Internet Engineering Task Force (IETF) J. Schaad
Request for Comments: 6664 Soaring Hawk Consulting
Category: Informational July 2012
ISSN: 2070-1721
S/MIME Capabilities for Public Key Definitions
Abstract
This document defines a set of Secure/Multipurpose Internet Mail
Extensions (S/MIME) Capability types for ASN.1 encoding for the
current set of public keys defined by the PKIX working group. This
facilitates the ability for a requester to specify information on the
public keys and signature algorithms to be used in responses.
"Online Certificate Status Protocol Algorithm Agility" (RFC 6277)
details an example of where this is used.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see 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/rfc6664.
Copyright Notice
Copyright (c) 2012 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.
Schaad Informational [Page 1]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. ASN.1 Notation . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Terminology . . . . . . . . . . . . . . . . . 4
2. RSA Public Keys . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Generic RSA Public Keys . . . . . . . . . . . . . . . . . 4
2.2. RSASSA-PSS Signature Public Keys . . . . . . . . . . . . . 5
2.3. RSAES-OAEP Key Transport Public Keys . . . . . . . . . . . 6
3. Diffie-Hellman Keys . . . . . . . . . . . . . . . . . . . . . 7
3.1. DSA Signature Public Key . . . . . . . . . . . . . . . . . 7
3.2. DH Key Agreement Keys . . . . . . . . . . . . . . . . . . 8
4. Elliptic Curve Keys . . . . . . . . . . . . . . . . . . . . . 8
4.1. Generic Elliptic Curve Keys . . . . . . . . . . . . . . . 9
4.2. Elliptic Curve DH Keys . . . . . . . . . . . . . . . . . . 10
4.3. Elliptic Curve MQV Keys . . . . . . . . . . . . . . . . . 10
5. RSASSA-PSS Signature Algorithm Capability . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1. Normative References . . . . . . . . . . . . . . . . . . . 13
7.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. 2008 ASN.1 Module . . . . . . . . . . . . . . . . . . 15
Appendix B. 1988 ASN.1 Module . . . . . . . . . . . . . . . . . . 18
Appendix C. Future Work . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
In the process of dealing with the Online Certificate Status Protocol
(OCSP) agility issues in [RFC6277], it was noted that we really
wanted to describe information to be used in selecting a public key,
but we did not have any way of doing so. This document fills that
hole by defining a set of Secure/Multipurpose Internet Mail
Extensions (S/MIME) Capability types for a small set of public key
representations.
S/MIME capabilities were originally defined in [SMIMEv3-MSG] as a way
for the sender of an S/MIME message to tell the recipient of the
message the set of encryption algorithms that were supported by the
sender's system. In the beginning, the focus was primarily on
communicating the set of encryption algorithms that were supported by
the sender. Over time, it was expanded to allow for an S/MIME client
to state that it supported new features such as the compression data
type and binary encoded contents. The structure was defined so that
parameters can be passed in as part of the capability to allow for
subsets of algorithms to be used. This was used for the RC2
encryption algorithm, although only two values out of the set of
values were ever used. The goal of restricting the set of values is
to allow a client to use a simple binary comparison in order to check
Schaad Informational [Page 2]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
for equality. The client should never need to decode the capability
and do an element-by-element comparison. Historically, this has not
been a problem as the vast majority of S/MIME capabilities consist of
just the algorithm identifier for the algorithm.
Many people are under the impression that only a single data
structure can be assigned to an object identifier, but this is not
the case. As an example, the OID rsaEncryption is used in multiple
locations for different data. It represents a public key, a key
transport algorithm (in S/MIME), and was originally used in the
Public-Key Cryptography Standards (PKCS) #7 specification as a
signature value identifier (this has since been changed by the S/MIME
specifications). One of the implications is that when mapping an
object identifier to a data type structure, the location in the ASN.1
structure needs to be taken into consideration as well.
1.1. ASN.1 Notation
The main body of the text is written using snippets of ASN.1 that are
extracted from the ASN.1 2008 module in Appendix A. ASN.1 2008 is
used in this document because it directly represents the metadata
that is not representable in the 1988 version of ASN.1 but instead is
part of the text. In keeping with the current policy of the PKIX
working group, the 1988 module along with the text is the normative
module. In the event of a conflict between the content of the two
modules, the 1988 module is authoritative.
