Internet-Draft | Digest Headers | April 2021 |
Polli & Pardue | Expires 15 October 2021 | [Page] |
- Workgroup:
- HTTP
- Internet-Draft:
- draft-ietf-httpbis-digest-headers-05
- Obsoletes:
- 3230 (if approved)
- Published:
- Intended Status:
- Standards Track
- Expires:
Digest Headers
Abstract
This document defines the HTTP Digest and Want-Digest fields, thus allowing client and server to negotiate an integrity checksum of the exchanged resource representation data.¶
This document obsoletes RFC 3230. It replaces the term "instance" with "representation", which makes it consistent with the HTTP Semantic and Context defined in draft-ietf-httpbis-semantics.¶
Note to Readers
RFC EDITOR: please remove this section before publication¶
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/.¶
The source code and issues list for this draft can be found at https://github.com/httpwg/http-extensions.¶
Status of This Memo
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶
This Internet-Draft will expire on 15 October 2021.¶
Copyright Notice
Copyright (c) 2021 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 (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶
1. Introduction
The core specification of HTTP does not define a means to protect the integrity of resources. When HTTP messages are transferred between endpoints, the protocol might choose to make use of features of the lower layer in order to provide some integrity protection; for instance TCP checksums or TLS records [RFC2818].¶
However, there are cases where relying on this alone is insufficient. An HTTP-level integrity mechanism that operates independent of transfer can be used to detect programming errors and/or corruption of data in flight or at rest, be used across multiple hops in order to provide end-to-end integrity guarantees, can aid fault diagnosis across hops and system boundaries, and can be used to validate integrity when reconstructing a resource fetched using different HTTP connections.¶
This document defines a mechanism that acts on HTTP representation-data. It can be combined with other mechanisms that protect representation-metadata, such as digital signatures, in order to protect the desired parts of an HTTP exchange in whole or in part.¶
1.1. A Brief History of HTTP Integrity Fields
The Content-MD5 header field was originally introduced to provide integrity, but HTTP/1.1 ([RFC7231], Appendix B) obsoleted it:¶
- The Content-MD5 header field has been removed because it was inconsistently implemented with respect to partial responses.¶
[RFC3230] provided a more flexible solution introducing the concept of
"instance", and the fields Digest
and Want-Digest
.¶
1.2. This Proposal
The concept of selected representation
defined in Section 3.2 of
[SEMANTICS] makes [RFC3230] definitions inconsistent with
current HTTP semantics. This document updates the Digest
and Want-Digest
field definitions to align with [SEMANTICS] concepts.¶
Basing Digest
on the selected representation makes it straightforward to
apply it to use-cases where the transferred data does require some sort of
manipulation to be considered a representation, or conveys a partial
representation of a resource eg. Range Requests (see Section 14.2 of
[SEMANTICS]).¶
This document replaces [RFC3230] to better align with [SEMANTICS] and to
provide more detailed description of Digest
usage in request and response
cases. Changes are intended to be semantically compatible with existing
implementations but note that negotiation of Content-MD5
is deprecated
Section 7, Digest
field parameters are obsoleted
Section 8, "md5" and "sha" digest-algorithms are obsoleted
Section 12.2 and the "adler32" algorithm is deprecated
Section 12.3.¶
The value of Digest
is calculated on selected representation, which is tied to
the value contained in any Content-Encoding
or Content-Type
header fields.
Therefore, a given resource may have multiple different digest values.¶
To allow both parties to exchange a Digest of a representation with no content codings (see Section 8.4.1 of [SEMANTICS]) two more digest-algorithms are added ("id-sha-256" and "id-sha-512").¶
1.3. Goals
The goals of this proposal are:¶
- Digest coverage for either the resource's
representation data
orselected representation data
communicated via HTTP.¶ - Support for multiple digest-algorithms.¶
- Negotiation of the use of digests.¶
The goals do not include:¶
- HTTP message integrity:
-
Digest mechanisms do not cover the full HTTP message nor its semantic, as representation metadata is not included in the checksum.¶
- HTTP field integrity:
-
Digest mechanisms cover only representation and selected representation data, and do not protect the integrity of associated representation metadata or other message fields.¶
- Authentication:
-
Digest mechanisms do not support authentication of the source of a digest, message or anything else. These mechanisms, therefore, are not a sufficient defense against many kinds of malicious attacks.¶
- Privacy:
-
Digest mechanisms do not provide message privacy.¶
- Authorization:
-
Digest mechanisms do not support authorization or other kinds of access controls.¶
1.4. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This document uses the Augmented BNF defined in [RFC5234] and updated by [RFC7405] along with the "#rule" extension defined in Section 5.6.1 of [SEMANTICS].¶
The definitions "representation", "selected representation", "representation data", "representation metadata", and "content" in this document are to be interpreted as described in [SEMANTICS].¶
Algorithm names respect the casing used in their definition document (eg. SHA-1, CRC32c) whereas digest-algorithm tokens are quoted (eg. "sha", "crc32c").¶
2. Representation Digest
The representation digest is an integrity mechanism for HTTP resources which uses a checksum that is calculated independently of the content (see Section 6.4 of [SEMANTICS]). It uses the representation data (see Section 8.1 of [SEMANTICS]), that can be fully or partially contained in the content, or not contained at all:¶
representation-data := Content-Encoding( Content-Type( bits ) )¶
This takes into account the effect of the HTTP semantics on the messages;
for example, the content can be affected by Range Requests or methods such as HEAD,
while the way the content is transferred "on the wire" is dependent on other
transformations (e.g. transfer codings for HTTP/1.1 - see Section 6.1 of
[HTTP11]). To help illustrate how such things affect Digest
