME capabilites attribute specify a partial   list as to what the client announcing the SMIMECapabilites can   support. A client does not have to list every capability it supports,   and probably should not list all its capabilities so that the   capabilities list doesn't get too long. In an SMIMECapabilities   encoding, the OIDs are listed in order of their preference, but   SHOULD be logically separated along the lines of their categories   (signature algorithms, symmetric algorithms, key encipherment   algorithms, etc.)   The structure of  SMIMECapabilities was designed to facilitate simple   table lookups and binary comparisons in order to determine matches.   For instance, the DER-encoding for the SMIMECapability for DES EDE3   CBC MUST be identically encoded regardless of the implementation.   In the case of symmetric algorithms, the associated parameters for   the OID MUST specify all of the parameters necessary to differentiate   between two instances of the same algorithm. For instance, the number   of rounds and block size for RC5 must be specified in addition to the   key length.   There is a list of OIDs (the registered SMIMECapability list) that is   centrally maintained and is separate from this memo. The list of OIDs   is maintained by the Internet Mail Consortium at   <http://www.imc.org/ietf-smime/oids.html>.   The OIDs that correspond to algorithms SHOULD use the same OID as the   actual algorithm, except in the case where the algorithm usage is   ambiguous from the OID. For instance, in an earlier memo,   rsaEncryption was ambiguous because it could refer to either a   signature algorithm or a key encipherment algorithm. In the event   that an OID is ambiguous, it needs to be arbitrated by the maintainer   of the registered S/MIME capabilities list as to which type of   algorithm will use the OID, and a new OID MUST be allocated under the   smimeCapabilities OID to satisfy the other use of the OID.   The registered S/MIME capabilities list specifies the parameters for   OIDs that need them, most notably key lengths in the case of   variable-length symmetric ciphers. In the event that there are no   differentiating parameters for a particular OID, the parameters MUST   be omitted, and MUST NOT be encoded as NULL.Dusse, et. al.               Informational                      [Page 6]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   Additional values for SMIMECapability may be defined in the future.   Receiving agents MUST handle a SMIMECapabilities object that has   values that it does not recognize in a graceful manner.2.6 ContentEncryptionAlgorithmIdentifier   Receiving agents MUST support decryption using the RC2 [RC2] or a   compatible algorithm at a key size of 40 bits, hereinafter called   "RC2/40".  Receiving agents SHOULD support decryption using DES EDE3   CBC, hereinafter called "tripleDES" [3DES] [DES].   Sending agents SHOULD support encryption with RC2/40 and tripleDES.2.6.1 Deciding Which Encryption Method To Use   When a sending agent creates an encrypted message, it has to decide   which type of encryption to use. The decision process involves using   information garnered from the capabilities lists included in messages   received from the recipient, as well as out-of-band information such   as private agreements, user preferences, legal restrictions, and so   on.Section 2.5 defines a method by which a sending agent can optionally   announce, among other things, its decrypting capabilities in its   order of preference. The following method for processing and   remembering the encryption capabilities attribute in incoming signed   messages SHOULD be used.    - 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   messages.   Before sending a message, the sending agent MUST decide whether it is   willing to use weak encryption for the particular data in theDusse, et. al.               Informational                      [Page 7]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   message. If the sending agent decides that weak encryption is   unacceptable for this data, then the sending agent MUST NOT use a   weak algorithm such as RC2/40.  The decision to use or not use weak   encryption overrides any other decision in this section about which   encryption algorithm to use.   Sections2.6.2.1 through2.6.2.4 describe the decisions a sending   agent SHOULD use in deciding which type of encryption should be   applied to a message. These rules are ordered, so the sending agent   SHOULD make its decision in the order given.2.6.2.1 Rule 1: Known Capabilities   If the sending agent has received a set of capabilities from the   recipient for the message the agent is about to encrypt, then the   sending agent SHOULD use that information by selecting the first   capability in the list (that is, the capability most preferred by the   intended recipient) for which the sending agent knows how to encrypt.   The sending agent SHOULD use one of the capabilities in the list if   the agent reasonably expects the recipient to be able to decrypt the   message.2.6.2.2 Rule 2: Unknown Capabilities, Known Use of Encryption   If:    - the sending agent has no knowledge of the encryption capabilities      of the recipient,    - and the sending agent has received at least one message from the      recipient,    - and the last encrypted message received from the recipient had a      trusted signature on it,   then the outgoing message SHOULD use the same encryption algorithm as   was used on the last signed and encrypted message received from the   recipient.2.6.2.3 Rule 3: Unknown Capabilities, Risk of Failed Decryption   If:    - the sending agent has no knowledge of the encryption capabilities      of the recipient,    - and the sending agent is willing to risk that the recipient may      not be able to decrypt the message,   then the sending agent SHOULD use tripleDES.Dusse, et. al.               Informational                      [Page 8]

