Internet DRAFT - draft-housley-lamps-cms-cek-hkdf-sha256
draft-housley-lamps-cms-cek-hkdf-sha256
Network Working Group R. Housley
Internet-Draft Vigil Security
Intended status: Standards Track 3 January 2024
Expires: 6 July 2024
Encryption Key Derivation in the Cryptographic Message Syntax (CMS)
using HKDF with SHA-256
draft-housley-lamps-cms-cek-hkdf-sha256-01
Abstract
This document specifies the derivation of the content-encryption key
or the content-authenticated-encryption key in the Cryptographic
Message Syntax (CMS). The use of this mechanism provides protection
against where the attacker manipulates the content-encryption
algorithm identifier or the content-authenticated-encryption
algorithm identifier.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 6 July 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. ASN.1 . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Use of of HKDF with SHA-256 to Derive Encryption Keys . . . . 4
3. The id-alg-cek-hkdf-sha256 Algorithm Identifier . . . . . . . 4
4. SMIMECapabilities Attribute Conventions . . . . . . . . . . . 5
5. Use of HKDF with SHA-256 with CMS . . . . . . . . . . . . . . 5
5.1. Enveloped-Data Content Type . . . . . . . . . . . . . . . 5
5.2. Encrypted-Data Content Type . . . . . . . . . . . . . . . 6
5.3. Authenticated-Enveloped-Data Content Type . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9
8. Operations Considerations . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 12
Appendix B. CMS_CEK_HKDF_SHA256 Function Examples . . . . . . . 14
B.1. CMS_CEK_HKDF_SHA256 with AES-128-GCM . . . . . . . . . . 14
B.2. CMS_CEK_HKDF_SHA256 with AES-128-CBC . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
This document specifies the derivation of the content-encryption key
for the Cryptographic Message Syntax (CMS) enveloped-data content
type [RFC5652], the content-encryption key for the CMS encrypted-data
content type [RFC5652], or the content-authenticated-encryption key
for the authenticated-enveloped-data content type [RFC5083].
The use of this mechanism provides protection against where the
attacker manipulates the content-encryption algorithm identifier or
the content-authenticated-encryption algorithm identifier. Johannes
Roth and Falko Strenzke presented such an attack at IETF 118
[RS2023], where:
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1. The attacker intercepts a CMS Authenticated-Enveloped-Data
content [RFC5083] that uses either AES-CCM or AES-GCM [RFC5084].
2. The attacker turns the intercepted content into a "garbage" CMS
Enveloped-Data content Section 6 of [RFC5652] that is composed of
AES-CBC guess blocks.
3. The attacker sends the "garbage" message to the victim, and the
victim reveals the result of the decryption to the attacker.
4. If any of the transformed plaintext blocks match the guess for
that block, then the attacker learns the plaintext for that
block.
With highly structured messages, one block can reveal the only
sensitive part of the original message.
This attack is thwarted if the encryption key depends upon the
delivery of the unmodified algorithm identifier.
The mitigation for this attack has three parts:
* Potential recipients include the id-alg-cek-hkdf-sha256 algorithm
identifier (with no parameters) in S/MIME Capabilities to indicate
support for this mitigation.
* As a flag to the recipient that this mitigation is being used,
carry the id-alg-cek-hkdf-sha256 algorithm identifier as the
contentEncryptionAlgorithm in the EncryptedContentInfo structure.
This structure is used in the enveloped-data content type, the
encrypted-data content type, and the authenticated-enveloped-data
content type. The parameters field of the id-alg-cek-hkdf-sha256
algorithm identifier identifies the content-encryption algorithm
or the content-authenticated-encryption algorithm and any
associated parameters.
* Perform encryption with a derived content-encryption key or
content-authenticated-encryption key:
CEK' = HKDF(CEK, AlgorithmIdentifier)
1.1. ASN.1
CMS values are generated using ASN.1 [X680], using the Basic Encoding
Rules (BER) and the Distinguished Encoding Rules (DER) [X690].
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1.2. 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.
