Network Working Group M. Thomson
Internet-Draft Mozilla
Intended status: Standards Track October 30, 2016
Expires: May 3, 2017
Merkle Integrity Content Encoding
draft-thomson-http-mice-02
Abstract
This memo introduces a content-coding for HTTP that provides
progressive integrity for message contents. This integrity
protection can be evaluated on a partial representation, allowing a
recipient to process a message as it is delivered while retaining
strong integrity protection.
Status of This Memo
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. The "mi-sha256" HTTP Content Encoding . . . . . . . . . . . . 3
2.1. Content Encoding Structure . . . . . . . . . . . . . . . 4
2.2. Validating Integrity Proofs . . . . . . . . . . . . . . . 5
3. The MI HTTP Header Field . . . . . . . . . . . . . . . . . . 6
3.1. MI Header Field Parameters . . . . . . . . . . . . . . . 6
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Simple Example . . . . . . . . . . . . . . . . . . . . . 6
4.2. Example with Multiple Records . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5.1. Message Truncation . . . . . . . . . . . . . . . . . . . 7
5.2. Algorithm Agility . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6.1. The "mi-sha256" HTTP Content Encoding . . . . . . . . . . 8
6.2. MI Header Field . . . . . . . . . . . . . . . . . . . . . 8
6.3. The HTTP MI Parameter Registry . . . . . . . . . . . . . 8
6.3.1. mi-sha256 parameter . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Appendix B. FAQ . . . . . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Integrity protection for HTTP content is highly valuable. HTTPS
[RFC2818] is the most common form of integrity protection deployed,
but that requires a direct TLS [RFC5246] connection to a host.
However, additional integrity protection might be desirable for some
use cases. This might be for additional protection against failures
or attack (see [SRI]) or because content needs to remain unmodified
throughout multiple HTTPS-protected exchanges.
This document describes a "mi-sha256" content-encoding (see
Section 2) that is a progressive, hash-based integrity check based on
Merkle Hash Trees [MERKLE].
The means of conveying the root integrity proof used by this content
encoding will depend on deployment requirements. This document
defines an MI header field (see Section 3) that can carry an
integrity proof.
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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 "mi-sha256" HTTP Content Encoding
A Merkle Hash Tree [MERKLE] is a structured integrity mechanism that
collates multiple integrity checks into a tree. The leaf nodes of
the tree contain data (or hashes of data) and non-leaf nodes contain
hashes of the nodes below them.
A balanced Merkle Hash Tree is used to efficiently prove membership
in large sets (such as in [RFC6962]). However, in this case, a
right-skewed tree is used to provide a progressive integrity proof.
This integrity proof is used to establish that a given record is part
of a message.
The hash function used for "mi-sha256" content encoding is SHA-256
[FIPS180-4]. The integrity proof for all records other than the last
is the hash of the concatenation of the record, the integrity proof
of all subsequent records, and a single octet with a value of 0x1:
proof(r[i]) = SHA-256(r[i] || proof(r[i+1]) || 0x1)
The integrity proof for the final record is the hash of the record
with a single octet with a value 0x0 appended:
proof(r[last]) = SHA-256(r[last] || 0x0)
Figure 1 shows the structure of the integrity proofs for a message
that is split into 4 blocks: A, B, C, D). As shown, the integrity
proof for the entire message (that is, "proof(A)") is derived from
the content of the first block (A), plus the value of the proof for
the second and subsequent blocks.
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proof(A)
/\
/ \
/ \
A proof(B)
/\
/ \
/ \
B proof(C)
/\
/ \
/ \
C proof(D)
|
|
D
Figure 1: Proof structure for a message with 4 blocks
The final encoded message is formed from the first record, followed
by an arbitrary number of tuples of the integrity proof of the next
record and then the record itself. Thus, in Figure 1, the body is:
rs || A || proof(B) || B || proof(C) || C || proof(D) || D
Note: The "||" operator is used to represent concatenation.
A message that has a content length less than or equal to the content
size does not include any inline proofs. The proof for a message
with a single record is simply the hash of the body plus a trailing
zero octet.
2.1. Content Encoding Structure
In order to produce the final content encoding the content of the
message is split into equal-sized records. The final record can
contain less than the defined record size.
The record size is included in the first 8 octets of the message as
an unsigned 64-bit integer. This refers to the length of each data
block.