RFC 2311         S/MIME Version 2 Message Specification       March 19982.6.2.4 Rule 4: Unknown Capabilities, No Risk of Failed Decryption   If:    - the sending agent has no knowledge of the encryption capabilities      of the recipient,    - and the sending agent is not willing to risk that the recipient      may not be able to decrypt the message,   then the sending agent MUST use RC2/40.2.6.3 Choosing Weak Encryption   Like all algorithms that use 40 bit keys, RC2/40 is considered by   many to be weak encryption. A sending agent that is controlled by a   human SHOULD allow a human sender to determine the risks of sending   data using RC2/40 or a similarly weak encryption algorithm before   sending the data, and possibly allow the human to use a stronger   encryption method such as tripleDES.2.6.4 Multiple Recipients   If a sending agent is composing an encrypted message to a group of   recipients where the encryption capabilities of some of the   recipients do not overlap, the sending agent is forced to send more   than one message. It should be noted that if the sending agent   chooses to send a message encrypted with a strong algorithm, and then   send the same message encrypted with a weak algorithm, someone   watching the communications channel can decipher the contents of the   strongly-encrypted message simply by decrypting the weakly-encrypted   message.3. Creating S/MIME Messages   This section describes the S/MIME message formats and how they are   created.  S/MIME messages are a combination of MIME bodies and PKCS   objects. Several MIME types as well as several PKCS objects are used.   The data to be secured is always a canonical MIME entity. The MIME   entity and other data, such as certificates and algorithm   identifiers, are given to PKCS processing facilities which produces a   PKCS object. The PKCS object is then finally wrapped in MIME.   S/MIME provides one format for enveloped-only data, several formats   for signed-only data, and several formats for signed and enveloped   data.  Several formats are required to accommodate several   environments, in particular for signed messages. The criteria for   choosing among these formats are also described.   The reader of this section is expected to understand MIME as   described in [MIME-SPEC] and [MIME-SECURE].Dusse, et. al.               Informational                      [Page 9]

RFC 2311         S/MIME Version 2 Message Specification       March 19983.1 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 theRFC-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   inSection 3.2, 3.4 and elsewhere.   The procedure for preparing a MIME entity is given in [MIME-SPEC].   The same procedure is used here with some additional restrictions   when signing.  Description of the procedures from [MIME-SPEC] are   repeated here, but the reader should refer to that document for the   exact procedure. This section also describes additional requirements.   A single procedure is used for creating MIME entities that are to be   signed, enveloped, or both signed and enveloped. Some additional   steps are recommended to defend against known corruptions that can   occur during mail transport that are of particular importance for   clear-signing using the multipart/signed format. It is recommended   that these additional steps be performed on enveloped messages, or   signed and enveloped messages in order that the message can be   forwarded to any environment without modification.   These steps are descriptive rather than prescriptive. The implementor   is free to use any procedure as long as the result is the same.     Step 1. The MIME entity is prepared according to the local             conventions     Step 2. The leaf parts of the MIME entity are converted to             canonical form     Step 3. Appropriate transfer encoding is applied to the leaves of             the MIME entity   When an S/MIME message is received, the security services on the   message are removed, and the result is the MIME entity. That MIME   entity is typically passed to a MIME-capable user agent where, it is   further decoded and presented to the user or receiving application.Dusse, et. al.               Informational                     [Page 10]

RFC 2311         S/MIME Version 2 Message Specification       March 19983.1.1 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   insection 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 endDusse, et. al.               Informational                     [Page 11]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   recipient so that they can verify the signatures directly. If the   transport internal to the site is not 8-bit clean, such as on a   wide-area network with a single mail gateway, verifying the signature   will not be possible unless the original MIME entity was only 7-bit   data.3.1.3 Transfer Encoding for Signing Using multipart/signed   If a multipart/signed entity is EVER to be transmitted over the   standard Internet SMTP infrastructure or other transport that is   constrained to 7-bit text, it MUST have transfer encoding applied so   that it is represented as 7-bit text. MIME entities that are 7-bit   data already need no transfer encoding. Entities such as 8-bit text   and binary data can be encoded with quoted-printable or base-64   transfer encoding.   