2. Use of of HKDF with SHA-256 to Derive Encryption Keys
The mitigation uses the HMAC-based Extract-and-Expand Key Derivation
Function (HKDF) [RFC5869] to derive output keying materiam (OKM) from
input key material (IKM). HKDF is used with the SHA-256 hash
function [FIPS180]. The derivation includes the DER-encoded
AlgoritmIdentifier as the optional info input value. This
AlgoritmIdentifier is carried as the parameter to the id-alg-cek-
hkdf-sha256 algorithm identifier. If an attacker were to change the
originator-provided AlgoritmIdentifier, then the recipient will
derive a different content-encryption key or content-authenticated-
encryption key.
The CMS_CEK_HKDF_SHA256 function uses the HKDF-Extract and HKDF-
Expand functions to derive the OKM from the IKM:
Inputs:
IKM input keying material
info DER-encoded AlgoritmIdentifier
Output:
OKM output keying material (same size as IKM)
The output OKM is calculated as follows:
OKM_SIZE = len(IKM)
IF OKM_SIZE > 8160 THEN raise error
salt = "The Cryptographic Message Syntax"
PRK = HKDF-Extract(salt, IKM)
OKM = HKDF-Expand(PRK, info, OKM_SIZE)
3. The id-alg-cek-hkdf-sha256 Algorithm Identifier
The id-alg-cek-hkdf-sha256 algoritm identifier indicates that the
CMS_CEK_HKDF_SHA256 function defined in Section 2 is used to derive
the content-encryption key or the content-authenticated-encryption
key.
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The following object identifier identifies the id-alg-cek-hkdf-sha256
algorithm:
id-alg-cek-hkdf-sha256 OBJECT IDENTIFIER ::= { TBD1 }
The id-alg-cek-hkdf-sha256 parameters field has an ASN.1 type of
AlgorithmIdentifier.
Using the conventions from [RFC5911], the id-alg-cek-hkdf-sha256
algorithm identifier is defined as:
ContentEncryptionAlgorithmIdentifier ::=
AlgorithmIdentifier{CONTENT-ENCRYPTION, { ... } }
cea-CEKHKDFSHA256 CONTENT-ENCRYPTION ::= {
IDENTIFIER id-alg-cek-hkdf-sha256
PARAMS TYPE ContentEncryptionAlgorithmIdentifier ARE required
SMIME-CAPS { IDENTIFIED BY id-alg-cek-hkdf-sha256 } }
4. SMIMECapabilities Attribute Conventions
The SMIMECapabilities Attribute is defined in Section 2.5.2 of
[RFC8551]. An S/MIME client announces the set of cryptographic
functions it supports using the SMIMECapabilities attribute.
If an S/MIME client supports the mechanism in this document, the id-
alg-cek-hkdf-sha256 object identifier SHOULD be included in the set
of cryptographic functions. The parameter with this encoding MUST be
absent.
The encoding for id-alg-cek-hkdf-sha256, in hexadecimal, is:
30 TBD
5. Use of HKDF with SHA-256 with CMS
This section describes the originator and recipient processing to
implement this mitigation for each of the CMS encrypting content
types.
5.1. Enveloped-Data Content Type
The fourth step of constructing an Enveloped-data is repeated below
from Section 6 of [RFC5652]:
4. The content is encrypted with the content-encryption key.
Content encryption may require that the content be padded to a
multiple of some block size; see Section 6.3 of [RFC5652].
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To implement this mitigation, the originator expands this step as
follows:
* Include the id-alg-cek-hkdf-sha256 algorithm identifier in the
contentEncryptionAlgorithm.algorithm field of the
EncryptedContentInfo structure, and set the
contentEncryptionAlgorithm.parameters field to the
AlgorithmIdentifier for the content-encryption algorithm that will
be used to encrypt the content, including both the algorithm and
optional parameters.
* Derive the new content-encryption key (CEK') from the original
content-encryption key (CEK) and the
ContentEncryptionAlgorithmIdentifier, which is carried in the
contentEncryptionAlgorithm.parameters field:
CEK' = CMS_CEK_HKDF_SHA256(CEK, ContentEncryptionAlgorithmIdentifier)
* The content is encrypted with the new content-encryption key
(CEK'). Content encryption may require that the content be padded
to a multiple of some block size; see Section 6.3 of [RFC5652].
