rfc2660.txt

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   In the former case, the symmetric-key cryptosystem parameter is
   passed encrypted under the receiver's public key.






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   In the latter mode, we encrypt the content using a prearranged
   session key, with key identification information specified on one of
   the header lines.

1.4.3.  Message Integrity and Sender Authentication

   Secure HTTP provides a means to verify message integrity and sender
   authenticity for a message via the computation of a Message
   Authentication Code (MAC), computed as a keyed hash over the document
   using a shared secret -- which could potentially have been arranged
   in a number of ways, e.g.: manual arrangement or 'inband' key
   management.  This technique requires neither the use of public key
   cryptography nor encryption.

   This mechanism is also useful for cases where it is appropriate to
   allow parties to identify each other reliably in a transaction
   without providing (third-party) non-repudiability for the
   transactions themselves. The provision of this mechanism is motivated
   by our bias that the action of "signing" a transaction should be
   explicit and conscious for the user, whereas many authentication
   needs (i.e., access control) can be met with a lighter-weight
   mechanism that retains the scalability advantages of public-key
   cryptography for key exchange.

1.4.4.  Freshness

   The protocol provides a simple challenge-response mechanism, allowing
   both parties to insure the freshness of transmissions. Additionally,
   the integrity protection provided to HTTP headers permits
   implementations to consider the Date: header allowable in HTTP
   messages as a freshness indicator, where appropriate (although this
   requires implementations to make allowances for maximum clock skew
   between parties, which we choose not to specify).

1.5.  Implementation Options

   In order to encourage widespread adoption of secure documents for the
   World-Wide Web in the face of the broad scope of application
   requirements, variability of user sophistication, and disparate
   implementation constraints, Secure HTTP deliberately caters to a
   variety of implementation options.  See Section 8 for implementation
   recommendations and requirements.

2.  Message Format

   Syntactically, Secure HTTP messages are the same as HTTP, consisting
   of a request or status line followed by headers and a body. However,
   the range of headers is different and the bodies are typically



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   cryptographically enhanced.

2.1.  Notational Conventions

   This document uses the augmented BNF from HTTP [RFC-2616]. You should
   refer to that document for a description of the syntax.

2.2.  Request Line

   In order to differentiate S-HTTP messages from HTTP messages and
   allow for special processing, the request line should use the special
   Secure" method and use the protocol designator "Secure-HTTP/1.4".
   Consequently, Secure-HTTP and HTTP processing can be intermixed on
   the same TCP port, e.g. port 80.  In order to prevent leakage of
   potentially sensitive information Request-URI should be "*". For
   example:

           Secure * Secure-HTTP/1.4

   When communicating via a proxy, the Request-URI should be consist of
   the AbsoluteURI. Typically, the rel path section should be replaced
   by "*" to minimize the information passed to in the clear.  (e.g.
   http://www.terisa.com/*); proxies should remove the appropriate
   amount of this information to minimize the threat of traffic
   analysis.  See Section 7.2.2.1 for a situation where providing more
   information is appropriate.

2.3.  The Status Line

   S-HTTP responses should use the protocol designator "Secure-
   HTTP/1.4".  For example:

           Secure-HTTP/1.4 200 OK

   Note that the status in the Secure HTTP response line does not
   indicate anything about the success or failure of the unwrapped HTTP
   request. Servers should always use 200 OK provided that the Secure
   HTTP processing is successful. This prevents analysis of success or
   failure for any request, which the correct recipient can determine
   from the encapsulated data. All case variations should be accepted.

2.4.  Secure HTTP Header Lines

   The header lines described in this section go in the header of a
   Secure HTTP message. All except 'Content-Type' and 'Content-Privacy-
   Domain' are optional. The message body shall be separated from the
   header block by two successive CRLFs.




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   All data and fields in header lines should be treated as case
   insensitive unless otherwise specified. Linear whitespace [RFC-822]
   should be used only as a token separator unless otherwise quoted.
   Long header lines may be line folded in the style of [RFC-822].

   This document refers to the header block following the S-HTTP
   request/response line and preceding the successive CRLFs collectively
   as "S-HTTP headers".

2.4.1.  Content-Privacy-Domain

   The two values defined by this document are 'MOSS' and 'CMS'.  CMS
   refers to the privacy enhancement specified in section 2.6.1. MOSS
   refers to the format defined in [RFC-1847] and [RFC-1848].

2.4.2.  Content-Type for CMS

   Under normal conditions, the terminal encapsulated content (after all
   privacy enhancements have been removed) would be an HTTP message. In
   this case, there shall be a Content-Type line reading:

           Content-Type: message/http

   The message/http content type is defined in RFC-2616.

   If the inner message is an S-HTTP message, then the content type
   shall be 'application/s-http'. (See Appendix for the definition of
   this.)

   It is intended that these types be registered with IANA as MIME
   content types.

   The terminal content may be of some other type provided that the type
   is properly indicated by the use of an appropriate Content-Type
   header line. In this case, the header fields for the encapsulation of
   the terminal content apply to the terminal content (the 'final
   headers'). But in any case, final headers should themselves always be
   S-HTTP encapsulated, so that the applicable S-HTTP/HTTP headers are
   never passed unenhanced.

   S-HTTP encapsulation of non-HTTP data is a useful mechanism for
   passing pre-enhanced data (especially presigned data) without
   requiring that the HTTP headers themselves be pre-enhanced.








