rfc2660.txt

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Network Working Group                                       E. Rescorla
Request for Comments: 2660                                   RTFM, Inc.
Category: Experimental                                     A. Schiffman
                                                   Terisa Systems, Inc.
                                                            August 1999


                 The Secure HyperText Transfer Protocol

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   This memo describes a syntax for securing messages sent using the
   Hypertext Transfer Protocol (HTTP), which forms the basis for the
   World Wide Web. Secure HTTP (S-HTTP) provides independently
   applicable security services for transaction confidentiality,
   authenticity/integrity and non-repudiability of origin.

   The protocol emphasizes maximum flexibility in choice of key
   management mechanisms, security policies and cryptographic algorithms
   by supporting option negotiation between parties for each
   transaction.

Table of Contents

   1. Introduction .................................................. 3
   1.1. Summary of Features ......................................... 3
   1.2. Changes ..................................................... 4
   1.3. Processing Model ............................................ 5
   1.4. Modes of Operation .......................................... 6
   1.5. Implementation Options ...................................... 7
   2. Message Format ................................................ 7
   2.1. Notational Conventions ...................................... 8
   2.2. The Request Line ............................................ 8
   2.3. The Status Line ............................................. 8
   2.4. Secure HTTP Header Lines .................................... 8
   2.5. Content .....................................................12
   2.6. Encapsulation Format Options ................................13



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   2.6.1. Content-Privacy-Domain: CMS ...............................13
   2.6.2. Content-Privacy-Domain: MOSS ..............................14
   2.6.3. Permitted HTTP headers ....................................14
   2.6.3.2. Host ....................................................15
   2.6.3.3. Connection ..............................................15
   3. Cryptographic Parameters ......................................15
   3.1. Options Headers .............................................15
   3.2. Negotiation Options .........................................16
   3.2.1. Negotiation Overview ......................................16
   3.2.2. Negotiation Option Format .................................16
   3.2.3. Parametrization for Variable-length Key Ciphers ...........18
   3.2.4. Negotiation Syntax ........................................18
   3.3. Non-Negotiation Headers .....................................23
   3.3.1. Encryption-Identity .......................................23
   3.3.2. Certificate-Info ..........................................23
   3.3.3. Key-Assign ................................................24
   3.3.4. Nonces ....................................................25
   3.4. Grouping Headers With SHTTP-Cryptopts .......................26
   3.4.1. SHTTP-Cryptopts ...........................................26
   4. New Header Lines for HTTP .....................................26
   4.1. Security-Scheme .............................................26
   5. (Retriable) Server Status Error Reports .......................27
   5.1. Retry for Option (Re)Negotiation ............................27
   5.2. Specific Retry Behavior .....................................28
   5.3. Limitations On Automatic Retries ............................29
   6. Other Issues ..................................................30
   6.1. Compatibility of Servers with Old Clients ...................30
   6.2. URL Protocol Type ...........................................30
   6.3. Browser Presentation ........................................31
   7. Implementation Notes ..........................................32
   7.1. Preenhanced Data ............................................32
   7.2. Note:Proxy Interaction ......................................34
   7.2.1. Client-Proxy Authentication ...............................34
   8. Implementation Recommendations and Requirements ...............34
   9. Protocol Syntax Summary .......................................35
   10. An Extended Example ..........................................36
   Appendix: A Review of CMS ........................................40
   Bibliography and References ......................................41
   Security Considerations ..........................................43
   Authors' Addresses ...............................................44
   Full Copyright Statement..........................................45










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1.  Introduction

   The World Wide Web (WWW) is a distributed hypermedia system which has
   gained widespread acceptance among Internet users.  Although WWW
   browsers support other, preexisting Internet application protocols,
   the native and primary protocol used between WWW clients and servers
   is the HyperText Transfer Protocol (HTTP) [RFC-2616].  The ease of
   use of the Web has prompted its widespread employment as a
   client/server architecture for many applications.  Many such
   applications require the client and server to be able to authenticate
   each other and exchange sensitive information confidentially. The
   original HTTP specification had only modest support for the
   cryptographic mechanisms appropriate for such transactions.

   Secure HTTP (S-HTTP) provides secure communication mechanisms between
   an HTTP client-server pair in order to enable spontaneous commercial
   transactions for a wide range of applications.  Our design intent is
   to provide a flexible protocol that supports multiple orthogonal
   operation modes, key management mechanisms, trust models,
   cryptographic algorithms and encapsulation formats through option
   negotiation between parties for each transaction.

1.1.  Summary of Features

   Secure HTTP is a secure message-oriented communications protocol
   designed for use in conjunction with HTTP. It is designed to coexist
   with HTTP's messaging model and to be easily integrated with HTTP
   applications.

   Secure HTTP provides a variety of security mechanisms to HTTP clients
   and servers, providing the security service options appropriate to
   the wide range of potential end uses possible for the World-Wide Web.
   The protocol provides symmetric capabilities to both client and
   server (in that equal treatment is given to both requests and
   replies, as well as for the preferences of both parties) while
   preserving the transaction model and implementation characteristics
   of HTTP.

   Several cryptographic message format standards may be incorporated
   into S-HTTP clients and servers, particularly, but in principle not
   limited to, [CMS] and [MOSS]. S-HTTP supports interoperation among a
   variety of implementations, and is compatible with HTTP.  S-HTTP
   aware clients can communicate with S-HTTP oblivious servers and
   vice-versa, although such transactions obviously would not use S-HTTP
   security features.

