CBOR Object Signing and Encryption (COSE): Hash Algorithms
draft-ietf-cose-hash-algs-02
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (cose WG) | |
|---|---|---|---|
| Author | Jim Schaad | ||
| Last updated | 2020-01-28 (Latest revision 2019-11-04) | ||
| Replaces | draft-schaad-cose-hash-algs | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text html xml htmlized pdfized bibtex | ||
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| Stream | WG state | WG Consensus: Waiting for Write-Up | |
| Document shepherd | Ivaylo Petrov | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | Ivaylo Petrov <ivaylo@ackl.io> |
draft-ietf-cose-hash-algs-02
Network Working Group J. Schaad
Internet-Draft August Cellars
Intended status: Informational 4 November 2019
Expires: 7 May 2020
CBOR Object Signing and Encryption (COSE): Hash Algorithms
draft-ietf-cose-hash-algs-02
Abstract
The CBOR Object Signing and Encryption (COSE) syntax
[I-D.ietf-cose-rfc8152bis-struct] does not define any direct methods
for using hash algorithms. There are however circumstances where
hash algorithms are used: Indirect signatures where the hash of one
or more contents are signed. X.509 certificate or other object
identification by the use of a thumbprint. This document defines a
set of hash algorithms that are identified by COSE Algorithm
Identifiers.
Contributing to this document
This note is to be removed before publishing as an RFC.
The source for this draft is being maintained in GitHub. Suggested
changes should be submitted as pull requests at https://github.com/
cose-wg/X509 Editorial changes can be managed in GitHub, but any
substantial issues need to be discussed on the COSE mailing list.
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 7 May 2020.
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Copyright Notice
Copyright (c) 2019 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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Terminology . . . . . . . . . . . . . . . . 3
1.2. Open Issues . . . . . . . . . . . . . . . . . . . . . . . 3
2. Hash Algorithm Usage . . . . . . . . . . . . . . . . . . . . 3
2.1. Example CBOR hash structure . . . . . . . . . . . . . . . 4
3. Hash Algorithm Identifiers . . . . . . . . . . . . . . . . . 5
3.1. SHA-1 Hash Algorithm . . . . . . . . . . . . . . . . . . 5
3.2. SHA-2 Hash Algorithms . . . . . . . . . . . . . . . . . . 6
3.3. SHAKE Algorithms . . . . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
4.1. COSE Algorithm Registry . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. Normative References . . . . . . . . . . . . . . . . . . . . 9
7. Informative References . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The CBOR Object Signing and Encryption (COSE) syntax does not define
any direct methods for the use of hash algorithms. It also does not
define a structure syntax that is used to encode a digested object
structure along the lines of the DigestedData ASN.1 structure in
[CMS]. This omission was intentional as a structure consisting of
just a digest identifier, the content, and a digest value does not by
itself provide any strong security service. Additionally, an
application is going to be better off defining this type of structure
so that it can include any additional data that needs to be hashed,
as well as methods of obtaining the data.
While the above is true, there are some cases where having some
standard hash algorithms defined for COSE with a common identifier
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makes a great deal of sense. Two of the cases where these are going
to be used are:
* Indirect signing of content, and
* Object identification.
Indirect signing of content is a paradigm where the content is not
directly signed, but instead a hash of the content is computed and
that hash value, along with the hash algorithm, is included in the
content that will be signed. Doing indirect signing allows for a
signature to be validated without first downloading all of the
content associated with the signature. This capability can be of
even greater importance in a constrained environment as not all of
the content signed may be needed by the device.
The use of hashes to identify objects is something that has been very
common. One of the primary things that has been identified by a hash
function for secure message is a certificate. Two examples of this
can be found in [ESS] and the newly defined COSE equivalents in
[I-D.ietf-cose-x509].
1.1. Requirements Terminology
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.
1.2. Open Issues
This section is to be removed before publishing as an RFC.