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2.4.3.  Content-Type for MOSS

   The Content-Type for MOSS shall be an acceptable MIME content type
   describing the cryptographic processing applied. (e.g.
   multipart/signed). The content type of the inner content is described
   in the content type line corresponding to that inner content, and for
   HTTP messages shall be 'message/http'.

2.4.4.  Prearranged-Key-Info

   This header line is intended to convey information about a key which
   has been arranged outside of the internal cryptographic format. One
   use of this is to permit in-band communication of session keys for
   return encryption in the case where one of the parties does not have
   a key pair. However, this should also be useful in the event that the
   parties choose to use some other mechanism, for instance, a one-time
   key list.

   This specification defines two methods for exchanging named keys,
   Inband, Outband. Inband indicates that the session key was exchanged
   previously, using a Key-Assign header of the corresponding method.
   Outband arrangements imply that agents have external access to key
   materials corresponding to a given name, presumably via database
   access or perhaps supplied immediately by a user from keyboard input.
   The syntax for the header line is:

     Prearranged-Key-Info =
      "Prearranged-Key-Info" ":" Hdr-Cipher "," CoveredDEK "," CoverKey-ID
     CoverKey-ID = method ":" key-name
     CoveredDEK = *HEX
     method = "inband" |  "outband"

   While chaining ciphers require an Initialization Vector (IV) [FIPS-
   81] to start off the chaining, that information is not carried by
   this field. Rather, it should be passed internal to the cryptographic
   format being used. Likewise, the bulk cipher used is specified in
   this fashion.

   <Hdr-Cipher> should be the name of the block cipher used to encrypt
   the session key (see section 3.2.4.7)

   <CoveredDEK> is the protected Data Encryption Key (a.k.a. transaction
   key) under which the encapsulated message was encrypted. It should be
   appropriately (randomly) generated by the sending agent, then
   encrypted under the cover of the negotiated key (a.k.a. session key)
   using the indicated header cipher, and then converted into hex.





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   In order to avoid name collisions, cover key namespaces must be
   maintained separately by host and port.

   Note that some Content-Privacy-Domains, notably likely future
   revisions of MOSS and CMS may have support for symmetric key
   management.

   The Prearranged-Key-Info field need not be used in such
   circumstances.  Rather, the native syntax is preferred. Keys
   exchanged with Key-Assign, however, may be used in this situation.

2.4.5.  MAC-Info

   This header is used to supply a Message Authenticity Check, providing
   both message authentication and integrity, computed from the message
   text, the time (optional -- to prevent replay attack), and a shared
   secret between client and server. The MAC should be computed over the
   encapsulated content of the S-HTTP message.  S-HTTP/1.1 defined that
   MACs should be computed using the following algorithm ('||' means
   concatenation):

        MAC = hex(H(Message||[<time>]||<shared key>))

   The time should be represented as an unsigned 32 bit quantity
   representing seconds since 00:00:00 GMT January 1, 1970 (the UNIX
   epoch), in network byte order. The shared key format is a local
   matter.

   Recent research [VANO95] has demonstrated some weaknesses in this
   approach, and this memo introduces a new construction, derived from
   [RFC-2104]. In the name of backwards compatibility, we retain the
   previous constructions with the same names as before. However, we
   also introduce a new series of names (See Section 3.2.4.8 for the
   names) that obey a different (hopefully stronger) construction. (^
   means bitwise XOR)

   HMAC = hex(H(K' ^ pad2 || H(K' ^ pad1 ||[<time>]|| Message)))
   pad1 = the byte 0x36 repeated enough times to fill out a
                hash input block. (I.e. 64 times for both MD5 and SHA-1)
   pad2 = the byte 0x5c repeated enough times to fill out a
                hash input block.
   K' = H(<shared key>)

   The original HMAC construction is for the use of a key with length
   equal to the length of the hash output. Although it is considered
   safe to use a key of a different length (Note that strength cannot be
   increased past the length of the hash function itself, but can be
   reduced by using a shorter key.) [KRAW96b] we hash the original key



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   to permit the use of shared keys (e.g. passphrases) longer than the
   length of the hash. It is noteworthy (though obvious) that this
   technique does not increase the strength of short keys.

   The format of the MAC-Info line is:

   MAC-Info =
   "MAC-Info" ":"  [hex-time],
   hash-alg, hex-hash-data, key-spec
   hex-time = <unsigned seconds since Unix epoch represented as HEX>
   hash-alg = <hash algorithms from section 3.2.4.8>
   hex-hash-data = <computation as described above represented as HEX>
   Key-Spec = "null" | "dek" | Key-ID

   Key-Ids can refer either to keys bound using the Key-Assign header
   line or those bound in the same fashion as the Outband method
   described later. The use of a 'Null' key-spec implies that a zero
   length key was used, and therefore that the MAC merely represents a
   hash of the message text and (optionally) the time.  The special
   key-spec 'DEK' refers to the Data Exchange Key used to encrypt the
   following message body (it is an error to use the DEK key-spec in
   situations where the following message body is unencrypted).

   If the time is omitted from the MAC-Info line, it should simply not
   be included in the hash.

   Note that this header line can be used to provide a more advanced
   equivalent of the original HTTP Basic authentication mode in that the
   user can be asked to provide a username and password. However, the
   password remains private and message integrity can be assured.
   Moreover, this can be accomplished without encryption of any kind.

   In addition, MAC-Info permits fast message integrity verification (at
   the loss of non-repudiability) for messages, provided that the
   participants share a key (possibly passed using Key-Assign in a
   previous message).