,
several examples are provided in Appendix A.¶
A representation digest consists of
the value of a checksum computed on the entire selected representation data
(see Section 8.1 of [SEMANTICS]) of a resource identified according to Section 6.4.2 of [SEMANTICS]
together with an indication of the algorithm used:¶
representation-data-digest = digest-algorithm "=" <encoded digest output>¶
When a message has no representation data it is still possible to assert that no representation data was sent computing the representation digest on an empty string (see Section 12.6).¶
The checksum is computed using one of the digest-algorithms listed in Section 5 and then encoded in the associated format.¶
The example below shows the "sha-256" digest-algorithm that uses base64 encoding.¶
sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=¶
3. The Digest Field
The Digest
field contains a list of one or more representation digest values as
defined in Section 2. It can be used in both requests and
responses.¶
Digest = 1#representation-data-digest¶
For example:¶
The relationship between Content-Location
(see Section 8.7 of
[SEMANTICS]) and Digest
is demonstrated in
Section 10.7. A comprehensive set of examples showing the impacts of
representation metadata, payload transformations and HTTP methods on Digest is
provided in Section 10 and Section 11.¶
A Digest
field MAY contain multiple representation-data-digest values.
For example, a server may provide representation-data-digest values using different algorithms,
allowing it to support a population of clients with different evolving capabilities;
this is particularly useful in support of transitioning away
from weaker algorithms should the need arise (see Section 12.9).¶
A recipient MAY ignore any or all of the representation-data-digests in a Digest field. This allows the recipient to choose which digest-algorithm(s) to use for validation instead of verifying every received representation-data-digest.¶
A sender MAY send a representation-data-digest using a digest-algorithm without knowing whether the recipient supports the digest-algorithm, or even knowing that the recipient will ignore it.¶
Digest
can be sent in a trailer section. When an incremental digest-algorithms
is used, the sender and the receiver can dynamically compute the digest value
while streaming the content.¶
4. The Want-Digest Field
The Want-Digest
field indicates the sender's desire to receive a representation
digest on messages associated with the request URI and representation metadata.¶
Want-Digest = 1#want-digest-value want-digest-value = digest-algorithm [ ";" "q" "=" qvalue] qvalue = ( "0" [ "." 0*1DIGIT ] ) / ( "1" [ "." 0*1( "0" ) ] )¶
If a digest-algorithm is not accompanied by a "qvalue", it is treated as if its associated "qvalue" were 1.0.¶
The sender is willing to accept a digest-algorithm if and only if it is listed
in a Want-Digest
field of a message, and its "qvalue" is non-zero.¶
If multiple acceptable digest-algorithm values are given, the sender's preferred digest-algorithm is the one (or ones) with the highest "qvalue".¶
Two examples of its use are:¶
5. Digest Algorithm Values
Digest-algorithm values are used to indicate a specific digest computation.¶
digest-algorithm = token¶
All digest-algorithm values are case-insensitive but lower case is preferred.¶
The Internet Assigned Numbers Authority (IANA) acts as a registry for digest-algorithm values. The registry contains the tokens listed below.¶
Some digest-algorithms, although registered, rely on vulnerable algorithms and MUST not be used:¶
- "md5", see [CMU-836068] and [NO-MD5];¶
- "sha", see [IACR-2020-014] and [NO-SHA1].¶
See the references above for further information.¶
- sha-256
- sha-512
- md5
-
- Description: The MD5 algorithm, as specified in [RFC1321]. The output of this algorithm is encoded using the base64 encoding [RFC4648]. This digest-algorithm MUST NOT be used as it's now vulnerable to collision attacks. See [NO-MD5] and [CMU-836068].¶
- Reference: [RFC1321], [RFC4648], this document.¶
- Status: deprecated¶
- sha
-
- Description: The SHA-1 algorithm [RFC3174]. The output of this algorithm is encoded using the base64 encoding [RFC4648]. This digest-algorithm MUST NOT be used as it's now vulnerable to collision attacks. See [NO-SHA1] and [IACR-2020-014].¶
- Reference: [RFC3174], [RFC6234], [RFC4648], this document.¶
- Status: deprecated¶
- unixsum
-
- Description: The algorithm computed by the UNIX "sum" command, as defined by the Single UNIX Specification, Version 2 [UNIX]. The output of this algorithm is an ASCII decimal-digit string representing the 16-bit checksum, which is the first word of the output of the UNIX "sum" command.¶
- Reference: [UNIX], this document.¶
- Status: standard¶
- unixcksum
-
- Description: The algorithm computed by the UNIX "cksum" command, as defined by the Single UNIX Specification, Version 2 [UNIX]. The output of this algorithm is an ASCII digit string representing the 32-bit CRC, which is the first word of the output of the UNIX "cksum" command.¶
- Reference: [UNIX], this document.¶
- Status: standard¶
To allow sender and recipient to provide a checksum which is independent from
Content-Encoding
, the following additional digest-algorithms are defined:¶
- id-sha-512
- id-sha-256
If other digest-algorithm values are defined, the associated encoding MUST either be represented as a quoted string or MUST NOT include ";" or "," in the character sets used for the encoding.¶
6. Use of Digest when acting on resources
POST and PATCH requests can appear to convey partial representations but are semantically acting on resources. The enclosed representation, including its metadata, refers to that action.¶
In these requests the representation digest MUST be computed on the representation-data of that action. This is the only possible choice because representation digest requires complete representation metadata (see Section 2).¶
In responses,¶
- if the representation describes the status of the request,
Digest
MUST be computed on the enclosed representation (see Section 10.8 );¶ - if there is a referenced resource
Digest
MUST be computed on the selected representation of the referenced resource even if that is different from the target resource. That might or might not result in computingDigest
on the enclosed representation.¶
The latter case might be done according to the HTTP semantics of the given
method, for example using the Content-Location
header field.