The primary reason for the 7-bit requirement is that the Internet   mail transport infrastructure cannot guarantee transport of 8-bit or   binary data. Even though many segments of the transport   infrastructure now handle 8-bit and even binary data, it is sometimes   not possible to know whether the transport path is 8-bit clear. If a   mail message with 8-bit data were to encounter a message transfer   agent that can not transmit 8-bit or binary data, the agent has three   options, none of which are acceptable for a clear-signed message:    - The agent could change the transfer encoding; this would      invalidate the signature.    - The agent could transmit the data anyway, which would most likely      result in the 8th bit being corrupted; this too would invalidate      the signature.    - The agent could return the message to the sender.   [MIME-SECURE] prohibits an agent from changing the transfer encoding   of the first part of a multipart/signed message. If a compliant agent   that can not transmit 8-bit or binary data encounters a   multipart/signed message with 8-bit or binary data in the first part,   it would have to return the message to the sender as undeliverable.3.1.4 Sample Canonical MIME Entity   This example shows a multipart/mixed message with full transfer   encoding.  This message contains a text part and an attachment. The   sample message text includes characters that are not US-ASCII and   thus must be transfer encoded. Though not shown here, the end of each   line is <CR><LF>. The line ending of the MIME headers, the text, and   transfer encoded parts, all must be <CR><LF>.   Note that this example is not of an S/MIME message.Dusse, et. al.               Informational                     [Page 12]

RFC 2311         S/MIME Version 2 Message Specification       March 1998       Content-Type: multipart/mixed; boundary=bar       --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       --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 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 insection 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.Dusse, et. al.               Informational                     [Page 13]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   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 insection 3.1.   Because there are several types of application/pkcs7-mime objects, a   sending agent SHOULD do as much as possible to help a receiving agent   know about the contents of the object without forcing the receiving   agent to decode the ASN.1 for the object. The MIME headers of all   application/pkcs7-mime objects SHOULD include the optional "smime-   type" parameter, as described in the following sections.3.2.1 The name and filename Parameters   For the application/pkcs7-mime, sending agents SHOULD emit the   optional "name" parameter to the Content-Type field for compatibility   with older systems. Sending agents SHOULD also emit the optional   Content-Disposition field [CONTDISP] with the "filename" parameter.   If a sending agent emits the above parameters, the value of the   parameters SHOULD be a file name with the appropriate extension:   MIME Type                      File Extension   application/pkcs7-mime              .p7m   (signedData, envelopedData)   application/pkcs7-mime              .p7c   (degenerate signedData   "certs-only" message)   application/pkcs7-signature         .p7s   application/pkcs10                  .p10   In addition, the file name SHOULD be limited to eight characters   followed by a three letter extension. The eight character filename   base can be any distinct name; the use of the filename base "smime"   SHOULD be used to indicate that the MIME entity is associated with   S/MIME.   Including a file name serves two purposes. It facilitates easier use   of S/MIME objects as files on disk. It also can convey type   information across gateways. When a MIME entity of type   application/pkcs7-mime (for example) arrives at a gateway that has no   special knowledge of S/MIME, it will default the entity's MIME type   to application/octet-stream and treat it as a generic attachment,   thus losing the type information. However, the suggested filename forDusse, et. al.               Informational                     [Page 14]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   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 tosection 3.1.     Step 2. The MIME entity and other required data is processed into a             PKCS #7 object of type envelopedData.     Step 3. The PKCS #7 object is inserted into an application/pkcs7-             mime MIME entity.   The smime-type parameter for enveloped-only messages is "enveloped-   data".  The file extension for this type of message is ".p7m".   A sample message would be:       Content-Type: application/pkcs7-mime; smime-type=enveloped-data;            name=smime.p7m       Content-Transfer-Encoding: base64       Content-Disposition: attachment; filename=smime.p7m       rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6       7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H       f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4       0GhIGfHfQbnj756YT64V3.4 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 theDusse, et. al.               Informational                     [Page 15]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   receivers and the relative importance of receivers with S/MIME   facilities being able to verify the signature versus the importance   of receivers without S/MIME software being able to view the message.   