When reading this document, it is assumed that you will have a degree
of familiarity with the basic object module that is presented in
Section 3 of RFC 5912 [RFC5912]. We use the SMIME-CAPS object in
this document; it associates two fields together in a single object.
SMIME-CAPS ::= CLASS {
&id OBJECT IDENTIFIER UNIQUE,
&Type OPTIONAL
}
WITH SYNTAX { [TYPE &Type] IDENTIFIED BY &id }
These fields are:
&id contains an object identifier. When placed in an object set,
this element is tagged so that no two elements can be placed in
the set that have the same value in the &id field. Note that this
is not a restriction saying that only a single object can exist
with a single object identifier.
Schaad Informational [Page 3]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
&Type optionally contains an ASN.1 type identifier. If the field
&Type is not defined, then the optional parameters field of the
AlgorithmIdentifier type would be omitted.
The class also has a specialized syntax for how to define an object
in this class. The all uppercase words TYPE IDENTIFIER and BY are
syntactic sugar to make it easier to read. The square brackets
define optional pieces of the syntax.
The ASN.1 syntax permits any field in an object to be referenced in
another location. This means that if an object called foo has a
field named &value, the value can be directly referenced as foo.&
value. This means that any updates to values or types are
automatically propagated, and we do not need to replicate the data.
1.2. Requirements Terminology
When capitalized, 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. RSA Public Keys
There are currently three different public key object identifiers for
RSA public keys. These are RSA, RSA Encryption Scheme - Optimal
Asymmetric Encryption Padding (RSAES-OAEP), and RSA Signature Scheme
with Appendix - Probabilistic Signature Scheme (RSASSA-PSS).
2.1. Generic RSA Public Keys
Almost all RSA keys that are contained in certificates today use the
generic RSA public key format and identifier. This allows for the
public key to be used both for key transport and for signature
validation (assuming it is compatible with the bits in the key usage
extension). The only reason for using one of the more specific
public key identifiers is if the user wants to restrict the usage of
the RSA public key to a specific algorithm.
For the generic RSA public key, the S/MIME capability that is
advertised is a request for a specific key size to be used. This
would normally be used for dealing with a request on the key to be
used for a signature that the client would then verify. In general,
the user would provide a specific key when a key transport algorithm
is being considered.
Schaad Informational [Page 4]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
The ASN.1 that is used for the generic RSA public key is defined as
below:
scap-pk-rsa SMIME-CAPS ::= {
TYPE RSAKeyCapabilities
IDENTIFIED BY pk-rsa.&id
}
RSAKeyCapabilities ::= SEQUENCE {
minKeySize RSAKeySize,
maxKeySize RSAKeySize OPTIONAL
}
RSAKeySize ::= INTEGER (1024 | 2048 | 3072 | 4096 | 7680 |
8192 | 15360, ...)
In the above ASN.1, we have defined the following:
scap-pk-rsa is a new SMIME-CAPS object. This object associates the
existing object identifier (rsaEncryption) used for the public key
in certificates (defined in [RFC3279] and [RFC5912]) with a new
type defined in this document.
RSAKeyCapabilities carries the set of desired capabilities for an
RSA key. The fields of this type are:
minKeySize contains the minimum length of the RSA modulus to be
used. This field SHOULD NOT contain a value less than 1024.
maxKeySize contains the maximum length of the RSA modules that
should be used. If this field is absent, then no maximum
length is requested/expected. This value is normally selected
so as not to cause the current code to run unacceptably long
when processing signatures.
RSAKeySize provides a set of suggested values to be used. The
values 1024, 2048, 3072, 7680, and 15360 are from the NIST guide
on signature sizes [NIST-SIZES] while the others are common powers
of two that are used. The list is not closed, and other values
can be used.
2.2. RSASSA-PSS Signature Public Keys
While one will use the generic RSA public key identifier in a
certificate most of the time, the RSASSA-PSS identifier can be used
if the owner of the key desires to restrict the usage of the key to
just this algorithm. This algorithm does have the ability to place a
Schaad Informational [Page 5]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
set of algorithm parameters in the public key info structure, but
they have not been included in this location as the same information
should be carried in the signature S/MIME capabilities instead.
