Algorithms for Asymmetric Key Package Content Type
Which cryptographic algorithms to use when protecting private keys and key packages with CMS.
Turner, S., "Algorithms for Asymmetric Key Package Content Type", RFC 5959, DOI 10.17487/RFC5959, August 2010, <https://www.rfc-editor.org/info/rfc5959>.
@misc{rfc5959,
series = {Request for Comments},
number = 5959,
howpublished = {RFC 5959},
publisher = {RFC Editor},
doi = {10.17487/RFC5959},
url = {https://www.rfc-editor.org/info/rfc5959},
author = {S. Turner},
title = {{Algorithms for Asymmetric Key Package Content Type}},
pagetotal = 7,
year = 2010,
month = aug,
} RFC text
Contents
Internet Engineering Task Force (IETF) S. Turner
Request for Comments: 5959 IECA
Category: Standards Track August 2010
ISSN: 2070-1721
Algorithms for Asymmetric Key Package Content Type
Abstract
This document describes the conventions for using several
cryptographic algorithms with the EncryptedPrivateKeyInfo structure,
as defined in RFC 5958. It also includes conventions necessary to
protect the AsymmetricKeyPackage content type with SignedData,
EnvelopedData, EncryptedData, AuthenticatedData, and
AuthEnvelopedData.
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5959.
Copyright Notice
Copyright (c) 2010 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
(http://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
This document describes the conventions for using several
cryptographic algorithms with the EncryptedPrivateKeyInfo structure
[RFC5958]. The EncryptedPrivateKeyInfo is used by [P12] to encrypt
PrivateKeyInfo [RFC5958]. It is similar to EncryptedData [RFC5652]
in that it has no recipients, no originators, and no content
encryption keys and requires keys to be managed by other means.
This document also includes conventions necessary to protect the
AsymmetricKeyPackage content type [RFC5958] with Cryptographic
Message Syntax (CMS) protecting content types: SignedData [RFC5652],
EnvelopedData [RFC5652], EncryptedData [RFC5652], AuthenticatedData
[RFC5652], and AuthEnvelopedData [RFC5083]. Implementations of
AsymmetricKeyPackage do not require support for any CMS protecting
content type; however, if the AsymmetricKeyPackage is CMS protected
it is RECOMMENDED that conventions defined herein be followed.
This document does not define any new algorithms instead it refers to
previously defined algorithms.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. EncryptedPrivateKeyInfo
The de facto standard used to encrypt the PrivateKeyInfo structure,
which is subsequently placed in the EncryptedPrivateKeyInfo
encryptedData field, is Password Based Encryption (PBE) based on PKCS
#5 [RFC2898] and PKCS #12 [P12]. The major difference between PKCS
#5 and PKCS #12 is the supported encoding for the password: ASCII for
PKCS #5 and Unicode for PKCS #12, encoded as specified in Section B.1
of [P12]. [RFC2898] specifies two PBE Schemes (PBES) 1 and 2;
[RFC2898] recommends PBES2 for new specification. PBES2 with a key
derivation algorithm of PBKDF2 using HMAC with SHA-256 [RFC5754] and
an encryption algorithm of AES Key Wrap with Padding as defined in
[RFC5649] MUST be supported. AES-256 Key Wrap with Padding [RFC5649]
MAY also be supported as an encryption algorithm.
3. AsymmetricKeyPackage
As noted in Asymmetric Key Packages [RFC5958], CMS can be used to
protect the AsymmetricKeyPackage. The following provides guidance
for SignedData [RFC5652], EnvelopedData [RFC5652], EncryptedData
[RFC5652], AuthenticatedData [RFC5652], and AuthEnvelopedData
[RFC5083].
3.1. SignedData
If an implementation supports SignedData, then it MUST support the
signature scheme RSA [RFC3370] [RFC5754] and SHOULD support the
signature schemes RSASSA-PSS [RFC4056] and DSA [RFC3370] [RFC5754].
Additionally, implementations MUST support in concert with these
signature schemes the hash function SHA-256 [RFC5754] and SHOULD
support the hash function SHA-1 [RFC3370].
3.2. EnvelopedData
If an implementation supports EnvelopedData, then it MUST implement
key transport and it MAY implement key agreement.