Messages signed using the multipart/signed format can always be   viewed by the receiver whether they have S/MIME software or not. They   can also be viewed whether they are using a MIME-native user agent or   they have messages translated by a gateway. In this context, "be   viewed" means the ability to process the message essentially as if it   were not a signed message, including any other MIME structure the   message might have.   Messages signed using the signedData format cannot be viewed by a   recipient unless they have S/MIME facilities. However, if they have   S/MIME facilities, these messages can always be verified if they were   not changed in transit.3.4.2 Signing Using application/pkcs7-mime and SignedData   This signing format uses the application/pkcs7-mime MIME type. The   steps to create this format are:     Step 1. The MIME entity is prepared according tosection 3.1     Step 2. The MIME entity and other required data is processed into a             PKCS #7 object of type signedData     Step 3. The PKCS #7 object is inserted into an             application/pkcs7-mime MIME entity   The smime-type parameter for messages using application/pkcs7-mime   and SignedData is "signed-data". The file extension for this type of   message is ".p7m".   A sample message would be:       Content-Type: application/pkcs7-mime; smime-type=signed-data;            name=smime.p7m       Content-Transfer-Encoding: base64       Content-Disposition: attachment; filename=smime.p7m       567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7       77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH       HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh       6YT64V0GhIGfHfQbnj75Dusse, et. al.               Informational                     [Page 16]

RFC 2311         S/MIME Version 2 Message Specification       March 19983.4.3 Signing Using the multipart/signed Format   This format is a clear-signing format. Recipients without any S/MIME   or PKCS processing facilities are able to view the message. It makes   use of the multipart/signed MIME type described in [MIME-SECURE]. The   multipart/signed MIME type has two parts. The first part contains the   MIME entity that is to be signed; the second part contains the   signature, which is a PKCS #7 detached signature.3.4.3.1 The application/pkcs7-signature MIME Type   This MIME type always contains a single PKCS #7 object of type   signedData.  The contentInfo field of the PKCS #7 object must be   empty. The signerInfos field contains the signatures for the MIME   entity. The details of the registered type are given inAppendix D.   The file extension for signed-only messages using application/pkcs7-   signature  is ".p7s".3.4.3.2 Creating a multipart/signed Message     Step 1. The MIME entity to be signed is prepared according tosection 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 insection 3.1.     Step 4. Transfer encoding is applied to the detached signature and             it is inserted into a MIME entity of type             application/pkcs7-signature     Step 5. The MIME entity of the application/pkcs7-signature is             inserted into the second part of the multipart/signed             entity   The multipart/signed Content type has two required parameters: the   protocol parameter and the micalg parameter.   The protocol parameter MUST be "application/pkcs7-signature". Note   that quotation marks are required around the protocol parameter   because MIME requires that the "/" character in the parameter value   MUST be quoted.Dusse, et. al.               Informational                     [Page 17]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   The micalg parameter allows for one-pass processing when the   signature is being verified. The value of the micalg parameter is   dependent on the message digest algorithm used in the calculation of   the Message Integrity Check. The value of the micalg parameter SHOULD   be one of the following:   Algorithm used     Value   --------------     ---------   MD5                md5   SHA-1              sha1   any other          unknown   (Historical note: some early implementations of S/MIME emitted and   expected "rsa-md5" and "rsa-sha1" for the micalg parameter.)   Receiving agents SHOULD be able to recover gracefully from a micalg   parameter value that they do not recognize.3.4.3.3 Sample multipart/signed Message       Content-Type: multipart/signed;          protocol="application/pkcs7-signature";          micalg=sha1; boundary=boundary42       --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.Dusse, et. al.               Informational                     [Page 18]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   An S/MIME implementation MUST be able to receive and process   arbitrarily nested S/MIME within reasonable resource limits of the   recipient computer.   It is possible to either sign a message first, or to envelope the   message first. It is up to the implementor and the user to choose.   When signing first, the signatories are then securely obscured by the   enveloping. When enveloping first the signatories are exposed, but it   is possible to verify signatures without removing the enveloping.   This may be useful in an environment were automatic signature   verification is desired, as no private key material is required to   verify a signature.3.6 Creating a Certificates-only Message   The certificates only message or MIME entity is used to transport   certificates, such as in response to a registration request. This   format can also be used to convey CRLs.     