Alternative name forms may also appear in the issuerAltName extension
(see sec. 5.2.2). The issuer name field MUST contain an X.500
distinguished name (DN). The issuer name field is defined as the
X.501 type Name, and MUST follow the encoding rules for the issuer
name field in the certificate (see sec. 4.1.2.4).
5.1.2.4 This Update
This field indicates the issue date of this CRL. ThisUpdate may be
encoded as UTCTime or GeneralizedTime.
CAs conforming to this profile that issue CRLs MUST encode thisUpdate
as UTCTime for dates through the year 2049. CAs conforming to this
profile that issue CRLs MUST encode thisUpdate as GeneralizedTime for
dates in the year 2050 or later.
Where encoded as UTCTime, thisUpdate MUST be specified and
interpreted as defined in section 4.1.2.5.1. Where encoded as
GeneralizedTime, thisUpdate MUST be specified and interpreted as
defined in section 4.1.2.5.2.
5.1.2.5 Next Update
This field indicates the date by which the next CRL will be issued.
The next CRL could be issued before the indicated date, but it will
not be issued any later than the indicated date. CAs SHOULD issue
CRLs with a nextUpdate time equal to or later than all previous CRLs.
nextUpdate may be encoded as UTCTime or GeneralizedTime.
This profile requires inclusion of nextUpdate in all CRLs issued by
conforming CAs. Note that the ASN.1 syntax of TBSCertList describes
this field as OPTIONAL, which is consistent with the ASN.1 structure
defined in [X.509]. The behavior of clients processing CRLs which
omit nextUpdate is not specified by this profile.
CAs conforming to this profile that issue CRLs MUST encode nextUpdate
as UTCTime for dates through the year 2049. CAs conforming to this
profile that issue CRLs MUST encode nextUpdate as GeneralizedTime for
dates in the year 2050 or later.
Where encoded as UTCTime, nextUpdate MUST be specified and
interpreted as defined in section 4.1.2.5.1. Where encoded as
GeneralizedTime, nextUpdate MUST be specified and interpreted as
defined in section 4.1.2.5.2.
5.1.2.6 Revoked Certificates
Revoked certificates are listed. The revoked certificates are named
by their serial numbers. Certificates revoked by the CA are uniquely
identified by the certificate serial number. The date on which the
revocation occurred is specified. The time for revocationDate MUST
be expressed as described in section 5.1.2.4. Additional information
may be supplied in CRL entry extensions; CRL entry extensions are
discussed in section 5.3.
5.1.2.7 Extensions
This field may only appear if the version is 2 (see sec. 5.1.2.1).
If present, this field is a SEQUENCE of one or more CRL extensions.
CRL extensions are discussed in section 5.2.
5.2 CRL Extensions
The extensions defined by ANSI X9 and ISO/IEC/ITU for X.509 v2 CRLs
[X.509] [X9.55] provide methods for associating additional attributes
with CRLs. The X.509 v2 CRL format also allows communities to define
private extensions to carry information unique to those communities.
Each extension in a CRL may be designated as critical or non-
critical. A CRL validation MUST fail if it encounters a critical
extension which it does not know how to process. However, an
unrecognized non-critical extension may be ignored. The following
subsections present those extensions used within Internet CRLs.
Communities may elect to include extensions in CRLs which are not
defined in this specification. However, caution should be exercised
in adopting any critical extensions in CRLs which might be used in a
general context.
Conforming CAs that issue CRLs are required to include the authority
key identifier (see sec. 5.2.1) and the CRL number (see sec. 5.2.3)
extensions in all CRLs issued.
5.2.1 Authority Key Identifier
The authority key identifier extension provides a means of
identifying the public key corresponding to the private key used to
sign a CRL. The identification can be based on either the key
identifier (the subject key identifier in the CRL signer's
certificate) or on the issuer name and serial number. This extension
is especially useful where an issuer has more than one signing key,
either due to multiple concurrent key pairs or due to changeover.
Conforming CAs MUST use the key identifier method, and MUST include
this extension in all CRLs issued.
The syntax for this CRL extension is defined in section 4.2.1.1.
