HKDF Algorithm IDs: Algorithm Identifiers for the HMAC-based Extract-and-Expand Key Derivation Function (HKDF)
Gives HKDF, a key derivation function, three official names so protocols can say which hash it uses.
Housley, R., "Algorithm Identifiers for the HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC 8619, DOI 10.17487/RFC8619, June 2019, <https://www.rfc-editor.org/info/rfc8619>.
@misc{rfc8619,
series = {Request for Comments},
number = 8619,
howpublished = {RFC 8619},
publisher = {RFC Editor},
doi = {10.17487/RFC8619},
url = {https://www.rfc-editor.org/info/rfc8619},
author = {R. Housley},
title = {{Algorithm Identifiers for the HMAC-based Extract-and-Expand Key Derivation Function (HKDF)}},
pagetotal = 6,
year = 2019,
month = jun,
} RFC text
Contents
Internet Engineering Task Force (IETF) R. Housley
Request for Comments: 8619 Vigil Security
Category: Standards Track June 2019
ISSN: 2070-1721
Algorithm Identifiers for the
HMAC-based Extract-and-Expand Key Derivation Function (HKDF)
Abstract
RFC 5869 specifies the HMAC-based Extract-and-Expand Key Derivation
Function (HKDF) algorithm. This document assigns algorithm
identifiers to the HKDF algorithm when used with three common one-way
hash functions.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8619.
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. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. ASN.1 . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. HKDF Algorithm Identifiers . . . . . . . . . . . . . . . . . 2
3. ASN.1 Module . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
The HKDF algorithm [RFC5869] is a key derivation function based on
the Hashed Message Authentication Code (HMAC). This document assigns
algorithm identifiers to the HKDF algorithm when used with three
common one-way hash functions. These algorithm identifiers are
needed to make use of the HKDF in some security protocols, such as
the Cryptographic Message Syntax (CMS) [RFC5652].
1.1. 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. ASN.1
In this specification, values are generated using ASN.1 [X.680] using
the Basic Encoding Rules (BER) and the Distinguished Encoding Rules
(DER) [X.690].
2. HKDF Algorithm Identifiers
This section assigns three algorithm identifiers to HKDF [RFC5869]
used with three common one-way hash functions that are specified in
[SHS]: SHA-256, SHA-384, and SHA-512. When any of these three object
identifiers appear within the ASN.1 type AlgorithmIdentifier, the
parameters component of that type SHALL be absent.
The specification of AlgorithmIdentifier is available in [RFC5911],
which evolved from the original definition in X.509 [X.509-88].
The assigned object identifiers are:
id-alg-hkdf-with-sha256 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 28 }
id-alg-hkdf-with-sha384 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 29 }
id-alg-hkdf-with-sha512 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 30 }
3. ASN.1 Module
This section contains the ASN.1 module for the HKDF algorithm
identifiers. This module imports types from other ASN.1 modules that
are defined in [RFC5912].
HKDF-OID-2019
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) modules(0) id-mod-hkdf-oid-2019(68) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORTS All
IMPORTS
AlgorithmIdentifier{}, KEY-DERIVATION
FROM AlgorithmInformation-2009 -- [RFC5912]
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-algorithmInformation-02(58) } ;
--
-- Object Identifiers
--
id-alg-hkdf-with-sha256 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 28 }
id-alg-hkdf-with-sha384 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 29 }
id-alg-hkdf-with-sha512 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 30 }
--
-- Key Derivation Algorithm Identifiers
--
KeyDevAlgs KEY-DERIVATION ::= {
kda-hkdf-with-sha256 |
kda-hkdf-with-sha384 |
kda-hkdf-with-sha512,
... }
kda-hkdf-with-sha256 KEY-DERIVATION ::= {
IDENTIFIER id-alg-hkdf-with-sha256
PARAMS ARE absent
SMIME-CAPS { IDENTIFIED BY id-alg-hkdf-with-sha256 } }
kda-hkdf-with-sha384 KEY-DERIVATION ::= {
IDENTIFIER id-alg-hkdf-with-sha384
PARAMS ARE absent
SMIME-CAPS { IDENTIFIED BY id-alg-hkdf-with-sha384 } }
kda-hkdf-with-sha512 KEY-DERIVATION ::= {
IDENTIFIER id-alg-hkdf-with-sha512
PARAMS ARE absent
SMIME-CAPS { IDENTIFIED BY id-alg-hkdf-with-sha512 } }
END
4. Security Considerations
Despite the simplicity of HKDF, there are many security
considerations that have been taken into account in the design and
analysis of this construction. An exposition of all of these aspects
is well beyond the scope of this document. Please refer to [EPRINT]
for detailed information, including rationale for the HKDF design.
5. IANA Considerations
One object identifier for the ASN.1 module in Section 3 was assigned
in the "SMI Security for S/MIME Module Identifiers
(1.2.840.113549.1.9.16.0)" registry [IANA-MOD]:
id-mod-hkdf-oid-2019 OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-9(9) smime(16) mod(0) 68 }
Three object identifiers for the HKDF algorithm identifiers were
assigned in the "SMI Security for S/MIME Algorithms
(1.2.840.113549.1.9.16.3)" registry [IANA-ALG]:
id-alg-hkdf-with-sha256 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 28 }
id-alg-hkdf-with-sha384 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 29 }
id-alg-hkdf-with-sha512 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 30 }
6. References
6.1. Normative References
[SHS] National Institute of Standards and Technology (NIST),
"Secure Hash Standard (SHS)", FIPS PUB 180-4,
DOI 10.6028/NIST.FIPS.180-4, August 2015.
