draft-ietf-httpbis-encryption-encoding-04.txt   draft-ietf-httpbis-encryption-encoding-05.txt 
HTTP Working Group M. Thomson HTTP Working Group M. Thomson
Internet-Draft Mozilla Internet-Draft Mozilla
Intended status: Standards Track October 31, 2016 Intended status: Standards Track December 21, 2016
Expires: May 4, 2017 Expires: June 24, 2017
Encrypted Content-Encoding for HTTP Encrypted Content-Encoding for HTTP
draft-ietf-httpbis-encryption-encoding-04 draft-ietf-httpbis-encryption-encoding-05
Abstract Abstract
This memo introduces a content coding for HTTP that allows message This memo introduces a content coding for HTTP that allows message
payloads to be encrypted. payloads to be encrypted.
Note to Readers Note to Readers
Discussion of this draft takes place on the HTTP working group Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at mailing list (ietf-http-wg@w3.org), which is archived at
skipping to change at page 1, line 41 skipping to change at page 1, line 41
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 4, 2017. This Internet-Draft will expire on June 24, 2017.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 19 skipping to change at page 2, line 19
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. The "aes128gcm" HTTP Content Coding . . . . . . . . . . . . . 3 2. The "aes128gcm" HTTP Content Coding . . . . . . . . . . . . . 3
2.1. Encryption Content Coding Header . . . . . . . . . . . . 5 2.1. Encryption Content Coding Header . . . . . . . . . . . . 5
2.2. Content Encryption Key Derivation . . . . . . . . . . . . 6 2.2. Content Encryption Key Derivation . . . . . . . . . . . . 6
2.3. Nonce Derivation . . . . . . . . . . . . . . . . . . . . 7 2.3. Nonce Derivation . . . . . . . . . . . . . . . . . . . . 6
3. Crypto-Key Header Field . . . . . . . . . . . . . . . . . . . 7 3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Encryption of a Response . . . . . . . . . . . . . . . . 7
4.1. Encryption of a Response . . . . . . . . . . . . . . . . 8 3.2. Encryption with Multiple Records . . . . . . . . . . . . 8
4.2. Encryption with Multiple Records . . . . . . . . . . . . 9 4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 4.1. Key and Nonce Reuse . . . . . . . . . . . . . . . . . . . 9
5.1. Key and Nonce Reuse . . . . . . . . . . . . . . . . . . . 10 4.2. Data Encryption Limits . . . . . . . . . . . . . . . . . 9
5.2. Data Encryption Limits . . . . . . . . . . . . . . . . . 10 4.3. Content Integrity . . . . . . . . . . . . . . . . . . . . 9
5.3. Content Integrity . . . . . . . . . . . . . . . . . . . . 10 4.4. Leaking Information in Headers . . . . . . . . . . . . . 10
5.4. Leaking Information in Headers . . . . . . . . . . . . . 11 4.5. Poisoning Storage . . . . . . . . . . . . . . . . . . . . 10
5.5. Poisoning Storage . . . . . . . . . . . . . . . . . . . . 11 4.6. Sizing and Timing Attacks . . . . . . . . . . . . . . . . 11
5.6. Sizing and Timing Attacks . . . . . . . . . . . . . . . . 12 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 5.1. The "aes128gcm" HTTP Content Coding . . . . . . . . . . . 11
6.1. The "aes128gcm" HTTP Content Coding . . . . . . . . . . . 12 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.2. Crypto-Key Header Field . . . . . . . . . . . . . . . . . 12 6.1. Normative References . . . . . . . . . . . . . . . . . . 11
6.3. The HTTP Crypto-Key Parameter Registry . . . . . . . . . 12 6.2. Informative References . . . . . . . . . . . . . . . . . 12
6.3.1. keyid . . . . . . . . . . . . . . . . . . . . . . . . 13 Appendix A. JWE Mapping . . . . . . . . . . . . . . . . . . . . 13
6.3.2. aes128gcm . . . . . . . . . . . . . . . . . . . . . . 13 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 14
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Normative References . . . . . . . . . . . . . . . . . . 13
7.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A. JWE Mapping . . . . . . . . . . . . . . . . . . . . 15
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction 1. Introduction
It is sometimes desirable to encrypt the contents of a HTTP message 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 (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. 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 For example, it might be necessary to store a file on a server
without exposing its contents to that server. Furthermore, that same without exposing its contents to that server. Furthermore, that same
file could be replicated to other servers (to make it more resistant file could be replicated to other servers (to make it more resistant
skipping to change at page 3, line 29 skipping to change at page 3, line 26
[RFC5116]. [RFC5116].