In contrast, the Location
header field does not affect Digest
because
it is not representation metadata.¶
6.1. Digest and PATCH
In PATCH requests, the representation digest MUST be computed on the patch document because the representation metadata refers to the patch document and not to the target resource (see Section 2 of [PATCH]).¶
In PATCH responses, the representation digest MUST be computed on the selected representation of the patched resource.¶
Digest
usage with PATCH is thus very similar to POST, but with the
resource's own semantic partly implied by the method and by the patch document.¶
7. Deprecate Negotiation of Content-MD5
This RFC deprecates the negotiation of Content-MD5 as it has been obsoleted by
[RFC7231].
The contentMD5
token defined in Section 5 of [RFC3230] MUST NOT be used as a digest-algorithm.¶
8. Obsolete Digest Field Parameters
Section 4.1.1 and 4.2 of [RFC3230] defined field parameters. This document
obsoletes the usage of parameters with Digest
because this feature has not
been widely deployed and complicates field-value processing.¶
[RFC3230] intended field parameters to provide a common way to attach additional information to a representation-data-digest. However, if parameters are used as an input to validate the checksum, an attacker could alter them to steer the validation behavior.¶
A digest-algorithm can still be parameterized by defining its own way to encode parameters into the representation-data-digest, in such a way as to mitigate security risks related to its computation.¶
9. Relationship to Subresource Integrity (SRI)
Subresource Integrity [SRI] is an integrity mechanism that shares some similarities to the present document's mechanism. However, there are differences in motivating factors, threat model and specification of integrity digest generation, signalling and validation.¶
SRI allows a first-party authority to declare an integrity assertion on a
resource served by a first or third party authority. This is done via the
integrity
attribute that can be added to script
or link
HTML elements.
Therefore, the integrity assertion is always made out-of-band to the resource
fetch. In contrast, the Digest
field is supplied in-band alongside the
selected representation, meaning that an authority can only declare an integrity
assertion for itself. Methods to improve the security properties of
representation digests are presented in Section 12. This
contrast is interesting because on one hand self-assertion is less likely to be
affected by coordination problems such as the first-party holding stale
information about the third party, but on the other hand the self-assertion is
only as trustworthy as the authority that provided it.¶
The SRI integrity
attribute contains a cryptographic hash algorithm and digest
value which is similar to representation-data-digest
(see
Section 2). The major differences are in serialization format.¶
SRI does not specify handling of partial representation data (e.g. Range requests). In contrast, this document specifies handling in terms that are fully compatible with core HTTP concepts (an example is provided in Section 10.3).¶
SRI specifies strong requirements on the selection of algorithm for generation and validation of digests. In contrast, the requirements in this document are weaker.¶
SRI defines no method for a client to declare an integrity assertion on
resources it transfers to a server. In contrast, the Digest
field can
appear on requests.¶
9.1. Supporting Both SRI and Representation Digest
The SRI and Representation Digest mechanisms are different and complementary but one is not capable of replacing the other because they have different threat, security and implementation properties.¶
A user agent that supports both mechanisms is expected to apply the rules specified for each but since the two mechanisms are independent, the ordering is not important. However, a user agent supporting both could benefit from performing representation digest validation first because it does not always require a conversion into identity encoding.¶
There is a chance that a user agent supporting both mechanisms may find one validates successfully while the other fails. This document specifies no requirements or guidance for user agents that experience such cases.¶
10. Examples of Unsolicited Digest
The following examples demonstrate interactions where a server responds with a
Digest
field even though the client did not solicit one using
Want-Digest
.¶
Some examples include JSON objects in the content. For presentation purposes, objects that fit completely within the line-length limits are presented on a single line using compact notation with no leading space. Objects that would exceed line-length limits are presented across multiple lines (one line per key-value pair) with 2 spaced of leading indentation.¶
Digest
is media-type agnostic
and does not provide canonicalization algorithms for specific formats.
