HTTP Working Group M. Thomson
Internet-Draft Mozilla
Intended status: Standards Track December 21, 2016
Expires: June 24, 2017
Encrypted Content-Encoding for HTTP
draft-ietf-httpbis-encryption-encoding-05
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
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This Internet-Draft will expire on June 24, 2017.
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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. The "aes128gcm" HTTP Content Coding . . . . . . . . . . . . . 3
2.1. Encryption Content Coding Header . . . . . . . . . . . . 5
2.2. Content Encryption Key Derivation . . . . . . . . . . . . 6
2.3. Nonce Derivation . . . . . . . . . . . . . . . . . . . . 6
3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Encryption of a Response . . . . . . . . . . . . . . . . 7
3.2. Encryption with Multiple Records . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
4.1. Key and Nonce Reuse . . . . . . . . . . . . . . . . . . . 9
4.2. Data Encryption Limits . . . . . . . . . . . . . . . . . 9
4.3. Content Integrity . . . . . . . . . . . . . . . . . . . . 9
4.4. Leaking Information in Headers . . . . . . . . . . . . . 10
4.5. Poisoning Storage . . . . . . . . . . . . . . . . . . . . 10
4.6. Sizing and Timing Attacks . . . . . . . . . . . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5.1. The "aes128gcm" HTTP Content Coding . . . . . . . . . . . 11
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Normative References . . . . . . . . . . . . . . . . . . 11
6.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. JWE Mapping . . . . . . . . . . . . . . . . . . . . 13
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
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.
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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. In particular, a key
management system is not described.
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].
Base64url encoding is defined in Section 2 of [RFC7515].
2. The "aes128gcm" HTTP Content Coding
The "aes128gcm" 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.
Using this content coding requires knowledge of a key. How this key
is acquired is not defined in this document.
The "aes128gcm" 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 "aes128gcm" content coding uses a fixed record size. The final
encoding consists of a header (see Section 2.1), zero or more fixed
size encrypted records, and a partial record. The partial record
MUST be shorter than the fixed record size.
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 ("rs") is included in the
content coding header (see Section 2.1).
+-----------+ content is rs octets minus padding
| data | of between 2 and 65537 octets;
+-----------+ the last record is smaller
|
v
+-----+-----------+ add padding to get rs octets;
| pad | data | the last record contains
+-----+-----------+ up to rs minus 1 octets
|
v
+--------------------+ encrypt with AEAD_AES_128_GCM;
| ciphertext | final size is rs plus 16 octets
+--------------------+ the last record is smaller
AEAD_AES_128_GCM produces ciphertext 16 octets longer than its input
plaintext. Therefore, the length of each enciphered record other
than the last is equal to the value of the "rs" parameter plus 16
octets. If the final record ends on a record boundary, the encoder
MUST append a record that contains contains only padding and is
smaller than the full record size. A receiver MUST fail to decrypt
if the final record ciphertext is less than 18 octets in size or
equal to the record size plus 16 (that is, the size of a full
encrypted record). Valid records always contain at least two octets
of padding and a 16 octet authentication tag.
Each record contains a 2 octet padding length field and between 0 and
65535 octets of padding, inserted into a record before the enciphered
content. The padding length is a two octet unsigned integer in
network byte order; padding is that number of zero-valued octets. A
receiver MUST fail to decrypt if any padding octet is 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 2.3.
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The additional data passed to each invocation of AEAD_AES_128_GCM is
a zero-length octet sequence.
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. Partial records at the ends
of a range cannot be decrypted. Thus, it is best if range requests
start and end on record boundaries. Note however that random access
to specific parts of encrypted data could be confounded by the
presence of padding.
Selecting the record size most appropriate for a given situation
requires a trade-off. A smaller record size allows decrypted octets
to be released more rapidly, which can be appropriate for
applications that depend on responsiveness. Smaller records also
reduce the additional data required if random access into the
ciphertext is needed. Applications that depend on being able to pad
by arbitrary amounts cannot increase the record size beyond 65537
octets.
Applications that don't depending on streaming, random access, or
arbitrary padding can use larger records, or even a single record. A
larger record size reduces the processing and data overheads.