When key transport is used, RSA encryption [RFC3370] MUST be
supported and RSAES-OAEP (RSA Encryption Scheme - Optimal Asymmetric
Encryption Padding) [RFC3560] SHOULD be supported.
When key agreement is used, Diffie-Hellman (DH) ephemeral-static
[RFC3370] MUST be supported.
Since the content type is used to carry a cryptographic key and its
attributes, an algorithm that is traditionally used to encrypt one
key with another is employed. Regardless of the key management
technique choice, implementations MUST support AES-128 Key Wrap with
Padding [RFC5649] as the content encryption algorithm.
Implementations SHOULD support AES-256 Key Wrap with Padding
[RFC5649] as the content encryption algorithm.
When key agreement is used, a key wrap algorithm is also specified to
wrap the content encryption key. If the content encryption algorithm
is AES-128 Key Wrap with Padding, then the key wrap algorithm MUST be
AES-128 Key Wrap with Padding [RFC5649]. If the content encryption
algorithm is AES-256 Key Wrap with Padding, then the key wrap
algorithm MUST be AES-256 Key Wrap with Padding [RFC5649].
3.3. EncryptedData
If an implementation supports EncryptedData, then it MUST implement
AES-128 Key Wrap with Padding [RFC5649] and SHOULD implement AES-256
Key Wrap with Padding [RFC5649].
NOTE: EncryptedData requires that keys be managed by other means;
therefore, the only algorithm specified is the content encryption
algorithm. Since the content type is used to carry a cryptographic
key and its attributes, an algorithm that is traditionally used to
encrypt one key with another is employed.
3.4. AuthenticatedData
If an implementation supports AuthenticatedData, then it MUST
implement SHA-256 [RFC5754] and SHOULD support SHA-1 [RFC3370] as the
message digest algorithm. Additionally, HMAC with SHA-256 [RFC4231]
MUST be supported and HMAC with SHA-1 [RFC3370] SHOULD be supported.
3.5. AuthEnvelopedData
If an implementation supports AuthEnvelopedData, then it MUST
implement the EnvelopedData recommendations except for the content
encryption algorithm, which in this case MUST be AES-GCM [RFC5084];
the 128-bit version MUST be implemented and the 256-bit version
SHOULD be implemented. Implementations MAY also support for AES-CCM
[RFC5084].
4. Public Key Sizes
The easiest way to implement the SignedData, EnvelopedData, and
AuthEnvelopedData is with public key certificates [RFC5280]. If an
implementation support RSA, RSASSA-PSS, DSS, RSAES-OAEP, or DH, then
it MUST support key lengths from 1024-bit to 2048-bit, inclusive.
5. SMIMECapabilities Attribute
[RFC5751] defines the SMIMECapabilities attribute as a mechanism for
recipients to indicate their supported capabilities including the
algorithms they support. The following are values for the
SMIMECapabilities attribute for AES Key Wrap with Padding [RFC5649]
when used as a content encryption algorithm:
AES-128 KW with Padding: 30 0d 06 09 60 86 48 01 65 03 04 01 08
AES-192 KW with Padding: 30 0d 06 09 60 86 48 01 65 03 04 01 1C
AES-256 KW with Padding: 30 0d 06 09 60 86 48 01 65 03 04 01 30
6. Security Considerations
The security considerations from [RFC3370], [RFC3560], [RFC4056],
[RFC4231], [RFC5083], [RFC5084], [RFC5649], [RFC5652], [RFC5754], and
[RFC5958] apply.
The strength of any encryption scheme is only as good as its weakest
link, which in the case of a PBES is the password. Passwords need to
provide sufficient entropy to ensure they cannot be easily guessed.
The U.S. National Institute of Standards and Technology (NIST)
Electronic Authentication Guidance [SP800-63] provides some
information on password entropy. [SP800-63] indicates that a user-
chosen 20-character password from a 94-character keyboard with no
checks provides 36 bits of entropy. If the 20-character password is
randomly chosen, then the amount of entropy is increased to roughly
131 bits of entropy. The amount of entropy in the password does not
correlate directly to bits of security but in general the more than
the better.
