Internet Engineering Task Force (IETF) B. Ramsdell
Request for Comments: 5750 Brute Squad Labs
Obsoletes: 3850 S. Turner
Category: Standards Track IECA
ISSN: 2070-1721 January 2010
Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2
Certificate Handling
Abstract
This document specifies conventions for X.509 certificate usage by
Secure/Multipurpose Internet Mail Extensions (S/MIME) v3.2 agents.
S/MIME provides a method to send and receive secure MIME messages,
and certificates are an integral part of S/MIME agent processing.
S/MIME agents validate certificates as described in RFC 5280, the
Internet X.509 Public Key Infrastructure Certificate and CRL Profile.
S/MIME agents must meet the certificate processing requirements in
this document as well as those in RFC 5280. This document obsoletes
RFC 3850.
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/rfc5750.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Ramsdell & Turner Standards Track [Page 1]RFC 5750 S/MIME 3.2 Certificate Handling January 2010
include Simplified BSD License text as described in Section 4.e of
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Table of Contents
1. Introduction ....................................................3
1.1. Definitions ................................................3
1.2. Conventions Used in This Document ..........................4
1.3. Compatibility with Prior Practice S/MIME ...................4
1.4. Changes from S/MIME v3 to S/MIME v3.1 ......................5
1.5. Changes since S/MIME v3.1 ..................................5
2. CMS Options .....................................................6
2.1. Certificate Revocation Lists ...............................6
2.2. Certificate Choices ........................................6
2.3. CertificateSet .............................................7
3. Using Distinguished Names for Internet Mail .....................8
4. Certificate Processing ..........................................9
4.1. Certificate Revocation Lists ..............................10
4.2. Certificate Path Validation ...............................11
4.3. Certificate and CRL Signing Algorithms and Key Sizes ......11
4.4. PKIX Certificate Extensions ...............................12
5. Security Considerations ........................................15
6. References .....................................................17
6.1. Reference Conventions .....................................17
6.2. Normative References ......................................17
6.3. Informative References ....................................19
Appendix A. Moving S/MIME v2 Certificate Handling to Historic
Status.................................................21
Appendix B. Acknowledgments........................................21
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1. Introduction
S/MIME (Secure/Multipurpose Internet Mail Extensions) v3.2, described
in [SMIME-MSG], provides a method to send and receive secure MIME
messages. Before using a public key to provide security services,
the S/MIME agent MUST verify that the public key is valid. S/MIME
agents MUST use PKIX certificates to validate public keys as
described in the Internet X.509 Public Key Infrastructure (PKIX)
Certificate and CRL Profile [KEYM]. S/MIME agents MUST meet the
certificate processing requirements documented in this document in
addition to those stated in [KEYM].
This specification is compatible with the Cryptographic Message
Syntax (CMS) RFC 5652 [CMS] in that it uses the data types defined by
CMS. It also inherits all the varieties of architectures for
certificate-based key management supported by CMS.
1.1. Definitions
For the purposes of this document, the following definitions apply.
ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.680
[X.680].
Attribute certificate (AC): An X.509 AC is a separate structure from
a subject's public key X.509 certificate. A subject may have
multiple X.509 ACs associated with each of its public key X.509
certificates. Each X.509 AC binds one or more attributes with one of
the subject's public key X.509 certificates. The X.509 AC syntax is
defined in [ACAUTH].
Certificate: A type that binds an entity's name to a public key with
a digital signature. This type is defined in the Internet X.509
Public Key Infrastructure (PKIX) Certificate and CRL Profile [KEYM].
This type also contains the distinguished name of the certificate
issuer (the signer), an issuer-specific serial number, the issuer's
signature algorithm identifier, a validity period, and extensions
also defined in that document.
Certificate Revocation List (CRL): A type that contains information
about certificates whose validity an issuer has prematurely revoked.
The information consists of an issuer name, the time of issue, the
next scheduled time of issue, a list of certificate serial numbers
and their associated revocation times, and extensions as defined in
[KEYM]. The CRL is signed by the issuer. The type intended by this
specification is the one defined in [KEYM].
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RFC 5750 S/MIME 3.2 Certificate Handling January 2010
Receiving agent: Software that interprets and processes S/MIME CMS
objects, MIME body parts that contain CMS objects, or both.
Sending agent: Software that creates S/MIME CMS objects, MIME body
parts that contain CMS objects, or both.