The ASN.1 that is used for the RSASSA-PSS public key is defined
below:
scap-pk-rsaSSA-PSS SMIME-CAPS ::= {
TYPE RSAKeyCapabilities
IDENTIFIED BY pk-rsaSSA-PSS.&id
}
In the above ASN.1, we have defined the following:
scap-pk-rsaSSA-PSS is a new SMIME-CAPS object. This object
associates the existing object identifier (id-RSASSA-PSS) used for
the public key certificates (defined in [RFC4055] and [RFC5912])
with type RSAKeyCapabilities.
2.3. RSAES-OAEP Key Transport Public Keys
While one will use the generic RSA public key identifier in a
certificate most of the time, the RSAES-OAEP identifier can be used
if the owner of the key desires to restrict the usage of the key to
just this algorithm. This algorithm does have the ability to place a
set of algorithm parameters in the public key info structure, but
they have not been included in this location as the same information
should be carried in the key transport S/MIME capabilities instead.
The ASN.1 that is used for the RSAES-OAEP public key is defined
below:
scap-pk-rsaES-OAEP SMIME-CAPS ::= {
TYPE RSAKeyCapabilities
IDENTIFIED BY pk-rsaES-OAEP.&id
}
In the above ASN.1, we have defined the following:
scap-pk-rsaES-OAEP is a new SMIME-CAPS object. This object
associates the existing object identifier (id-RSAES-OAEP) used for
the public key certificates (defined in [RFC4055] and [RFC5912])
with type RSAKeyCapabilities.
Schaad Informational [Page 6]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
3. Diffie-Hellman Keys
There are currently two Diffie-Hellman (DH) public key object
identifiers. These are DH key agreement and Digital Signature
Standard (DSA).
3.1. DSA Signature Public Key
This public key type is used for the validation of DSA signatures.
The ASN.1 that is used for DSA keys is defined below:
scap-pk-dsa SMIME-CAPS ::= {
TYPE DSAKeyCapabilities
IDENTIFIED BY pk-dsa.&id
}
DSAKeyCapabilities ::= CHOICE {
keySizes [0] SEQUENCE {
minKeySize DSAKeySize,
maxKeySize DSAKeySize OPTIONAL,
maxSizeP [1] INTEGER OPTIONAL,
maxSizeQ [2] INTEGER OPTIONAL,
maxSizeG [3] INTEGER OPTIONAL
},
keyParams [1] pk-dsa.&Params
}
DSAKeySize ::= INTEGER (1024 | 2048 | 3072 | 7680 | 15360 )
In the above ASN.1, we have defined the following:
scap-pk-dsa is a new SMIME-CAPS object. This object associates the
existing object identifier (id-dsa) used for the public key in
certificates (defined in [RFC3279] and [RFC5912]) with a new type
defined here, DSAKeyCapabilities.
DSAKeyCapabilities carries the desired set of capabilities for the
DSA key. The fields of this type are:
keySizes is used when only a key size is needed to be specified
and not a specific group. It is expected that this would be
the most commonly used of the two options. In key sizes, the
fields are used as follows:
minKeySize contains the minimum length of the DSA modulus to
be used.
Schaad Informational [Page 7]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
maxKeySize contains the maximum length of the DSA modules that
should be used. If this field is absent, then no maximum
length is requested/expected.
maxSizeP contains the maximum length of the value p that
should be used. If this field is absent, then no maximum
length is imposed.
maxSizeQ contains the maximum length of the value q that
should be used. If this field is absent, then no maximum
length is imposed.
maxSizeG contains the maximum length of the value g that
should be used. If this field is absent, then no maximum
length is imposed.
keyParams contains the exact set of DSA for the key used to sign
the message. This field is provided for completeness and to
match the fields for Elliptic Curve; however, it is expected
that usage of this field will be extremely rare.
3.2. DH Key Agreement Keys
This public key type is used with the DH key agreement algorithm.
The ASN.1 that is used for DH keys is defined below:
scap-pk-dh SMIME-CAPS ::= {
TYPE DSAKeyCapabilities
IDENTIFIED BY pk-dh.&id
}
In the above ASN.1, we have defined the following:
scap-pk-dh is a new SMIME-CAPS object. This object associates the
existing object identifier (dhpublicnumber) used for the public
key algorithm in the certificates (defined in [RFC3279] and
[RFC5912]) with a new type defined above, DSAKeyCapabilities.