The choice of content encryption algorithms for this document was
based on [RFC5649]: "In the design of some high assurance
cryptographic modules, it is desirable to segregate cryptographic
keying material from other data. The use of a specific cryptographic
mechanism solely for the protection of cryptographic keying material
can assist in this goal". Unfortunately, there is no AES-GCM or AES-
CCM mode that provides the same properties. If an AES-GCM and AES-
CCM mode that provides the same properties is defined, then this
document will be updated to adopt that algorithm.
[SP800-57] provides comparable bits of security for some algorithms
and key sizes. [SP800-57] also provides time frames during which
certain numbers of bits of security are appropriate and some
environments may find these time frames useful.
7. References
7.1. Normative References
[P12] RSA Laboratories, "PKCS #12 v1.0: Personal Information
Exchange Syntax", June 1999.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898, September 2000.
[RFC3370] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
[RFC3560] Housley, R., "Use of the RSAES-OAEP Key Transport
Algorithm in Cryptographic Message Syntax (CMS)", RFC
3560, July 2003.
[RFC4056] Schaad, J., "Use of the RSASSA-PSS Signature Algorithm in
Cryptographic Message Syntax (CMS)", RFC 4056, June 2005.
[RFC4231] Nystrom, M., "Identifiers and Test Vectors for HMAC-
SHA-224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",
RFC 4231, December 2005.
[RFC5083] Housley, R., "Cryptographic Message Syntax (CMS)
Authenticated-Enveloped-Data Content Type", RFC 5083,
November 2007.
[RFC5084] Housley, R., "Using AES-CCM and AES-GCM Authenticated
Encryption in the Cryptographic Message Syntax (CMS)",
RFC 5084, November 2007.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 5280, May 2008.
[RFC5649] Housley, R. and M. Dworkin, "Advanced Encryption Standard
(AES) Key Wrap with Padding Algorithm", RFC 5649,
September 2009.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD
70, RFC 5652, September 2009.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic
Message Syntax", RFC 5754, January 2010.
[RFC5958] Turner, S., "Asymmetric Key Packages", RFC 5958, August
2010.
7.2. Informative References
[SP800-57] National Institute of Standards and Technology (NIST),
Special Publication 800-57: Recommendation for Key
Management - Part 1 (Revised), March 2007.
[SP800-63] National Institute of Standards and Technology (NIST),
Special Publication 800-63: Electronic Authentication
Guidance, April 2006.
Author's Address
Sean Turner
IECA, Inc.
3057 Nutley Street, Suite 106
Fairfax, VA 22031
USA
EMail: turners@ieca.com
AI ELI5
On this page
§1Purpose and Scope
RFC 5959 describes conventions for using several cryptographic algorithms with the EncryptedPrivateKeyInfo structure from RFC 5958. That structure is used by PKCS #12 to encrypt a PrivateKeyInfo. Like EncryptedData in RFC 5652, it has no recipients, no originators and no content encryption keys, so keys must be managed by other means.
It also covers the conventions needed to protect the AsymmetricKeyPackage content type with Cryptographic Message Syntax (CMS) types: SignedData, EnvelopedData, EncryptedData, AuthenticatedData and AuthEnvelopedData. Implementations of AsymmetricKeyPackage do not have to support any CMS protecting type, but if a package is CMS protected, following these conventions is RECOMMENDED.
The document defines no new algorithms. It refers to previously defined ones.
When you put a secret key in a locked box, you need to agree on which locks to use. This paper only picks from locks that already exist.
§2Encrypting a Private Key with a Password
The de facto standard for encrypting the PrivateKeyInfo that goes into the encryptedData field is Password Based Encryption (PBE) from PKCS #5 (RFC 2898) and PKCS #12. The main difference between the two is how the password is encoded: ASCII for PKCS #5 and Unicode for PKCS #12.
RFC 2898 defines two schemes, PBES1 and PBES2, and recommends PBES2 for new specifications. Implementations MUST support PBES2 with the PBKDF2 key derivation function using HMAC with SHA-256 and with AES Key Wrap with Padding as the encryption algorithm. They MAY also support AES-256 Key Wrap with Padding.
A password is turned into a strong key, and that key locks up the secret. Everyone must at least know the standard recipe for doing this.