S/MIME agent: User software that is a receiving agent, a sending
agent, or both.
1.2. Conventions Used in This Document
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 [MUSTSHOULD].
We define some additional terms here:
SHOULD+ This term means the same as SHOULD. However, the authors
expect that a requirement marked as SHOULD+ will be
promoted at some future time to be a MUST.
SHOULD- This term means the same as SHOULD. However, the authors
expect that a requirement marked as SHOULD- will be
demoted to a MAY in a future version of this document.
MUST- This term means the same as MUST. However, the authors
expect that this requirement will no longer be a MUST in a
future document. Although its status will be determined
at a later time, it is reasonable to expect that if a
future revision of a document alters the status of a MUST-
requirement, it will remain at least a SHOULD or a
SHOULD-.
1.3. Compatibility with Prior Practice S/MIME
S/MIME version 3.2 agents ought to attempt to have the greatest
interoperability possible with agents for prior versions of S/MIME.
S/MIME version 2 is described in RFC 2311 through RFC 2315 inclusive
[SMIMEv2], S/MIME version 3 is described in RFC 2630 through RFC 2634
inclusive and RFC 5035 [SMIMEv3], and S/MIME version 3.1 is described
in RFC 3850, RFC 3851, RFC 3852, RFC 2634, and RFC 5035 [SMIMEv3.1].
RFC 2311 also has historical information about the development of
S/MIME.
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1.4. Changes from S/MIME v3 to S/MIME v3.1
Version 1 and version 2 CRLs MUST be supported.
Multiple certification authority (CA) certificates with the same
subject and public key, but with overlapping validity periods, MUST
be supported.
Version 2 attribute certificates SHOULD be supported, and version 1
attributes certificates MUST NOT be used.
The use of the MD2 digest algorithm for certificate signatures is
discouraged, and security language was added.
Clarified use of email address use in certificates. Certificates
that do not contain an email address have no requirements for
verifying the email address associated with the certificate.
Receiving agents SHOULD display certificate information when
displaying the results of signature verification.
Receiving agents MUST NOT accept a signature made with a certificate
that does not have the digitalSignature or nonRepudiation bit set.
Clarifications for the interpretation of the key usage and extended
key usage extensions.
1.5. Changes since S/MIME v3.1
Conventions Used in This Document: Moved to Section 1.2. Added
definitions for SHOULD+, SHOULD-, and MUST-.
Section 1.1: Updated ASN.1 definition and reference.
Section 1.3: Added text about v3.1 RFCs.
Section 3: Aligned email address text with RFC 5280. Updated
note to indicate emailAddress IA5String upper bound is
255 characters. Added text about matching email
addresses.
Section 4.2: Added text to indicate how S/MIME agents locate the
correct user certificate.
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Section 4.3: RSA with SHA-256 (PKCS #1 v1.5) added as MUST; DSA
with SHA-256 added as SHOULD+; RSA with SHA-1, DSA
with SHA-1, and RSA with MD5 changed to SHOULD-; and
RSASSA-PSS with SHA-256 added as SHOULD+. Updated key
sizes and changed pointer to PKIX RFCs.
Section 4.4.1: Aligned with PKIX on use of basic constraints
extension in CA certificates. Clarified which
extension is used to constrain end entities from using
their keys to perform issuing authority operations.
Section 5: Updated security considerations.
Section 7: Moved references from Appendix B to Section 6.
Updated the references.
Appendix A: Moved Appendix A to Appendix B. Added Appendix A to
move S/MIME v2 Certificate Handling to Historic
Status.
2. CMS Options
The CMS message format allows for a wide variety of options in
content and algorithm support. This section puts forth a number of
support requirements and recommendations in order to achieve a base
level of interoperability among all S/MIME implementations. Most of
the CMS format for S/MIME messages is defined in [SMIME-MSG].
2.1. Certificate Revocation Lists
Receiving agents MUST support the Certificate Revocation List (CRL)
format defined in [KEYM]. If sending agents include CRLs in outgoing
messages, the CRL format defined in [KEYM] MUST be used. In all
cases, both v1 and v2 CRLs MUST be supported.
All agents MUST be capable of performing revocation checks using CRLs
as specified in [KEYM]. All agents MUST perform revocation status
checking in accordance with [KEYM]. Receiving agents MUST recognize
CRLs in received S/MIME messages.
Agents SHOULD store CRLs received in messages for use in processing
later messages.