* No Open Issues
2. Hash Algorithm Usage
As noted in the previous section, hash functions can be used for a
variety of purposes. Some of these purposes require that a hash
function be cryptographically strong, these include direct and
indirect signatures. That is, using the hash as part of the
signature or using the hash as part of the body to be signed. Other
uses of hash functions do not require the same level of strength.
This document contains some hash functions that are not designed to
be used for cryptographic operations. An application that is using a
hash function needs to carefully evaluate exactly what hash
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properties are needed and which hash functions are going to provide
them. Applications should also make sure that the ability to change
hash functions is part of the base design as cryptographic advances
are sure to reduce the strength of a hash function.
A hash function is a map from one, normally large, bit string to a
second, usually smaller, bit string. There are going to be
collisions by a hash function, the trick is to make sure that it is
difficult to find two values that are going to map to the same output
value. The standard "Collision Attack" is one where an attacker can
find two different messages that have the same hash value. If a
collision attack exists, then the function SHOULD NOT be used for a
cryptographic purpose. The only reason why such a hash function is
used is when there is absolutely no other choice (e.g. a Hardware
Security Module (HSM) that cannot be replaced), and only after
looking at the possible security issues. Cryptographic purposes
would include the creation of signatures or the use of hashes for
indirect signatures. These functions may still be usable for non-
cryptographic purposes.
An example of a non-cryptographic use of a hash is for filtering from
a collection of values to find possible candidates that can later be
checked to see if they are the correct one. A simple example of this
is the classic thumbprint of a certificate. If the thumbprint is
used to verify that it is the correct certificate, then that usage is
subject to a collision attack as above. If however, the thumbprint
is used to sort through a collection of certificates to find those
that might be used for the purpose of verifying a signature, a simple
filter capability is sufficient. In this case, one still needs to
validate that the public key validates the signature (and the
certificate is trusted), and all certificates that don't contain a
key that validates the signature can be discarded as false positives.
To distinguish between these two cases, a new value in the
recommended column of the COSE Algorithms registry is to be added.
"Filter Only" indicates that the only purpose of a hash function
should be to filter results and not those which require collision
resistance.
2.1. Example CBOR hash structure
[COSE] did not provide a default structure for holding a hash value
not only because no separate hash algorithms were defined, but
because how the structure is setup is frequently application
specific. There are four fields that are often included as part of a
hash structure:
* The hash algorithm identifier.
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* The hash value.
* A pointer to the value that was hashed, this could be a pointer to
a file, an object that can be obtained from the network, or a
pointer to someplace in the message, or something very application
specific.
* Additional data, this can be something as simple as a random value
to make finding hash collisions slightly harder as the value
handed to the application cannot have been selected to have a
collision, or as complicated as a set of processing instructions
that are used with the object that is pointed to. The additional
data can be dealt with in a number of ways, prepending or
appending to the content, but it is strongly suggested to it
either be a fixed known size, or the lengths of the pieces being
hashed be included. (Encoding as a CBOR array accomplished this
requirement.)
An example of a structure which permits all of the above fields to
exist would look like the following.
COSE_Hash_V = ( 1 : int / tstr, # Algorithm identifier 2 : bstr, # Hash value 3 : tstr ?, # Location of object hashed 4 : any ? # object containing other details and things )
An alternate structure that could be used for situations where one is
searching a group of objects for a match. In this case, the location
would not be needed and adding extra data to the hash would be
counterproductive. This results in a structure that looks like this:
COSE_Hash_Find = [ hashAlg : int / tstr, hashValue : bstr ]
3. Hash Algorithm Identifiers
3.1. SHA-1 Hash Algorithm
The SHA-1 hash algorithm [RFC3174] was designed by the United States
National Security Agency and published in 1995. Since that time a
large amount of cryptographic analysis has been applied to this
algorithm and a successful collision attack has been created
([SHA-1-collision]). The IETF formally started discouraging the use
of SHA-1 with the publishing of [RFC6194].