To the extent that message-based encryption formats use the same To the extent that message-based encryption formats use the same
primitives, the format can be considered as sequence of encrypted primitives, the format can be considered as sequence of encrypted
messages with a particular profile. For instance, Appendix A messages with a particular profile. For instance, Appendix A
explains how the format is congruent with a sequence of JSON Web explains how the format is congruent with a sequence of JSON Web
Encryption [RFC7516] values with a fixed header. Encryption [RFC7516] values with a fixed header.
This mechanism is likely only a small part of a larger design that This mechanism is likely only a small part of a larger design that
uses content encryption. How clients and servers acquire and uses content encryption. How clients and servers acquire and
identify keys will depend on the use case. Though a complete key identify keys will depend on the use case. In particular, a key
management system is not described, this document defines an Crypto- management system is not described.
Key header field that can be used to convey keying material.
1.1. Notational Conventions 1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
Base64url encoding is defined in Section 2 of [RFC7515]. Base64url encoding is defined in Section 2 of [RFC7515].
2. The "aes128gcm" HTTP Content Coding 2. The "aes128gcm" HTTP Content Coding
The "aes128gcm" HTTP content coding indicates that a payload has been The "aes128gcm" HTTP content coding indicates that a payload has been
encrypted using Advanced Encryption Standard (AES) in Galois/Counter encrypted using Advanced Encryption Standard (AES) in Galois/Counter
Mode (GCM) as identified as AEAD_AES_128_GCM in [RFC5116], 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 Section 5.1. The AEAD_AES_128_GCM algorithm uses a 128 bit content
encryption key. encryption key.
Using this content coding requires knowledge of a key. The Crypto- Using this content coding requires knowledge of a key. How this key
Key header field (Section 3) can be included to describe how the is acquired is not defined in this document.
content encryption key is derived or retrieved. Keys might be
provided in messages that are separate from those with encrypted
content using Crypto-Key, or provided through external mechanisms.
The "aes128gcm" content coding uses a single fixed set of encryption The "aes128gcm" content coding uses a single fixed set of encryption
primitives. Cipher suite agility is achieved by defining a new primitives. Cipher suite agility is achieved by defining a new
content coding scheme. This ensures that only the HTTP Accept- content coding scheme. This ensures that only the HTTP Accept-
Encoding header field is necessary to negotiate the use of Encoding header field is necessary to negotiate the use of
encryption. encryption.
The "aes128gcm" content coding uses a fixed record size. The final The "aes128gcm" content coding uses a fixed record size. The final
encoding consists of a header (see Section 2.1), zero or more fixed encoding consists of a header (see Section 2.1), zero or more fixed
size encrypted records, and a partial record. The partial record size encrypted records, and a partial record. The partial record
MUST be shorter than the fixed record size. 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 +-----------+ content is rs octets minus padding
| data | of between 2 and 65537 octets; | data | of between 2 and 65537 octets;
+-----------+ the last record is smaller +-----------+ the last record is smaller
| |
v v
+-----+-----------+ add padding to get rs octets; +-----+-----------+ add padding to get rs octets;
| pad | data | the last record contains | pad | data | the last record contains
+-----+-----------+ up to rs minus 1 octets +-----+-----------+ up to rs minus 1 octets
| |
v v
+--------------------+ encrypt with AEAD_AES_128_GCM; +--------------------+ encrypt with AEAD_AES_128_GCM;
| ciphertext | final size is rs plus 16 octets | ciphertext | final size is rs plus 16 octets
+--------------------+ the last record is smaller +--------------------+ the last record is smaller
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).