Step 1. The certificates are made available to the PKCS #7             generating process which creates a PKCS #7 object of type             signedData.  The contentInfo and signerInfos fields must be             empty.     Step 2. The PKCS #7 signedData object is enclosed in an             application/pkcs7-mime MIME entity   The smime-type parameter for a certs-only message is "certs-only".   The file extension for this type of message is ".p7c".3.7 Creating a Registration Request   A typical application which allows a user to generate cryptographic   information has to submit that information to a certification   authority, who transforms it into a certificate. PKCS #10 describes a   syntax for certification requests. The application/pkcs10 body type   MUST be used to transfer a PKCS #10 certification request.   The details of certification requests and the process of obtaining a   certificate are beyond the scope of this memo. Instead, only the   format of data used in application/pkcs10 is defined.3.7.1 Format of the application/pkcs10 Body   PKCS #10 defines the ASN.1 type CertificationRequest for use in   submitting a certification request. Therefore, when the MIME content   type application/pkcs10 is used, the body MUST be a   CertificationRequest, encoded using the Basic Encoding Rules (BER).Dusse, et. al.               Informational                     [Page 19]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   Although BER is specified, instead of the more restrictive DER, a   typical application will use DER since the CertificationRequest's   CertificationRequestInfo has to be DER-encoded in order to be signed.   A robust application SHOULD output DER, but allow BER or DER on   input.   Data produced by BER or DER is 8-bit, but many transports are limited   to 7-bit data. Therefore, a suitable 7-bit Content-Transfer-Encoding   SHOULD be applied. The base64 Content-Transfer-Encoding SHOULD be   used with application/pkcs10, although any 7-bit transfer encoding   may work.3.7.2 Sending and Receiving an application/pkcs10 Body Part   For sending a certificate-signing request, the application/pkcs10   message format MUST be used to convey a PKCS #10 certificate-signing   request. Note that for sending certificates and CRLs messages without   any signed content, the application/pkcs7-mime message format MUST be   used to convey a degenerate PKCS #7 signedData "certs-only" message.   To send an application/pkcs10 body, the application generates the   cryptographic information for the user. The details of the   cryptographic information are beyond the scope of this memo.     Step 1. The cryptographic information is placed within a PKCS #10             CertificationRequest.     Step 2. The CertificationRequest is encoded according to BER or DER             (typically, DER).     Step 3. As a typical step, the DER-encoded CertificationRequest is             also base64 encoded so that it is 7-bit data suitable for             transfer in SMTP. This then becomes the body of an             application/pkcs10 body part.   The result might look like this:       Content-Type: application/pkcs10; name=smime.p10       Content-Transfer-Encoding: base64       Content-Disposition: attachment; filename=smime.p10       rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6       7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H       f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4       0GhIGfHfQbnj756YT64V   A typical application only needs to send a certification request. It   is a certification authority that has to receive and process theDusse, et. al.               Informational                     [Page 20]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   request. The steps for recovering the CertificationRequest from the   message are straightforward but are not presented here. The   procedures for processing the certification request are beyond the   scope of this document.3.8 Identifying an S/MIME Message   Because S/MIME takes into account interoperation in non-MIME   environments, several different mechanisms are employed to carry the   type information, and it becomes a bit difficult to identify S/MIME   messages. The following table lists criteria for determining whether   or not a message is an S/MIME message. A message is considered an   S/MIME message if it matches any below.   The file suffix in the table below comes from the "name" parameter in   the content-type header, or the "filename" parameter on the content-   disposition header. These parameters that give the file suffix are   not listed below as part of the parameter section.   MIME type:   application/pkcs7-mime   parameters:  any   file suffix: any   MIME type:   application/pkcs10   parameters:  any   file suffix: any   MIME type:   multipart/signed   parameters:  protocol="application/pkcs7-signature"   file suffix: any   MIME type:   application/octet-stream   parameters:  any   file suffix: p7m, p7s, aps, p7c, p104. 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 toDusse, et. al.               Informational                     [Page 21]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   "store and protect" certificates for correspondents in such a way so   as to guarantee their later retrieval.4.1 Key Pair Generation   An S/MIME agent or some related administrative utility or function   MUST be capable of generating RSA key pairs on behalf of the user.   Each key pair MUST be generated from a good source of non-   deterministic random input and protected in a secure fashion.   