4. Elliptic Curve Keys
There are currently three Elliptic Curve Cryptography (ECC) public
key object identifiers. These are EC, EC-DH, and Elliptic Curve
Menezes-Qu-Vanstone (EC-MQV).
Schaad Informational [Page 8]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
4.1. Generic Elliptic Curve Keys
Almost all ECC keys that are contained in certificates today use the
generic ECC public key format and identifier. This allows for the
public key to be used both for key agreement and for signature
validation (assuming the appropriate bits are in the certificate).
The only reason for using one of the more specific public key
identifier is if the user wants to restrict the usage of the ECC
public key to a specific algorithm.
For the generic ECC public key, the S/MIME capability that is
advertised is a request for a specific group to be used.
The ASN.1 that is used for the generic ECC public key is defined
below:
scap-pk-ec SMIME-CAPS ::= {
TYPE EC-SMimeCaps
IDENTIFIED BY pk-ec.&id
}
EC-SMimeCaps ::= SEQUENCE (SIZE (1..MAX)) OF ECParameters
In the above ASN.1, we have defined the following:
scap-pk-ec is a new SMIME-CAPS object. This object associates the
existing object identifier (id-ecPublicKey) used for the public
key algorithm in the certificates (defined in [RFC5480] and
[RFC5912]) with the new type EC-SMimeCaps.
EC-SMimeCaps carries a sequence of at least one ECParameters
structure. This allows for multiple curves to be requested in a
single capability request. A maximum/minimum style of specifying
sizes is not provided as much greater care is required in
selecting a specific curve than is needed to create the parameters
for a DSA/DH key. As specified in [RFC5480], for PKIX-compliant
certificates, only the namedCurve choice of ECParameters is
expected to be used.
Schaad Informational [Page 9]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
4.2. Elliptic Curve DH Keys
This public key type is used with the Elliptic Curve Diffie-Hellman
key agreement algorithm.
The ASN.1 that is used for EC-DH keys is defined below:
scap-pk-ecDH SMIME-CAPS ::= {
TYPE EC-SMimeCaps
IDENTIFIED BY pk-ecDH.&id
}
In the above ASN.1, we have defined the following:
scap-pk-ecDH is a new SMIME-CAPS object. This object associates the
existing object identifier (id-ecDH) used for the public key
algorithm in the certificate (defined in [RFC5480] and [RFC5912])
with the same type structure used for public keys.
4.3. Elliptic Curve MQV Keys
This public key type is used with the Elliptic Curve MQV key
agreement algorithm.
The ASN.1 that is used for EC-MQV keys is defined below:
scap-pk-ecMQV SMIME-CAPS ::= {
TYPE EC-SMimeCaps
IDENTIFIED BY pk-ecMQV.&id
}
In the above ASN.1, we have defined the following:
scap-pk-ecMQV is a new SMIME-CAPS object. This object associates
the existing object identifier (id-ecMQV) used for the public key
algorithm in the certificate (defined in [RFC5480] and [RFC5912])
with the same type structure used for public keys.
5. RSASSA-PSS Signature Algorithm Capability
This document defines a new SMIMECapability for the RSASSA-PSS
signature algorithm. One already exists in [RFC4055] where the
parameters field is not used.
When the S/MIME group defined an S/MIME capability for the RSASSA-PSS
signature algorithm, it was done in the context of how S/MIME defines
and uses S/MIME capabilities. When placed in an S/MIME message
[SMIME-MSG] or in a certificate [RFC4262], it is always placed in a
Schaad Informational [Page 10]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
sequence of capabilities. This means that one could place the
identifier for RSASSA-PSS in the sequence along with the identifier
for MD5, SHA-1, and SHA-256. The assumption was then made that one
could compute the matrix of all answers, and the publisher would
support all elements in the matrix. This has the possibility that
the publisher could accidentally publish a point in the matrix that
is not supported.
In this situation, there is only a single item that is published.