AEAD_AES_128_GCM produces ciphertext 16 octets longer than its input AEAD_AES_128_GCM produces ciphertext 16 octets longer than its input
plaintext. Therefore, the length of each enciphered record other plaintext. Therefore, the length of each enciphered record other
than the last is equal to the value of the "rs" parameter plus 16 than the last is equal to the value of the "rs" parameter plus 16
octets. To prevent an attacker from truncating a stream, an encoder octets. If the final record ends on a record boundary, the encoder
MUST append a record that contains only padding and is smaller than MUST append a record that contains contains only padding and is
the full record size if the final record ends on a record boundary. smaller than the full record size. A receiver MUST fail to decrypt
A receiver MUST fail to decrypt if the final record ciphertext is if the final record ciphertext is less than 18 octets in size or
less than 18 octets in size or equal to the record size plus 16 (that equal to the record size plus 16 (that is, the size of a full
is, the size of a full encrypted record). Valid records always encrypted record). Valid records always contain at least two octets
contain at least two octets of padding and a 16 octet authentication of padding and a 16 octet authentication tag.
tag.
Each record contains between 2 and 65537 octets of padding, inserted Each record contains a 2 octet padding length field and between 0 and
into a record before the enciphered content. Padding consists of a 65535 octets of padding, inserted into a record before the enciphered
two octet unsigned integer in network byte order, followed that content. The padding length is a two octet unsigned integer in
number of zero-valued octets. A receiver MUST fail to decrypt if any network byte order; padding is that number of zero-valued octets. A
padding octet other than the first two are non-zero, or a record has receiver MUST fail to decrypt if any padding octet is non-zero, or a
more padding than the record size can accommodate. record has more padding than the record size can accommodate.
The nonce for each record is a 96-bit value constructed from the The nonce for each record is a 96-bit value constructed from the
record sequence number and the input keying material. Nonce record sequence number and the input keying material. Nonce
derivation is covered in Section 2.3. derivation is covered in Section 2.3.
The additional data passed to each invocation of AEAD_AES_128_GCM is The additional data passed to each invocation of AEAD_AES_128_GCM is
a zero-length octet sequence. a zero-length octet sequence.
A consequence of this record structure is that range requests A consequence of this record structure is that range requests
[RFC7233] and random access to encrypted payload bodies are possible [RFC7233] and random access to encrypted payload bodies are possible
at the granularity of the record size. Partial records at the ends 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 of a range cannot be decrypted. Thus, it is best if range requests
start and end on record boundaries. 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 Selecting the record size most appropriate for a given situation
requires a trade-off. A smaller record size allows decrypted octets requires a trade-off. A smaller record size allows decrypted octets
to be released more rapidly, which can be appropriate for to be released more rapidly, which can be appropriate for
applications that depend on responsiveness. Smaller records also applications that depend on responsiveness. Smaller records also
reduce the additional data required if random access into the reduce the additional data required if random access into the
ciphertext is needed. Applications that depend on being able to pad ciphertext is needed. Applications that depend on being able to pad
by arbitrary amounts cannot increase the record size beyond 65537 by arbitrary amounts cannot increase the record size beyond 65537
octets. octets.
skipping to change at page 6, line 12 skipping to change at page 6, line 6
every application of the content coding ensures that content every application of the content coding ensures that content
encryption key reuse is highly unlikely. encryption key reuse is highly unlikely.
rs: The "rs" or record size parameter contains an unsigned 32-bit rs: The "rs" or record size parameter contains an unsigned 32-bit
integer in network byte order that describes the record size in integer in network byte order that describes the record size in
octets. Note that it is therefore impossible to exceed the octets. Note that it is therefore impossible to exceed the
2^36-31 limit on plaintext input to AEAD_AES_128_GCM. Values 2^36-31 limit on plaintext input to AEAD_AES_128_GCM. Values
smaller than 3 are invalid. smaller than 3 are invalid.
keyid: The "keyid" parameter can be used to identify the keying keyid: The "keyid" parameter can be used to identify the keying
material that is used. When the Crypto-Key header field is used, material that is used. Recipients that receive a message are
the "keyid" identifies a matching value in that field. The expected to know how to retrieve keys; the "keyid" parameter might
"keyid" parameter MUST be used if keying material included in an be input to that process.