The presence of the id-alg-cek-hkdf-sha256 algorithm identifier in
the contentEncryptionAlgorithm.algorithm field of the
EncryptedContentInfo structure tells the recipient to derive the new
content-encryption key (CEK') as shown above, and then use it for
decryption of the EncryptedContent. If the id-alg-cek-hkdf-sha256
algorithm identifier is not present in the
contentEncryptionAlgorithm.algorithm field of the
EncryptedContentInfo structure, then the recipient uses the original
content-encryption key (CEK) for decryption of the EncryptedContent.
5.2. Encrypted-Data Content Type
As specified in Section 8 of [RFC5652], the content-encryption key is
managed by other means.
To implement this mitigation, the originator performs the following:
* Include the id-alg-cek-hkdf-sha256 algorithm identifier in the
contentEncryptionAlgorithm.algorithm field of the
EncryptedContentInfo structure, and set the
contentEncryptionAlgorithm.parameters field to the
AlgorithmIdentifier for the content-encryption algorithm that will
be used to encrypt the content, including both the algorithm and
optional parameters.
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* Derive the new content-encryption key (CEK') from the original
content-encryption key (CEK) and the
ContentEncryptionAlgorithmIdentifier, which is carried in the
contentEncryptionAlgorithm.parameters field:
CEK' = CMS_CEK_HKDF_SHA256(CEK, ContentEncryptionAlgorithmIdentifier)
* The content is encrypted with the new content-encryption key
(CEK'). Content encryption may require that the content be padded
to a multiple of some block size; see Section 6.3 of [RFC5652].
The presence of the id-alg-cek-hkdf-sha256 algorithm identifier in
the contentEncryptionAlgorithm.algorithm field of the
EncryptedContentInfo structure tells the recipient to derive the new
content-encryption key (CEK') as shown above, and then use it for
decryption of the EncryptedContent. If the id-alg-cek-hkdf-sha256
algorithm identifier is not present in the
contentEncryptionAlgorithm.algorithm field of the
EncryptedContentInfo structure, then the recipient uses the original
content-encryption key (CEK) for decryption of the EncryptedContent.
5.3. Authenticated-Enveloped-Data Content Type
The fifth step of constructing an Authenticated-Enveloped-Data is
repeated below from Section 2 of [RFC5083]:
5. The attributes collected in step 4 are authenticated and the CMS
content is authenticated and encrypted with the content-
authenticated-encryption key. If the authenticated encryption
algorithm requires either the additional authenticated data (AAD)
or the content to be padded to a multiple of some block size,
then the padding is added as described in Section 6.3 of
[RFC5652].
To implement this mitigation, the originator expands this step as
follows:
* Include the id-alg-cek-hkdf-sha256 algorithm identifier in the
contentEncryptionAlgorithm.algorithm field of the
EncryptedContentInfo structure, and set the
contentEncryptionAlgorithm.parameters field to the
AlgorithmIdentifier for the content-authenticated-encryption
algorithm that will be used for authenticated encryption of the
content, including both the algorithm and optional parameters.
* Derive the new content-authenticated-encryption key (CEK') from
the original content-authenticated-encryption key (CEK) and the
ContentEncryptionAlgorithmIdentifier:
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CEK' = CMS_CEK_HKDF_SHA256(CEK, ContentEncryptionAlgorithmIdentifier)
* The attributes collected in step 4 are authenticated and the CMS
content is authenticated and encrypted with the new content-
authenticated-encryption key (CEK'). If the authenticated
encryption algorithm requires either the additional authenticated
data (AAD) or the content to be padded to a multiple of some block
size, then the padding is added as described in Section 6.3 of
[RFC5652].
The presence of the id-alg-cek-hkdf-sha256 algorithm identifier in
the contentEncryptionAlgorithm.algorithm field of the
EncryptedContentInfo structure tells the recipient to derive the new
content-authenticated-encryption key (CEK') as shown above, and then
use it for authenticated decryption of the EncryptedContent and the
authentication of the AAD. If the id-alg-cek-hkdf-sha256 algorithm
identifier is not present in the contentEncryptionAlgorithm.algorithm
field of the EncryptedContentInfo structure, then the recipient uses
the original content-authenticated-encryption (CEK) for decryption
and authentication of the EncryptedContent and the authentication of
the AAD.
6. Security Considerations
This mitigation always uses HKDF with SHA-256. One KDF algorithm was
selected to avoid the need for negotiation. In the future, if a
weakness is found in the KDF algorithm, a new attribute will need to
be assigned for use with an alternative KDF algorithm.