A user agent SHOULD generate RSA key pairs at a minimum key size of   768 bits and a maximum key size of 1024 bits. A user agent MUST NOT   generate RSA key pairs less than 512 bits long. Some agents created   in the United States have chosen to create 512 bit keys in order to   get more advantageous export licenses. However, 512 bit keys are   considered by many to be cryptographically insecure.   Implementors should be aware that multiple (active) key pairs may be   associated with a single individual. For example, one key pair may be   used to support confidentiality, while a different key pair may be   used for authentication.5. Security Considerations   This entire memo discusses security. Security issues not covered in   other parts of the memo include:   40-bit encryption is considered weak by most cryptographers. Using   weak cryptography in S/MIME offers little actual security over   sending plaintext. However, other features of S/MIME, such as the   specification of tripleDES and the ability to announce stronger   cryptographic capabilities to parties with whom you communicate,   allow senders to create messages that use strong encryption. Using   weak cryptography is never recommended unless the only alternative is   no cryptography. When feasible, sending and receiving agents should   inform senders and recipients the relative cryptographic strength of   messages.   It is impossible for most software or people to estimate the value of   a message. Further, it is impossible for most software or people to   estimate the actual cost of decrypting a message that is encrypted   with a key of a particular size. Further, it is quite difficult to   determine the cost of a failed decryption if a recipient cannot   decode a message. Thus, choosing between different key sizes (or   choosing whether to just use plaintext) is also impossible. However,   decisions based on these criteria are made all the time, and   therefore this memo gives a framework for using those estimates in   choosing algorithms.Dusse, et. al.               Informational                     [Page 22]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   If a sending agent is sending the same message using different   strengths of cryptography, an attacker watching the communications   channel can determine the contents of the strongly-encrypted message   by decrypting the weakly-encrypted version. In other words, a sender   should not send a copy of a message using weaker cryptography than   they would use for the original of the message.Dusse, et. al.               Informational                     [Page 23]

RFC 2311         S/MIME Version 2 Message Specification       March 1998A. Object Identifiers and Syntax   The syntax for SMIMECapability is:   SMIMECapability ::= SEQUENCE {       capabilityID OBJECT IDENTIFIER,       parameters OPTIONAL ANY DEFINED BY capabilityID }   SMIMECapabilities ::= SEQUENCE OF SMIMECapabilityA.1 Content Encryption AlgorithmsRC2-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 than256, 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, therc2ParameterVersion values are 160, 120, 58 respectively.DES-EDE3-CBC OBJECT IDENTIFIER ::= {iso(1) member-body(2) us(840) rsadsi(113549) encryptionAlgorithm(3) 7}For DES-CBC and DES-EDE3-CBC, the parameter should be encoded as:CBCParameter :: IVwhere IV ::= OCTET STRING -- 8 octets.A.2 Digest Algorithmsmd5 OBJECT IDENTIFIER ::= {iso(1) member-body(2) us(840) rsadsi(113549) digestAlgorithm(2) 5}sha-1 OBJECT IDENTIFIER ::= {iso(1) identified-organization(3) oiw(14) secsig(3) algorithm(2) 26}A.3 Asymmetric Encryption AlgorithmsrsaEncryption OBJECT IDENTIFIER ::= {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1}Dusse, et. al.               Informational                     [Page 24]

RFC 2311         S/MIME Version 2 Message Specification       March 1998rsa OBJECT IDENTIFIER ::=     {joint-iso-ccitt(2) ds(5) algorithm(8) encryptionAlgorithm(1) 1}A.4 Signature Algorithmsmd2WithRSAEncryption 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 AttributessigningTime OBJECT IDENTIFIER ::=     {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 5}smimeCapabilities OBJECT IDENTIFIER ::=    {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15}Dusse, et. al.               Informational                     [Page 25]

RFC 2311         S/MIME Version 2 Message Specification       March 1998B. References   [3DES] W. Tuchman, "Hellman Presents No Shortcut Solutions To DES,"   IEEE Spectrum, v. 16, n. 7, July 1979, pp40-41.   [CHARSETS] Character sets assigned by IANA. See   <ftp://ftp.isi.edu/in-notes/iana/assignments/character-sets>.   [CONTDISP] Troost, R., Dorner, S and K. Moore, "Communicating   Presentation Information in Internet Messages:  The Content-   Disposition Header Field",RFC 2183, August 1997.   [DES] ANSI X3.106, "American National Standard for Information   Systems-Data Link Encryption," American National Standards Institute,   1983.   [MD5] Rivest, R., "The MD5 Message Digest Algorithm",RFC 1321, April   1992.   [MIME-SPEC] The primary definition of MIME.   Freed, N., and N. Borenstein, "MIME Part 1: Format of Internet   Message Bodies",RFC 2045, November 1996.   Freed, N., and N. Borenstein, "MIME Part 2: Media Types",RFC 2046,   November 1996.   Moore, K., "MIME Part 3: Message Header Extensions for Non-ASCII   Text",RFC 2047, November 1996.   Freed, N., Klensin, J., and J. Postel, "MIME Part 4: Registration   Procedures",RFC 2048, November 1996.   Freed, N., and N. Borenstein, "MIME Part 5: Conformance Criteria and   Examples",RFC 2049, November 1996.   [MIME-SECURE] Galvin, J., Murphy, S., Crocker, S., and N. Freed,   "Security Multiparts for MIME: Multipart/Signed and   Multipart/Encrypted",RFC 1847, October 1995.   [MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate   Requirement Levels",BCP 14,RFC 2119, March 1997.   [PKCS-1] Kaliski, B., "PKCS #1: RSA Encryption Version 1.5",RFC2313, March 1998.   [PKCS-7] Kaliski, B., "PKCS #7: Cryptographic Message Syntax Version   1.5",RFC 2315, March 1998.Dusse, et. al.               Informational                     [Page 26]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   [PKCS-10] Kaliski, B., "PKCS #10: Certification Request Syntax   Version 1.5",RFC 2314, March 1998.   [RC2] Rivest, R., "Description of the RC2(r) Encryption Algorithm",RFC 2268, January 1998.   [SHA1] NIST FIPS PUB 180-1, "Secure Hash Standard," National   Institute of Standards and Technology, U.S. Department of Commerce,   DRAFT, 31 May 1994.Dusse, et. al.               Informational                     [Page 27]

RFC 2311         S/MIME Version 2 Message Specification       March 1998C. Compatibility with Prior Practice in S/MIME   S/MIME was originally developed by RSA Data Security, Inc. Many   developers implemented S/MIME agents before this document was   published. All S/MIME receiving agents SHOULD make every attempt to   interoperate with these earlier implementations of S/MIME.C.1 Early MIME Types   Some early implementations of S/MIME agents used the following MIME   types:   application/x-pkcs7-mime   application/x-pkcs7-signature   application/x-pkcs10   In each case, the "x-" subtypes correspond to the subtypes described   in this document without the "x-".C.2 Profiles   Early S/MIME documentation had two profiles for encryption:   "restricted" and "unrestricted". The difference between these   profiles historically came about due to US Government export   regulations, as described at the end of this section. It is expected   that in the future, there will be few agents that only use the   restricted profile.   Briefly, the restricted profile required the ability to encrypt and   decrypt using RSA's trade-secret RC2 algorithm in CBC mode with 40-   bit keys. The unrestricted profile required the ability to encrypt   and decrypt using RSA's trade-secret RC2 algorithm in CBC mode with   40-bit keys, and to encrypt and decrypt using tripleDES. The   restricted profile also had non-mandatory suggestions for other   algorithms, but these were not widely implemented.   It is important to note that many current implementations of S/MIME   use the restricted profile.C.2.1 Historical Reasons for the Existence of Two Encryption Profiles   Due to US Government export regulations, an S/MIME agent which   supports a strong content encryption algorithm such as DES would not   be freely exportable outside of North America. US software   manufacturers have been compelled to incorporate an exportable or   "restricted" content encryption algorithm in order to create a widely   exportable version of their product.  S/MIME agents created in the US   and intended for US domestic use (or use under special StateDusse, et. al.               Informational                     [Page 28]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   Department export licenses) can utilize stronger, "unrestricted"   content encryption. However, in order to achieve interoperability,   such agents need to support whatever exportable algorithm is   incorporated in restricted S/MIME agents.   The RC2 symmetric encryption algorithm has been approved by the US   Government for "expedited" export licensing at certain key sizes.   Consequently, support for the RC2 algorithm in CBC mode is required   for baseline interoperability in all S/MIME implementations. Support   for other strong symmetric encryption algorithms such as RC5 CBC, DES   CBC and DES EDE3-CBC for content encryption is strongly encouraged   where possible.Dusse, et. al.               Informational                     [Page 29]

RFC 2311         S/MIME Version 2 Message Specification       March 1998D. Request for New MIME SubtypesD.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: COMMOND.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: noneDusse, et. al.               Informational                     [Page 30]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   Optional parameters: name, filename   Encoding considerations: Will be binary data, therefore should use   base64 encoding   Security considerations: Described in [PKCS-7]   Interoperability considerations: Designed to carry digital   signatures with PKCS-7, as described in [PKCS-7]   Published specification:RFC 2311   Applications which use this media type: Secure Internet mail and   other secure data transports.   Additional information:   File extension(s): .p7s   Macintosh File Type Code(s):   Person & email address to contact for further information:   Steve Dusse, spock@rsa.com   Intended usage: COMMOND.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 andDusse, et. al.               Informational                     [Page 31]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   other transports where certificates are required.   Additional information:   File extension(s): .p10   Macintosh File Type Code(s):   Person & email address to contact for further information:   Steve Dusse, spock@rsa.com   Intended usage: COMMONDusse, et. al.               Informational                     [Page 32]

