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Versions: 00 draft-thomson-http-encryption

Network Working Group                                      M. Nottingham
Internet-Draft
Intended status: Informational                                M. Thomson
Expires: September 6, 2015                                       Mozilla
                                                           March 5, 2015


                  Encrypted Content-Encoding for HTTP
              draft-nottingham-http-encryption-encoding-00

Abstract

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

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 6, 2015.

Copyright Notice

   Copyright (c) 2015 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
   (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.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  The "aesgcm-128" HTTP content-coding  . . . . . . . . . . . .   3
   3.  The "Encryption" HTTP header field  . . . . . . . . . . . . .   5
     3.1.  Encryption Header Field Parameters  . . . . . . . . . . .   5
     3.2.  Content Encryption Key Derivation . . . . . . . . . . . .   6
   4.  Encryption-Key Header Field . . . . . . . . . . . . . . . . .   6
     4.1.  Explicit Key  . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Diffie-Hellman  . . . . . . . . . . . . . . . . . . . . .   7
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Successful GET Response . . . . . . . . . . . . . . . . .   8
     5.2.  Encryption and Compression  . . . . . . . . . . . . . . .   8
     5.3.  Encryption with More Than One Key . . . . . . . . . . . .   8
     5.4.  Encryption with Explicit Key  . . . . . . . . . . . . . .   9
     5.5.  Diffie-Hellman Encryption . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  The "aesgcm-128" HTTP content-coding  . . . . . . . . . .  10
     6.2.  Encryption Header Fields  . . . . . . . . . . . . . . . .  10
     6.3.  The HTTP Encryption Parameter Registry  . . . . . . . . .  10
       6.3.1.  keyid . . . . . . . . . . . . . . . . . . . . . . . .  11
       6.3.2.  salt  . . . . . . . . . . . . . . . . . . . . . . . .  11
       6.3.3.  rs  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.4.  The HTTP Encryption-Key Parameter Registry  . . . . . . .  11
       6.4.1.  keyid . . . . . . . . . . . . . . . . . . . . . . . .  12
       6.4.2.  key . . . . . . . . . . . . . . . . . . . . . . . . .  12
       6.4.3.  dh  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
     7.1.  Key and Nonce Reuse . . . . . . . . . . . . . . . . . . .  13
     7.2.  Content Integrity . . . . . . . . . . . . . . . . . . . .  13
     7.3.  Leaking Information in Headers  . . . . . . . . . . . . .  13
     7.4.  Poisoning Storage . . . . . . . . . . . . . . . . . . . .  14
     7.5.  Sizing and Timing Attacks . . . . . . . . . . . . . . . .  14
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

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





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   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.

   Message-based encryption formats - such as those that are described
   by [RFC4880], [RFC5652], [I-D.ietf-jose-json-web-encryption], and
   [XMLENC] - are not suited to stream processing, which is necessary
   for HTTP messages.  While virtually any of these alternatives could
   be profiled and adapted to suit, the overhead and complexity that
   would introduce is sub-optimal.

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

   This mechanism is likely only a small part of a larger design that
   uses content encryption.  In particular, this document does not
   describe key management practices.  How clients and servers acquire
   and identify keys will depend on the use case.

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].

2.  The "aesgcm-128" HTTP content-coding

   The "aesgcm-128" 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 Encryption-
   Key header field Section 4 can be included to describe how the the
   content encryption key is derived or retrieved.

   The "aesgcm-128" 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.



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   The "aesgcm-128" content-coding uses a fixed record size.  The
   resulting encoding is a series of fixed-size records, though the
   final record can contain any amount of data.

          +------+
          | data |         input of between rs-256
          +------+            and rs-1 octets
              |
              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.  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 is
   equal to the value of the "rs" parameter plus 16 octets.  It is a
   fatal decryption error to have a remainder of 16 octets or less in
   size (though AEAD_AES_128_GCM permits input plaintext to be zero
   length, records always contain at least one padding octet).

   Each record contains between 0 and 255 octets of padding, inserted
   into a record before the enciphered content.  The length of the
   padding is stored in the first octet of the payload.  All padding
   octets MUST be set to zero.  It is a fatal decryption error to have a
   record with more padding than the record size.

   The nonce used for each record is a 96-bit value containing the index
   of the current record in network byte order.  Records are indexed
   starting at zero.

   The additional data passed to the AEAD algorithm is a zero-length
   octet sequence.

   Issue:  Double check that having no AAD is safe.








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3.  The "Encryption" HTTP header field

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

     Encryption-val = #encryption_params
     encryption_params = [ param *( ";" param ) ]

   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.

