[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits] [IPR]

Versions: (draft-thomson-http-encryption) 00 01 02 03 04 05 06 07 08 09 RFC 8188

HTTP Working Group                                            M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Standards Track                          March 20, 2016
Expires: September 21, 2016


                  Encrypted Content-Encoding for HTTP
               draft-ietf-httpbis-encryption-encoding-01

Abstract

   This memo introduces a content-coding for HTTP that allows message
   payloads to be encrypted.

Note to Readers

   Discussion of this draft takes place on the HTTP working group
   mailing list (ietf-http-wg@w3.org), which is archived at
   https://lists.w3.org/Archives/Public/ietf-http-wg/ .

   Working Group information can be found at http://httpwg.github.io/ ;
   source code and issues list for this draft can be found at
   https://github.com/httpwg/http-extensions/labels/encryption .

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 21, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents



Thomson                Expires September 21, 2016               [Page 1]


Internet-Draft           HTTP encryption coding               March 2016


   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  The "aesgcm" HTTP Content Encoding  . . . . . . . . . . . . .   4
   3.  The Encryption HTTP Header Field  . . . . . . . . . . . . . .   5
     3.1.  Encryption Header Field Parameters  . . . . . . . . . . .   6
     3.2.  Content Encryption Key Derivation . . . . . . . . . . . .   7
     3.3.  Nonce Derivation  . . . . . . . . . . . . . . . . . . . .   7
   4.  Crypto-Key Header Field . . . . . . . . . . . . . . . . . . .   8
     4.1.  Explicit Key  . . . . . . . . . . . . . . . . . . . . . .   9
     4.2.  Diffie-Hellman  . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Pre-shared Authentication Secrets . . . . . . . . . . . .  10
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  Successful GET Response . . . . . . . . . . . . . . . . .  11
     5.2.  Encryption and Compression  . . . . . . . . . . . . . . .  12
     5.3.  Encryption with More Than One Key . . . . . . . . . . . .  12
     5.4.  Encryption with Explicit Key  . . . . . . . . . . . . . .  12
     5.5.  Encryption with Multiple Records  . . . . . . . . . . . .  13
     5.6.  Diffie-Hellman Encryption . . . . . . . . . . . . . . . .  13
     5.7.  Diffie-Hellman with Authentication Secret . . . . . . . .  14
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     6.1.  Key and Nonce Reuse . . . . . . . . . . . . . . . . . . .  15
     6.2.  Content Integrity . . . . . . . . . . . . . . . . . . . .  15
     6.3.  Leaking Information in Headers  . . . . . . . . . . . . .  15
     6.4.  Poisoning Storage . . . . . . . . . . . . . . . . . . . .  16
     6.5.  Sizing and Timing Attacks . . . . . . . . . . . . . . . .  16
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     7.1.  The "aesgcm" HTTP Content Encoding  . . . . . . . . . . .  16
     7.2.  Encryption Header Fields  . . . . . . . . . . . . . . . .  17
     7.3.  The HTTP Encryption Parameter Registry  . . . . . . . . .  17
       7.3.1.  keyid . . . . . . . . . . . . . . . . . . . . . . . .  18
       7.3.2.  salt  . . . . . . . . . . . . . . . . . . . . . . . .  18
       7.3.3.  rs  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     7.4.  The HTTP Crypto-Key Parameter Registry  . . . . . . . . .  18
       7.4.1.  keyid . . . . . . . . . . . . . . . . . . . . . . . .  19
       7.4.2.  aesgcm  . . . . . . . . . . . . . . . . . . . . . . .  19
       7.4.3.  dh  . . . . . . . . . . . . . . . . . . . . . . . . .  19
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  19



Thomson                Expires September 21, 2016               [Page 2]


Internet-Draft           HTTP encryption coding               March 2016


     8.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Appendix A.  JWE Mapping  . . . . . . . . . . . . . . . . . . . .  21
   Appendix B.  Intermediate Values for Encryption . . . . . . . . .  22
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  23
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   It is sometimes desirable to encrypt the contents of a HTTP message
   (request or response) so that when the payload is stored (e.g., with
   a HTTP PUT), only someone with the appropriate key can read it.

   For example, it might be necessary to store a file on a server
   without exposing its contents to that server.  Furthermore, that same
   file could be replicated to other servers (to make it more resistant
   to server or network failure), downloaded by clients (to make it
   available offline), etc.  without exposing its contents.

   These uses are not met by the use of TLS [RFC5246], since it only
   encrypts the channel between the client and server.

   This document specifies a content-coding (Section 3.1.2 of [RFC7231])
   for HTTP to serve these and other use cases.

   This content-coding is not a direct adaptation of message-based
   encryption formats - such as those that are described by [RFC4880],
   [RFC5652], [RFC7516], and [XMLENC] - which are not suited to stream
   processing, which is necessary for HTTP.  The format described here
   cleaves more closely to the lower level constructs described in
   [RFC5116].

   To the extent that message-based encryption formats use the same
   primitives, the format can be considered as sequence of encrypted
   messages with a particular profile.  For instance, Appendix A
   explains how the format is congruent with a sequence of JSON Web
   Encryption [RFC7516] values with a fixed header.