Examples of Digest
are calculated inclusive of any space.¶
10.2. Server Returns No Representation Data
In this example, a HEAD request is used to retrieve the checksum of a resource.¶
The response Digest
field-value is calculated over the JSON object
{"hello": "world"}
, which is not shown because there is no payload
data.¶
Request:¶
Response:¶
10.3. Server Returns Partial Representation Data
In this example, the client makes a range request and the server
responds with partial content. The Digest
field-value represents
the entire JSON object {"hello": "world"}
.¶
Request:¶
Response:¶
10.4. Client and Server Provide Full Representation Data
The request contains a Digest
field-value calculated on the enclosed
representation. It also includes an Accept-Encoding: br
header field that advertises the
client supports brotli encoding.¶
The response includes a Content-Encoding: br
that indicates the selected
representation is brotli encoded. The Digest
field-value is therefore
different compared to the request.¶
For presentation purposes, the response body is displayed as a base64-encoded string because it contains non-printable characters.¶
Request:¶
Response:¶
10.5. Client Provides Full Representation Data, Server Provides No Representation Data
The request Digest
field-value is calculated on the enclosed payload.¶
The response Digest
field-value
depends on the representation metadata header fields, including
Content-Encoding: br
even when the response does not contain content.¶
Request:¶
Response:¶
10.6. Client and Server Provide Full Representation Data, Client Uses id-sha-256.
The response contains two digest values:¶
- one with no content coding applied, which in this case accidentally matches the unencoded digest-value sent in the request;¶
- one taking into account the
Content-Encoding
.¶
As the response body contains non-printable characters, it is displayed as a base64-encoded string.¶
Request:¶
Response:¶
10.7. POST Response does not Reference the Request URI
The request Digest
field-value is computed on the enclosed representation (see
Section 6).¶
The representation enclosed in the response refers to the resource identified by
Content-Location
(see [SEMANTICS], Section 6.4.2). Digest
is thus computed on the enclosed representation.¶
Request:¶
Response:¶
Note that a 204 No Content
response without content but with the same
Digest
field-value would have been legitimate too.¶
10.8. POST Response Describes the Request Status
The request Digest
field-value is computed on the enclosed representation (see
Section 6).¶
The representation enclosed in the response describes the status of the request,
so Digest
is computed on that enclosed representation.¶
Response Digest
has no explicit relation with the resource referenced by
Location
.¶
Request:¶
Response:¶
10.9. Digest with PATCH
This case is analogous to a POST request where the target resource reflects the effective request URI.¶
The PATCH request uses the application/merge-patch+json
media type defined in
[RFC7396].¶
Digest
is calculated on the enclosed payload, which corresponds to the patch
document.¶
The response Digest
field-value is computed on the complete representation of the patched
resource.¶
Request:¶
Response:¶
Note that a 204 No Content
response without content but with the same
Digest
field-value would have been legitimate too.¶
10.10. Error responses
In error responses, the representation-data does not necessarily refer to the target resource. Instead, it refers to the representation of the error.¶
In the following example a client attempts to patch the resource located at /books/123. However, the resource does not exist and the server generates a 404 response with a body that describes the error in accordance with [RFC7807].¶
The response Digest
field-value is computed on this enclosed representation.¶
Request:¶
Response:¶
10.11. Use with Trailer Fields and Transfer Coding
An origin server sends Digest
as trailer field, so it can calculate digest-value
while streaming content and thus mitigate resource consumption.
The Digest
field-value is the same as in Section 10.1 because Digest
is designed to
be independent from the use of one or more transfer codings (see Section 2).¶
Request:¶
Response:¶
11. Examples of Want-Digest Solicited Digest
The following examples demonstrate interactions where a client solicits a
Digest
using Want-Digest
.¶
Some examples include JSON objects in the content. For presentation purposes, objects that fit completely within the line-length limits are presented on a single line using compact notation with no leading space. Objects that would exceed line-length limits are presented across multiple lines (one line per key-value pair) with 2 spaced of leading indentation.¶
Digest
is media-type agnostic
and does not provide canonicalization algorithms for specific formats.