Despite the above, there are still times where SHA-1 needs to be used
and therefore it makes sense to assign a point for the use of this
hash algorithm. Some of these situations are with historic HSMs
where only SHA-1 is implemented or where the SHA-1 value is used for
the purpose of filtering and thus the collision resistance property
is not needed.
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Because of the known issues for SHA-1 and the fact that is should no
longer be used, the algorithm will be registered with the
recommendation of "Filter Only".
+-------+-------+-------------+-----------------+-------------+
| Name | Value | Description | Reference | Recommended |
+=======+=======+=============+=================+=============+
| SHA-1 | TBD6 | SHA-1 Hash | [This Document] | Filter Only |
+-------+-------+-------------+-----------------+-------------+
Table 1: SHA-1 Hash Algorithm
3.2. SHA-2 Hash Algorithms
The family of SHA-2 hash algorithms [FIPS-180-4] was designed by the
United States National Security Agency and published in 2001. Since
that time some additional algorithms have been added to the original
set to deal with length extension attacks and some performance
issues. While the SHA-3 hash algorithms have been published since
that time, the SHA-2 algorithms are still broadly used.
There are a number of different parameters for the SHA-2 hash
functions. The set of hash functions which have been chosen for
inclusion in this document are based on those different parameters
and some of the trade-offs involved.
* *SHA-256/64* provides a truncated hash. The length of the
truncation is designed to allow for smaller transmission size.
The trade-off is that the odds that a collision will occur
increase proportionally. Locations that use this hash function
need either to analysis the potential problems with having a
collision occur, or where the only function of the hash is to
narrow the possible choices.
The latter is the case for [I-D.ietf-cose-x509], the hash value is
used to select possible certificates and, if there are multiple
choices then, each choice can be tested by using the public key.
* *SHA-256* is probably the most common hash function used
currently. SHA-256 is an efficient hash algorithm for 32-bit
hardware.
* *SHA-384* and *SHA-512* hash functions are efficient for 64-bit
hardware.
* *SHA-512/256* provides a hash function that runs more efficiently
on 64-bit hardware, but offers the same security levels as SHA-
256.
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+-------------+-------+----------------+-----------+-------------+
| Name | Value | Description | Reference | Recommended |
+=============+=======+================+===========+=============+
| SHA-256/64 | TBD1 | SHA-2 256-bit | [This | Filter Only |
| | | Hash truncated | Document] | |
| | | to 64-bits | | |
+-------------+-------+----------------+-----------+-------------+
| SHA-256 | TBD2 | SHA-2 256-bit | [This | Yes |
| | | Hash | Document] | |
+-------------+-------+----------------+-----------+-------------+
| SHA-384 | TBD3 | SHA-2 384-bit | [This | Yes |
| | | Hash | Document] | |
+-------------+-------+----------------+-----------+-------------+
| SHA-512 | TBD4 | SHA-2 512-bit | [This | Yes |
| | | Hash | Document] | |
+-------------+-------+----------------+-----------+-------------+
| SHA-512/256 | TBD5 | SHA-2 512-bit | [This | Yes |
| | | Hash truncated | Document] | |
| | | to 256-bits | | |
+-------------+-------+----------------+-----------+-------------+
Table 2: SHA-2 Hash Algorithms
3.3. SHAKE Algorithms
The family SHA-3 hash algorithms [FIPS-202] was the result of a
competition run by NIST. The pair of algorithms known as SHAKE-128
and SHAKE-256 are the instances of SHA-3 that are currently being
standardized in the IETF.
The SHA-3 hash algorithms have a significantly different structure
than the SHA-2 hash algorithms. One of the benefits of this
differences is that when computing a shorter SHAKE hash value, the
value is not a prefix of the result of computing the longer hash.