2.1. Encryption Content Coding Header
The content coding uses a header block that includes all parameters
needed to decrypt the content (other than the key). The header block
is placed in the body of a message ahead of the sequence of records.
+-----------+--------+-----------+---------------+
| salt (16) | rs (4) | idlen (1) | keyid (idlen) |
+-----------+--------+-----------+---------------+
salt: The "salt" parameter comprises the first 16 octets of the
"aes128gcm" content coding header. The same "salt" parameter
value 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 coding ensures that content
encryption key reuse is highly unlikely.
rs: The "rs" or record size parameter contains an unsigned 32-bit
integer in network byte order that describes the record size in
octets. Note that it is therefore impossible to exceed the
2^36-31 limit on plaintext input to AEAD_AES_128_GCM. Values
smaller than 3 are invalid.
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keyid: The "keyid" parameter can be used to identify the keying
material that is used. Recipients that receive a message are
expected to know how to retrieve keys; the "keyid" parameter might
be input to that process.
2.2. 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 "salt" parameter using
the HMAC-based key derivation function (HKDF) described in [RFC5869]
using the SHA-256 hash algorithm [FIPS180-4].
The 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 is expected to
be provided to recipients separately. The extract phase of HKDF
therefore produces a pseudorandom key (PRK) as follows:
PRK = HMAC-SHA-256(salt, IKM)
The info parameter to HKDF is set to the ASCII-encoded string
"Content-Encoding: aes128gcm" and a single zero octet:
cek_info = "Content-Encoding: aes128gcm" || 0x00
Note: Concatenation of octet sequences is represented by the "||"
operator.
AEAD_AES_128_GCM requires a 16 octet (128 bit) content encryption key
(CEK), 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)
2.3. 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 that 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.
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The length (L) parameter is 12 octets. The info parameter for the
nonce is the ASCII-encoded string "Content-Encoding: nonce",
terminated by a a single zero octet:
nonce_info = "Content-Encoding: nonce" || 0x00
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
This nonce construction prevents removal or reordering of records.
However, it permits truncation of the tail of the sequence (see
Section 2 for how this is avoided).
3. Examples
This section shows a few examples of the encrypted content coding.
Note: All binary values in the examples in this section use base64url
encoding [RFC7515]. This includes the bodies of requests.
Whitespace and line wrapping is added to fit formatting constraints.
3.1. Encryption of a Response
Here, a successful HTTP GET response has been encrypted. This uses a
record size of 4096 and no padding (just the 2 octet padding length),
so only a partial record is present. The input keying material is
identified by an empty string (that is, the "keyid" field in the
header is zero octets in length).
The encrypted data in this example is the UTF-8 encoded string "I am
the walrus". The input keying material is the value
"B33e_VeFrOyIHwFTIfmesA" (in base64url). The content body contains a
single record and is shown here using base64url encoding for
presentation reasons.
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HTTP/1.1 200 OK
Content-Type: application/octet-stream
Content-Length: 54
Content-Encoding: aes128gcm
sJvlboCWzB5jr8hI_q9cOQAAEAAANSmxkSVa0-MiNNuF77YHSs-iwaNe_OK0qfmO
c7NT5WSW
Note that the media type has been changed to "application/octet-
stream" to avoid exposing information about the content.
Alternatively (and equivalently), the Content-Type header field can
be omitted.
Intermediate values for this example (all shown in base64):
salt (from header) = sJvlboCWzB5jr8hI_q9cOQ
PRK = MLAQxt_DHjM15cdlyU1oUnjq7TFlzToGTkdRmvvxVBw
CEK = v31u7VGV3soO3wNaMaIdhg
NONCE = XOaygzko98zjUFTJ
plaintext = AABJIGFtIHRoZSB3YWxydXM
3.2. Encryption with Multiple Records
This example shows the same message with input keying material of
"BO3ZVPxUlnLORbVGMpbT1Q". In this example, the plaintext is split
into records of 10 octets each (that is, the "rs" field in the header
is 10). The first record includes a single octet of padding. This
means that there are 7 octets of message in the first record, and 8
in the second. This causes the end of the content to align with a
record boundary, forcing the creation of a third record that contains
only two octets of the padding length.