5.2.2 Issuer Alternative Name
The issuer alternative names extension allows additional identities
to be associated with the issuer of the CRL. Defined options include
an rfc822 name (electronic mail address), a DNS name, an IP address,
and a URI. Multiple instances of a name and multiple name forms may
be included. Whenever such identities are used, the issuer
alternative name extension MUST be used.
The issuerAltName extension SHOULD NOT be marked critical.
The OID and syntax for this CRL extension are defined in section
4.2.1.8.
5.2.3 CRL Number
The CRL number is a non-critical CRL extension which conveys a
monotonically increasing sequence number for each CRL issued by a CA.
This extension allows users to easily determine when a particular CRL
supersedes another CRL. CAs conforming to this profile MUST include
this extension in all CRLs.
id-ce-cRLNumber OBJECT IDENTIFIER ::= { id-ce 20 }
cRLNumber ::= INTEGER (0..MAX)
5.2.4 Delta CRL Indicator
The delta CRL indicator is a critical CRL extension that identifies a
delta-CRL. The use of delta-CRLs can significantly improve
processing time for applications which store revocation information
in a format other than the CRL structure. This allows changes to be
added to the local database while ignoring unchanged information that
is already in the local database.
When a delta-CRL is issued, the CAs MUST also issue a complete CRL.
The value of BaseCRLNumber identifies the CRL number of the base CRL
that was used as the starting point in the generation of this delta-
CRL. The delta-CRL contains the changes between the base CRL and the
current CRL issued along with the delta-CRL. It is the decision of a
CA as to whether to provide delta-CRLs. Again, a delta-CRL MUST NOT
be issued without a corresponding complete CRL. The value of
CRLNumber for both the delta-CRL and the corresponding complete CRL
MUST be identical.
A CRL user constructing a locally held CRL from delta-CRLs MUST
consider the constructed CRL incomplete and unusable if the CRLNumber
of the received delta-CRL is more than one greater than the CRLnumber
of the delta-CRL last processed.
id-ce-deltaCRLIndicator OBJECT IDENTIFIER ::= { id-ce 27 }
deltaCRLIndicator ::= BaseCRLNumber
BaseCRLNumber ::= CRLNumber
5.2.5 Issuing Distribution Point
The issuing distribution point is a critical CRL extension that
identifies the CRL distribution point for a particular CRL, and it
indicates whether the CRL covers revocation for end entity
certificates only, CA certificates only, or a limitied set of reason
codes. Although the extension is critical, conforming
implementations are not required to support this extension.
The CRL is signed using the CA's private key. CRL Distribution
Points do not have their own key pairs. If the CRL is stored in the
X.500 Directory, it is stored in the Directory entry corresponding to
the CRL distribution point, which may be different than the Directory
entry of the CA.
The reason codes associated with a distribution point shall be
specified in onlySomeReasons. If onlySomeReasons does not appear, the
distribution point shall contain revocations for all reason codes.
CAs may use CRL distribution points to partition the CRL on the basis
of compromise and routine revocation. In this case, the revocations
with reason code keyCompromise (1) and cACompromise (2) appear in one
distribution point, and the revocations with other reason codes
appear in another distribution point.
Where the issuingDistributionPoint extension contains a URL, the
following semantics MUST be assumed: the object is a pointer to the
most current CRL issued by this CA. The URI schemes ftp, http,
mailto [RFC1738] and ldap [RFC1778] are defined for this purpose.
The URI MUST be an absolute, not relative, pathname and MUST specify
the host.
id-ce-issuingDistributionPoint OBJECT IDENTIFIER ::= { id-ce 28 }
issuingDistributionPoint ::= SEQUENCE {
distributionPoint [0] DistributionPointName OPTIONAL,
onlyContainsUserCerts [1] BOOLEAN DEFAULT FALSE,
onlyContainsCACerts [2] BOOLEAN DEFAULT FALSE,
onlySomeReasons [3] ReasonFlags OPTIONAL,
indirectCRL [4] BOOLEAN DEFAULT FALSE }
5.3 CRL Entry Extensions
The CRL entry extensions already defined by ANSI X9 and ISO/IEC/ITU
for X.509 v2 CRLs provide methods for associating additional
attributes with CRL entries [X.509] [X9.55]. The X.509 v2 CRL format
also allows communities to define private CRL entry extensions to
carry information unique to those communities. Each extension in a
CRL entry may be designated as critical or non-critical. A CRL
validation MUST fail if it encounters a critical CRL entry extension
which it does not know how to process. However, an unrecognized
non-critical CRL entry extension may be ignored. The following
subsections present recommended extensions used within Internet CRL
entries and standard locations for information. Communities may
elect to use additional CRL entry extensions; however, caution should
be exercised in adopting any critical extensions in CRL entries which
might be used in a general context.