Examples of Digest
are calculated inclusive of any space.¶
11.1. Server Selects Client's Least Preferred Algorithm
The client requests a digest, preferring "sha". The server is free to reply with "sha-256" anyway.¶
Request:¶
Response:¶
11.2. Server Selects Algorithm Unsupported by Client
The client requests a "sha" digest only. The server is currently free to reply with a Digest containing an unsupported algorithm.¶
Request:¶
Response:¶
12. Security Considerations
12.1. Digest Does Not Protect the Full HTTP Message
This document specifies a data integrity mechanism that protects HTTP
representation data
, but not HTTP representation metadata
fields, from
certain kinds of accidental corruption.¶
Digest
is not intended to be a general protection against malicious tampering with
HTTP messages. This can be achieved by combining it with other approaches such
as transport-layer security or digital signatures.¶
12.2. Broken Cryptographic Algorithms
Cryptographic algorithms are intended to provide a proof of integrity suited towards cryptographic constructions such as signatures.¶
However, these rely on collision-resistance for their security proofs
[CMU-836068]. The "md5" and "sha" digest-algorithms are vulnerable to collisions attacks,
so they MUST NOT be used with Digest
.¶
12.3. Other Deprecated Algorithms
The ADLER32 algorithm defined in [RFC1950] has been deprecated by [RFC3309]
because, under certain conditions, it provides weak detection of errors. It is now
NOT RECOMMENDED for use with Digest
.¶
12.4. Digest for End-to-End Integrity
Digest
only covers the representation data
and not the
representation metadata
. Digest
could help protect the representation data
from buggy manipulation, undesired "transforming proxies" (see Section 7.7 of [SEMANTICS])
or other actions as the data passes across multiple hops or system boundaries.
Even a simple mechanism for end-to-end representation data
integrity is valuable
because user-agent can validate that resource retrieval succeeded before handing off to a
HTML parser, video player etc. for parsing.¶
Identity digest-algorithms (e.g. "id-sha-256" and "id-sha-512") are particularly useful for end-to-end integrity because they allow piecing together a resource from different sources with different HTTP messaging characteristics. For example, different servers that apply different content codings.¶
Note that using Digest
alone does not provide end-to-end integrity of HTTP messages over
multiple hops, since metadata could be manipulated at any stage. Methods to protect
metadata are discussed in Section 12.6.¶
12.5. Digest and Content-Location in Responses
When a state-changing method returns the Content-Location
header field, the
enclosed representation refers to the resource identified by its value and
Digest
is computed accordingly.¶
12.6. Usage in Signatures
Digital signatures are widely used together with checksums to provide the
certain identification of the origin of a message [NIST800-32]. Such signatures
can protect one or more HTTP fields and there are additional considerations when
Digest
is included in this set.¶
Since the Digest
field is a hash of a resource representation, it explicitly
depends on the representation metadata
(eg. the values of Content-Type
,
Content-Encoding
etc). A signature that protects Digest
but not other
representation metadata
can expose the communication to tampering. For
example, an actor could manipulate the Content-Type
field-value and cause a
digest validation failure at the recipient, preventing the application from
accessing the representation. Such an attack consumes the resources of both
endpoints. See also Section 12.5.¶
Digest
SHOULD always be used over a connection that provides integrity at
the transport layer that protects HTTP fields.¶
A Digest
field using NOT RECOMMENDED digest-algorithms SHOULD NOT be used in
signatures.¶
Using signatures to protect the Digest
of an empty representation
allows receiving endpoints to detect if an eventual payload has been stripped or added.¶
Any mangling of Digest
, including de-duplication of representation-data-digest values
or combining different field values (see Section 5.2 of [SEMANTICS])
might affect signature validation.¶
12.7. Usage in Trailer Fields
When Digest
is used in trailer fields, the receiver gets the digest value after the content
and may thus be tempted to process the data before validating the digest value.
It is prefereable that data is only be processed after validating the Digest.¶
If received in trailers, Digest
MUST NOT be discarded;
instead, it MAY be merged in the header section (See Section 6.5.1 of [SEMANTICS]).¶
Not every digest-algorithm is suitable for use in the trailer section, some may require to pre-process the whole payload before sending a message (eg. see [I-D.thomson-http-mice]).¶
12.8. Usage with Encryption
Digest
may expose details of encrypted payload when the checksum
is computed on the unencrypted data.
For example, the use of the "id-sha-256" digest-algorithm
in conjunction with the encrypted content-coding [RFC8188].¶
The representation-data-digest of an encrypted payload can change between different messages depending on the encryption algorithm used; in those cases its value could not be used to provide a proof of integrity "at rest" unless the whole (e.g. encoded) content is persisted.¶
12.9. Algorithm Agility
The security properties of digest-algorithms are not fixed. Algorithm Agility (see [RFC7696]) is achieved by providing implementations with flexibility choose digest-algorithms from the IANA Digest Algorithm Values registry in Section 13.1.¶
To help endpoints understand weaker algorithms from stronger ones, this document adds to the IANA Digest Algorithm Values registry a new "Status" field containing the most-recent appraisal of the digest-algorithm; the allowed values are specified in Section 13.2.¶
An endpoint might have a preference for algorithms,
such as preferring "standard" algorithms over "deprecated" ones.
Transition from weak algorithms is supported
by negotiation of digest-algorithm using Want-Digest
(see Section 4)
or by sending multiple representation-data-digest values from which the receiver chooses.
Endpoints are advised that sending multiple values consumes resources,
which may be wasted if the receiver ignores them (see Section 3).¶
12.9.1. Duplicate digest-algorithm in field value
An endpoint might receive multiple representation-data-digest values (see Section 3) that use the same digest-algorithm with different or identical digest-values. For example:¶
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=, sha-256=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=¶
A receiver is permitted to ignore any representation-data-digest value, so validation of duplicates is left as an implementation decision. Endpoints might select all, some or none of the values for checksum comparison and, based on the intersection of those results, conditionally pass or fail digest validation.¶
12.10. Resource exhaustion
Digest
validation consumes computational resources.