   This mechanism is likely only a small part of a larger design that
   uses content encryption.  How clients and servers acquire and
   identify keys will depend on the use case.  Though a complete key
   management system is not described, this document defines an Crypto-
   Key header field that can be used to convey keying material.

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].



Thomson                Expires September 21, 2016               [Page 3]


Internet-Draft           HTTP encryption coding               March 2016


   Base64url encoding is defined in Section 2 of [RFC7515].

2.  The "aesgcm" HTTP Content Encoding

   The "aesgcm" HTTP content-coding indicates that a payload has been
   encrypted using Advanced Encryption Standard (AES) in Galois/Counter
   Mode (GCM) as identified as AEAD_AES_128_GCM in [RFC5116],
   Section 5.1.  The AEAD_AES_128_GCM algorithm uses a 128 bit content
   encryption key.

   When this content-coding is in use, the Encryption header field
   (Section 3) describes how encryption has been applied.  The Crypto-
   Key header field (Section 4) can be included to describe how the
   content encryption key is derived or retrieved.

   The "aesgcm" content-coding uses a single fixed set of encryption
   primitives.  Cipher suite agility is achieved by defining a new
   content-coding scheme.  This ensures that only the HTTP Accept-
   Encoding header field is necessary to negotiate the use of
   encryption.

   The "aesgcm" content-coding uses a fixed record size.  The resulting
   encoding is a series of fixed-size records, with a final record that
   is one or more octets shorter than a fixed sized record.


          +------+         input of between rs-65537
          | data |            and rs-2 octets
          +------+      (one fewer for the last record)
              |
              v
   +-----+-----------+
   | pad |   data    |     add padding to form plaintext
   +-----+-----------+
            |
            v
   +--------------------+
   |    ciphertext      |  encrypt with AEAD_AES_128_GCM
   +--------------------+     expands by 16 octets

   The record size determines the length of each portion of plaintext
   that is enciphered, with the exception of the final record, which is
   necessarily smaller.  The record size defaults to 4096 octets, but
   can be changed using the "rs" parameter on the Encryption header
   field.

   AEAD_AES_128_GCM expands ciphertext to be 16 octets longer than its
   input plaintext.  Therefore, the length of each enciphered record



Thomson                Expires September 21, 2016               [Page 4]


Internet-Draft           HTTP encryption coding               March 2016


   other than the last is equal to the value of the "rs" parameter plus
   16 octets.  A receiver MUST fail to decrypt if the final record
   ciphertext is 16 octets or less in size.  Valid records always
   contain at least one byte of padding and a 16 octet authentication
   tag.

   Each record contains between 2 and 65537 octets of padding, inserted
   into a record before the enciphered content.  Padding consists of a
   two octet unsigned integer in network byte order, followed that
   number of zero-valued octets.  A receiver MUST fail to decrypt if any
   padding octet other than the first two are non-zero, or a record has
   more padding than the record size can accommodate.

   The nonce for each record is a 96-bit value constructed from the
   record sequence number and the input keying material.  Nonce
   derivation is covered in Section 3.3.

   The additional data passed to each invocation of AEAD_AES_128_GCM is
   a zero-length octet sequence.

   A sequence of full-sized records can be truncated to produce a
   shorter sequence of records with valid authentication tags.  To
   prevent an attacker from truncating a stream, an encoder MUST append
   a record that contains only padding and is smaller than the full
   record size if the final record ends on a record boundary.  A
   receiver MUST treat the stream as failed due to truncation if the
   final record is the full record size.

   A consequence of this record structure is that range requests
   [RFC7233] and random access to encrypted payload bodies are possible
   at the granularity of the record size.  However, without data from
   adjacent ranges, partial records cannot be used.  Thus, it is best if
   records start and end on multiples of the record size, plus the 16
   octet authentication tag size.

3.  The Encryption HTTP Header Field

   The "Encryption" HTTP header field describes the encrypted content
   encoding(s) that have been applied to a payload body, and therefore
   how those content encoding(s) can be removed.

   The "Encryption" header field uses the extended ABNF syntax defined
   in Section 1.2 of [RFC7230] and the "parameter" rule from [RFC7231]


     Encryption = #encryption_params
     encryption_params = [ parameter *( ";" parameter ) ]




Thomson                Expires September 21, 2016               [Page 5]


Internet-Draft           HTTP encryption coding               March 2016


   If the payload is encrypted more than once (as reflected by having
   multiple content-codings that imply encryption), each application of
   the content encoding is reflected in the Encryption header field, in
   the order in which they were applied.

   Encryption header field values with multiple instances of the same
   parameter name are invalid.

   The Encryption header MAY be omitted if the sender does not intend
   for the immediate recipient to be able to decrypt the payload body.
   Alternatively, the Encryption header field MAY be omitted if the
   sender intends for the recipient to acquire the header field by other
   means.

   Servers processing PUT requests MUST persist the value of the
   Encryption header field, unless they remove the content-coding by
   decrypting the payload.