All CRL entry extensions used in this specification are non-critical.
Support for these extensions is optional for conforming CAs and
applications. However, CAs that issue CRLs SHOULD include reason
codes (see sec. 5.3.1) and invalidity dates (see sec. 5.3.3) whenever
this information is available.
5.3.1 Reason Code
The reasonCode is a non-critical CRL entry extension that identifies
the reason for the certificate revocation. CAs are strongly
encouraged to include meaningful reason codes in CRL entries;
however, the reason code CRL entry extension SHOULD be absent instead
of using the unspecified (0) reasonCode value.
id-ce-cRLReason OBJECT IDENTIFIER ::= { id-ce 21 }
-- reasonCode ::= { CRLReason }
CRLReason ::= ENUMERATED {
unspecified (0),
keyCompromise (1),
cACompromise (2),
affiliationChanged (3),
superseded (4),
cessationOfOperation (5),
certificateHold (6),
removeFromCRL (8) }
5.3.2 Hold Instruction Code
The hold instruction code is a non-critical CRL entry extension that
provides a registered instruction identifier which indicates the
action to be taken after encountering a certificate that has been
placed on hold.
id-ce-holdInstructionCode OBJECT IDENTIFIER ::= { id-ce 23 }
holdInstructionCode ::= OBJECT IDENTIFIER
The following instruction codes have been defined. Conforming
applications that process this extension MUST recognize the following
instruction codes.
holdInstruction OBJECT IDENTIFIER ::=
{ iso(1) member-body(2) us(840) x9-57(10040) 2 }
id-holdinstruction-none OBJECT IDENTIFIER ::= {holdInstruction 1}
id-holdinstruction-callissuer
OBJECT IDENTIFIER ::= {holdInstruction 2}
id-holdinstruction-reject OBJECT IDENTIFIER ::= {holdInstruction 3}
Conforming applications which encounter an id-holdinstruction-
callissuer MUST call the certificate issuer or reject the
certificate. Conforming applications which encounter an id-
holdinstruction-reject MUST reject the certificate. The hold
instruction id-holdinstruction-none is semantically equivalent to the
absence of a holdInstructionCode, and its use is strongly deprecated
for the Internet PKI.
5.3.3 Invalidity Date
The invalidity date is a non-critical CRL entry extension that
provides the date on which it is known or suspected that the private
key was compromised or that the certificate otherwise became invalid.
This date may be earlier than the revocation date in the CRL entry,
which is the date at which the CA processed the revocation. When a
revocation is first posted by a CA in a CRL, the invalidity date may
precede the date of issue of earlier CRLs, but the revocation date
SHOULD NOT precede the date of issue of earlier CRLs. Whenever this
information is available, CAs are strongly encouraged to share it
with CRL users.
The GeneralizedTime values included in this field MUST be expressed
in Greenwich Mean Time (Zulu), and MUST be specified and interpreted
as defined in section 4.1.2.5.2.
id-ce-invalidityDate OBJECT IDENTIFIER ::= { id-ce 24 }
invalidityDate ::= GeneralizedTime
5.3.4 Certificate Issuer
This CRL entry extension identifies the certificate issuer associated
with an entry in an indirect CRL, i.e. a CRL that has the indirectCRL
indicator set in its issuing distribution point extension. If this
extension is not present on the first entry in an indirect CRL, the
certificate issuer defaults to the CRL issuer. On subsequent entries
in an indirect CRL, if this extension is not present, the certificate
issuer for the entry is the same as that for the preceding entry.