In order to avoid resource exhaustion, implementations can restrict
validation of the algorithm types, number of validations, or the size of content.¶
13. IANA Considerations
13.1. Establish the HTTP Digest Algorithm Values Registry
This memo sets this specification to be the establishing document for the HTTP Digest Algorithm Values registry.¶
13.2. The "status" Field in the HTTP Digest Algorithm Values Registry
This memo adds the field "Status" to the HTTP Digest Algorithm Values registry. The allowed values for the "Status" fields are described below.¶
- Status
13.3. Deprecate "MD5" Digest Algorithm
This memo updates the "MD5" digest-algorithm in the HTTP Digest Algorithm Values registry:¶
13.4. Update "UNIXsum" Digest Algorithm
This memo updates the "UNIXsum" digest-algorithm in the HTTP Digest Algorithm Values registry:¶
13.5. Update "UNIXcksum" Digest Algorithm
This memo updates the "UNIXcksum" digest-algorithm in the HTTP Digest Algorithm Values registry:¶
13.6. Update "CRC32c" Digest Algorithm
This memo updates the "CRC32c" digest-algorithm in the HTTP Digest Algorithm Values registry:¶
- Digest Algorithm: crc32c¶
- Description: The CRC32c algorithm is a 32-bit cyclic redundancy check. It achieves a better hamming distance (for better error-detection performance) than many other 32-bit CRC functions. Other places it is used include iSCSI and SCTP. The 32-bit output is encoded in hexadecimal (using between 1 and 8 ASCII characters from 0-9, A-F, and a-f; leading 0's are allowed). For example, crc32c=0a72a4df and crc32c=A72A4DF are both valid checksums for the 3-byte message "dog".¶
- Reference: [RFC4960] appendix B, this document.¶
- Status: standard.¶
13.7. Deprecate "SHA" Digest Algorithm
This memo updates the "SHA" digest-algorithm in the HTTP Digest Algorithm Values registry:¶
13.8. Obsolete "ADLER32" Digest Algorithm
This memo updates the "ADLER32" digest-algorithm in the HTTP Digest Algorithm Values registry:¶
- Digest Algorithm: adler32¶
- Description: The ADLER32 algorithm is a checksum specified in [RFC1950] "ZLIB Compressed Data Format". The 32-bit output is encoded in hexadecimal (using between 1 and 8 ASCII characters from 0-9, A-F, and a-f; leading 0's are allowed). For example, adler32=03da0195 and adler32=3DA0195 are both valid checksums for the 4-byte message "Wiki". This algorithm is obsoleted and SHOULD NOT be used.¶
- Status: obsoleted¶
13.9. Obsolete "contentMD5" token in Digest Algorithm
This memo adds the "contentMD5" token in the HTTP Digest Algorithm Values registry:¶
13.10. The "id-sha-256" Digest Algorithm
This memo registers the "id-sha-256" digest-algorithm in the HTTP Digest Algorithm Values registry:¶
13.11. The "id-sha-512" Digest Algorithm
This memo registers the "id-sha-512" digest-algorithm in the HTTP Digest Algorithm Values registry:¶
13.12. Changes Compared to RFC5843
The digest-algorithm values for "MD5", "SHA", "SHA-256", "SHA-512", "UNIXcksum", "UNIXsum", "ADLER32" and "CRC32c" have been updated to lowercase.¶
The status of "MD5" has been updated to "deprecated", and its description states that this algorithm MUST NOT be used.¶
The status of "SHA" has been updated to "deprecated", and its description states that this algorithm MUST NOT be used.¶
The status for "CRC2c", "UNIXsum" and "UNIXcksum" has been updated to "standard".¶
The "id-sha-256" and "id-sha-512" algorithms have been added to the registry.¶
14. References
14.1. Normative References
- [CMU-836068]
- Carnagie Mellon University, Software Engineering Institute, "MD5 Vulnerable to collision attacks", , <https://www.kb.cert.org/vuls/id/836068/>.
- [IACR-2020-014]
- Leurent, G. and T. Peyrin, "SHA-1 is a Shambles", , <https://eprint.iacr.org/2020/014.pdf>.
- [NIST800-32]
- National Institute of Standards and Technology, U.S. Department of Commerce, "Introduction to Public Key Technology and the Federal PKI Infrastructure", , <https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-32.pdf>.
- [RFC1321]
- Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, DOI 10.17487/RFC1321, , <https://www.rfc-editor.org/rfc/rfc1321>.
- [RFC1950]
- Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format Specification version 3.3", RFC 1950, DOI 10.17487/RFC1950, , <https://www.rfc-editor.org/rfc/rfc1950>.
- [RFC2119]
- Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
- [RFC3174]
- Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, DOI 10.17487/RFC3174, , <https://www.rfc-editor.org/rfc/rfc3174>.