3.1.  Encryption Header Field Parameters

   The following parameters are used in determining the content
   encryption key that is used for encryption:

   keyid:  The "keyid" parameter contains a string that identifies the
      keying material that is used.  The "keyid" parameter SHOULD be
      included, unless key identification is guaranteed by other means.
      The "keyid" parameter MUST be used if keying material included in
      an Crypto-Key header field is needed to derive the content
      encryption key.

   salt:  The "salt" parameter contains a base64url-encoded octets
      [RFC7515] that is used as salt in deriving a unique content
      encryption key (see Section 3.2).  The "salt" parameter MUST be
      present, and MUST be exactly 16 octets long when decoded.  The
      "salt" parameter MUST NOT be reused for two different payload
      bodies that have the same input keying material; generating a
      random salt for every application of the content encoding ensures
      that content encryption key reuse is highly unlikely.

   rs:  The "rs" parameter contains a positive decimal integer that
      describes the record size in octets.  This value MUST be greater
      than 1.  If the "rs" parameter is absent, the record size defaults
      to 4096 octets.








Thomson                Expires September 21, 2016               [Page 6]


Internet-Draft           HTTP encryption coding               March 2016


3.2.  Content Encryption Key Derivation

   In order to allow the reuse of keying material for multiple different
   HTTP messages, a content encryption key is derived for each message.
   The content encryption key is derived from the decoded value of the
   "salt" parameter using the HMAC-based key derivation function (HKDF)
   described in [RFC5869] using the SHA-256 hash algorithm [FIPS180-4].

   The decoded value of the "salt" parameter is the salt input to HKDF
   function.  The keying material identified by the "keyid" parameter is
   the input keying material (IKM) to HKDF.  Input keying material can
   either be prearranged, or can be described using the Crypto-Key
   header field (Section 4).  The first step of HKDF is therefore:


      PRK = HMAC-SHA-256(salt, IKM)

   The info parameter to HKDF is set to the ASCII-encoded string
   "Content-Encoding: aesgcm", a single zero octet and an optional
   context string:


      cek_info = "Content-Encoding: aesgcm" || 0x00 || context

   Unless otherwise specified, the context is a zero length octet
   sequence.  Specifications that use this content encoding MAY specify
   the use of an expanded context to cover additional inputs in the key
   derivation.

   AEAD_AES_128_GCM requires a 16 octet (128 bit) content encryption
   key, so the length (L) parameter to HKDF is 16.  The second step of
   HKDF can therefore be simplified to the first 16 octets of a single
   HMAC:


      CEK = HMAC-SHA-256(PRK, cek_info || 0x01)

3.3.  Nonce Derivation

   The nonce input to AEAD_AES_128_GCM is constructed for each record.
   The nonce for each record is a 12 octet (96 bit) value is produced
   from the record sequence number and a value derived from the input
   keying material.

   The input keying material and salt values are input to HKDF with
   different info and length parameters.





Thomson                Expires September 21, 2016               [Page 7]


Internet-Draft           HTTP encryption coding               March 2016


   The length (L) parameter is 12 octets.  The info parameter for the
   nonce is the ASCII-encoded string "Content-Encoding: nonce", a single
   zero octet and an context:


      nonce_info = "Content-Encoding: nonce" || 0x00 || context

   The context for nonce derivation SHOULD be the same as is used for
   content encryption key derivation.

   The result is combined with the record sequence number - using
   exclusive or - to produce the nonce.  The record sequence number
   (SEQ) is a 96-bit unsigned integer in network byte order that starts
   at zero.

   Thus, the final nonce for each record is a 12 octet value:


      NONCE = HMAC-SHA-256(PRK, nonce_info || 0x01) XOR SEQ

4.  Crypto-Key Header Field

   An Crypto-Key header field can be used to describe the input keying
   material used in the Encryption header field.

   The Crypto-Key header field uses the extended ABNF syntax defined in
   Section 1.2 of [RFC7230] and the "parameter" rule from [RFC7231].


     Crypto-Key = #crypto_key_params
     crypto_key_params = [ parameter *( ";" parameter ) ]

   keyid:  The "keyid" parameter corresponds to the "keyid" parameter in
      the Encryption header field.

   aesgcm:  The "aesgcm" parameter contains the base64url-encoded octets
      [RFC7515] of the input keying material.

   dh:  The "dh" parameter contains an ephemeral Diffie-Hellman share.
      This form of the header field can be used to encrypt content for a
      specific recipient.

   Crypto-Key header field values with multiple instances of the same
   parameter name are invalid.

   The input keying material used by the key derivation (see
   Section 3.2) can be determined based on the information in the




Thomson                Expires September 21, 2016               [Page 8]


Internet-Draft           HTTP encryption coding               March 2016


   Crypto-Key header field.  The method for key derivation depends on
   the parameters that are present in the header field.

   The value or values provided in the Crypto-Key header field is valid
   only for the current HTTP message unless additional information
   indicates a greater scope.

   Note that different methods for determining input keying material
   will produce different amounts of data.  The HKDF process ensures
   that the final content encryption key is the necessary size.

   Alternative methods for determining input keying material MAY be
   defined by specifications that use this content-encoding.