2.2. Certificate Choices
Receiving agents MUST support v1 X.509 and v3 X.509 certificates as
profiled in [KEYM]. End-entity certificates MAY include an Internet
mail address, as described in Section 3.
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Receiving agents SHOULD support X.509 version 2 attribute
certificates. See [ACAUTH] for details about the profile for
attribute certificates.
2.2.1. Historical Note about CMS Certificates
The CMS message format supports a choice of certificate formats for
public key content types: PKIX, PKCS #6 extended certificates
[PKCS6], and PKIX attribute certificates.
The PKCS #6 format is not in widespread use. In addition, PKIX
certificate extensions address much of the same functionality and
flexibility as was intended in the PKCS #6. Thus, sending and
receiving agents MUST NOT use PKCS #6 extended certificates.
X.509 version 1 attribute certificates are also not widely
implemented, and have been superseded with version 2 attribute
certificates. Sending agents MUST NOT send version 1 attribute
certificates.
2.3. CertificateSet
Receiving agents MUST be able to handle an arbitrary number of
certificates of arbitrary relationship to the message sender and to
each other in arbitrary order. In many cases, the certificates
included in a signed message may represent a chain of certification
from the sender to a particular root. There may be, however,
situations where the certificates in a signed message may be
unrelated and included for convenience.
Sending agents SHOULD include any certificates for the user's public
key(s) and associated issuer certificates. This increases the
likelihood that the intended recipient can establish trust in the
originator's public key(s). This is especially important when
sending a message to recipients that may not have access to the
sender's public key through any other means or when sending a signed
message to a new recipient. The inclusion of certificates in
outgoing messages can be omitted if S/MIME objects are sent within a
group of correspondents that has established access to each other's
certificates by some other means such as a shared directory or manual
certificate distribution. Receiving S/MIME agents SHOULD be able to
handle messages without certificates using a database or directory
lookup scheme.
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A sending agent SHOULD include at least one chain of certificates up
to, but not including, a certification authority (CA) that it
believes that the recipient may trust as authoritative. A receiving
agent MUST be able to handle an arbitrarily large number of
certificates and chains.
Agents MAY send CA certificates, that is, cross-certificates, self-
issued certificates, and self-signed certificates. Note that
receiving agents SHOULD NOT simply trust any self-signed certificates
as valid CAs, but SHOULD use some other mechanism to determine if
this is a CA that should be trusted. Also note that when
certificates contain Digital Signature Algorithm (DSA) public keys
the parameters may be located in the root certificate. This would
require that the recipient possess both the end-entity certificate
and the root certificate to perform a signature verification, and is
a valid example of a case where transmitting the root certificate may
be required.
Receiving agents MUST support chaining based on the distinguished
name fields. Other methods of building certificate chains MAY be
supported.
Receiving agents SHOULD support the decoding of X.509 attribute
certificates included in CMS objects. All other issues regarding the
generation and use of X.509 attribute certificates are outside of the
scope of this specification. One specification that addresses
attribute certificate use is defined in [SECLABEL].
3. Using Distinguished Names for Internet Mail
End-entity certificates MAY contain an Internet mail address as
described in [KEYM], Section 4.2.1.6. The email address SHOULD be in
the subjectAltName extension, and SHOULD NOT be in the subject
distinguished name.
Receiving agents MUST recognize and accept certificates that contain
no email address. Agents are allowed to provide an alternative
mechanism for associating an email address with a certificate that
does not contain an email address, such as through the use of the
agent's address book, if available. Receiving agents MUST recognize
email addresses in the subjectAltName field. Receiving agents MUST
recognize email addresses in the Distinguished Name field in the PKCS
#9 [PKCS9] emailAddress attribute:
pkcs-9-at-emailAddress OBJECT IDENTIFIER ::=
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1 }
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Note that this attribute MUST be encoded as IA5String and has an
upper bound of 255 characters. The right side of the email address
SHOULD be treated as ASCII-case-insensitive.
Sending agents SHOULD make the address in the From or Sender header
in a mail message match an Internet mail address in the signer's
certificate. Receiving agents MUST check that the address in the
From or Sender header of a mail message matches an Internet mail
address, if present, in the signer's certificate, if mail addresses
are present in the certificate. A receiving agent SHOULD provide
some explicit alternate processing of the message if this comparison
fails, which may be to display a message that shows the recipient the
addresses in the certificate or other certificate details.