- [RFC3230]
- Mogul, J. and A. Van Hoff, "Instance Digests in HTTP", RFC 3230, DOI 10.17487/RFC3230, , <https://www.rfc-editor.org/rfc/rfc3230>.
- [RFC3309]
- Stone, J., Stewart, R., and D. Otis, "Stream Control Transmission Protocol (SCTP) Checksum Change", RFC 3309, DOI 10.17487/RFC3309, , <https://www.rfc-editor.org/rfc/rfc3309>.
- [RFC4648]
- Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, , <https://www.rfc-editor.org/rfc/rfc4648>.
- [RFC4960]
- Stewart, R., Ed., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, , <https://www.rfc-editor.org/rfc/rfc4960>.
- [RFC5234]
- Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, , <https://www.rfc-editor.org/rfc/rfc5234>.
- [RFC5843]
- Bryan, A., "Additional Hash Algorithms for HTTP Instance Digests", RFC 5843, DOI 10.17487/RFC5843, , <https://www.rfc-editor.org/rfc/rfc5843>.
- [RFC6234]
- Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI 10.17487/RFC6234, , <https://www.rfc-editor.org/rfc/rfc6234>.
- [RFC7405]
- Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC 7405, DOI 10.17487/RFC7405, , <https://www.rfc-editor.org/rfc/rfc7405>.
- [RFC8174]
- Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/rfc8174>.
- [SEMANTICS]
- Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP Semantics", Work in Progress, Internet-Draft, draft-ietf-httpbis-semantics-15, , <https://tools.ietf.org/html/draft-ietf-httpbis-semantics-15>.
- [UNIX]
- The Open Group, "The Single UNIX Specification, Version 2 - 6 Vol Set for UNIX 98", .
14.2. Informative References
- [HTTP11]
- Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP/1.1", Work in Progress, Internet-Draft, draft-ietf-httpbis-messaging-15, , <https://tools.ietf.org/html/draft-ietf-httpbis-messaging-15>.
- [I-D.ietf-httpbis-header-structure]
- Nottingham, M. and P. Kamp, "Structured Field Values for HTTP", Work in Progress, Internet-Draft, draft-ietf-httpbis-header-structure-19, , <https://tools.ietf.org/html/draft-ietf-httpbis-header-structure-19>.
- [I-D.thomson-http-mice]
- Thomson, M. and J. Yasskin, "Merkle Integrity Content Encoding", Work in Progress, Internet-Draft, draft-thomson-http-mice-03, , <https://tools.ietf.org/html/draft-thomson-http-mice-03>.
- [NO-MD5]
- Turner, S. and L. Chen, "Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms", RFC 6151, DOI 10.17487/RFC6151, , <https://www.rfc-editor.org/rfc/rfc6151>.
- [NO-SHA1]
- Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security Considerations for the SHA-0 and SHA-1 Message-Digest Algorithms", RFC 6194, DOI 10.17487/RFC6194, , <https://www.rfc-editor.org/rfc/rfc6194>.
- [PATCH]
- Dusseault, L. and J. Snell, "PATCH Method for HTTP", RFC 5789, DOI 10.17487/RFC5789, , <https://www.rfc-editor.org/rfc/rfc5789>.
- [RFC2818]
- Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/RFC2818, , <https://www.rfc-editor.org/rfc/rfc2818>.
- [RFC7231]
- Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, , <https://www.rfc-editor.org/rfc/rfc7231>.
- [RFC7396]
- Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396, DOI 10.17487/RFC7396, , <https://www.rfc-editor.org/rfc/rfc7396>.
- [RFC7696]
- Housley, R., "Guidelines for Cryptographic Algorithm Agility and Selecting Mandatory-to-Implement Algorithms", BCP 201, RFC 7696, DOI 10.17487/RFC7696, , <https://www.rfc-editor.org/rfc/rfc7696>.
- [RFC7807]
- Nottingham, M. and E. Wilde, "Problem Details for HTTP APIs", RFC 7807, DOI 10.17487/RFC7807, , <https://www.rfc-editor.org/rfc/rfc7807>.
- [RFC8188]
- Thomson, M., "Encrypted Content-Encoding for HTTP", RFC 8188, DOI 10.17487/RFC8188, , <https://www.rfc-editor.org/rfc/rfc8188>.
- [SRI]
- Akhawe, D., Braun, F., Marier, F., and J. Weinberger, "Subresource Integrity", W3C Recommendation REC-SRI-20160623, , <https://www.w3.org/TR/2016/REC-SRI-20160623/>.