4.1.  Explicit Key

   The "aesgcm" parameter is decoded and used as the input keying
   material for the "aesgcm" content encoding.  The "aesgcm" parameter
   MUST decode to at least 16 octets in order to be used as input keying
   material for "aesgcm" content encoding.

   Other key determination parameters can be ignored if the "aesgcm"
   parameter is present.

4.2.  Diffie-Hellman

   The "dh" parameter is included to describe a Diffie-Hellman share,
   either modp (or finite field) Diffie-Hellman [DH] or elliptic curve
   Diffie-Hellman (ECDH) [RFC4492].

   This share is combined with other information at the recipient to
   determine the HKDF input keying material.  In order for the exchange
   to be successful, the following information MUST be established out
   of band:

   o  Which Diffie-Hellman form is used.

   o  The modp group or elliptic curve that will be used.

   o  A label that uniquely identifies the group.  This label will be
      expressed as a sequence of octets and MUST NOT include a zero-
      valued octet.

   o  The format of the ephemeral public share that is included in the
      "dh" parameter.  This encoding MUST result in a single, canonical
      sequence of octets.  For instance, using ECDH both parties need to
      agree whether this is an uncompressed or compressed point.




Thomson                Expires September 21, 2016               [Page 9]


Internet-Draft           HTTP encryption coding               March 2016


   In addition to identifying which content-encoding this input keying
   material is used for, the "keyid" parameter is used to identify this
   additional information at the receiver.

   The intended recipient recovers their private key and are then able
   to generate a shared secret using the designated Diffie-Hellman
   process.

   The context for content encryption key and nonce derivation (see
   Section 3.2) is set to include the means by which the keys were
   derived.  The context is formed from the concatenation of group
   label, a single zero octet, the length of the public key of the
   recipient, the public key of the recipient, the length of the public
   key of the sender, and the public key of the sender.  The public keys
   are encoded into octets as defined for the group when determining the
   context string.


      context = label || 0x00 ||
                  length(recipient_public) || recipient_public ||
                  length(sender_public) || sender_public

   The two length fields are encoded as a two octet unsigned integer in
   network byte order.

   Specifications that rely on an Diffie-Hellman exchange for
   determining input keying material MUST either specify the parameters
   for Diffie-Hellman (label, group parameters, or curves and point
   format) that are used, or describe how those parameters are
   negotiated between sender and receiver.

4.3.  Pre-shared Authentication Secrets

   Key derivation MAY be extended to include an additional
   authentication secret.  Such a secret is shared between the sender
   and receiver of a message using other means.

   A pre-shared authentication secret is not explicitly signaled in
   either the Encryption or Crypto-Key header fields.  Use of this
   additional step depends on prior agreement.

   When a shared authentication secret is used, the keying material
   produced by the key agreement method (e.g., Diffie-Hellman, explicit
   key, or otherwise) is combined with the authentication secret using
   HKDF.  The output of HKDF is the input keying material used to derive
   the content encryption key and nonce Section 3.2.





Thomson                Expires September 21, 2016              [Page 10]


Internet-Draft           HTTP encryption coding               March 2016


   The authentication secret is used as the "salt" parameter to HKDF,
   the raw keying material (e.g., Diffie-Hellman output) is used as the
   "IKM" parameter, the ASCII-encoded string "Content-Encoding: auth"
   with a terminal zero octet is used as the "info" parameter, and the
   length of the output is 32 octets (i.e., the entire output of the
   underlying SHA-256 HMAC function):


      auth_info = "Content-Encoding: auth" || 0x00
      IKM = HKDF(authentication, raw_key, auth_info, 32)

   This invocation of HKDF does not take the same context that is
   provided to the final key derivation stages.  Alternatively, this
   phase can be viewed as always having a zero-length context.

   Note that in the absence of an authentication secret, the input
   keying material is simply the raw keying material:


      IKM = raw_key

5.  Examples

   This section shows a few examples of the content encoding.

   Note: All binary values in the examples in this section use the URL
   and filename safe variant of base64 [RFC4648].  This includes the
   bodies of requests.  Whitespace in these values is added to fit
   formatting constraints.

5.1.  Successful GET Response


   HTTP/1.1 200 OK
   Content-Type: application/octet-stream
   Content-Encoding: aesgcm
   Connection: close
   Encryption: keyid="http://example.org/bob/keys/123";
               salt="XZwpw6o37R-6qoZjw6KwAw"

   [encrypted payload]

   Here, a successful HTTP GET response has been encrypted using input
   keying material that is identified by a URI.

   Note that the media type has been changed to "application/octet-
   stream" to avoid exposing information about the content.




Thomson                Expires September 21, 2016              [Page 11]


Internet-Draft           HTTP encryption coding               March 2016


5.2.  Encryption and Compression

   In this example, a response is first compressed, then encrypted.
   Note that this particular encoding might compromise confidentiality
   if the contents of the response could be influenced by an attacker.


   HTTP/1.1 200 OK
   Content-Type: text/html
   Content-Encoding: gzip, aesgcm
   Transfer-Encoding: chunked
   Encryption: keyid="mailto:me@example.com";
               salt="m2hJ_NttRtFyUiMRPwfpHA"

   [encrypted payload]

5.3.  Encryption with More Than One Key

   Here, a PUT request has been encrypted twice with different input
   keying material; decrypting twice is necessary to read the content.
   The outer layer of encryption uses a 1200 octet record size.