Crypto-Key header field is needed to derive the content encryption
key. The "keyid" parameter can also be used to identify keys in
an application-specific fashion.
2.2. Content Encryption Key Derivation 2.2. Content Encryption Key Derivation
In order to allow the reuse of keying material for multiple different In order to allow the reuse of keying material for multiple different
HTTP messages, a content encryption key is derived for each message. HTTP messages, a content encryption key is derived for each message.
The content encryption key is derived from the decoded value of the The content encryption key is derived from the "salt" parameter using
"salt" parameter using the HMAC-based key derivation function (HKDF) the HMAC-based key derivation function (HKDF) described in [RFC5869]
described in [RFC5869] using the SHA-256 hash algorithm [FIPS180-4]. using the SHA-256 hash algorithm [FIPS180-4].
The value of the "salt" parameter is the salt input to HKDF function. The value of the "salt" parameter is the salt input to HKDF function.
The keying material identified by the "keyid" parameter is the input The keying material identified by the "keyid" parameter is the input
keying material (IKM) to HKDF. Input keying material can either be keying material (IKM) to HKDF. Input keying material is expected to
prearranged, or can be described using the Crypto-Key header field be provided to recipients separately. The extract phase of HKDF
(Section 3). The extract phase of HKDF therefore produces a therefore produces a pseudorandom key (PRK) as follows:
pseudorandom key (PRK) as follows:
PRK = HMAC-SHA-256(salt, IKM) PRK = HMAC-SHA-256(salt, IKM)
The info parameter to HKDF is set to the ASCII-encoded string The info parameter to HKDF is set to the ASCII-encoded string
"Content-Encoding: aes128gcm" and a single zero octet: "Content-Encoding: aes128gcm" and a single zero octet:
cek_info = "Content-Encoding: aes128gcm" || 0x00 cek_info = "Content-Encoding: aes128gcm" || 0x00
Note: Concatenation of octet sequences is represented by the "||" Note: Concatenation of octet sequences is represented by the "||"
operator. operator.
skipping to change at page 7, line 34 skipping to change at page 7, line 24
at zero. at zero.
Thus, the final nonce for each record is a 12 octet value: Thus, the final nonce for each record is a 12 octet value:
NONCE = HMAC-SHA-256(PRK, nonce_info || 0x01) XOR SEQ NONCE = HMAC-SHA-256(PRK, nonce_info || 0x01) XOR SEQ
This nonce construction prevents removal or reordering of records. This nonce construction prevents removal or reordering of records.
However, it permits truncation of the tail of the sequence (see However, it permits truncation of the tail of the sequence (see
Section 2 for how this is avoided). Section 2 for how this is avoided).
3. Crypto-Key Header Field 3. Examples
A Crypto-Key header field can be used to describe the input keying
material used by the "aes128gcm" content coding.
Ordinarily, this header field will not appear in the same message as
the encrypted content. Including the encryption key with the
encrypted payload reduces the value of using encryption to a somewhat
complicated checksum. However, the Crypto-Key header field could be
used in one message to provision keys for other messages.
The Crypto-Key header field uses the extended ABNF syntax defined in
Section 1.2 of [RFC7230] and the "parameter" and "OWS" rules from
[RFC7231].
Crypto-Key = #crypto-key-params
crypto-key-params = [ parameter *( OWS ";" OWS parameter ) ]
keyid: The "keyid" parameter corresponds to the "keyid" parameter in
the content coding.
aes128gcm: The "aes128gcm" parameter contains the base64url-encoded
octets [RFC7515] of the input keying material for the "aes128gcm"
content coding.
Crypto-Key header field values with multiple instances of the same
parameter name in a single crypto-key-params production are invalid.
The input keying material used by the key derivation (see
Section 2.2) can be determined based on the information in the
Crypto-Key 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.
Alternative methods for determining input keying material MAY be
defined by specifications that use this content coding. This
document only defines the use of the "aes128gcm" parameter which
describes an explicit key.
The "aes128gcm" parameter MUST decode to at least 16 octets in order
to be used as input keying material for "aes128gcm" content coding.
4. Examples
This section shows a few examples of the encrypted content coding. This section shows a few examples of the encrypted content coding.