[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>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/info/rfc5652>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.
[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>.
[X.680] ITU-T, "Information technology -- Abstract Syntax Notation
One (ASN.1): Specification of basic notation",
ITU-T Recommendation X.680, ISO/IEC 8824-1:2015, August
2015.
[X.690] ITU-T, "Information technology -- ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ITU-T Recommendation X.690, ISO/IEC 8825-1:2015,
August 2015.
6.2. Informative References
[EPRINT] Krawczyk, H., "Cryptographic Extraction and Key
Derivation: The HKDF Scheme", Proceedings of CRYPTO 2010,
August 2010, <https://eprint.iacr.org/2010/264.pdf>.
[IANA-ALG] IANA, "SMI Security for S/MIME Algorithms
(1.2.840.113549.1.9.16.3)",
<https://www.iana.org/assignments/smi-numbers/>.
[IANA-MOD] IANA, "SMI Security for S/MIME Module Identifier
(1.2.840.113549.1.9.16.0)",
<https://www.iana.org/assignments/smi-numbers/>.
[RFC5911] Hoffman, P. and J. Schaad, "New ASN.1 Modules for
Cryptographic Message Syntax (CMS) and S/MIME", RFC 5911,
DOI 10.17487/RFC5911, June 2010,
<https://www.rfc-editor.org/info/rfc5911>.
[RFC5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
DOI 10.17487/RFC5912, June 2010,
<https://www.rfc-editor.org/info/rfc5912>.
[X.509-88] CCITT, "Recommendation X.509: The Directory -
Authentication Framework", 1988.
Author's Address
Russell Housley
Vigil Security, LLC
515 Dranesville Road
Herndon, VA 20170
United States of America
Email: housley@vigilsec.com
AI ELI5
On this page
§1Why HKDF Needs Identifiers
The HMAC-based Extract-and-Expand Key Derivation Function (HKDF) is specified in RFC 5869. It is a key derivation function built on the Hashed Message Authentication Code (HMAC).
RFC 8619 does not change HKDF. It only assigns algorithm identifiers to HKDF when it is used with three common one-way hash functions. The RFC says these identifiers are needed to make use of HKDF in some security protocols, such as the Cryptographic Message Syntax (CMS) defined in RFC 5652.
Values in the document are generated with ASN.1 using the Basic Encoding Rules (BER) and the Distinguished Encoding Rules (DER).
A recipe book already describes how HKDF works. This document just writes a label on each version so other programs can point at the one they mean.
§2The Three Algorithm Identifiers
The RFC assigns three object identifiers, one for HKDF used with each hash function from the Secure Hash Standard: SHA-256, SHA-384 and SHA-512. All three sit under the same S/MIME algorithms arc (smime(16) alg(3)) and differ only in the last number: 28, 29 and 30.
When any of these identifiers appears in the ASN.1 type AlgorithmIdentifier, the parameters component of that type SHALL be absent. In other words, the identifier alone says everything: which key derivation function and which hash.
The definition of AlgorithmIdentifier is available in RFC 5911, which evolved from the original definition in X.509.
There are three name tags, one for each size of hash. The tag already says everything, so nothing else needs to be attached to it.
- 28id-alg-hkdf-with-sha256 — HKDF used with SHA-256.
- 29id-alg-hkdf-with-sha384 — HKDF used with SHA-384.
- 30id-alg-hkdf-with-sha512 — HKDF used with SHA-512.
§3The ASN.1 Module
Section 3 holds an ASN.1 module named HKDF-OID-2019. It imports AlgorithmIdentifier{} and KEY-DERIVATION from the AlgorithmInformation-2009 module, which is defined in RFC 5912.
The module repeats the three object identifiers, then defines kda-hkdf-with-sha256, kda-hkdf-with-sha384 and kda-hkdf-with-sha512 as KEY-DERIVATION objects. Each one gives its identifier, states PARAMS ARE absent and lists S/MIME capabilities identified by the same OID. A set called KeyDevAlgs collects the three and is marked extensible.
This is the official form that lists the three name tags in a way computers can read, and says none of them carries extra notes.
§4Security Considerations
The RFC is brief here. It notes that, despite the simplicity of HKDF, many security considerations were taken into account in its design and analysis, and that an exposition of all of them is well beyond the scope of the document.
Readers who want the details, including the rationale for the HKDF design, are pointed to Krawczyk's paper "Cryptographic Extraction and Key Derivation: The HKDF Scheme" (CRYPTO 2010).
The authors say the safety reasoning is long and lives somewhere else, and they tell you which paper to read.
§5IANA Registrations
One object identifier for the ASN.1 module was assigned in the "SMI Security for S/MIME Module Identifiers (1.2.840.113549.1.9.16.0)" registry: id-mod-hkdf-oid-2019, number 68.
The three algorithm identifiers (28, 29 and 30) were assigned in the "SMI Security for S/MIME Algorithms (1.2.840.113549.1.9.16.3)" registry.
The official list keeper wrote down the module and the three name tags so nobody else can use the same numbers.