A receiving agent SHOULD display a subject name or other certificate
details when displaying an indication of successful or unsuccessful
signature verification.
All subject and issuer names MUST be populated (i.e., not an empty
SEQUENCE) in S/MIME-compliant X.509 certificates, except that the
subject distinguished name (DN) in a user's (i.e., end-entity)
certificate MAY be an empty SEQUENCE in which case the subjectAltName
extension will include the subject's identifier and MUST be marked as
critical.
4. Certificate Processing
S/MIME agents need to provide some certificate retrieval mechanism in
order to gain access to certificates for recipients of digital
envelopes. There are many ways to implement certificate retrieval
mechanisms. [X.500] directory service is an excellent example of a
certificate retrieval-only mechanism that is compatible with classic
X.500 Distinguished Names. Another method under consideration by the
IETF is to provide certificate retrieval services as part of the
existing Domain Name System (DNS). Until such mechanisms are widely
used, their utility may be limited by the small number of the
correspondent's certificates that can be retrieved. At a minimum,
for initial S/MIME deployment, a user agent could automatically
generate a message to an intended recipient requesting the
recipient's certificate in a signed return message.
Receiving and sending agents SHOULD also provide a mechanism to allow
a user to "store and protect" certificates for correspondents in such
a way so as to guarantee their later retrieval. In many
environments, it may be desirable to link the certificate
retrieval/storage mechanisms together in some sort of certificate
database. In its simplest form, a certificate database would be
local to a particular user and would function in a similar way as an
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RFC 5750 S/MIME 3.2 Certificate Handling January 2010
"address book" that stores a user's frequent correspondents. In this
way, the certificate retrieval mechanism would be limited to the
certificates that a user has stored (presumably from incoming
messages). A comprehensive certificate retrieval/storage solution
may combine two or more mechanisms to allow the greatest flexibility
and utility to the user. For instance, a secure Internet mail agent
may resort to checking a centralized certificate retrieval mechanism
for a certificate if it cannot be found in a user's local certificate
storage/retrieval database.
Receiving and sending agents SHOULD provide a mechanism for the
import and export of certificates, using a CMS certs-only message.
This allows for import and export of full certificate chains as
opposed to just a single certificate. This is described in
[SMIME-MSG].
Agents MUST handle multiple valid certification authority (CA)
certificates containing the same subject name and the same public
keys but with overlapping validity intervals.
4.1. Certificate Revocation Lists
In general, it is always better to get the latest CRL information
from a CA than to get information stored away from incoming messages.
A receiving agent SHOULD have access to some CRL retrieval mechanism
in order to gain access to certificate revocation information when
validating certification paths. A receiving or sending agent SHOULD
also provide a mechanism to allow a user to store incoming
certificate revocation information for correspondents in such a way
so as to guarantee its later retrieval.
Receiving and sending agents SHOULD retrieve and utilize CRL
information every time a certificate is verified as part of a
certification path validation even if the certificate was already
verified in the past. However, in many instances (such as off-line
verification) access to the latest CRL information may be difficult
or impossible. The use of CRL information, therefore, may be
dictated by the value of the information that is protected. The
value of the CRL information in a particular context is beyond the
scope of this specification but may be governed by the policies
associated with particular certification paths.
All agents MUST be capable of performing revocation checks using CRLs
as specified in [KEYM]. All agents MUST perform revocation status
checking in accordance with [KEYM]. Receiving agents MUST recognize
CRLs in received S/MIME messages.
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RFC 5750 S/MIME 3.2 Certificate Handling January 2010
4.2. Certificate Path Validation
In creating a user agent for secure messaging, certificate, CRL, and
certification path validation SHOULD be highly automated while still
acting in the best interests of the user. Certificate, CRL, and path
validation MUST be performed as per [KEYM] when validating a
correspondent's public key. This is necessary before using a public
key to provide security services such as verifying a signature,
encrypting a content-encryption key (e.g., RSA), or forming a
pairwise symmetric key (e.g., Diffie-Hellman) to be used to encrypt
or decrypt a content-encryption key.
Certificates and CRLs are made available to the path validation
procedure in two ways: a) incoming messages, and b) certificate and
CRL retrieval mechanisms. Certificates and CRLs in incoming messages
are not required to be in any particular order nor are they required
to be in any way related to the sender or recipient of the message
(although in most cases they will be related to the sender).