The final encoded stream comprises of the record size ("rs"), plus a
sequence of records, each "rs" octets in length. Each record, other
than the last, is followed by a 32 octet proof for the record that
follows. This allows a receiver to validate and act upon each record
after receiving the proof that precedes it. The final record is not
followed by a proof.
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Note: This content encoding increases the size of a message by 8
plus 32 octets times the length of the message divided by the
record size, rounded up, less one. That is, 8 + 32 * (ceil(length
/ rs) - 1).
Constructing a message with the "mi-sha256" content encoding requires
processing of the records in reverse order, inserting the proof
derived from each record before that record.
This structure permits the use of range requests [RFC7233]. However,
to validate a given record, a contiguous sequence of records back to
the start of the message is needed.
2.2. Validating Integrity Proofs
A receiver of a message with the "mi-sha256" content-encoding applied
first attempts to acquire the integrity proof for the first record.
If the MI header field is present, a value might be included there.
The first 8 octets are read as an unsigned 64-bit integer, "rs". The
remainder of the message is read into records of size "rs" (based on
the value in the MI header field) plus 32 octets. The last record is
between 1 and "rs" octets in length, if not then validation fails.
For each record:
1. Hash the record using SHA-256 with a single octet appended:
a. All records other than the last have an octet with a value of
0x1 appended.
b. The last record has an octet with a value of 0x0 appended.
2. Compare the hash with the expected value:
a. For the first record, the expected value might found in the
MI header field and is otherwise provided through some external
means.
b. For records after the first, the expected value is the last
32 octets of the previous record.
3. If the hash is different, then this record and all subsequent
records do not have integrity protection and this process ends.
4. If a record is valid, up to "rs" octets is passed on for
processing. In other words, the trailing 32 octets is removed
from every record other than the last before being used.
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If an integrity check fails, the message SHOULD be discarded and the
exchange treated as an error unless explicitly configured otherwise.
For clients, treat this as equivalent to a server error; servers
SHOULD generate a 400 or other 4xx status code. However, if the
integrity proof for the first record is not known, this check SHOULD
NOT fail unless explicitly configured to do so.
3. The MI HTTP Header Field
The MI HTTP header field carries message integrity proofs
corresponding to content encoding(s) that have been applied to a
payload body.
The MI header field uses the extended ABNF syntax defined in
Section 1.2 of [RFC7230] and the "parameter" rule from [RFC7231]:
MI = #mi_params
mi_params = [ parameter *( ";" parameter ) ]
If the payload is encoded more than once (as reflected by having
multiple content-codings that use the message integrity header
field), each application of the content encoding is reflected in the
MI header field in the order in which they were applied.
The MI header MAY be omitted if the sender intends for the receiver
to acquire the integrity proof for the first record by other means.
3.1. MI Header Field Parameters
The following parameters are used in validating content encoded with
the "mi-sha256" content encoding:
mi-sha256: The "mi-sha256" parameter carries an integrity proof for
the first record of the message. This provides integrity for the
entire message body. This value is encoded using base64url
encoding [RFC7515].
4. Examples
4.1. Simple Example
The following example contains a short message. This contains just a
single record, so there are no inline integrity proofs, just a single
value in a MI header field. The record size is prepended to the
message body (shown here in angle brackets).
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HTTP/1.1 200 OK
MI: mi-sha256=dcRDgR2GM35DluAV13PzgnG6-pvQwPywfFvAu1UeFrs
Content-Encoding: mi-sha256
Content-Length: 49
<0x0000000000000029>When I grow up, I want to be a watermelon
4.2. Example with Multiple Records
This example shows the same message as above, but with a smaller
record size (16 octets). This results in two integrity proofs being
included in the representation.
PUT /test HTTP/1.1
Host: example.com
MI: mi-sha256=IVa9shfs0nyKEhHqtB3WVNANJ2Njm5KjQLjRtnbkYJ4
Content-Encoding: mi-sha256
Content-Length: 113
<0x0000000000000010>When I grow up,
OElbplJlPK-Rv6JNK6p5_515IaoPoZo-2elWL7OQ60A
I want to be a w
iPMpmgExHPrbEX3_RvwP4d16fWlK4l--p75PUu_KyN0
atermelon
Since the inline integrity proofs contain non-printing characters,
these are shown here using the base64url encoding [RFC7515] with new
lines between the original text and integrity proofs. Note that
there is a single trailing space (0x20) on the first line.