Appendix A. Resource Representation and Representation-Data
The following examples show how representation metadata, payload transformations and method impacts on the message and content. When the content contains non-printable characters (eg. when it is compressed) it is shown as base64-encoded string.¶
A request with a JSON object without any content coding.¶
Request:¶
Here is a gzip-compressed JSON object using a content coding.¶
Request:¶
Now the same content conveys a malformed JSON object.¶
Request:¶
A Range-Request alters the content, conveying a partial representation.¶
Request:¶
Response:¶
Now the method too alters the content.¶
Request:¶
Response:¶
Finally the semantics of an HTTP response might decouple the effective request URI
from the enclosed representation. In the example response below, the
Content-Location
header field indicates that the enclosed representation
refers to the resource available at /authors/123
.¶
Request:¶
Response:¶
Appendix B. FAQ
-
Why remove all references to content-md5?¶
Those were unnecessary to understanding and using this specification.¶
-
Why remove references to instance manipulation?¶
Those were unnecessary for correctly using and applying the specification. An example with Range Request is more than enough. This document uses the term "partial representation" which should group all those cases.¶
-
How to use
Digest
withPATCH
method?¶ -
Why remove references to delta-encoding?¶
Unnecessary for a correct implementation of this specification. The revised specification can be nicely adapted to "delta encoding", but all the references here to delta encoding don't add anything to this RFC. Another job would be to refresh delta encoding.¶
-
Why remove references to Digest Authentication?¶
This specification seems to me completely unrelated to Digest Authentication but for the word "Digest".¶
-
What changes in
Want-Digest
?¶The contentMD5 token defined in Section 5 of [RFC3230] is deprecated by Section 7.¶
To clarify that
Digest
andWant-Digest
can be used in both requests and responses - [RFC3230] carefully usessender
andreceiver
in their definition - we added examples on usingWant-Digest
in responses to advertise the supported digest-algorithms and the inability to accept requests with unsupported digest-algorithms.¶ -
Does this specification change supported algorithms?¶
Yes. This RFC updates [RFC5843] which is still delegated for all algorithms updates, and adds two more algorithms: "id-sha-256" and "id-sha-512" which allows to send a checksum of a resource representation with no content codings applied. To simplify a future transition to Structured Fields [I-D.ietf-httpbis-header-structure] we suggest to use lowercase for digest-algorithms.¶
-
What about mid-stream trailer fields?¶
While mid-stream trailer fields are interesting, since this specification is a rewrite of [RFC3230] we do not think we should face that. As a first thought, nothing in this document precludes future work that would find a use for mid-stream trailers, for example an incremental digest-algorithm. A document defining such a digest-algorithm is best positioned to describe how it is used.¶
Acknowledgements
The vast majority of this document is inherited from [RFC3230], so thanks to J. Mogul and A. Van Hoff for their great work. The original idea of refreshing this document arose from an interesting discussion with M. Nottingham, J. Yasskin and M. Thomson when reviewing the MICE content coding.¶
Code Samples
RFC Editor: Please remove this section before publication.¶
How can I generate and validate the Digest
values shown in the examples
throughout this document?¶
The following python3 code can be used to generate digests for JSON objects using SHA algorithms for a range of encodings. Note that these are formatted as base64. This function could be adapted to other algorithms and should take into account their specific formatting rules.¶
import base64, json, hashlib, brotli, logging log = logging.getLogger() def encode_item(item, encoding=lambda x: x): indent = 2 if isinstance(item, dict) and len(item) > 1 else None json_bytes = json.dumps(item, indent=indent).encode() return encoding(json_bytes) def digest_bytes(bytes_, algorithm=hashlib.sha256): checksum_bytes = algorithm(bytes_).digest() log.warning("Log bytes: \n[%r]", bytes_) return base64.encodebytes(checksum_bytes).strip() def digest(item, encoding=lambda x: x, algorithm=hashlib.sha256): content_encoded = encode_item(item, encoding) return digest_bytes(content_encoded, algorithm) item = {"hello": "world"} print("Encoding | digest-algorithm | digest-value") print("Identity | sha256 |", digest(item)) # Encoding | digest-algorithm | digest-value # Identity | sha256 | X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE= print("Encoding | digest-algorithm | digest-value") print("Brotli | sha256 |", digest(item, encoding=brotli.compress)) # Encoding | digest-algorithm | digest-value # Brotli | sha256 | 4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo= print("Encoding | digest-algorithm | digest-value") print("Identity | sha512 |", digest(item, algorithm=hashlib.sha512)) # Encoding | digest-algorithm | digest-value # Identity | sha512 | b'WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm' # '+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew=='¶
Changes
RFC Editor: Please remove this section before publication.¶
Since draft-ietf-httpbis-digest-headers-02
- Deprecate SHA-1 #1154¶
- Avoid id-* with encrypted content¶
- Digest is independent from MESSAGING and HTTP/1.1 is not normative #1215¶
- Identity is not a valid field value for content-encoding #1223¶
- Mention trailers #1157¶
- Reference httpbis-semantics #1156¶
- Add contentMD5 as an obsoleted digest-algorithm #1249¶
- Use lowercase digest-algorithms names in the doc and in the digest-algorithm IANA table.¶
Since draft-ietf-httpbis-digest-headers-00
- Align title with document name¶
- Add id-sha-* algorithm examples #880¶
- Reference [RFC6234] and [RFC3174] instead of FIPS-1¶
- Deprecate MD5¶
- Obsolete ADLER-32 but don't forbid it #828¶
- Update CRC32C value in IANA table #828¶
- Use when acting on resources (POST, PATCH) #853¶
- Added Relationship with SRI, draft Use Cases #868, #971¶
- Warn about the implications of
Content-Location
¶