   S-HTTP does not require client-side public key certificates (or
   public keys), as it supports symmetric key-only operation modes.



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   This is significant because it means that spontaneous private
   transactions can occur without requiring individual users to have
   an established public key.  While S-HTTP is able to take advantage
   of ubiquitous certification infrastructures, its deployment does
   not require it.

   S-HTTP supports end-to-end secure transactions, in contrast with the
   original HTTP authorization mechanisms which require the client to
   attempt access and be denied before the security mechanism is
   employed.  Clients may be "primed" to initiate a secure transaction
   (typically using information supplied in message headers); this may
   be used to support encryption of fill-out forms, for example. With
   S-HTTP, no sensitive data need ever be sent over the network in the
   clear.

   S-HTTP provides full flexibility of cryptographic algorithms, modes
   and parameters. Option negotiation is used to allow clients and
   servers to agree on transaction modes (e.g., should the request be
   signed or encrypted or both -- similarly for the reply?);
   cryptographic algorithms (RSA vs. DSA for signing, DES vs.
   RC2 for encrypting, etc.); and certificate selection
   (please sign with your "Block-buster Video certificate").

   S-HTTP attempts to avoid presuming a particular trust model, although
   its designers admit to a conscious effort to facilitate
   multiply-rooted hierarchical trust, and anticipate that principals may
   have many public key certificates.

   S-HTTP differs from Digest-Authentication, described in [RFC-2617] in
   that it provides support for public key cryptography and consequently
   digital signature capability, as well as providing confidentiality.

1.2.  Changes

   This document describes S-HTTP/1.4. It differs from the previous
   memo in that it differs from the previous memo in its support of
   the Cryptographic Message Syntax (CMS) [CMS], a successor to PKCS-7;
   and hence now supports the Diffie-Hellman and the (NIST) Digital
   Signature Standard cryptosystems. CMS used in RSA mode is bits on the
   wire compatible with PKCS-7.











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1.3.  Processing Model

1.3.1.  Message Preparation

   The creation of an S-HTTP message can be thought of as a a function
   with three inputs:

      1. The cleartext message. This is either an HTTP message
      or some other data object. Note that since the cleartext message
      is carried transparently, headers and all, any version of HTTP
      can be carried within an S-HTTP wrapper.
      2. The receiver's cryptographic preferences and keying material.
      This is either explicitly specified by the receiver or subject
      to some default set of preferences.
      3. The sender's cryptographic preferences and keying material.
      This input to the function can be thought of as implicit
      since it exists only in the memory of the sender.

   In order to create an S-HTTP message, then, the sender integrates the
   sender's preferences with the receiver's preferences. The result of
   this is a list of cryptographic enhancements to be applied and keying
   material to be used to apply them. This may require some user
   intervention. For instance, there might be multiple keys available to
   sign the message. (See Section 3.2.4.9.3 for more on this topic.)
   Using this data, the sender applies the enhancements to the message
   clear-text to create the S-HTTP message.

   The processing steps required to transform the cleartext message into
   the S-HTTP message are described in Sections 2 and 3. The processing
   steps required to merge the sender's and receiver's preferences are
   described in Sections 3.2.

1.3.2.  Message Recovery

   The recovery of an S-HTTP message can be thought of as a function of
   four distinct inputs:

      1. The S-HTTP message.
      2. The receiver's stated cryptographic preferences and keying
      material. The receiver has the opportunity to remember what
      cryptographic preferences it provided in order for this
      document to be dereferenced.
      3. The receiver's current cryptographic preferences and
      keying material.
      4. The sender's previously stated cryptographic options.
      The sender may have stated that he would perform certain
      cryptographic operations in this message. (Again, see
      sections 4 and 5 for details on how to do this.)



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   In order to recover an S-HTTP message, the receiver needs to read the
   headers to discover which cryptographic transformations were
   performed on the message, then remove the transformations using some
   combination of the sender's and receiver's keying material, while
   taking note of which enhancements were applied.

   The receiver may also choose to verify that the applied enhancements
   match both the enhancements that the sender said he would apply
   (input 4 above) and that the receiver requested (input 2 above) as
   well as the current preferences to see if the S-HTTP message was
   appropriately transformed. This process may require interaction with
   the user to verify that the enhancements are acceptable to the user.
   (See Section 6.4 for more on this topic.)

1.4.  Modes of Operation

   Message protection may be provided on three orthogonal axes:
   signature, authentication, and encryption. Any message may be signed,
   authenticated, encrypted, or any combination of these (including no
   protection).

   Multiple key management mechanisms are supported, including
   password-style manually shared secrets and public-key key exchange.
   In particular, provision has been made for prearranged (in an earlier
   transaction or out of band) symmetric session keys in order to send
   confidential messages to those who have no public key pair.

   Additionally, a challenge-response ("nonce") mechanism is provided to
   allow parties to assure themselves of transaction freshness.

1.4.1.  Signature

   If the digital signature enhancement is applied, an appropriate
   certificate may either be attached to the message (possibly along
   with a certificate chain) or the sender may expect the recipient to
   obtain the required certificate (chain) independently.

1.4.2.  Key Exchange and Encryption

   In support of bulk encryption, S-HTTP defines two key transfer
   mechanisms, one using public-key enveloped key exchange and another
   with externally arranged keys.