If the attacker removes the id-alg-cek-hkdf-sha256 object identifier
from the contentEncryptionAlgorithm.algorithm field of the
EncryptedContentInfo structure prior to delivery to the recipient,
then the recipient will not attempt to derive CEK', which will deny
the recipient access to the content, but will not assist the attacker
in recovering the plaintext content.
If the attacker changes contentEncryptionAlgorithm.parameters field
of the EncryptedContentInfo structure prior to delivery to the
recipient, then the recipient will derive a different CEK', which
will not assist the attacker in recovering the plaintext content.
Providing the object identifier as an inout to the key derivation
function is sufficient to mitigate the attack described in [RS2023],
but this mitigation includes both the object identifier and the
parameters to protect against some yet-to-be-discovered attack that
only manipulates the parameters.
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Implementations MUST protect the content-encryption keys and content-
authenticated-encryption keys, this includes the CEK and CEK'.
Compromise of a content-encryption key may result in disclosure of
the associated encrypted content. Compromise of a content-
authenticated-encryption key may result in disclosure of the
associated encrypted content or allow modification of the
authenticated content and the additional authenticated data (AAD).
Implementations MUST randomly generate content-encryption keys and
content-authenticated-encryption keys. Using an inadequate pseudo-
random number generator (PRNG) to generate cryptographic keys can
result in little or no security. An attacker may find it much easier
to reproduce the PRNG environment that produced the keys, and then
searching the resulting small set of possibilities, rather than brute
force searching the whole key space. The generation of quality
random numbers is difficult. [RFC4086] offers important guidance on
this topic.
7. Privacy Considerations
If the message-digest attribute is included in the AuthAttributes,
then the attribute value will contain the unencrypted one-way hash
value of the plaintext of the content. Disclosure of this hash value
enables content tracking, and it can be used to determine if the
content matches one or more candidates. For these reasons, the
AuthAttributes SHOULD NOT contain the message-digest attribute.
8. Operations Considerations
CMS is often used to provide encryption in messaging environments,
where various forms of unsolicited messages (such as spam and
phishing) represent a significant volume of unwanted traffic.
Mitigation strategies for unwanted message traffic involve analysis
of message content plaintext. When recipients accept unsolicited
encrypted messages, they become even more vulnerable to unwanted
traffic since many mitigation strategies will be unable to access the
message content plaintext. Therefore, software that receives
messages that have been encrypted using CMS ought to provide
alternate mechanisms to handle the unwanted message traffic. One
approach that does not require disclosure of keying material to a
server is to reject or discard encrypted messages unless they purport
to come from a member of a previously approved originator list.
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9. IANA Considerations
For the ASN.1 Module in the Appendix A of this document, IANA is
requested to assign an object identifier (OID) for the module
identifier (TBD0) with a Description of "id-mod-CMS-CEK-HKDF-
SHA256-2023". The OID for the module should be allocated in the "SMI
Security for S/MIME Module Identifier" registry
(1.2.840.113549.1.9.16.0).
For the id-alg-cek-hkdf-sha256 algorithm identifier in Section 3 of
this document, IANA is requested to assign an object identifier (OID)
(TBD1) with a Description of "id-alg-cek-hkdf-sha256". The OID for
the algoritm should be allocated in the "SMI Security for S/MIME
Algorithms" registry (1.2.840.113549.1.9.16.3).
10. Acknowledgements
Thanks to Mike Ounsworth, Carl Wallace, and Joe Mandel their careful
review and constructive comments.
11. References
11.1. Normative References
[FIPS180] National Institute of Standards and Technology (NIST),
"Secure Hash Standard (SHS)", FIPS PUB 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/rfc/rfc2119>.
[RFC5083] Housley, R., "Cryptographic Message Syntax (CMS)
Authenticated-Enveloped-Data Content Type", RFC 5083,
DOI 10.17487/RFC5083, November 2007,
<https://www.rfc-editor.org/rfc/rfc5083>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/rfc/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/rfc/rfc5869>.
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[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/rfc/rfc8174>.
[RFC8551] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Message Specification", RFC 8551, DOI 10.17487/RFC8551,
April 2019, <https://www.rfc-editor.org/rfc/rfc8551>.