   PUT /thing HTTP/1.1
   Host: storage.example.com
   Content-Type: application/http
   Content-Encoding: aesgcm, aesgcm
   Content-Length: 1235
   Encryption: keyid="mailto:me@example.com";
               salt="NfzOeuV5USPRA-n_9s1Lag",
               keyid="http://example.org/bob/keys/123";
               salt="bDMSGoc2uobK_IhavSHsHA"; rs=1200

   [encrypted payload]

5.4.  Encryption with Explicit Key

   This example shows the UTF-8 encoded string "I am the walrus"
   encrypted using an directly provided value for the input keying
   material.  The content body contains a single record only and is
   shown here using base64url encoding for presentation reasons.










Thomson                Expires September 21, 2016              [Page 12]


Internet-Draft           HTTP encryption coding               March 2016


   HTTP/1.1 200 OK
   Content-Length: 33
   Content-Encoding: aesgcm
   Encryption: keyid="a1"; salt="vr0o6Uq3w_KDWeatc27mUg"
   Crypto-Key: keyid="a1"; aesgcm="csPJEXBYA5U-Tal9EdJi-w"

   VDeU0XxaJkOJDAxPl7h9JD5V8N43RorP7PfpPdZZQuwF

5.5.  Encryption with Multiple Records

   This example shows the same encrypted message, but split into records
   of 10 octets each.  The first record includes a single additional
   octet of padding, which causes the end of the content to align with a
   record boundary, forcing the creation of a third record that contains
   only padding.


   HTTP/1.1 200 OK
   Content-Length: 70
   Content-Encoding: aesgcm
   Encryption: keyid="a1"; salt="4pdat984KmT9BWsU3np0nw"; rs=10
   Crypto-Key: keyid="a1"; aesgcm="BO3ZVPxUlnLORbVGMpbT1Q"

   uzLfrZ4cbMTC6hlUqHz4NvWZshFlTN3o2RLr6FrIuOKEfl2VrM_jYgoiIyEo
   Zvc-ZGwV-RMJejG4M6ZfGysBAdhpPqrLzw

5.6.  Diffie-Hellman Encryption


   HTTP/1.1 200 OK
   Content-Length: 33
   Content-Encoding: aesgcm
   Encryption: keyid="dhkey"; salt="Qg61ZJRva_XBE9IEUelU3A"
   Crypto-Key: keyid="dhkey";
                   dh="BDgpRKok2GZZDmS4r63vbJSUtcQx4Fq1V58-6-3NbZzS
                       TlZsQiCEDTQy3CZ0ZMsqeqsEb7qW2blQHA4S48fynTk"

   yqD2bapcx14XxUbtwjiGx69eHE3Yd6AqXcwBpT2Kd1uy

   This example shows the same string, "I am the walrus", encrypted
   using ECDH over the P-256 curve [FIPS186], which is identified with
   the label "P-256" encoded in ASCII.  The content body is shown here
   encoded in URL-safe base64url for presentation reasons only.

   The receiver (in this case, the HTTP client) uses a key pair that is
   identified by the string "dhkey" and the sender (the server) uses a
   key pair for which the public share is included in the "dh" parameter
   above.  The keys shown below use uncompressed points [X9.62] encoded



Thomson                Expires September 21, 2016              [Page 13]


Internet-Draft           HTTP encryption coding               March 2016


   using base64url.  Line wrapping is added for presentation purposes
   only.


      Receiver:
         private key: 9FWl15_QUQAWDaD3k3l50ZBZQJ4au27F1V4F0uLSD_M
         public key: BCEkBjzL8Z3C-oi2Q7oE5t2Np-p7osjGLg93qUP0wvqR
                     T21EEWyf0cQDQcakQMqz4hQKYOQ3il2nNZct4HgAUQU
      Sender:
         private key: vG7TmzUX9NfVR4XUGBkLAFu8iDyQe-q_165JkkN0Vlw
         public key: <the value of the "dh" parameter>

5.7.  Diffie-Hellman with Authentication Secret

   This example shows the same receiver key pair from Section 5.6, but
   with a shared authentication secret of "R29vIGdvbyBnJyBqb29iIQ".


   HTTP/1.1 200 OK
   Content-Length: 33
   Content-Encoding: aesgcm
   Encryption: keyid="dhkey"; salt="lngarbyKfMoi9Z75xYXmkg"
   Crypto-Key: keyid="dhkey";
                   dh="BNoRDbb84JGm8g5Z5CFxurSqsXWJ11ItfXEWYVLE85Y7
                       CYkDjXsIEc4aqxYaQ1G8BqkXCJ6DPpDrWtdWj_mugHU"

   6nqAQUME8hNqw5J3kl8cpVVJylXKYqZOeseZG8UueKpA

   The sender's private key used in this example is "nCScek-QpEjmOOlT-
   rQ38nZzvdPlqa00Zy0i6m2OJvY".  Intermediate values for this example
   are included in Appendix B.