Incoming certificates and CRLs SHOULD be cached for use in path
validation and optionally stored for later use. This temporary
certificate and CRL cache SHOULD be used to augment any other
certificate and CRL retrieval mechanisms for path validation on
incoming signed messages.
When verifying a signature and the certificates that are included in
the message, if a signingCertificate attribute from RFC 2634 [ESS] or
a signingCertificateV2 attribute from RFC 5035 [ESS] is found in an
S/MIME message, it SHALL be used to identify the signer's
certificate. Otherwise, the certificate is identified in an S/MIME
message, either using the issuerAndSerialNumber, which identifies the
signer's certificate by the issuer's distinguished name and the
certificate serial number, or the subjectKeyIdentifier, which
identifies the signer's certificate by a key identifier.
When decrypting an encrypted message, if a
SMIMEEncryptionKeyPreference attribute is found in an encapsulating
SignedData, it SHALL be used to identify the originator's certificate
found in OriginatorInfo. See [CMS] for the CMS fields that reference
the originator's and recipient's certificates.
4.3. Certificate and CRL Signing Algorithms and Key Sizes
Certificates and Certificate Revocation Lists (CRLs) are signed by
the certificate issuer. Receiving agents:
- MUST support RSA with SHA-256
- SHOULD+ support DSA with SHA-256
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- SHOULD+ support RSASSA-PSS with SHA-256
- SHOULD- support RSA with SHA-1
- SHOULD- support DSA with SHA-1
- SHOULD- support RSA with MD5
The following are the RSA and RSASSA-PSS key size requirements for
S/MIME receiving agents during certificate and CRL signature
verification:
key size <= 1023 : MAY (see Section 5)
1024 <= key size <= 4096 : MUST (see Section 5)
4096 < key size : MAY (see Section 5)
The following are the DSA key size requirements for S/MIME receiving
agents during certificate and CRL signature verification:
key size <= 1023 : MAY (see Section 5)
1024 <= key size <= 3072 : MUST (see Section 5)
For 512-bit RSA with SHA-1 see [KEYMALG] and [FIPS186-2] without
Change Notice 1, for 512-bit RSA with SHA-256 see [RSAOAEP] and
[FIPS186-2] without Change Notice 1, for 1024-bit through 3072-bit
RSA with SHA-256 see [RSAOAEP] and [FIPS186-2] with Change Notice 1,
and for 4096-bit RSA with SHA-256 see [RSAOAEP] and [PKCS1]. In
either case, the first reference provides the signature algorithm's
object identifier and the second provides the signature algorithm's
definition.
For 512-bit DSA with SHA-1 see [KEYMALG] and [FIPS186-2] without
Change Notice 1, for 512-bit DSA with SHA-256 see [KEYMALG2] and
[FIPS186-2] without Change Notice 1, for 1024-bit DSA with SHA-1 see
[KEYMALG] and [FIPS186-2] with Change Notice 1, for 1024-bit through
3072 DSA with SHA-256 see [KEYMALG2] and [FIPS186-3]. In either
case, the first reference provides the signature algorithm's object
identifier and the second provides the signature algorithm's
definition.
For RSASSA-PSS with SHA-256 see [RSAPSS].
4.4. PKIX Certificate Extensions
PKIX describes an extensible framework in which the basic certificate
information can be extended and describes how such extensions can be
used to control the process of issuing and validating certificates.
The PKIX Working Group has ongoing efforts to identify and create
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extensions that have value in particular certification environments.
Further, there are active efforts underway to issue PKIX certificates
for business purposes. This document identifies the minimum required
set of certificate extensions that have the greatest value in the
S/MIME environment. The syntax and semantics of all the identified
extensions are defined in [KEYM].
Sending and receiving agents MUST correctly handle the basic
constraints, key usage, authority key identifier, subject key
identifier, and subject alternative names certificate extensions when
they appear in end-entity and CA certificates. Some mechanism SHOULD
exist to gracefully handle other certificate extensions when they
appear in end-entity or CA certificates.
Certificates issued for the S/MIME environment SHOULD NOT contain any
critical extensions (extensions that have the critical field set to
TRUE) other than those listed here. These extensions SHOULD be
marked as non-critical unless the proper handling of the extension is
deemed critical to the correct interpretation of the associated
certificate. Other extensions may be included, but those extensions
SHOULD NOT be marked as critical.
Interpretation and syntax for all extensions MUST follow [KEYM],
unless otherwise specified here.