This means that we need to publish all of the associated information
along with the identifier for the signature algorithm in a single
entity. For this reason, we now define a new parameter type to be
used as the SMIMECapability type, which contains a hash identifier
and a mask identifier. The ASN.1 used for this is as follows:
scap-sa-rsaSSA-PSS SMIME-CAPS ::= {
TYPE RsaSsa-Pss-sig-caps
IDENTIFIED BY sa-rsaSSA-PSS.&id
}
RsaSsa-Pss-sig-caps ::= SEQUENCE {
hashAlg SMIMECapability{{ MaskAlgorithmSet }},
maskAlg SMIMECapability{{ ... }} OPTIONAL,
trailerField INTEGER DEFAULT 1
}
scap-mf-mgf1 SMIME-CAPS ::= {
TYPE SMIMECapability{{ ... }}
IDENTIFIED BY id-mgf1
}
MaskAlgorithmSet SMIME-CAPS ::= {scap-mf-mgf1, ...}
In the above ASN.1, we have defined the following:
scap-sa-rsaSSA-PSS is a new SMIME-CAPS object. This object
associates the existing object identifier (id-RSASSA-PSS) used for
the signature algorithm (defined in [RFC4055] and [RFC5912]) with
the new type RsaSsa-Pss-sig-caps.
RsaSsa-Pss-sig-caps carries the desired set of capabilities for the
RSASSA-PSS signature algorithm. The fields of this type are:
hashAlg contains the S/MIME capability for the hash algorithm we
are declaring we support with the RSASSA-PSS signature
algorithm.
Schaad Informational [Page 11]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
maskAlg contains the S/MIME capability for the mask algorithm we
are declaring we support with the RSASSA-PSS signature
algorithm.
trailerField specifies which trailer field algorithm is being
supported. This MUST be the value 1.
NOTE: In at least one iteration of the design, we used a sequence of
hash identifiers and a sequence of masking functions and again made
the assumption that the entire matrix would be supported. This has
been removed at this point since the original intent of S/MIME
capabilities is that one should be able to do a binary comparison of
the DER encoding of the field and determine a specific capability was
published. We could return to using the sequence if we wanted to
lose the ability to do a binary compare but needed to shorten the
encodings. This does not currently appear to be an issue at this
point.
6. Security Considerations
This document provides new fields that can be placed in an S/MIME
capabilities sequence. There are number of considerations that need
to be taken into account when doing this.
As mentioned above, we have defined data structures to be associated
with object identifiers in cases where an association already exists.
When either encoding or decoding structures, care needs to be taken
that the association used is one appropriate for the location in the
surrounding ASN.1 structure. This means that one needs to make sure
that only public keys are placed in public key locations, signatures
are placed in signature locations, and S/MIME capabilities are placed
in SMIMECapability locations. Failure to do so will create decode
errors at best and can cause incorrect behavior at worst.
The more specific the information that is provided in an S/MIME
Capabilities field, the better the end results are going to be.
Specifying a signature algorithm means that there are no questions
for the receiver that the signature algorithm is supported.
Signature algorithms can be implied by specifying both public key
algorithms and hash algorithms together. If the list includes RSA
v1.5, EC-DSA, SHA-1, and SHA-256, the implication is that all four
values in the cross section are supported by the sender. If the
sender does not support EC-DSA with SHA-1, this would lead to a
situation where the recipient uses a signature algorithm that the
sender does not support. Omitting SHA-1 from the list may lead to
the problem where both entities support RSA v1.5 with SHA-1 as their
only common algorithm, but this is no longer discoverable by the
recipient.
Schaad Informational [Page 12]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
As a general rule, providing more information about the algorithms
that are supported is preferable. The more choices that are provided
the recipient, the greater the likelihood that a common algorithm
with good security can be used by both parties. However, one should
avoid being exhaustive in providing the list of algorithms to the
recipient. The greater the number of algorithms that are passed, the
more difficult it is for a recipient to make intelligent decisions
about which algorithm to use. This is a more significant problem
when there are more than two entities involved in the "negotiation"
of a common algorithm to be used (such as sending an encrypted S/MIME
message where a common content encryption algorithm is needed). The
larger the set of algorithms and the more recipients involved, the
more memory and processing time will be needed in order to complete
the decision-making process.
The S/MIME capabilities are defined so that the order of algorithms
in the sequence is meant to encode a preference order by the sender
of the sequence. Many entities will ignore the order preference when
making a decision either by using their own preferred order or using
a random decision from a matrix.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 3279, April 2002.