RFC 2311         S/MIME Version 2 Message Specification       March 1998E. Encapsulating Signed Messages for Internet Transport   The rationale behind the multiple formats for signing has to do with   the MIME subtype defaulting rules of the application and multipart   top-level types, and the behavior of currently deployed gateways and   mail user agents.   Ideally, the multipart/signed format would be the only format used   because it provides a truly backwards compatible way to sign MIME   entities. In a pure MIME environment with very capable user agents,   this would be possible. The world, however, is more complex than   this.   One problem with the multipart/signed format occurs with gateways to   non-MIME environments. In these environments, the gateway will   generally not be S/MIME aware, will not recognize the   multipart/signed type, and will default its treatment to   multipart/mixed as is prescribed by the MIME standard. The real   problem occurs when the gateway also applies conversions to the MIME   structure of the original message that is being signed and is   contained in the first part of the multipart/signed structure, such   as the gateway converting text and attachments to the local format.   Because the signature is over the MIME structure of the original   message, but the original message is now decomposed and transformed,   the signature cannot be verified. Because MIME encoding of a   particular set of body parts can be done in many different ways,   there is no way to reconstruct the original MIME entity over which   the signature was computed.   A similar problem occurs when an attempt is made to combine an   existing user agent with a stand-alone S/MIME facility. Typical user   agents do not have the ability to make a multipart sub-entity   available to a stand-alone application in the same way they make leaf   MIME entities available to "viewer" applications. This user agent   behavior is not required by the MIME standard and thus not widely   implemented. The result is that it is impossible for most user agents   to hand off the entire multipart/signed entity to a stand-alone   application.E.1 Solutions to the Problem   To work around these two problems, the application/pkcs7-mime type   can be used. When going through a gateway, it will be defaulted to   the MIME type of application/octet-stream and treated as a single   opaque entity. That is, the message will be treated as an attachment   of unknown type, converted into the local representation for an   attachment and thus can be made available to an S/MIME facility   completely intact. A similar result is achieved when a user agentDusse, et. al.               Informational                     [Page 33]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   similarly treats the application/pkcs7-mime MIME entity as a simple   leaf node of the MIME structure and makes it available to viewer   applications.   Another way to work around these problems is to encapsulate the   multipart/signed MIME entity in a MIME entity that will not be   damaged by the gateway. At the time that this memo is being written,   there is a proposal for a MIME entity "application/mime" for this   purpose. However, no implementations of S/MIME use this type of   wrapping.E.2 Encapsulation in an Non-MIME Environment   While this document primarily addresses the Internet, it is useful to   compose and receive S/MIME secured messages in non-MIME environments.   This is particularly the case when it is desired that security be   implemented end-to-end. Other discussion here addresses the receipt   of S/MIME messages in non-MIME environments. Here the composition of   multipart/signed entities is addressed.   When a message is to be sent in such an environment, the   multipart/signed entity is created as described above. That entity is   then treated as an opaque stream of bits and added to the message as   an attachment. It must have a file name that ends with ".aps", as   this is the sole mechanism for recognizing it as an S/MIME message by   the receiving agent.   When this message arrives in a MIME environment, it is likely to have   a MIME type of application/octet-stream, with MIME parameters giving   the filename for the attachment. If the intervening gateway has   carried the file type, it will end in ".aps" and be recognized as an   S/MIME message.Dusse, et. al.               Informational                     [Page 34]

RFC 2311         S/MIME Version 2 Message Specification       March 1998F. Acknowledgements   Significant contributions to the content of this memo were made by   many people, including Jim Schaad, Jeff Thompson, and Jeff Weinstein.G. Authors' Addresses   Steve Dusse   RSA Data Security, Inc.   100 Marine Parkway, #500   Redwood City, CA  94065  USA   Phone: (415) 595-8782   EMail: spock@rsa.com   Paul Hoffman   Internet Mail Consortium   127 Segre Place   Santa Cruz, CA  95060   Phone: (408) 426-9827   EMail: phoffman@imc.org   Blake Ramsdell   Worldtalk   13122 NE 20th St., Suite C   Bellevue, WA 98005   Phone: (425) 882-8861   EMail: blaker@deming.com   Laurence Lundblade   QUALCOMM Incorporated   Eudora Division   6455 Lusk Boulevard   San Diego, California 92121-2779   Phone: (800) 238-3672   EMail: lgl@qualcomm.comDusse, et. al.               Informational                     [Page 35]

RFC 2311         S/MIME Version 2 Message Specification       March 1998   Lisa Repka   Netscape Communications Corporation   501 East Middlefield Road   Mountain View, CA  94043   Phone: (415) 254-1900   EMail: repka@netscape.comDusse, et. al.               Informational                     [Page 36]

RFC 2311         S/MIME Version 2 Message Specification       March 1998H.  Full Copyright Statement   Copyright (C) The Internet Society (1998).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Dusse, et. al.               Informational                     [Page 37]