Asymmetric key packages: 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
ISO Plain Language
On this page
§1Introduction
This document describes the conventions for using several cryptographic algorithms with the EncryptedPrivateKeyInfo structure defined in RFC 5958. The EncryptedPrivateKeyInfo is used by PKCS #12 to encrypt a PrivateKeyInfo. It is similar to EncryptedData in that it has no recipients, no originators and no content encryption keys, and requires keys to be managed by other means.
The document also includes conventions necessary to protect the AsymmetricKeyPackage content type with the Cryptographic Message Syntax protecting content types: SignedData, EnvelopedData, EncryptedData, AuthenticatedData and AuthEnvelopedData. Implementations of AsymmetricKeyPackage do not require support for any CMS protecting content type; however, if the AsymmetricKeyPackage is CMS protected it is RECOMMENDED that the conventions defined here be followed.
The document does not define any new algorithms; it refers to previously defined ones.
A private key has to be stored and moved somewhere, and it must be protected on the way. RFC 5958 defined two containers for it but left the choice of algorithms open. This document closes that gap: it says which algorithms to use for each way of protecting a key package, so that two implementations do the same thing.
| CMS type | Must support | Should support |
|---|---|---|
| SignedData | RSA signature, SHA-256 hash | RSASSA-PSS and DSA signatures, SHA-1 hash |
| EnvelopedData | Key transport, RSA encryption, AES-128 Key Wrap with Padding | RSAES-OAEP, Diffie-Hellman key agreement, AES-256 Key Wrap with Padding |
| EncryptedData | AES-128 Key Wrap with Padding | AES-256 Key Wrap with Padding |
| AuthenticatedData | SHA-256 digest, HMAC with SHA-256 | SHA-1 digest, HMAC with SHA-1 |
| AuthEnvelopedData | AES-GCM, 128-bit version | AES-GCM, 256-bit version, AES-CCM |
§2EncryptedPrivateKeyInfo
The de facto standard used to encrypt the PrivateKeyInfo structure, which is subsequently placed in the EncryptedPrivateKeyInfo encryptedData field, is Password Based Encryption based on PKCS #5 and PKCS #12. 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. PKCS #5 specifies two PBE schemes, PBES 1 and 2, and recommends PBES2 for new specifications.
PBES2 with a key derivation algorithm of PBKDF2 using HMAC with SHA-256 and an encryption algorithm of AES Key Wrap with Padding MUST be supported. AES-256 Key Wrap with Padding MAY also be supported as an encryption algorithm.
The usual way to protect a private key with a password runs through PBES2. The password is turned into a key with PBKDF2 using HMAC with SHA-256, and that key wraps the private key with AES Key Wrap with Padding. Every implementation must support that combination.
- PBES2The password-based encryption scheme that turns a password into an encryption key.
- PBKDF2 with HMAC-SHA-256The key derivation function, which makes each guess of the password more expensive.
- AES Key Wrap with PaddingThe algorithm that encrypts the private key once the key is derived.
- The password itselfThe weakest link, so it needs enough entropy to resist guessing.
§3.1SignedData
CMS can be used to protect the AsymmetricKeyPackage, and this document gives guidance for each of the five protecting content types. If an implementation supports SignedData, then it MUST support the signature scheme RSA and SHOULD support the signature schemes RSASSA-PSS and DSA. Implementations MUST support, in concert with these signature schemes, the hash function SHA-256, and SHOULD support the hash function SHA-1.
SignedData adds a signature to the package. An implementation that supports it must be able to sign with RSA and hash with SHA-256; RSASSA-PSS, DSA and SHA-1 are recommended as well.
§3.2EnvelopedData
If an implementation supports EnvelopedData, then it MUST implement key transport and it MAY implement key agreement. When key transport is used, RSA encryption MUST be supported and RSAES-OAEP SHOULD be supported. When key agreement is used, Diffie-Hellman ephemeral-static 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 chosen, implementations MUST support AES-128 Key Wrap with Padding as the content encryption algorithm, and SHOULD support AES-256 Key Wrap with Padding.