This field is defined as follows:
id-ce-certificateIssuer OBJECT IDENTIFIER ::= { id-ce 29 }
certificateIssuer ::= GeneralNames
If used by conforming CAs that issue CRLs, this extension is always
critical. If an implementation ignored this extension it could not
correctly attribute CRL entries to certificates. This specification
RECOMMENDS that implementations recognize this extension.
6 Certification Path Validation
Certification path validation procedures for the Internet PKI are
based on section 12.4.3 of [X.509]. Certification path processing
verifies the binding between the subject distinguished name and/or
subject alternative name and subject public key. The binding is
limited by constraints which are specified in the certificates which
comprise the path. The basic constraints and policy constraints
extensions allow the certification path processing logic to automate
the decision making process.
This section describes an algorithm for validating certification
paths. Conforming implementations of this specification are not
required to implement this algorithm, but MUST be functionally
equivalent to the external behavior resulting from this procedure.
Any algorithm may be used by a particular implementation so long as
it derives the correct result.
In section 6.1, the text describes basic path validation. This text
assumes that all valid paths begin with certificates issued by a
single "most-trusted CA". The algorithm requires the public key of
the CA, the CA's name, the validity period of the public key, and any
constraints upon the set of paths which may be validated using this
key.
The "most-trusted CA" is a matter of policy: it could be a root CA in
a hierarchical PKI; the CA that issued the verifier's own
certificate(s); or any other CA in a network PKI. The path
validation procedure is the same regardless of the choice of "most-
trusted CA."
section 6.2 describes extensions to the basic path validation
algorithm. Two specific cases are discussed: the case where paths may
begin with one of several trusted CAs; and where compatibility with
the PEM architecture is required.
6.1 Basic Path Validation
The text assumes that the trusted public key (and related
information) is contained in a "self-signed" certificate. This
simplifies the description of the path processing procedure. Note
that the signature on the self-signed certificate does not provide
any security services. The trusted public key (and related
information) may be obtained in other formats; the information is
trusted because of other procedures used to obtain and protect it.
The goal of path validation is to verify the binding between a
subject distinguished name or subject alternative name and subject
public key, as represented in the "end entity" certificate, based on
the public key of the "most-trusted CA". This requires obtaining a
sequence of certificates that support that binding. The procedures
performed to obtain this sequence is outside the scope of this
section.
The following text also assumes that certificates do not use subject
or unique identifier fields or private critical extensions, as
recommended within this profile. However, if these components appear
in certificates, they MUST be processed. Finally, policy qualifiers
are also neglected for the sake of clarity.
A certification path is a sequence of n certificates where:
* for all x in {1,(n-1)}, the subject of certificate x is the
issuer of certificate x+1.
* certificate x=1 is the the self-signed certificate, and
* certificate x=n is the end entity certificate.
This section assumes the following inputs are provided to the path
processing logic:
(a) a certification path of length n;
(b) a set of initial policy identifiers (each comprising a
sequence of policy element identifiers), which identifies one or
more certificate policies, any one of which would be acceptable
for the purposes of certification path processing, or the special
value "any-policy";
(c) the current date/time (if not available internally to the
certification path processing module); and
(d) the time, T, for which the validity of the path should be
determined. (This may be the current date/time, or some point in
the past.)
From the inputs, the procedure intializes five state variables:
(a) acceptable policy set: A set of certificate policy
identifiers comprising the policy or policies recognized by the
public key user together with policies deemed equivalent through
policy mapping. The initial value of the acceptable policy set is
the special value "any-policy".
(b) constrained subtrees: A set of root names defining a set of
subtrees within which all subject names in subsequent certificates
in the certification path shall fall. The initial value is
"unbounded".
(c) excluded subtrees: A set of root names defining a set of
subtrees within which no subject name in subsequent certificates
in the certification path may fall. The initial value is "empty".
(d) explicit policy: an integer which indicates if an explicit
policy identifier is required. The integer indicates the first
certificate in the path where this requirement is imposed. Once
set, this variable may be decreased, but may not be increased.
(That is, if a certificate in the path requires explicit policy
identifiers, a later certificate can not remove this requirement.)
The initial value is n+1.