5. Security Considerations
The integrity of an entire message body depends on the means by which
the integrity proof for the first record is protected. If this value
comes from the same place as the message, then this provides only
limited protection against transport-level errors (something that TLS
provides adequate protection against).
Separate protection for header fields might be provided by other
means if the first record retrieved is the first record in the
message, but range requests do not allow for this option.
5.1. Message Truncation
This integrity scheme permits the detection of truncated messages.
However, it enables and even encourages processing of messages prior
to receiving an complete message. Actions taken on a partial message
can produce incorrect results. For example, a message could say "I
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need some 2mm copper cable, please send 100mm for evaluation
purposes" then be truncated to "I need some 2mm copper cable, please
send 100m". A network-based attacker might be able to force this
sort of truncation by delaying packets that contain the remainder of
the message.
Whether it is safe to act on partial messages will depend on the
nature of the message and the processing that is performed.
5.2. Algorithm Agility
A new content encoding type is needed in order to define the use of a
hash function other than SHA-256.
6. IANA Considerations
6.1. The "mi-sha256" HTTP Content Encoding
This memo registers the "mi-sha256" HTTP content-coding in the HTTP
Content Codings Registry, as detailed in Section 2.
o Name: mi-sha256
o Description: A Merkle Hash Tree based content encoding that
provides progressive integrity.
o Reference: this specification
6.2. MI Header Field
This memo registers the "MI" HTTP header field in the Permanent
Message Header Registry, as detailed in Section 3.
o Field name: MI
o Protocol: HTTP
o Status: Standard
o Reference: this specification
o Notes:
6.3. The HTTP MI Parameter Registry
This memo establishes a registry for parameters used by the "MI"
header field under the "Hypertext Transfer Protocol (HTTP)
Parameters" grouping. The "Hypertext Transfer Protocol (HTTP) MI
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Parameters" registry operates under an "Specification Required"
policy [RFC5226].
Entries in this registry are expected to include the following
information:
o Parameter Name: The name of the parameter.
o Purpose: A brief description of the purpose of the parameter.
o Reference: A reference to a specification that defines the
semantics of the parameter.
The initial contents of this registry are:
6.3.1. mi-sha256 parameter
o Parameter Name: mi-sha256
o Purpose: The value of the integrity proof for the first record.
o Reference: this document
7. References
7.1. 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>.
[MERKLE] Merkle, R., "A Digital Signature Based on a Conventional
Encryption Function", International Crytology Conference -
CRYPTO , 1987.
[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>.
[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>.
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[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>.
7.2. Informative References
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>.
[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>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<http://www.rfc-editor.org/info/rfc6962>.
[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>.
[SRI] Akhawe, D., Braun, F., Marier, F., and J. Weinberger,
"Subresource Integrity", W3C CR , November 2015,
<https://w3c.github.io/webappsec-subresource-integrity/>.
Appendix A. Acknowledgements
David Benjamin and Erik Nygren both separately suggested that
something like this might be valuable. James Manger and Eric
Rescorla provided useful feedback.
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Appendix B. FAQ
1. Why not include the first proof in the encoding?
The requirements for the integrity proof for the first record
require a great deal more flexibility than this allows for.
Transferring the proof separately is sometimes necessary.
Separating the value out allows for that to happen more easily.
2. Why do messages have to be processed in reverse to construct
them?
The final integrity value, no matter how it is derived, has to
depend on every bit of the message. That means that there are
three choices: both sender and receiver have to process the whole
message, the sender has to work backwards, or the receiver has to
work backwards. The current form is the best option of the
three. The expectation is that this will be useful for content
that is generated once and sent multiple times, since the onerous
backwards processing requirement can be amortized.
3. Why not just generate a table of hashes?
An alternative design includes a header that comprises hashes of
every block of the message. The final proof is a hash of that
table. This has the advantage that the table can be built in any
order. The disadvantage is that a receiver needs to store the
table while processing content, whereas a chained hash can be
processed with a single stored hash worth of state no matter how
many blocks are present. The chained hash is also smaller by 32
octets.
Author's Address
Martin Thomson
Mozilla
Email: martin.thomson@gmail.com
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