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. 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.
EnvelopedData encrypts the package for one or more recipients. RSA key transport is required and Diffie-Hellman key agreement may be added. Whatever way the content key is shared, the package itself is encrypted with AES-128 Key Wrap with Padding, and the 256-bit version is recommended. When key agreement is used, the content key is wrapped with the same algorithm that encrypts the package.
§3.3EncryptedData, AuthenticatedData and AuthEnvelopedData
If an implementation supports EncryptedData, then it MUST implement AES-128 Key Wrap with Padding and SHOULD implement AES-256 Key Wrap with Padding. EncryptedData requires that keys be managed by other means, so the only algorithm specified is the content encryption algorithm, and since the content type carries a cryptographic key and its attributes, an algorithm traditionally used to encrypt one key with another is employed.
If an implementation supports AuthenticatedData, then it MUST implement SHA-256 as the message digest algorithm and SHOULD support SHA-1. HMAC with SHA-256 MUST be supported, and HMAC with SHA-1 SHOULD be supported.
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; the 128-bit version MUST be implemented and the 256-bit version SHOULD be implemented. Implementations MAY also support AES-CCM.
The remaining three CMS types have simpler rules. EncryptedData encrypts the package with a key managed elsewhere, so only AES-128 Key Wrap with Padding is required. AuthenticatedData adds a message authentication code, and requires SHA-256 and HMAC with SHA-256. AuthEnvelopedData does both at once, using AES-GCM with 128-bit keys.
§4Public Key Sizes
The easiest way to implement the SignedData, EnvelopedData and AuthEnvelopedData content types is with public key certificates. If an implementation supports RSA, RSASSA-PSS, DSS, RSAES-OAEP or DH, then it MUST support key lengths from 1024-bit to 2048-bit, inclusive.
Whichever public key algorithm an implementation uses, it has to handle key lengths from 1024 to 2048 bits, inclusive.
§5The SMIMECapabilities Attribute
The SMIMECapabilities attribute is defined as a mechanism for recipients to indicate their supported capabilities, including the algorithms they support. This document gives the attribute values for AES Key Wrap with Padding when used as a content encryption algorithm: AES-128 KW with Padding is 30 0d 06 09 60 86 48 01 65 03 04 01 08, AES-192 KW with Padding is 30 0d 06 09 60 86 48 01 65 03 04 01 1C, and AES-256 KW with Padding is 30 0d 06 09 60 86 48 01 65 03 04 01 30.
The S/MIME capability attribute is how a recipient says which algorithms it can handle. The document gives the exact byte values to use for the three AES key wrap sizes.
- 30 0d 06 09 60 86 48 01 65 03 04 01 08AES-128 KW with Padding — The value that advertises the 128-bit key wrap algorithm.
- 30 0d 06 09 60 86 48 01 65 03 04 01 1CAES-192 KW with Padding — The value that advertises the 192-bit key wrap algorithm.
- 30 0d 06 09 60 86 48 01 65 03 04 01 30AES-256 KW with Padding — The value that advertises the 256-bit key wrap algorithm.
§6Security Considerations
The security considerations from the documents behind each of the algorithms 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 NIST Electronic Authentication Guidance 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, the amount of entropy is increased to roughly 131 bits. The amount of entropy in the password does not correlate directly to bits of security, but in general the more the better.
The choice of content encryption algorithms for this document was based on the reasoning that in the design of some high assurance cryptographic modules it is desirable to segregate cryptographic keying material from other data, and that 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 such a mode is defined, the document will be updated to adopt that algorithm. The NIST key management guidance provides comparable bits of security for some algorithms and key sizes, and also provides time frames during which certain numbers of bits of security are appropriate, which some environments may find useful.
The weakest link in password-based protection is the password itself. The RFC quotes the NIST guidance: a 20-character password that a person chose gives about 36 bits of entropy, and the same length chosen at random gives about 131. It also explains why key wrap was chosen over AES-GCM and AES-CCM for the content encryption: key wrap keeps keying material separate from other data, and the authenticated modes do not provide those same properties.