(e) policy mapping: an integer which indicates if policy mapping
is permitted. The integer indicates the last certificate on which
policy mapping may be applied. Once set, this variable may be
decreased, but may not be increased. (That is, if a certificate in
the path specifies policy mapping is not permitted, it can not be
overriden by a later certificate.) The initial value is n+1.
The actions performed by the path processing software for each
certificate i=1 through n are described below. The self-signed
certificate is certificate i=1, the end entity certificate is i=n.
The processing is performed sequentially, so that processing
certificate i affects the state variables for processing certificate
(i+1). Note that actions (h) through (m) are not applied to the end
entity certificate (certificate n).
The path processing actions to be performed are:
(a) Verify the basic certificate information, including:
(1) the certificate was signed using the subject public key
from certificate i-1 (in the special case i=1, this step may be
omitted; if not, use the subject public key from the same
certificate),
(2) the certificate validity period includes time T,
(3) the certificate had not been revoked at time T and is not
currently on hold status that commenced before time T, (this
may be determined by obtaining the appropriate CRL or status
information, or by out-of-band mechanisms), and
(4) the subject and issuer names chain correctly (that is, the
issuer of this certificate was the subject of the previous
certificate.)
(b) Verify that the subject name and subjectAltName extension
(critical or noncritical) is consistent with the constrained
subtrees state variables.
(c) Verify that the subject name and subjectAltName extension
(critical or noncritical) is consistent with the excluded subtrees
state variables.
(d) Verify that policy information is consistent with the initial
policy set:
(1) if the explicit policy state variable is less than or equal
to i, a policy identifier in the certificate shall be in the
initial policy set; and
(2) if the policy mapping variable is less than or equal to i,
the policy identifier may not be mapped.
(e) Verify that policy information is consistent with the
acceptable policy set:
(1) if the certificate policies extension is marked critical,
the intersection of the policies extension and the acceptable
policy set shall be non-null;
(2) the acceptable policy set is assigned the resulting
intersection as its new value.
(g) Verify that the intersection of the acceptable policy set and
the initial policy set is non-null.
(h) Recognize and process any other critical extension present in
the certificate.
(i) Verify that the certificate is a CA certificate (as specified
in a basicConstraints extension or as verified out-of-band).
(j) If permittedSubtrees is present in the certificate, set the
constrained subtrees state variable to the intersection of its
previous value and the value indicated in the extension field.
(k) If excludedSubtrees is present in the certificate, set the
excluded subtrees state variable to the union of its previous
value and the value indicated in the extension field.
(l) If a policy constraints extension is included in the
certificate, modify the explicit policy and policy mapping state
variables as follows:
(1) If requireExplicitPolicy is present and has value r, the
explicit policy state variable is set to the minimum of its
current value and the sum of r and i (the current certificate
in the sequence).
(2) If inhibitPolicyMapping is present and has value q, the
policy mapping state variable is set to the minimum of its
current value and the sum of q and i (the current certificate
in the sequence).
(m) If a key usage extension is marked critical, ensure the
keyCertSign bit is set.
If any one of the above checks fail, the procedure terminates,
returning a failure indication and an appropriate reason. If none of
the above checks fail on the end-entity certificate, the procedure
terminates, returning a success indication together with the set of
all policy qualifier values encountered in the set of certificates.
6.2 Extending Path Validation
The path validation algorithm presented in 6.1 is based on several
simplifying assumptions (e.g., a single trusted CA that starts all
valid paths). This algorithm may be extended for cases where the
assumptions do not hold.
This procedure may be extended for multiple trusted CAs by providing
a set of self-signed certificates to the validation module. In this
case, a valid path could begin with any one of the self-signed
certificates. Limitations in the trust paths for any particular key
may be incorporated into the self-signed certificate's extensions. In
this way, the self-signed certificates permit the path validation
module to automatically incorporate local security policy and
requirements.
It is also possible to specify an extended version of the above
certification path processing procedure which results in default
behavior identical to the rules of PEM [RFC1422]. In this extended
version, additional inputs to the procedure are a list of one or more
Policy Certification Authorities (PCAs) names and an indicator of the
position in the certification path where the PCA is expected. At the
nominated PCA position, the CA name is compared against this list.