Unlike the SHA-2 hash functions, no algorithm identifier is created
for shorter lengths. Applications can specify a minimum length for
any hash function. A validator can infer the actual length from the
hash value in these cases.
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+----------+-------+---------------+-----------------+-------------+
| Name | Value | Description | Reference | Recommended |
+==========+=======+===============+=================+=============+
| SHAKE128 | TBD10 | 128-bit SHAKE | [This Document] | Yes |
+----------+-------+---------------+-----------------+-------------+
| SHAKE256 | TBD11 | 256-bit SHAKE | [This Document] | Yes |
+----------+-------+---------------+-----------------+-------------+
Table 3: SHAKE Hash Functions
4. IANA Considerations
4.1. COSE Algorithm Registry
IANA is requested to register the following algorithms in the "COSE
Algorithms" registry.
* The SHA-1 hash function found in Table 1.
* The set of SHA-2 hash functions found in Table 2.
* The set of SHAKE hash functions found in Table 3.
Many of the hash values produced are relatively long and as such the
use of a two byte algorithm identifier seems reasonable. SHA-1 is
tagged as deprecated and thus a longer algorithm identifier is
appropriate even though it is a shorter hash value.
In addition, IANA is to add the value of 'Filter Only' to the set of
legal values for the 'Recommended' column. This value is only to be
used for hash functions and indicates that it is not to be used for
purposes which require collision resistance.
5. Security Considerations
The security considerations have already been called out as part of
the previous text. The following issues need to be dealt with:
* Protocols need to perform a careful analysis of the properties of
a hash function that are needed and how they map onto the possible
attacks. In particular, one needs to distinguish between those
uses that need the cryptographic properties, i.e. collision
resistance, and properties that correspond to possible object
identification. The different attacks correspond to who or what
is being protected, is it the originator that is the attacker or a
third party? This is the difference between collision resistance
and second pre-image resistance. As a general rule, longer hash
values are "better" than short ones, but trade-offs of
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transmission size, timeliness, and security all need to be
included as part of this analysis. In many cases the value being
hashed is a public value, as such pre-image resistance is not part
of this analysis.
* Algorithm agility needs to be considered a requirement for any use
of hash functions. As with any cryptographic function, hash
functions are under constant attack and the strength of hash
algorithms will be reduced over time.
6. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[I-D.ietf-cose-rfc8152bis-struct]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", Work in Progress, Internet-Draft,
draft-ietf-cose-rfc8152bis-struct-06, 11 September 2019,
<https://tools.ietf.org/html/draft-ietf-cose-rfc8152bis-
struct-06>.
[FIPS-180-4]
National Institute of Standards and Technology, "Secure
Hash Standard", FIPS PUB 180-4, August 2015.
[FIPS-202] National Institute of Standards and Technology, "SHA-3
Standard: Permutation-Based Hash and Extendable-Output
Functions", FIPS PUB 202, August 2015.
[COSE] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
7. Informative References
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/info/rfc5652>.
[ESS] Hoffman, P., Ed., "Enhanced Security Services for S/MIME",
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RFC 2634, DOI 10.17487/RFC2634, June 1999,
<https://www.rfc-editor.org/info/rfc2634>.
[I-D.ietf-cose-x509]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Headers for carrying and referencing X.509 certificates",
Work in Progress, Internet-Draft, draft-ietf-cose-x509-04,
12 September 2019,
<https://tools.ietf.org/html/draft-ietf-cose-x509-04>.
[RFC3174] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
(SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,
<https://www.rfc-editor.org/info/rfc3174>.
[RFC6194] 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, March 2011,
<https://www.rfc-editor.org/info/rfc6194>.
[SHA-1-collision]
Stevens, M., Bursztein, E., Karpman, P., Albertini, A.,
and Y. Markov, "The first collision for full SHA-1",
February 2017,
<https://shattered.io/static/shattered.pdf>.
Author's Address
Jim Schaad
August Cellars
Email: ietf@augustcellars.com
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