4.4.1. Basic Constraints
The basic constraints extension serves to delimit the role and
position that an issuing authority or end-entity certificate plays in
a certification path.
For example, certificates issued to CAs and subordinate CAs contain a
basic constraint extension that identifies them as issuing authority
certificates. End-entity certificates contain the key usage
extension that restrains end entities from using the key when
performing issuing authority operations (see Section 4.4.2).
As per [KEYM], certificates MUST contain a basicConstraints extension
in CA certificates, and SHOULD NOT contain that extension in end-
entity certificates.
4.4.2. Key Usage Certificate Extension
The key usage extension serves to limit the technical purposes for
which a public key listed in a valid certificate may be used.
Issuing authority certificates may contain a key usage extension that
restricts the key to signing certificates, certificate revocation
lists, and other data.
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RFC 5750 S/MIME 3.2 Certificate Handling January 2010
For example, a certification authority may create subordinate issuer
certificates that contain a key usage extension that specifies that
the corresponding public key can be used to sign end user
certificates and sign CRLs.
If a key usage extension is included in a PKIX certificate, then it
MUST be marked as critical.
S/MIME receiving agents MUST NOT accept the signature of a message if
it was verified using a certificate that contains the key usage
extension without either the digitalSignature or nonRepudiation bit
set. Sometimes S/MIME is used as a secure message transport for
applications beyond interpersonal messaging. In such cases, the
S/MIME-enabled application can specify additional requirements
concerning the digitalSignature or nonRepudiation bits within this
extension.
If the key usage extension is not specified, receiving clients MUST
presume that the digitalSignature and nonRepudiation bits are set.
4.4.3. Subject Alternative Name
The subject alternative name extension is used in S/MIME as the
preferred means to convey the email address(es) that correspond(s) to
the entity for this certificate. Any email addresses present MUST be
encoded using the rfc822Name CHOICE of the GeneralName type as
described in [KEYM], Section 4.2.1.6. Since the SubjectAltName type
is a SEQUENCE OF GeneralName, multiple email addresses MAY be
present.
4.4.4. Extended Key Usage Extension
The extended key usage extension also serves to limit the technical
purposes for which a public key listed in a valid certificate may be
used. The set of technical purposes for the certificate therefore
are the intersection of the uses indicated in the key usage and
extended key usage extensions.
For example, if the certificate contains a key usage extension
indicating digital signature and an extended key usage extension that
includes the email protection OID, then the certificate may be used
for signing but not encrypting S/MIME messages. If the certificate
contains a key usage extension indicating digital signature but no
extended key usage extension, then the certificate may also be used
to sign but not encrypt S/MIME messages.
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RFC 5750 S/MIME 3.2 Certificate Handling January 2010
If the extended key usage extension is present in the certificate,
then interpersonal message S/MIME receiving agents MUST check that it
contains either the emailProtection or the anyExtendedKeyUsage OID as
defined in [KEYM]. S/MIME uses other than interpersonal messaging
MAY require the explicit presence of the extended key usage extension
or other OIDs to be present in the extension or both.
5. Security Considerations
All of the security issues faced by any cryptographic application
must be faced by a S/MIME agent. Among these issues are protecting
the user's private key, preventing various attacks, and helping the
user avoid mistakes such as inadvertently encrypting a message for
the wrong recipient. The entire list of security considerations is
beyond the scope of this document, but some significant concerns are
listed here.
When processing certificates, there are many situations where the
processing might fail. Because the processing may be done by a user
agent, a security gateway, or other program, there is no single way
to handle such failures. Just because the methods to handle the
failures have not been listed, however, the reader should not assume
that they are not important. The opposite is true: if a certificate
is not provably valid and associated with the message, the processing
software should take immediate and noticeable steps to inform the end
user about it.
Some of the many places where signature and certificate checking
might fail include:
- no Internet mail addresses in a certificate match the sender of a
message, if the certificate contains at least one mail address
- no certificate chain leads to a trusted CA
- no ability to check the CRL for a certificate
- an invalid CRL was received
- the CRL being checked is expired
- the certificate is expired
- the certificate has been revoked
Ramsdell & Turner Standards Track [Page 15]RFC 5750 S/MIME 3.2 Certificate Handling January 2010
There are certainly other instances where a certificate may be
invalid, and it is the responsibility of the processing software to
check them all thoroughly, and to decide what to do if the check
fails.