If a recognized PCA name is found, then a constraint of
SubordinateToCA is implicitly assumed for the remainder of the
certification path and processing continues. If no valid PCA name is
found, and if the certification path cannot be validated on the basis
of identified policies, then the certification path is considered
invalid.
7 Algorithm Support
This section describes cryptographic algorithms which may be used
with this profile. The section describes one-way hash functions and
digital signature algorithms which may be used to sign certificates
and CRLs, and identifies OIDs for public keys contained in a
certificate.
Conforming CAs and applications are not required to support the
algorithms or algorithm identifiers described in this section.
However, conforming CAs and applications that use the algorithms
identified here MUST support them as specified.
7.1 One-way Hash Functions
This section identifies one-way hash functions for use in the
Internet PKI. One-way hash functions are also called message digest
algorithms. SHA-1 is the preferred one-way hash function for the
Internet PKI. However, PEM uses MD2 for certificates [RFC1422] [RFC
1423] and MD5 is used in other legacy applications. For this reason,
MD2 and MD5 are included in this profile.
7.1.1 MD2 One-way Hash Function
MD2 was developed by Ron Rivest for RSA Data Security. RSA Data
Security has not placed the MD2 algorithm in the public domain.
Rather, RSA Data Security has granted license to use MD2 for non-
commercial Internet Privacy-Enhanced Mail. For this reason, MD2 may
continue to be used with PEM certificates, but SHA-1 is preferred.
MD2 produces a 128-bit "hash" of the input. MD2 is fully described
in RFC1319 [RFC1319].
At the Selected Areas in Cryptography '95 conference in May 1995,
Rogier and Chauvaud presented an attack on MD2 that can nearly find
collisions [RC95]. Collisions occur when one can find two different
messages that generate the same message digest. A checksum operation
in MD2 is the only remaining obstacle to the success of the attack.
For this reason, the use of MD2 for new applications is discouraged.
It is still reasonable to use MD2 to verify existing signatures, as
the ability to find collisions in MD2 does not enable an attacker to
find new messages having a previously computed hash value.
7.1.2 MD5 One-way Hash Function
MD5 was developed by Ron Rivest for RSA Data Security. RSA Data
Security has placed the MD5 algorithm in the public domain. MD5
produces a 128-bit "hash" of the input. MD5 is fully described in
RFC1321 [RFC1321].
Den Boer and Bosselaers [DB94] have found pseudo-collisions for MD5,
but there are no other known cryptanalytic results. The use of MD5
for new applications is discouraged. It is still reasonable to use
MD5 to verify existing signatures.
7.1.3 SHA-1 One-way Hash Function
SHA-1 was developed by the U.S. Government. SHA-1 produces a 160-bit
"hash" of the input. SHA-1 is fully described in FIPS 180-1 [FIPS
180-1].
SHA-1 is the one-way hash function of choice for use with both the
RSA and DSA signature algorithms (see sec. 7.2).
7.2 Signature Algorithms
Certificates and CRLs described by this standard may be signed with
any public key signature algorithm. The certificate or CRL indicates
the algorithm through an algorithm identifier which appears in the
signatureAlgorithm field in a Certificate or CertificateList. This
algorithm identifier is an OID and has optionally associated
parameters. This section identifies algorithm identifiers and
parameters that shall be used in the signatureAlgorithm field in a
Certificate or CertificateList.
RSA and DSA are the most popular signature algorithms used in the
Internet. Signature algorithms are always used in conjunction with a
one-way hash function identified in section 7.1.
The signature algorithm and one-way hash function used to sign a
certificate or CRL is indicated by use of an algorithm identifier.
An algorithm identifier is an OID, and may include associated
parameters. This section identifies OIDS for RSA and DSA. The
contents of the parameters component for each algorithm vary; details
are provided for each algorithm.
The data to be signed (e.g., the one-way hash function output value)
is formatted for the signature algorithm to be used. Then, a private
key operation (e.g., RSA encryption) is performed to generate the
signature value. This signature value is then ASN.1 encoded as a BIT
STRING and included in the Certificate or CertificateList in the
signature field.
7.2.1 RSA Signature Algorithm
A patent statement regarding the RSA algorithm can be found at the
end of this profile.