HTTP/1.1 200 OK
Content-Length: 93
Content-Encoding: aes128gcm
uNCkWiNYzKTnBN9ji3-qWAAAAAoCYTGHOqYFz-0in3dpb-VE2GfBngkaPy6bZus_
qLF79s6zQyTSsA0iLOKyd3JqVIwprNzVatRCWZGUx_qsFbJBCQu62RqQuR2d
4. 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.
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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.
4.1. 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 coding.
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.
4.2. Data Encryption Limits
There are limits to the data that AEAD_AES_128_GCM can encipher. The
maximum value for the record size is limited by the size of the "rs"
field in the header (see Section 2.1), which ensures that the 2^36-31
limit for a single application of AEAD_AES_128_GCM is not reached
[RFC5116]. In order to preserve a 2^-40 probability of
indistinguishability under chosen plaintext attack (IND-CPA), the
total amount of plaintext that can be enciphered MUST be less than
2^44.5 blocks of 16 octets [AEBounds].
If rs is a multiple of 16 octets, this means 398 terabytes can be
encrypted safely, including padding and overhead. However, if the
record size is not a multiple of 16 octets, the total amount of data
that can be safely encrypted is reduced proportionally. The worst
case is a record size of 3 octets, for which at most 74 terabytes of
plaintext can be encrypted, of which at least two-thirds is padding.
4.3. 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.
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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.
4.4. 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:
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.
4.5. 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]).
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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.
4.6. 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.
Developing a padding strategy is difficult. A good padding strategy
can depend on context. Common strategies include padding to a small
set of fixed lengths, padding to multiples of a values, or padding to
powers of 2. Even a good strategy can still cause size information
to leak if processing activity of a recipient can be observed. This
is especially true if the trailing records of a message contain only
padding. Distributing non-padding data is recommended to avoid
leaking size information.
5. IANA Considerations
5.1. The "aes128gcm" HTTP Content Coding
This memo registers the "aes128gcm" HTTP content coding in the HTTP
Content Codings Registry, as detailed in Section 2.
o Name: aes128gcm
o Description: AES-GCM encryption with a 128-bit content encryption
key
o Reference: this specification
6. References
6.1. Normative References
[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>.
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[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>.
[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>.
6.2. Informative References
[AEBounds]
Luykx, A. and K. Paterson, "Limits on Authenticated
Encryption Use in TLS", March 2016,
<http://www.isg.rhul.ac.uk/~kp/TLS-AEbounds.pdf>.
[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>.
[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>.
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[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>.
[XMLENC] Eastlake, D., Reagle, J., Hirsch, F., Roessler, T.,
Imamura, T., Dillaway, B., Simon, E., Yiu, K., and M.
Nystroem, "XML Encryption Syntax and Processing", W3C
Recommendation REC-xmlenc-core1-20130411 , January 2013,
<https://www.w3.org/TR/2013/REC-xmlenc-core1-20130411>.
Appendix A. JWE Mapping
The "aes128gcm" content coding 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 the value { "alg": "dir",
"enc": "A128GCM" }, describing direct encryption using AES-GCM
with a 128-bit content encryption 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
exclusive or of the 96-bit record sequence number, starting at
zero, and a value derived from the input keying material (see
Section 2.3). This value is also not transmitted.
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o The final value is the concatenated header, JWE Ciphertext, and
JWE Authentication Tag, all expressed without base64url encoding.
The "." separator is omitted, since the length of these fields is
known.
Thus, the example in Section 3.1 can be rendered using the JWE
Compact Serialization as:
eyAiYWxnIjogImRpciIsICJlbmMiOiAiQTEyOEdDTSIgfQ..31iQYc1v4a36EgyJ.
NSmxkSVa0-MiNNuF77YHSs8.osGjXvzitKn5jnOzU-Vklg
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. Acknowledgements
Mark Nottingham was an original author of this document.
The following people provided valuable input: Richard Barnes, David
Benjamin, Peter Beverloo, JR Conlin, Mike Jones, Stephen Farrell,
Adam Langley, John Mattsson, Julian Reschke, Eric Rescorla, Jim
Schaad, and Magnus Westerlund.
Author's Address
Martin Thomson
Mozilla
Email: martin.thomson@gmail.com
Thomson Expires June 24, 2017 [Page 14]