Note: All binary values in the examples in this section use base64url Note: All binary values in the examples in this section use base64url
encoding [RFC7515]. This includes the bodies of requests. encoding [RFC7515]. This includes the bodies of requests.
Whitespace and line wrapping is added to fit formatting constraints. Whitespace and line wrapping is added to fit formatting constraints.
4.1. Encryption of a Response 3.1. Encryption of a Response
Here, a successful HTTP GET response has been encrypted using input Here, a successful HTTP GET response has been encrypted. This uses a
keying material that is identified by the string "a1". 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 encrypted data in this example is the UTF-8 encoded string "I am
the walrus". The input keying material is included in the Crypto-Key the walrus". The input keying material is the value
header field. The content body contains a single record only and is "B33e_VeFrOyIHwFTIfmesA" (in base64url). The content body contains a
shown here using base64url encoding for presentation reasons. single record and is shown here using base64url encoding for
presentation reasons.
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-Type: application/octet-stream Content-Type: application/octet-stream
Content-Length: 33 Content-Length: 54
Content-Encoding: aes128gcm Content-Encoding: aes128gcm
Crypto-Key: aes128gcm=B33e_VeFrOyIHwFTIfmesA
9Y1iaZMzICC05DO3y8dWiAAAopoAzpM9l8LHdpDaO9C-UvT4kttTI_edSsHv1o5b sJvlboCWzB5jr8hI_q9cOQAAEAAANSmxkSVa0-MiNNuF77YHSs-iwaNe_OK0qfmO
lWZ5mBYL c7NT5WSW
Note that the media type has been changed to "application/octet- Note that the media type has been changed to "application/octet-
stream" to avoid exposing information about the content. stream" to avoid exposing information about the content.
Alternatively (and equivalently), the Content-Type header field can Alternatively (and equivalently), the Content-Type header field can
be omitted. be omitted.
4.2. Encryption with Multiple Records Intermediate values for this example (all shown in base64):
This example shows the same encrypted message, but split into records salt (from header) = sJvlboCWzB5jr8hI_q9cOQ
of 10 octets each. The first record includes a single additional PRK = MLAQxt_DHjM15cdlyU1oUnjq7TFlzToGTkdRmvvxVBw
octet of padding, which causes the end of the content to align with a 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 record boundary, forcing the creation of a third record that contains
only padding. only two octets of the padding length.
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-Length: 70 Content-Length: 93
Content-Encoding: aes128gcm Content-Encoding: aes128gcm
Crypto-Key: keyid="a1"; aes128gcm="BO3ZVPxUlnLORbVGMpbT1Q"
_lgOPHdbKmIaLnZC7_8huQAAAAoCYTGkQWUSYylMKzMduBHDCFDwL2oODx8nkh0n uNCkWiNYzKTnBN9ji3-qWAAAAAoCYTGHOqYFz-0in3dpb-VE2GfBngkaPy6bZus_
uOTNrh48DaWSm02DiQPzQAOGe6xRAeBj588hH6jQRTh_szFRS2Nwx9Aeuiic qLF79s6zQyTSsA0iLOKyd3JqVIwprNzVatRCWZGUx_qsFbJBCQu62RqQuR2d
5. Security Considerations 4. Security Considerations
This mechanism assumes the presence of a key management framework This mechanism assumes the presence of a key management framework
that is used to manage the distribution of keys between valid senders that is used to manage the distribution of keys between valid senders
and receivers. Defining key management is part of composing this and receivers. Defining key management is part of composing this
mechanism into a larger application, protocol, or framework. mechanism into a larger application, protocol, or framework.
Implementation of cryptography - and key management in particular - Implementation of cryptography - and key management in particular -
can be difficult. For instance, implementations need to account for can be difficult. For instance, implementations need to account for
the potential for exposing keying material on side channels, such as the potential for exposing keying material on side channels, such as
might be exposed by the time it takes to perform a given operation. might be exposed by the time it takes to perform a given operation.
The requirements for a good implementation of cryptographic The requirements for a good implementation of cryptographic
algorithms can change over time. algorithms can change over time.