6.  Security Considerations

   This mechanism assumes the presence of a key management framework
   that is used to manage the distribution of keys between valid senders
   and receivers.  Defining key management is part of composing this
   mechanism into a larger application, protocol, or framework.

   Implementation of cryptography - and key management in particular -
   can be difficult.  For instance, implementations need to account for
   the potential for exposing keying material on side channels, such as
   might be exposed by the time it takes to perform a given operation.
   The requirements for a good implementation of cryptographic
   algorithms can change over time.






Thomson                Expires September 21, 2016              [Page 14]


Internet-Draft           HTTP encryption coding               March 2016


6.1.  Key and Nonce Reuse

   Encrypting different plaintext with the same content encryption key
   and nonce in AES-GCM is not safe [RFC5116].  The scheme defined here
   uses a fixed progression of nonce values.  Thus, a new content
   encryption key is needed for every application of the content
   encoding.  Since input keying material can be reused, a unique "salt"
   parameter is needed to ensure a content encryption key is not reused.

   If a content encryption key is reused - that is, if input keying
   material and salt are reused - this could expose the plaintext and
   the authentication key, nullifying the protection offered by
   encryption.  Thus, if the same input keying material is reused, then
   the salt parameter MUST be unique each time.  This ensures that the
   content encryption key is not reused.  An implementation SHOULD
   generate a random salt parameter for every message; a counter could
   achieve the same result.

6.2.  Content Integrity

   This mechanism only provides content origin authentication.  The
   authentication tag only ensures that an entity with access to the
   content encryption key produced the encrypted data.

   Any entity with the content encryption key can therefore produce
   content that will be accepted as valid.  This includes all recipients
   of the same HTTP message.

   Furthermore, any entity that is able to modify both the Encryption
   header field and the HTTP message body can replace the contents.
   Without the content encryption key or the input keying material,
   modifications to or replacement of parts of a payload body are not
   possible.

6.3.  Leaking Information in Headers

   Because only the payload body is encrypted, information exposed in
   header fields is visible to anyone who can read the HTTP message.
   This could expose side-channel information.

   For example, the Content-Type header field can leak information about
   the payload body.

   There are a number of strategies available to mitigate this threat,
   depending upon the application's threat model and the users'
   tolerance for leaked information:





Thomson                Expires September 21, 2016              [Page 15]


Internet-Draft           HTTP encryption coding               March 2016


   1.  Determine that it is not an issue.  For example, if it is
       expected that all content stored will be "application/json", or
       another very common media type, exposing the Content-Type header
       field could be an acceptable risk.

   2.  If it is considered sensitive information and it is possible to
       determine it through other means (e.g., out of band, using hints
       in other representations, etc.), omit the relevant headers, and/
       or normalize them.  In the case of Content-Type, this could be
       accomplished by always sending Content-Type: application/octet-
       stream (the most generic media type), or no Content-Type at all.

   3.  If it is considered sensitive information and it is not possible
       to convey it elsewhere, encapsulate the HTTP message using the
       application/http media type (Section 8.3.2 of [RFC7230]),
       encrypting that as the payload of the "outer" message.

6.4.  Poisoning Storage

   This mechanism only offers encryption of content; it does not perform
   authentication or authorization, which still needs to be performed
   (e.g., by HTTP authentication [RFC7235]).

   This is especially relevant when a HTTP PUT request is accepted by a
   server; if the request is unauthenticated, it becomes possible for a
   third party to deny service and/or poison the store.

6.5.  Sizing and Timing Attacks

   Applications using this mechanism need to be aware that the size of
   encrypted messages, as well as their timing, HTTP methods, URIs and
   so on, may leak sensitive information.

   This risk can be mitigated through the use of the padding that this
   mechanism provides.  Alternatively, splitting up content into
   segments and storing the separately might reduce exposure.  HTTP/2
   [RFC7540] combined with TLS [RFC5246] might be used to hide the size
   of individual messages.

7.  IANA Considerations

7.1.  The "aesgcm" HTTP Content Encoding

   This memo registers the "encrypted" HTTP content-coding in the HTTP
   Content Codings Registry, as detailed in Section 2.

   o  Name: aesgcm




Thomson                Expires September 21, 2016              [Page 16]


Internet-Draft           HTTP encryption coding               March 2016


   o  Description: AES-GCM encryption with a 128-bit content encryption
      key

   o  Reference: this specification

7.2.  Encryption Header Fields

   This memo registers the "Encryption" HTTP header field in the
   Permanent Message Header Registry, as detailed in Section 3.

   o  Field name: Encryption

   o  Protocol: HTTP

   o  Status: Standard

   o  Reference: this specification

   o  Notes:

   This memo registers the "Crypto-Key" HTTP header field in the
   Permanent Message Header Registry, as detailed in Section 4.

   o  Field name: Crypto-Key

   o  Protocol: HTTP

   o  Status: Standard

   o  Reference: this specification

   o  Notes:

7.3.  The HTTP Encryption Parameter Registry

   This memo establishes a registry for parameters used by the
   "Encryption" header field under the "Hypertext Transfer Protocol
   (HTTP) Parameters" grouping.  The "Hypertext Transfer Protocol (HTTP)
   Encryption Parameters" registry operates under an "Specification
   Required" policy [RFC5226].

