Network Working Group M. Thomson
Internet-Draft Mozilla
Intended status: Standards Track May 11, 2015
Expires: November 12, 2015
Encrypted Content-Encoding for HTTP
draft-thomson-http-encryption-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.
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This Internet-Draft will expire on November 12, 2015.
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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 . . . . . . . . . . . . 9
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 . . . . . . . . . 11
6.3.1. keyid . . . . . . . . . . . . . . . . . . . . . . . . 11
6.3.2. salt . . . . . . . . . . . . . . . . . . . . . . . . 11
6.3.3. rs . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.4. The HTTP Encryption-Key Parameter Registry . . . . . . . 12
6.4.1. keyid . . . . . . . . . . . . . . . . . . . . . . . . 12
6.4.2. key . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.4.3. dh . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
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 . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Normative References . . . . . . . . . . . . . . . . . . 15
8.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. JWE Mapping . . . . . . . . . . . . . . . . . . . . 16
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 17
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.
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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.
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], [I-D.ietf-jose-json-web-encryption], 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 [I-D.ietf-jose-json-web-encryption] values with a fixed
header.
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
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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.
The "aesgcm-128" 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.
+------+
| 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. A receiver
MUST fail to decrypt if the remainder is 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. A receiver MUST fail to decrypt if a
record has more padding than the record size can accommodate.
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.
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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.
Issue: Double check that this construction (with no AAD) is safe.
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.
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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.
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 "salt" 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.
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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.
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.
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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.
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]
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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.
5.4. Encryption with Explicit Key
HTTP/1.1 200 OK
Content-Length: 31
Content-Encoding: aesgcm-128
Encryption: keyid="a1"; salt="ibZx1RNz537h1XNkRcPpjA"
Encryption-Key: keyid="a1"; key="9Z57YCb3dK95dSsdFJbkag"
zK3kpG__Z8whjIkG6RYgPz11oUkTKcxPy9WP-VPMfuc
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="5hpuYfxDzG6nSs9-EQuaBg"
Encryption-Key: keyid="dhkey";
dh="BLsyIPbDn6bquEOwHaju2gj8kUVoflzTtPs_6fGoock_
dwxi1BcgFtObPVnic4alcEucx8I6G8HmEZCJnAl36Zg"
BmuHqRzdD4W1mibxglrPiRHZRSY49Dzdm6jHrWXzZrE
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.
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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: iCjNf8v4ox_g1rJuSs_gbNmYuUYx76ZRruQs_CHRzDg
public key: BPM1w41cSD4BMeBTY0Fz9ryLM-LeM22Dvt0gaLRukf05
rMhzFAvxVW_mipg5O0hkWad9ZWW0uMRO2Nrd32v8odQ
Sender:
private key: W0cxgeHDZkR3uMQYAbVgF5swKQUAR7DgoTaaQVlA-Fg
public key: <the value of the "dh" parameter>
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
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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].
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.
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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:
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
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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.
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 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 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 header fields is visible to anyone who can
read the message.
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For example, the Content-Type header field 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
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.
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.
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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.
[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.
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[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.
[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.
[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-128" content encoding can be considered as a sequence of
JSON Web Encryption (JWE) objects, each corresponding to a single
fixed size record. 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 key. This header is not 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
96-bit integer value of the record sequence number, starting at
zero. This value is also not transmitted.
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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..AAAAAAAAAAAAAAAA.
LwTC-fwdKh8de0smD2jfzA.eh1vURhu65M2lxhctbbntA
Where the first line represents the fixed JWE Protected Header, JWE
Encrypted Key, and JWE Initialization Vector, all of which are
determined algorithmically. The second line contains the encoded
body, split into JWE Ciphertext and JWE Authentication Tag.
Appendix B. Acknowledgements
Mark Nottingham was an original author of this document.
The following people provided valuable feedback and suggestions:
Richard Barnes, Mike Jones, Stephen Farrell, Eric Rescorla, and Jim
Schaad.
Author's Address
Martin Thomson
Mozilla
Email: martin.thomson@gmail.com
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