5.1. Key and Nonce Reuse 4.1. Key and Nonce Reuse
Encrypting different plaintext with the same content encryption key Encrypting different plaintext with the same content encryption key
and nonce in AES-GCM is not safe [RFC5116]. The scheme defined here and nonce in AES-GCM is not safe [RFC5116]. The scheme defined here
uses a fixed progression of nonce values. Thus, a new content uses a fixed progression of nonce values. Thus, a new content
encryption key is needed for every application of the content coding. encryption key is needed for every application of the content coding.
Since input keying material can be reused, a unique "salt" parameter Since input keying material can be reused, a unique "salt" parameter
is needed to ensure a content encryption key is not reused. is needed to ensure a content encryption key is not reused.
If a content encryption key is reused - that is, if input keying If a content encryption key is reused - that is, if input keying
material and salt are reused - this could expose the plaintext and material and salt are reused - this could expose the plaintext and
the authentication key, nullifying the protection offered by the authentication key, nullifying the protection offered by
encryption. Thus, if the same input keying material is reused, then encryption. Thus, if the same input keying material is reused, then
the salt parameter MUST be unique each time. This ensures that the the salt parameter MUST be unique each time. This ensures that the
content encryption key is not reused. An implementation SHOULD content encryption key is not reused. An implementation SHOULD
generate a random salt parameter for every message; a counter could generate a random salt parameter for every message; a counter could
achieve the same result. achieve the same result.
5.2. Data Encryption Limits 4.2. Data Encryption Limits
There are limits to the data that AEAD_AES_128_GCM can encipher. The 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" 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 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 limit for a single application of AEAD_AES_128_GCM is not reached
[RFC5116]. In order to preserve a 2^-40 probability of [RFC5116]. In order to preserve a 2^-40 probability of
indistinguishability under chosen plaintext attack (IND-CPA), the indistinguishability under chosen plaintext attack (IND-CPA), the
total amount of plaintext that can be enciphered MUST be less than total amount of plaintext that can be enciphered MUST be less than
2^44.5 blocks of 16 octets [AEBounds]. 2^44.5 blocks of 16 octets [AEBounds].
If rs is a multiple of 16 octets, this means 398 terabytes can be If rs is a multiple of 16 octets, this means 398 terabytes can be
encrypted safely, including padding. However, if the record size is encrypted safely, including padding and overhead. However, if the
not a multiple of 16 octets, the total amount of data that can be record size is not a multiple of 16 octets, the total amount of data
safely encrypted is reduced proportionally. The worst case is a that can be safely encrypted is reduced proportionally. The worst
record size of 3 octets, for which at most 74 terabytes of plaintext case is a record size of 3 octets, for which at most 74 terabytes of
can be encrypted, of which at least two-thirds is padding. plaintext can be encrypted, of which at least two-thirds is padding.
5.3. Content Integrity 4.3. Content Integrity
This mechanism only provides content origin authentication. The This mechanism only provides content origin authentication. The
authentication tag only ensures that an entity with access to the authentication tag only ensures that an entity with access to the
content encryption key produced the encrypted data. content encryption key produced the encrypted data.
Any entity with the content encryption key can therefore produce Any entity with the content encryption key can therefore produce
content that will be accepted as valid. This includes all recipients content that will be accepted as valid. This includes all recipients
of the same HTTP message. of the same HTTP message.
Furthermore, any entity that is able to modify both the Encryption Furthermore, any entity that is able to modify both the Encryption
skipping to change at page 11, line 4 skipping to change at page 10, line 11
This mechanism only provides content origin authentication. The This mechanism only provides content origin authentication. The
authentication tag only ensures that an entity with access to the authentication tag only ensures that an entity with access to the
content encryption key produced the encrypted data. content encryption key produced the encrypted data.
Any entity with the content encryption key can therefore produce Any entity with the content encryption key can therefore produce
content that will be accepted as valid. This includes all recipients content that will be accepted as valid. This includes all recipients
of the same HTTP message. of the same HTTP message.
Furthermore, any entity that is able to modify both the Encryption Furthermore, any entity that is able to modify both the Encryption
header field and the HTTP message body can replace the contents. header field and the HTTP message body can replace the contents.
Without the content encryption key or the input keying material, Without the content encryption key or the input keying material,
modifications to or replacement of parts of a payload body are not modifications to or replacement of parts of a payload body are not
possible. possible.