[RFC4055] Schaad, J., Kaliski, B., and R. Housley, "Additional
Algorithms and Identifiers for RSA Cryptography for
use in the Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL)
Profile", RFC 4055, June 2005.
[RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R., and T.
Polk, "Elliptic Curve Cryptography Subject Public Key
Information", RFC 5480, March 2009.
7.2. Informative References
[NIST-SIZES] Barker, E., Barker, W., Burr, W., Polk, W., and M.
Smid, "Recommendation for Key Management -- Part 1:
General", NIST Special Publication 800-57, March 2007.
Schaad Informational [Page 13]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
[RFC4262] Santesson, S., "X.509 Certificate Extension for
Secure/Multipurpose Internet Mail Extensions (S/MIME)
Capabilities", RFC 4262, December 2005.
[RFC5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
Public Key Infrastructure Using X.509 (PKIX)",
RFC 5912, June 2010.
[RFC6277] Santesson, S. and P. Hallam-Baker, "Online Certificate
Status Protocol Algorithm Agility", RFC 6277,
June 2011.
[SMIME-MSG] Ramsdell, B. and S. Turner, "Secure/Multipurpose
Internet Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[SMIMEv3-MSG] Ramsdell, B., "S/MIME Version 3 Message
Specification", RFC 2633, June 1999.
Schaad Informational [Page 14]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
Appendix A. 2008 ASN.1 Module
This appendix contains a module compatible with the work done to
update the PKIX ASN.1 modules to recent versions of the ASN.1
specifications [RFC5912]. This appendix is to be considered
informational per the current direction of the PKIX working group.
PUBLIC-KEY-SMIME-CAPABILITIES
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pubKeySMIMECaps-08(78) }
DEFINITIONS ::=
BEGIN
IMPORTS
SMIME-CAPS, PUBLIC-KEY, SMIMECapability
FROM AlgorithmInformation-2009
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-algorithmInformation-02(58)}
pk-rsa, pk-dsa, pk-dh, pk-ec, pk-ecDH, pk-ecMQV, ECParameters
FROM PKIXAlgs-2009
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkix1-algorithms2008-02(56) }
pk-rsaSSA-PSS, pk-rsaES-OAEP, sa-rsaSSA-PSS,
HashAlgorithms, id-mgf1
FROM PKIX1-PSS-OAEP-Algorithms-2009
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkix1-rsa-pkalgs-02(54)}
;
--
-- Define a set containing all of the S/MIME capabilities defined
-- by this document.
--
SMimeCaps SMIME-CAPS ::= {
PubKeys-SMimeCaps |
scap-sa-rsaSSA-PSS
}
PubKeys-SMimeCaps SMIME-CAPS ::= {
scap-pk-rsa | scap-pk-rsaSSA-PSS |
scap-pk-dsa |
scap-pk-ec | scap-pk-ecDH | scap-pk-ecMQV
Schaad Informational [Page 15]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
}
--
-- We defined RSA keys from the modules in RFC 3279 and RFC 4055.
--
scap-pk-rsa SMIME-CAPS ::= {
TYPE RSAKeyCapabilities
IDENTIFIED BY pk-rsa.&id
}
RSAKeyCapabilities ::= SEQUENCE {
minKeySize RSAKeySize,
maxKeySize RSAKeySize OPTIONAL
}
RSAKeySize ::= INTEGER (1024 | 2048 | 3072 | 4096 | 7680 |
8192 | 15360, ...)
scap-pk-rsaES-OAEP SMIME-CAPS ::= {
TYPE RSAKeyCapabilities
IDENTIFIED BY pk-rsaES-OAEP.&id
}
scap-pk-rsaSSA-PSS SMIME-CAPS ::= {
TYPE RSAKeyCapabilities
IDENTIFIED BY pk-rsaSSA-PSS.&id
}
scap-sa-rsaSSA-PSS SMIME-CAPS ::= {
TYPE RsaSsa-Pss-sig-caps
IDENTIFIED BY sa-rsaSSA-PSS.&id
}
RsaSsa-Pss-sig-caps ::= SEQUENCE {
hashAlg SMIMECapability{{ MaskAlgorithmSet }},
maskAlg SMIMECapability{{ ... }} OPTIONAL,
trailerField INTEGER DEFAULT 1
}
scap-mf-mgf1 SMIME-CAPS ::= {
TYPE SMIMECapability{{ ... }}
IDENTIFIED BY id-mgf1
}
MaskAlgorithmSet SMIME-CAPS ::= {scap-mf-mgf1, ...}
Schaad Informational [Page 16]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
--
-- We define DH/DSA keys from the module in RFC 3279.