It is possible for there to be multiple unexpired CRLs for a CA. If
an agent is consulting CRLs for certificate validation, it SHOULD
make sure that the most recently issued CRL for that CA is consulted,
since an S/MIME message sender could deliberately include an older
unexpired CRL in an S/MIME message. This older CRL might not include
recently revoked certificates, which might lead an agent to accept a
certificate that has been revoked in a subsequent CRL.
When determining the time for a certificate validity check, agents
have to be careful to use a reliable time. Unless it is from a
trusted agent, this time MUST NOT be the SigningTime attribute found
in an S/MIME message. For most sending agents, the SigningTime
attribute could be deliberately set to direct the receiving agent to
check a CRL that could have out-of-date revocation status for a
certificate, or cause an improper result when checking the Validity
field of a certificate.
In addition to the Security Considerations identified in [KEYM],
caution should be taken when processing certificates that have not
first been validated to a trust anchor. Certificates could be
manufactured by untrusted sources for the purpose of mounting denial
of service or other attacks. For example, keys selected to require
excessive cryptographic processing, or extensive lists of CRL
Distribution Point (CDP) and/or Authority Information Access (AIA)
addresses in the certificate, could be used to mount denial-of-
service attacks. Similarly, attacker-specified CDP and/or AIA
addresses could be included in fake certificates to allow the
originator to detect receipt of the message even if signature
verification fails.
The 4096-bit RSA key size requirement for certificate and CRL
verification is larger than the 2048-bit RSA key sizes for message
signature generation/verification or message encryption/decryption in
[SMIME-MSG] because many root CAs included in certificate stores have
already issued root certificates with the 4096-bit key. The standard
that defines comparable key sizes for DSA is not yet available. In
particular, [FIPS186-2] without Change Notice 1 allowed DSA key sizes
between 512 and 1024 bits, [FIPS186-2] with Change Notice 1 only
allowed DSA key sizes of 1024 bits, and [FIPS186-3] allowed DSA key
sizes from 1024 to 3072 bits. Further, 4096-bit keys are normally
only used by Root certificates and not by subordinate CA
certificates, thereby lengthening the root CA certificate's validity
period.
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RFC 5750 S/MIME 3.2 Certificate Handling January 2010
RSA and DSA keys of less than 1024 bits are now considered by many
experts to be cryptographically insecure (due to advances in
computing power), and should no longer be used to sign certificates
or CRLs. Such keys were previously considered secure, so processing
previously received signed and encrypted mail may require processing
certificates or CRLs signed with weak keys. Implementations that
wish to support previous versions of S/MIME or process old messages
need to consider the security risks that result from accepting
certificates and CRLs with smaller key sizes (e.g., spoofed
certificates) versus the costs of denial of service. If an
implementation supports verification of certificates or CRLs
generated with RSA and DSA keys of less than 1024 bits, it MUST warn
the user. Implementers should consider providing a stronger warning
for weak signatures on certificates and CRLs associated with newly
received messages than the one provided for certificates and CRLs
associated with previously stored messages. Server implementations
(e.g., secure mail list servers) where user warnings are not
appropriate SHOULD reject messages with weak cryptography.
If an implementation is concerned about compliance with National
Institute of Standards and Technology (NIST) key size
recommendations, then see [SP800-57].
6. References
6.1. Reference Conventions
[CMS] refers to [RFC5652].
[ESS] refers to [RFC2634] and [RFC5035].
[SMIMEv2] refers to [RFC2311], [RFC2312], [RFC2313], [RFC2314], and
[RFC2315].
[SMIMEv3] refers to [RFC2630], [RFC2631], [RFC2632], [RFC2633],
[RFC2634], and [RFC5035].
[SMIMv3.1] refers to [RFC2634], [RFC3850], [RFC3851], [RFC3852], and
[RFC5035].
6.2. Normative References
[ACAUTH] Farrell, S., Housley, R., and S. Turner, "An Internet
Attribute Certificate Profile for Authorization", RFC
5755, January 2010.
[RFC2634] Hoffman, P., Ed., "Enhanced Security Services for
S/MIME", RFC 2634, June 1999.
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[RFC5035] Schaad, J., "Enhanced Security Services (ESS) Update:
Adding CertID Algorithm Agility", RFC 5035, August 2007.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
5652, September 2009.