   The Encryption header MAY be omitted if the sender does not intend
   for the immediate recipient to be able to decrypt the message.
   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 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 is included
      in an Encryption-Key header field.

   salt:  The "salt" parameter contains a base64 URL-encoded octets 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.  The "salt" parameter MUST NOT be reused
      for two different messages that have the same content encryption
      key; generating a random nonce for each message ensures that 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.




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3.2.  Content Encryption Key Derivation

   In order to allow the reuse of keying material for multiple different
   messages, a content encryption key is derived for each message.  This
   key is derived from the decoded value of the "s" parameter using the
   HMAC-based key derivation function (HKDF) described in [RFC5869]
   using the SHA-256 hash algorithm [FIPS180-2].

   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 Encryption-Key
   header field Section 4.  The first step of HKDF is therefore:

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

   AEAD_AES_128_GCM requires 16 octets (128 bits) of key, so the length
   (L) parameter of HKDF is 16.  The info parameter is set to the ASCII-
   encoded string "Content-Encoding: aesgcm128".  The second step of
   HKDF can therefore be simplified to the first 16 octets of a single
   HMAC:

      OKM = HMAC-SHA-256(PRK, "Content-Encoding: aesgcm128" || 0x01)

4.  Encryption-Key Header Field

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

     Encryption-Key-val = #encryption_key_params
     encryption_key_params = [ param *( ";" param ) ]

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

   key:  The "key" parameter contains the URL-safe base64 [RFC4648]
      octets 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.

   The input keying material used by the content-encoding key derivation
   (see Section 3.2) can be determined based on the information in the
   Encryption-Key header field.  The method for key derivation depends
   on the parameters that are present in the header field.





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   Note that different methods for determining input keying materal 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 "key" parameter is decoded and used directly if present.  The
   "key" parameter MUST decode to exactly 16 octets in order to be used
   as input keying material for "aesgcm128" content encoding.

   Other key determination parameters can be ignored if the "key"
   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  The format of the ephemeral public share that is included in the
      "dh" parameter.  For instance, using ECDH both parties need to
      agree whether this is an uncompressed or compressed point.

   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 appropriate Diffie-Hellman
   process.

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



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5.  Examples

5.1.  Successful GET Response

   HTTP/1.1 200 OK
   Content-Type: application/octet-stream
   Content-Encoding: aesgcm-128
   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 a key
   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.

5.2.  Encryption and Compression

   HTTP/1.1 200 OK
   Content-Type: text/html
   Content-Encoding: aesgcm-128, gzip
   Transfer-Encoding: chunked
   Encryption: keyid="mailto:me@example.com";
               salt="m2hJ_NttRtFyUiMRPwfpHA"

   [encrypted payload]

5.3.  Encryption with More Than One Key

   PUT /thing HTTP/1.1
   Host: storage.example.com
   Content-Type: application/http
   Content-Encoding: aesgcm-128, aesgcm-128
   Content-Length: 1234
   Encryption: keyid="mailto:me@example.com";
               salt="NfzOeuV5USPRA-n_9s1Lag",
               keyid="http://example.org/bob/keys/123";
               salt="bDMSGoc2uobK_IhavSHsHA"; rs=1200

   [encrypted payload]

   Here, a PUT request has been encrypted with two keys; both will be
   necessary to read the content.  The outer layer of encryption uses a
   1200 octet record size.




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5.4.  Encryption with Explicit Key

   HTTP/1.1 200 OK
   Content-Length: 31
   Content-Encoding: aesgcm-128
   Encryption: keyid="a1"; salt="owIfQR647esVfrzCW_i9GQ"
   Encryption-Key: keyid="a1"; key="JcqK-OLkJZlJ3sJJWstJCA"

   LwTC-fwdKh8de0smD2jfzHodb1EYbuuTNpcYXLW257Q

   This example shows the string "I am the walrus" encrypted using an
   explicit key.  The content body contains a single record only and is
   shown here encoded in URL-safe base64 for presentation reasons only.

5.5.  Diffie-Hellman Encryption

   HTTP/1.1 200 OK
   Content-Length: 31
   Content-Encoding: aesgcm-128
   Encryption: keyid="dhkey"; salt="XYFSCgMVjc45IMfLOcMfiw"
   Encryption-Key: keyid="dhkey";
                   dh="BELKqvZ7n3p5C9_ipP_6X9DBNAGuJujSN7YWbtcGZMMH
                       3urZM-zlii3mGGCMjlqR-yWwiPlMdKRdOL8gQSdHw8E"

   P6ikHE_wyKnYHXxLswvuFBO3JJOZpM1Bg3KikQEmczU

   This example shows the same string, "I am the walrus", encrypted
   using ECDH over the P-256 curve [FIPS186].  The content body is shown
   here encoded in URL-safe base64 for presentation reasons only.