--
scap-pk-dsa SMIME-CAPS ::= {
TYPE DSAKeyCapabilities
IDENTIFIED BY pk-dsa.&id
}
DSAKeyCapabilities ::= CHOICE {
keySizes [0] SEQUENCE {
minKeySize DSAKeySize,
maxKeySize DSAKeySize OPTIONAL,
maxSizeP [1] INTEGER OPTIONAL,
maxSizeQ [2] INTEGER OPTIONAL,
maxSizeG [3] INTEGER OPTIONAL
},
keyParams [1] pk-dsa.&Params
}
DSAKeySize ::= INTEGER (1024 | 2048 | 3072 | 7680 | 15360 )
scap-pk-dh SMIME-CAPS ::= {
TYPE DSAKeyCapabilities
IDENTIFIED BY pk-dh.&id
}
--
-- We define Elliptic Curve keys from the module in RFC 3279.
--
scap-pk-ec SMIME-CAPS ::= {
TYPE EC-SMimeCaps
IDENTIFIED BY pk-ec.&id
}
EC-SMimeCaps ::= SEQUENCE (SIZE (1..MAX)) OF ECParameters
scap-pk-ecDH SMIME-CAPS ::= {
TYPE EC-SMimeCaps
IDENTIFIED BY pk-ecDH.&id
}
scap-pk-ecMQV SMIME-CAPS ::= {
TYPE EC-SMimeCaps
IDENTIFIED BY pk-ecMQV.&id
}
Schaad Informational [Page 17]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
END
Appendix B. 1988 ASN.1 Module
This appendix contains the normative ASN.1 module for this document.
PUBLIC-KEY-SMIME-CAPABILITIES-88
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pubKeySMIMECaps-88(77) }
DEFINITIONS ::=
BEGIN
IMPORTS
ECParameters
FROM PKIX1Algorithms2008
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
45 }
id-mgf1
FROM PKIX1-PSS-OAEP-Algorithms
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkix1-rsa-pkalgs(33) }
AlgorithmIdentifier
FROM PKIX1Explicit88
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-pkix1-explicit(18) }
;
--
-- We define RSA keys from the modules in RFC 3279 and RFC 4055.
--
RSAKeyCapabilities ::= SEQUENCE {
minKeySize RSAKeySize,
maxKeySize RSAKeySize OPTIONAL
}
RSAKeySize ::= INTEGER (1024 | 2048 | 3072 | 4096 | 7680 |
8192 | 15360, ...)
RsaSsa-Pss-sig-caps ::= SEQUENCE {
Schaad Informational [Page 18]
RFC 6664 S/MIME Capabilities for Public Keys July 2012
hashAlg AlgorithmIdentifier,
maskAlg AlgorithmIdentifier OPTIONAL,
trailerField INTEGER DEFAULT 1
}
--
-- We define DH/DSA keys from the module in RFC 3279.
--
DSAKeyCapabilities ::= CHOICE {
keySizes [0] SEQUENCE {
minKeySize DSAKeySize,
maxKeySize DSAKeySize OPTIONAL,
maxSizeP [1] INTEGER OPTIONAL,
maxSizeQ [2] INTEGER OPTIONAL,
maxSizeG [3] INTEGER OPTIONAL
},
keyParams [1] pk-dsa.&Params
}
DSAKeySize ::= INTEGER (1024 | 2048 | 3072 | 7680 | 15360 )
--
-- We define Elliptic Curve keys from the module in RFC 3279.
--
EC-SMimeCaps ::= SEQUENCE (SIZE (1..MAX)) OF ECParameters
END
Appendix C. Future Work
A future revision of [RFC5912] should be done at some point to expand
the definition of the PUBLIC-KEY class and allow for an
SMIMECapability to be included in the class definition. This would
encourage people to think about this as an issue when defining new
public key structures in the future.
Author's Address
Jim Schaad
Soaring Hawk Consulting
EMail: ietf@augustcellars.com
Schaad Informational [Page 19]