[X680] ITU-T, "Information technology -- Abstract Syntax Notation
One (ASN.1): Specification of basic notation", ITU-T
Recommendation X.680, ISO/IEC 8824-1:2021, February 2021,
<https://www.itu.int/rec/T-REC-X.680>.
[X690] 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 8825-1-2021,
February 2021, <https://www.itu.int/rec/T-REC-X.690>.
11.2. Informative References
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/rfc/rfc4086>.
[RFC5084] Housley, R., "Using AES-CCM and AES-GCM Authenticated
Encryption in the Cryptographic Message Syntax (CMS)",
RFC 5084, DOI 10.17487/RFC5084, November 2007,
<https://www.rfc-editor.org/rfc/rfc5084>.
[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/rfc/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/rfc/rfc5912>.
[RS2023] Roth, J. and F. Strenzke, "AEAD-to-CBC Downgrade Attacks
on CMS", 8 November 2023,
<https://datatracker.ietf.org/meeting/118/materials/
slides-118-lamps-attack-against-aead-in-cms>.
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Appendix A. ASN.1 Module
This ASN.1 Module builds upon the conventions established in
[RFC5911] and [RFC5912].
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<CODE STARTS>
CMS-CEK-HKDF-SHA256-Module-2024
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
id-smime(16) id-mod(0) id-mod-CMS-CEK-HKDF-SHA256-2024(TBD0) }
DEFINITIONS IMPLICIT TAGS ::= BEGIN
EXPORTS ALL;
IMPORTS
AlgorithmIdentifier{}, CONTENT-ENCRYPTION, SMIME-CAPS
FROM AlgorithmInformation-2009 -- in [FRC5911]
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-algorithmInformation-02(58) } ;
--
-- CEK-HKDF-SHA256 Algorithm
--
id-alg-cek-hkdf-sha256 OBJECT IDENTIFIER ::= { TBD1 }
ContentEncryptionAlgorithmIdentifier ::=
AlgorithmIdentifier{CONTENT-ENCRYPTION, { ... } }
cea-CEKHKDFSHA256 CONTENT-ENCRYPTION ::= {
IDENTIFIER id-alg-cek-hkdf-sha256
PARAMS TYPE ContentEncryptionAlgorithmIdentifier ARE required
SMIME-CAPS { IDENTIFIED BY id-alg-cek-hkdf-sha256 } }
--
-- S/MIIME Capability for CEK-HKDF-SHA256 Algorithm
--
SMimeCaps SMIME-CAPS ::= { cap-CMSCEKHKDFSHA256, ... }
cap-CMSCEKHKDFSHA256 SMIME-CAPS ::=
{ -- No value -- IDENTIFIED BY id-alg-cek-hkdf-sha256 }
END
<CODE ENDS>
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Appendix B. CMS_CEK_HKDF_SHA256 Function Examples
This appendix provides two test vectores for the CMS_CEK_HKDF_SHA256
function.
B.1. CMS_CEK_HKDF_SHA256 with AES-128-GCM
This test vector uses includes an AlgorithmIdentifier for
AES-128-GCM.
IKM = c702e7d0a9e064b09ba55245fb733cf3
The AES-128-CGM AlgorithmIdentifier:
algorithm=2.16.840.1.101.3.4.1.6
parameters=GCMParameters:
aes-nonce=0x5c79058ba2f43447639d29e2
aes-ICVlen is ommited; it indicates the DEFAULT of 12
DER-encoded AlgorithmIdentifier:
301b0609608648016503040106300e040c5c79058ba2f43447639d29e2
OKM = 2124ffb29fac4e0fbbc7d5d87492bff3
B.2. CMS_CEK_HKDF_SHA256 with AES-128-CBC
This test vector uses includes an AlgorithmIdentifier for
AES-128-CBC.
IKM = c702e7d0a9e064b09ba55245fb733cf3
The AES-128-CBC AlgorithmIdentifier:
algorithm=2.16.840.1.101.3.4.1.2
parameters=AES-IV=0x651f722ffd512c52fe072e507d72b377
DER-encoded AlgorithmIdentifier:
301d06096086480165030401020410651f722ffd512c52fe072e507d72b377
OKM = 9cd102c52f1e19ece8729b35bfeceb50
Author's Address
Russ Housley
Vigil Security, LLC
Herndon, VA,
United States of America
Email: housley@vigilsec.com
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