   Entries in this registry are expected to include the following
   information:

   o  Parameter Name: The name of the parameter.

   o  Purpose: A brief description of the purpose of the parameter.




Thomson                Expires September 21, 2016              [Page 17]


Internet-Draft           HTTP encryption coding               March 2016


   o  Reference: A reference to a specification that defines the
      semantics of the parameter.

   The initial contents of this registry are:

7.3.1.  keyid

   o  Parameter Name: keyid

   o  Purpose: Identify the key that is in use.

   o  Reference: this document

7.3.2.  salt

   o  Parameter Name: salt

   o  Purpose: Provide a source of entropy for derivation of a content
      encryption key.  This value is mandatory.

   o  Reference: this document

7.3.3.  rs

   o  Parameter Name: rs

   o  Purpose: The size of the encrypted records.

   o  Reference: this document

7.4.  The HTTP Crypto-Key Parameter Registry

   This memo establishes a registry for parameters used by the "Crypto-
   Key" header field under the "Hypertext Transfer Protocol (HTTP)
   Parameters" grouping.  The "Hypertext Transfer Protocol (HTTP)
   Crypto-Key Parameters" operates under an "Specification Required"
   policy [RFC5226].

   Entries in this registry are expected to include the following
   information:

   o  Parameter Name: The name of the parameter.

   o  Purpose: A brief description of the purpose of the parameter.

   o  Reference: A reference to a specification that defines the
      semantics of the parameter.




Thomson                Expires September 21, 2016              [Page 18]


Internet-Draft           HTTP encryption coding               March 2016


   The initial contents of this registry are:

7.4.1.  keyid

   o  Parameter Name: keyid

   o  Purpose: Identify the key that is in use.

   o  Reference: this document

7.4.2.  aesgcm

   o  Parameter Name: aesgcm

   o  Purpose: Provide an explicit input keying material value for the
      aesgcm content encoding.

   o  Reference: this document

7.4.3.  dh

   o  Parameter Name: dh

   o  Purpose: Carry a modp or elliptic curve Diffie-Hellman share used
      to derive input keying material.

   o  Reference: this document

8.  References

8.1.  Normative References

   [DH]       Diffie, W. and M. Hellman, "New Directions in
              Cryptography", IEEE Transactions on Information Theory,
              V.IT-22 n.6 , June 1977.

   [FIPS180-4]
              Department of Commerce, National., "NIST FIPS 180-4,
              Secure Hash Standard", March 2012,
              <http://csrc.nist.gov/publications/fips/fips180-4/
              fips-180-4.pdf>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.





Thomson                Expires September 21, 2016              [Page 19]


Internet-Draft           HTTP encryption coding               March 2016


   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492,
              DOI 10.17487/RFC4492, May 2006,
              <http://www.rfc-editor.org/info/rfc4492>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <http://www.rfc-editor.org/info/rfc5116>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <http://www.rfc-editor.org/info/rfc5869>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <http://www.rfc-editor.org/info/rfc7231>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <http://www.rfc-editor.org/info/rfc7515>.

8.2.  Informative References

   [FIPS186]  National Institute of Standards and Technology (NIST),
              "Digital Signature Standard (DSS)", NIST PUB 186-4 , July
              2013.

   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880,
              DOI 10.17487/RFC4880, November 2007,
              <http://www.rfc-editor.org/info/rfc4880>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.







Thomson                Expires September 21, 2016              [Page 20]


Internet-Draft           HTTP encryption coding               March 2016


   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <http://www.rfc-editor.org/info/rfc5652>.

   [RFC7233]  Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
              "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
              RFC 7233, DOI 10.17487/RFC7233, June 2014,
              <http://www.rfc-editor.org/info/rfc7233>.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <http://www.rfc-editor.org/info/rfc7235>.

   [RFC7516]  Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              RFC 7516, DOI 10.17487/RFC7516, May 2015,
              <http://www.rfc-editor.org/info/rfc7516>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <http://www.rfc-editor.org/info/rfc7540>.

   [X9.62]    ANSI, "Public Key Cryptography For The Financial Services
              Industry: The Elliptic Curve Digital Signature Algorithm
              (ECDSA)", ANSI X9.62 , 1998.

   [XMLENC]   Eastlake, D., Reagle, J., Imamura, T., Dillaway, B., and
              E. Simon, "XML Encryption Syntax and Processing", W3C
              REC , December 2002, <http://www.w3.org/TR/xmlenc-core/>.