§3Algorithms for Each CMS Protecting Type
For each CMS type an implementation chooses to support, RFC 5959 sets MUST and SHOULD algorithms.
- SignedData: RSA MUST be supported; RSASSA-PSS and DSA SHOULD be. SHA-256 MUST be supported, and SHA-1 SHOULD be.
- EnvelopedData: key transport MUST be implemented and key agreement MAY be. For key transport, RSA encryption MUST be supported and RSAES-OAEP SHOULD be. For key agreement, ephemeral-static Diffie-Hellman MUST be supported. In every case AES-128 Key Wrap with Padding MUST be supported as the content encryption algorithm, and AES-256 Key Wrap with Padding SHOULD be. With key agreement, the key wrap algorithm must match the content encryption algorithm.
- EncryptedData: AES-128 Key Wrap with Padding MUST be implemented and AES-256 Key Wrap with Padding SHOULD be.
- AuthenticatedData: SHA-256 MUST be implemented and SHA-1 SHOULD be, as the digest. HMAC with SHA-256 MUST be supported and HMAC with SHA-1 SHOULD be.
- AuthEnvelopedData: follow the EnvelopedData recommendations, except that the content encryption algorithm MUST be AES-GCM, with the 128-bit version required and the 256-bit version recommended. AES-CCM MAY also be supported.
The key-wrap choice is deliberate: since the content is itself a cryptographic key, the RFC uses an algorithm traditionally used to encrypt one key with another.
There are five kinds of box. For each one, the paper says which locks every maker must know, which they should know and which are optional extras.
| CMS type | MUST support | SHOULD support |
|---|---|---|
| SignedData | RSA; SHA-256 | RSASSA-PSS; DSA; SHA-1 |
| EnvelopedData | Key transport with RSA; ephemeral-static DH if key agreement is used; AES-128 Key Wrap with Padding | RSAES-OAEP; AES-256 Key Wrap with Padding |
| EncryptedData | AES-128 Key Wrap with Padding | AES-256 Key Wrap with Padding |
| AuthenticatedData | SHA-256; HMAC with SHA-256 | SHA-1; HMAC with SHA-1 |
| AuthEnvelopedData | EnvelopedData rules, but with AES-GCM (128-bit) | AES-GCM (256-bit) |
§4Public Key Sizes
The easiest way to implement SignedData, EnvelopedData and AuthEnvelopedData is with public key certificates (RFC 5280). An implementation that supports RSA, RSASSA-PSS, DSS, RSAES-OAEP or DH MUST support key lengths from 1024 bits to 2048 bits, inclusive.
Locks come in different sizes. Every maker must at least be able to handle the small and medium ones named in the paper.
§5SMIMECapabilities Values
RFC 5751 defines the SMIMECapabilities attribute, which lets a recipient advertise the algorithms it supports. RFC 5959 supplies the encoded values for AES Key Wrap with Padding when used as a content encryption algorithm, one each for the AES-128, AES-192 and AES-256 variants. They differ only in their last byte: 08, 1C and 30 respectively.
It is like a name tag that says which locks you can handle, so that people sending you a box can pick one you understand.
AES-128 KW with Padding: 30 0d 06 09 60 86 48 01 65 03 04 01 08
AES-192 KW with Padding: 30 0d 06 09 60 86 48 01 65 03 04 01 1C
AES-256 KW with Padding: 30 0d 06 09 60 86 48 01 65 03 04 01 30§6Security Considerations
The security considerations of the referenced algorithm and CMS documents apply. The RFC adds that password-based encryption is only as strong as its weakest link, the password, which must have enough entropy to resist guessing. Citing NIST SP 800-63, it says a user-chosen 20-character password from a 94-character keyboard with no checks gives 36 bits of entropy, and a randomly chosen one gives roughly 131 bits.
The key-wrap algorithms were chosen following RFC 5649, which values using a mechanism solely for protecting keying material. The RFC notes that no AES-GCM or AES-CCM mode provides the same property, and says the document will be updated if one is defined. NIST SP 800-57 is cited for comparing key sizes and security levels.
A strong lock does not help if the password is easy to guess. Long, random passwords are much harder to guess than ones people make up.