[FIPS186-2] National Institute of Standards and Technology (NIST),
"Digital Signature Standard (DSS)", FIPS Publication
186-3, January 2000. [With Change Notice 1]
[FIPS186-3] National Institute of Standards and Technology (NIST),
FIPS Publication 186-3: Digital Signature Standard, June
2009.
[KEYM] 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.
[KEYMALG] 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.
[KEYMALG2] Dang, Q., Santesson, S., Moriarty, K., Brown, D., and T.
Polk, "Internet X.509 Public Key Infrastructure:
Additional Algorithms and Identifiers for DSA and
ECDSA", RFC 5758, January 2010.
[MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[PKCS1] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[PKCS9] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
Classes and Attribute Types Version 2.0", RFC 2985,
November 2000.
[RSAPSS] Schaad, J., "Use of the RSASSA-PSS Signature Algorithm
in Cryptographic Message Syntax (CMS)", RFC 4056, June
2005.
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[RSAOAEP] 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.
[SMIME-MSG] Ramsdell, B. and S. Turner, "Secure/Multipurpose
Internet Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[X.680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002.
Information Technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation.
6.3. Informative References
[PKCS6] RSA Laboratories, "PKCS #6: Extended-Certificate Syntax
Standard", November 1993.
[SECLABEL] Nicolls, W., "Implementing Company Classification Policy
with the S/MIME Security Label", RFC 3114, May 2002.
[RFC2311] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and
L. Repka, "S/MIME Version 2 Message Specification", RFC
2311, March 1998.
[RFC2312] Dusse, S., Hoffman, P., Ramsdell, B., and J. Weinstein,
"S/MIME Version 2 Certificate Handling", RFC 2312, March
1998.
[RFC2313] Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", RFC
2313, March 1998.
[RFC2314] Kaliski, B., "PKCS #10: Certification Request Syntax
Version 1.5", RFC 2314, March 1998.
[RFC2315] Kaliski, B., "PKCS #7: Cryptographic Message Syntax
Version 1.5", RFC 2315, March 1998.
[RFC2630] Housley, R., "Cryptographic Message Syntax", RFC 2630,
June 1999.
[RFC2631] Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC
2631, June 1999.
[RFC2632] Ramsdell, B., Ed., "S/MIME Version 3 Certificate
Handling", RFC 2632, June 1999.
Ramsdell & Turner Standards Track [Page 19]RFC 5750 S/MIME 3.2 Certificate Handling January 2010
[RFC2633] Ramsdell, B., Ed., "S/MIME Version 3 Message
Specification", RFC 2633, June 1999.
[RFC3850] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Certificate Handling",
RFC 3850, July 2004.
[RFC3851] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.
[RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
3852, July 2004.
[SP800-57] National Institute of Standards and Technology (NIST),
Special Publication 800-57: Recommendation for Key
Management, August 2005.
[X.500] ITU-T Recommendation X.500 (1997) | ISO/IEC 9594-
1:1997, Information technology - Open Systems
Interconnection - The Directory: Overview of concepts,
models and services.
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Appendix A. Moving S/MIME v2 Certificate Handling to Historic Status
The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], and v3.2 (this document)
are backwards compatible with the S/MIME v2 Certificate Handling
Specification [SMIMEv2], with the exception of the algorithms
(dropped RC2/40 requirement and added DSA and RSASSA-PSS
requirements). Therefore, it is recommended that RFC 2312 [SMIMEv2]
be moved to Historic status.
Appendix B. Acknowledgments
Many thanks go out to the other authors of the S/MIME v2 RFC: Steve
Dusse, Paul Hoffman, and Jeff Weinstein. Without v2, there wouldn't
be a v3, v3.1, or v3.2.
A number of the members of the S/MIME Working Group have also worked
very hard and contributed to this document. Any list of people is
doomed to omission, and for that I apologize. In alphabetical order,
the following people stand out in my mind because they made direct
contributions to this document.
Bill Flanigan, Trevor Freeman, Elliott Ginsburg, Alfred Hoenes, Paul
Hoffman, Russ Housley, David P. Kemp, Michael Myers, John Pawling,
Denis Pinkas, and Jim Schaad.
Authors' Addresses
Blake Ramsdell
Brute Squad Labs, Inc.
EMail: blaker@gmail.com
Sean Turner
IECA, Inc.
3057 Nutley Street, Suite 106
Fairfax, VA 22031
USA
EMail: turners@ieca.com
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