5.4. Leaking Information in Headers 4.4. Leaking Information in Headers
Because only the payload body is encrypted, information exposed in Because only the payload body is encrypted, information exposed in
header fields is visible to anyone who can read the HTTP message. header fields is visible to anyone who can read the HTTP message.
This could expose side-channel information. This could expose side-channel information.
For example, the Content-Type header field can leak information about For example, the Content-Type header field can leak information about
the payload body. the payload body.
There are a number of strategies available to mitigate this threat, There are a number of strategies available to mitigate this threat,
depending upon the application's threat model and the users' depending upon the application's threat model and the users'
skipping to change at page 11, line 39 skipping to change at page 10, line 45
in other representations, etc.), omit the relevant headers, and/ in other representations, etc.), omit the relevant headers, and/
or normalize them. In the case of Content-Type, this could be or normalize them. In the case of Content-Type, this could be
accomplished by always sending Content-Type: application/octet- accomplished by always sending Content-Type: application/octet-
stream (the most generic media type), or no Content-Type at all. stream (the most generic media type), or no Content-Type at all.
3. If it is considered sensitive information and it is not possible 3. If it is considered sensitive information and it is not possible
to convey it elsewhere, encapsulate the HTTP message using the to convey it elsewhere, encapsulate the HTTP message using the
application/http media type (Section 8.3.2 of [RFC7230]), application/http media type (Section 8.3.2 of [RFC7230]),
encrypting that as the payload of the "outer" message. encrypting that as the payload of the "outer" message.
5.5. Poisoning Storage 4.5. Poisoning Storage
This mechanism only offers encryption of content; it does not perform This mechanism only offers encryption of content; it does not perform
authentication or authorization, which still needs to be performed authentication or authorization, which still needs to be performed
(e.g., by HTTP authentication [RFC7235]). (e.g., by HTTP authentication [RFC7235]).
This is especially relevant when a HTTP PUT request is accepted by a This is especially relevant when a HTTP PUT request is accepted by a
server; if the request is unauthenticated, it becomes possible for a server; if the request is unauthenticated, it becomes possible for a
third party to deny service and/or poison the store. third party to deny service and/or poison the store.
5.6. Sizing and Timing Attacks 4.6. Sizing and Timing Attacks
Applications using this mechanism need to be aware that the size of Applications using this mechanism need to be aware that the size of
encrypted messages, as well as their timing, HTTP methods, URIs and encrypted messages, as well as their timing, HTTP methods, URIs and
so on, may leak sensitive information. so on, may leak sensitive information.
This risk can be mitigated through the use of the padding that this This risk can be mitigated through the use of the padding that this
mechanism provides. Alternatively, splitting up content into mechanism provides. Alternatively, splitting up content into
segments and storing the separately might reduce exposure. HTTP/2 segments and storing the separately might reduce exposure. HTTP/2
[RFC7540] combined with TLS [RFC5246] might be used to hide the size [RFC7540] combined with TLS [RFC5246] might be used to hide the size
of individual messages. of individual messages.
6. IANA Considerations 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.
6.1. The "aes128gcm" HTTP Content Coding 5. IANA Considerations
5.1. The "aes128gcm" HTTP Content Coding
This memo registers the "aes128gcm" HTTP content coding in the HTTP This memo registers the "aes128gcm" HTTP content coding in the HTTP
Content Codings Registry, as detailed in Section 2. Content Codings Registry, as detailed in Section 2.
o Name: aes128gcm o Name: aes128gcm
o Description: AES-GCM encryption with a 128-bit content encryption o Description: AES-GCM encryption with a 128-bit content encryption
key key
o Reference: this specification o Reference: this specification
6.2. Crypto-Key Header Field 6. References
This memo registers the "Crypto-Key" HTTP header field in the
Permanent Message Header Registry, as detailed in Section 3.
o Field name: Crypto-Key
o Protocol: HTTP
o Status: Standard
o Reference: this specification
o Notes:
6.3. 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.