   The receiver (in this case, the HTTP client) uses the key 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 [X.692] encoded using URL-
   safe base64.  Line wrapping is added for presentation purposes only.

      Receiver:
         private key: QjGwenE3vCg8Eajo-PukGgUkYq8Vu-SQn04Cc9DR-YA
         public key: BBM3pYS4nXG6bQYnZbGDY7l6CVrQTZ-1u00h7XV6A_TD
                   v7mXvv5k29uoLid8SdDycw341PJTW4hNCe2FNysN52U
      Sender:
         private key: wlC-qzKBWO6jYq32nlD0ZZVsI5jGVBC1gN7zkXjaPks
         public key: <the value of the "dh" parameter>








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6.  IANA Considerations

6.1.  The "aesgcm-128" HTTP content-coding

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

   o  Name: aesgcm-128

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

   o  Reference [this specification]

6.2.  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 "Encryption-Key" HTTP header field in the
   Permanent Message Header Registry, as detailed in Section 4.

   o  Field name: Encryption-Key

   o  Protocol: HTTP

   o  Status: Standard

   o  Reference: [this specification]

   o  Notes:

6.3.  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" operates under an "Specification Required"
   policy [RFC5226].



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   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.

   The initial contents of this registry are:

6.3.1.  keyid

   o  Parameter Name: keyid

   o  Purpose: Identify the key that is in use.

   o  Reference: [this document]

6.3.2.  salt

   o  Parameter Name: salt

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

   o  Reference: [this document]

6.3.3.  rs

   o  Parameter Name: rs

   o  Purpose: The size of the encrypted records.

   o  Reference: [this document]

6.4.  The HTTP Encryption-Key Parameter Registry

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

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




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   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.

   The initial contents of this registry are:

6.4.1.  keyid

   o  Parameter Name: keyid

   o  Purpose: Identify the key that is in use.

   o  Reference: [this document]

6.4.2.  key

   o  Parameter Name: key

   o  Purpose: Provide an explicit key.

   o  Reference: [this document]

6.4.3.  dh

   o  Parameter Name: dh

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

   o  Reference: [this document]

7.  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.




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7.1.  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
   relies on the uniqueness of the "nonce" parameter to ensure that the
   content encryption key is different for every message.

   If a key and nonce are reused, this could expose the content
   encryption key and it makes message modification trivial.  If the
   same key is used for multiple messages, then the nonce parameter MUST
   be unique for each.  An implementation SHOULD generate a random nonce
   parameter for every message, though using a counter could achieve the
   desired result.

7.2.  Content Integrity

   This mechanism only provides content origin authentication.  The
   authentication tag only ensures that those with access to the content
   encryption key produce a message that will be accepted as valid.

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

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

7.3.  Leaking Information in Headers

   Because "encrypted" only operates upon the message payload, any
   information exposed in headers is visible to anyone who can read the
   message.

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

   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:

   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
       could be an acceptable risk.





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   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).

   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 [RFC7230], encrypting that as the
       payload of the "outer" message.

7.4.  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.

7.5.  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
   [I-D.ietf-httpbis-http2] combined with TLS [RFC5246] might be used to
   hide the size of individual messages.

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.

   [FIPS180-2]
              Department of Commerce, National., "NIST FIPS 180-2,
              Secure Hash Standard", August 2002.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.



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   [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, May 2006.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, January 2008.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869, May 2010.

   [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", RFC 7230, June
              2014.

   [RFC7231]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content", RFC 7231, June 2014.

8.2.  Informative References

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

   [I-D.ietf-httpbis-http2]
              Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer
              Protocol version 2", draft-ietf-httpbis-http2-17 (work in
              progress), February 2015.

   [I-D.ietf-jose-json-web-encryption]
              Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              draft-ietf-jose-json-web-encryption-40 (work in progress),
              January 2015.

   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880, November 2007.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, September 2009.



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   [RFC7235]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Authentication", RFC 7235, June 2014.

   [X.692]    ANSI, "Public Key Cryptography For The Financial Services
              Industry: The Elliptic Curve Digital Signature Algorithm
              (ECDSA)", ANSI X9.62, 1998. , n.d..

   [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.  Acknowledgements

   The following people provided valuable feedback and suggestions:
   Richard Barnes, Stephen Farrell, Eric Rescorla, and Jim Schaad.

Authors' Addresses

   Mark Nottingham

   Email: mnot@mnot.net
   URI:   http://www.mnot.net/


   Martin Thomson
   Mozilla

   Email: martin.thomson@gmail.com























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