Appendix A.  JWE Mapping

   The "aesgcm" content encoding can be considered as a sequence of JSON
   Web Encryption (JWE) objects [RFC7516], each corresponding to a
   single fixed size record that includes leading padding.  The
   following transformations are applied to a JWE object that might be
   expressed using the JWE Compact Serialization:

   o  The JWE Protected Header is fixed to a value { "alg": "dir",
      "enc": "A128GCM" }, describing direct encryption using AES-GCM
      with a 128-bit content encryption key.  This header is not




Thomson                Expires September 21, 2016              [Page 21]


Internet-Draft           HTTP encryption coding               March 2016


      transmitted, it is instead implied by the value of the Content-
      Encoding header field.

   o  The JWE Encrypted Key is empty, as stipulated by the direct
      encryption algorithm.

   o  The JWE Initialization Vector ("iv") for each record is set to the
      exclusive or of the 96-bit record sequence number, starting at
      zero, and a value derived from the input keying material (see
      Section 3.3).  This value is also not transmitted.

   o  The final value is the concatenated JWE Ciphertext and the JWE
      Authentication Tag, both expressed without URL-safe Base 64
      encoding.  The "." separator is omitted, since the length of these
      fields is known.

   Thus, the example in Section 5.4 can be rendered using the JWE
   Compact Serialization as:


   eyAiYWxnIjogImRpciIsICJlbmMiOiAiQTEyOEdDTSIgfQ..31iQYc1v4a36EgyJ.
   VDeU0XxaJkOJDAxPl7h9JD4.VfDeN0aKz-z36T3WWULsBQ

   Where the first line represents the fixed JWE Protected Header, an
   empty JWE Encrypted Key, and the algorithmically-determined JWE
   Initialization Vector.  The second line contains the encoded body,
   split into JWE Ciphertext and JWE Authentication Tag.

Appendix B.  Intermediate Values for Encryption

   The intermediate values calculated for the example in Section 5.7 are
   shown here.  The following are inputs to the calculation:

   Plaintext:  SSBhbSB0aGUgd2FscnVz

   Sender public key:  BNoRDbb84JGm8g5Z5CFxurSqsXWJ11ItfXEWYVLE85Y7
      CYkDjXsIEc4aqxYaQ1G8BqkXCJ6DPpDrWtdWj_mugHU

   Sender private key:  nCScek-QpEjmOOlT-rQ38nZzvdPlqa00Zy0i6m2OJvY

   Receiver public key:  BCEkBjzL8Z3C-oi2Q7oE5t2Np-p7osjGLg93qUP0wvqR
      T21EEWyf0cQDQcakQMqz4hQKYOQ3il2nNZct4HgAUQU

   Receiver private key:  9FWl15_QUQAWDaD3k3l50ZBZQJ4au27F1V4F0uLSD_M

   Salt:  lngarbyKfMoi9Z75xYXmkg





Thomson                Expires September 21, 2016              [Page 22]


Internet-Draft           HTTP encryption coding               March 2016


   Note that knowledge of just one of the private keys is necessary.
   The sender randomly generates the salt value, whereas salt is input
   to the receiver.

   This produces the following intermediate values:

   Shared secret (raw_key):  RNjC-NVW4BGJbxWPW7G2mowsLeDa53LYKYm4-NOQ6Y

   Input keying material (IKM):  EhpZec37Ptm4IRD5-jtZ0q6r1iK5vYmY1tZwtN8
      fbZY

   Context for content encryption key derivation:
      Q29udGVudC1FbmNvZGluZzogYWVzZ2NtAFAtMjU2AABB BCEkBjzL8Z3C-
      oi2Q7oE5t2Np-p7osjGLg93qUP0wvqR
      T21EEWyf0cQDQcakQMqz4hQKYOQ3il2nNZct4HgAUQUA
      QQTaEQ22_OCRpvIOWeQhcbq0qrF1iddSLX1xFmFSxPOW
      OwmJA417CBHOGqsWGkNRvAapFwiegz6Q61rXVo_5roB1

   Content encryption key (CEK):  AN2-xhvFWeYh5z0fcDu0Ww

   Context for nonce derivation:  Q29udGVudC1FbmNvZGluZzogbm9uY2UAUC0yNT
      YAAEEE ISQGPMvxncL6iLZDugTm3Y2n6nuiyMYuD3epQ_TC-pFP
      bUQRbJ_RxANBxqRAyrPiFApg5DeKXac1ly3geABRBQBB
      BNoRDbb84JGm8g5Z5CFxurSqsXWJ11ItfXEWYVLE85Y7
      CYkDjXsIEc4aqxYaQ1G8BqkXCJ6DPpDrWtdWj_mugHU

   Base nonce:  JY1Okw5rw1Drkg9J

   When the CEK and nonce are used with AES GCM and the padded plaintext
   of AABJIGFtIHRoZSB3YWxydXM, the final ciphertext is
   6nqAQUME8hNqw5J3kl8cpVVJylXKYqZOeseZG8UueKpA, as shown in the
   example.

Appendix C.  Acknowledgements

   Mark Nottingham was an original author of this document.

   The following people provided valuable input: Richard Barnes, David
   Benjamin, Peter Beverloo, Mike Jones, Stephen Farrell, Adam Langley,
   John Mattsson, Eric Rescorla, and Jim Schaad.

Author's Address

   Martin Thomson
   Mozilla

   Email: martin.thomson@gmail.com




Thomson                Expires September 21, 2016              [Page 23]


Html markup produced by rfcmarkup 1.129b, available from https://tools.ietf.org/tools/rfcmarkup/