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. aes128gcm
o Parameter Name: aes128gcm
o Purpose: Provide an explicit input keying material value for the
aes128gcm content coding.
o Reference: this document
7. References
7.1. Normative References 6.1. Normative References
[FIPS180-4] [FIPS180-4]
Department of Commerce, National., "NIST FIPS 180-4, Department of Commerce, National., "NIST FIPS 180-4,
Secure Hash Standard", March 2012, Secure Hash Standard", March 2012,
<http://csrc.nist.gov/publications/fips/fips180-4/ <http://csrc.nist.gov/publications/fips/fips180-4/
fips-180-4.pdf>. fips-180-4.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008, Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<http://www.rfc-editor.org/info/rfc5116>. <http://www.rfc-editor.org/info/rfc5116>.
[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>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869, Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010, DOI 10.17487/RFC5869, May 2010,
<http://www.rfc-editor.org/info/rfc5869>. <http://www.rfc-editor.org/info/rfc5869>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing", Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014, RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>. <http://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014, DOI 10.17487/RFC7231, June 2014,
<http://www.rfc-editor.org/info/rfc7231>. <http://www.rfc-editor.org/info/rfc7231>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web [RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <http://www.rfc-editor.org/info/rfc7515>. 2015, <http://www.rfc-editor.org/info/rfc7515>.
7.2. Informative References 6.2. Informative References
[AEBounds] [AEBounds]
Luykx, A. and K. Paterson, "Limits on Authenticated Luykx, A. and K. Paterson, "Limits on Authenticated
Encryption Use in TLS", March 2016, Encryption Use in TLS", March 2016,
<http://www.isg.rhul.ac.uk/~kp/TLS-AEbounds.pdf>. <http://www.isg.rhul.ac.uk/~kp/TLS-AEbounds.pdf>.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007, DOI 10.17487/RFC4880, November 2007,
<http://www.rfc-editor.org/info/rfc4880>. <http://www.rfc-editor.org/info/rfc4880>.
skipping to change at page 16, line 10 skipping to change at page 14, line 10
o The JWE Initialization Vector ("iv") for each record is set to the o The JWE Initialization Vector ("iv") for each record is set to the
exclusive or of the 96-bit record sequence number, starting at exclusive or of the 96-bit record sequence number, starting at
zero, and a value derived from the input keying material (see zero, and a value derived from the input keying material (see
Section 2.3). This value is also not transmitted. Section 2.3). This value is also not transmitted.
o The final value is the concatenated header, JWE Ciphertext, and o The final value is the concatenated header, JWE Ciphertext, and
JWE Authentication Tag, all expressed without base64url encoding. JWE Authentication Tag, all expressed without base64url encoding.
The "." separator is omitted, since the length of these fields is The "." separator is omitted, since the length of these fields is
known. known.
Thus, the example in Section 4.1 can be rendered using the JWE Thus, the example in Section 3.1 can be rendered using the JWE
Compact Serialization as: Compact Serialization as:
eyAiYWxnIjogImRpciIsICJlbmMiOiAiQTEyOEdDTSIgfQ..31iQYc1v4a36EgyJ. eyAiYWxnIjogImRpciIsICJlbmMiOiAiQTEyOEdDTSIgfQ..31iQYc1v4a36EgyJ.
AM6TPZfCx3aQ2jvQvlL0-JLb.21Mj951Kwe_WjluVZnmYFgs NSmxkSVa0-MiNNuF77YHSs8.osGjXvzitKn5jnOzU-Vklg
Where the first line represents the fixed JWE Protected Header, an Where the first line represents the fixed JWE Protected Header, an
empty JWE Encrypted Key, and the algorithmically-determined JWE empty JWE Encrypted Key, and the algorithmically-determined JWE
Initialization Vector. The second line contains the encoded body, Initialization Vector. The second line contains the encoded body,
split into JWE Ciphertext and JWE Authentication Tag. split into JWE Ciphertext and JWE Authentication Tag.
Appendix B. Acknowledgements Appendix B. Acknowledgements
Mark Nottingham was an original author of this document. Mark Nottingham was an original author of this document.
 End of changes. 44 change blocks. 
210 lines changed or deleted 118 lines changed or added

This html diff was produced by rfcdiff 1.45. The latest version is available from http://tools.ietf.org/tools/rfcdiff/