The RSA algorithm is named for its inventors: Rivest, Shamir, and
Adleman. This profile includes three signature algorithms based on
the RSA asymmetric encryption algorithm. The signature algorithms
combine RSA with either the MD2, MD5, or the SHA-1 one-way hash
functions.
The signature algorithm with MD2 and the RSA encryption algorithm is
defined in PKCS #1 [RFC2313]. As defined in RFC2313, the ASN.1 OID
used to identify this signature algorithm is:
md2WithRSAEncryption OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-1(1) 2 }
The signature algorithm with MD5 and the RSA encryption algorithm is
defined in PKCS #1 [RFC2313]. As defined in RFC2313, the ASN.1 OID
used to identify this signature algorithm is:
md5WithRSAEncryption OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-1(1) 4 }
The signature algorithm with SHA-1 and the RSA encryption algorithm
is implemented using the padding and encoding conventions described
in PKCS #1 [RFC2313]. The message digest is computed using the SHA-1
hash algorithm. The ASN.1 object identifier used to identify this
signature algorithm is:
sha-1WithRSAEncryption OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-1(1) 5 }
When any of these three OIDs appears within the ASN.1 type
AlgorithmIdentifier, the parameters component of that type shall be
the ASN.1 type NULL.
The RSA signature generation process and the encoding of the result
is described in detail in RFC2313.
7.2.2 DSA Signature Algorithm
A patent statement regarding the DSA can be found at the end of this
profile.
The Digital Signature Algorithm (DSA) is also called the Digital
Signature Standard (DSS). DSA was developed by the U.S. Government,
and DSA is used in conjunction with the the SHA-1 one-way hash
function. DSA is fully described in FIPS 186 [FIPS 186]. The ASN.1
OIDs used to identify this signature algorithm are:
id-dsa-with-sha1 ID ::= {
iso(1) member-body(2) us(840) x9-57 (10040)
x9cm(4) 3 }
Where the id-dsa-with-sha1 algorithm identifier appears as the
algorithm field in an AlgorithmIdentifier, the encoding shall omit
the parameters field. That is, the AlgorithmIdentifier shall be a
SEQUENCE of one component - the OBJECT IDENTIFIER id-dsa-with-sha1.
The DSA parameters in the subjectPublicKeyInfo field of the
certificate of the issuer shall apply to the verification of the
signature.
When signing, the DSA algorithm generates two values. These values
are commonly referred to as r and s. To easily transfer these two
values as one signature, they shall be ASN.1 encoded using the
following ASN.1 structure:
Dss-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER }
7.3 Subject Public Key Algorithms
Certificates described by this profile may convey a public key for
any public key algorithm. The certificate indicates the algorithm
through an algorithm identifier. This algorithm identifier is an OID
and optionally associated parameters.
This section identifies preferred OIDs and parameters for the RSA,
DSA, and Diffie-Hellman algorithms. Conforming CAs shall use the
identified OIDs when issuing certificates containing public keys for
these algorithms. Conforming applications supporting any of these
algorithms shall, at a minimum, recognize the OID identified in this
section.
7.3.1 RSA Keys
The OID rsaEncryption identifies RSA public keys.
pkcs-1 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) 1 }
rsaEncryption OBJECT IDENTIFIER ::= { pkcs-1 1}
The rsaEncryption OID is intended to be used in the algorithm field
of a value of type AlgorithmIdentifier. The parameters field shall
have ASN.1 type NULL for this algorithm identifier.
The RSA public key shall be encoded using the ASN.1 type
RSAPublicKey:
RSAPublicKey ::= SEQUENCE {
modulus INTEGER, -- n
publicExponent INTEGER -- e -- }
where modulus is the modulus n, and publicExponent is the public
exponent e. The DER encoded RSAPublicKey is the value of the BIT
STRING subjectPublicKey.
This OID is used in public key certificates for both RSA signature
keys and RSA encryption keys. The intended application for the key
may be indicated in the key usage field (see sec. 4.2.1.3). The use
of a single key for both signature and encryption purposes is not
recommended, but is not forbidden.
If the keyUsage extension is present in an end entity certificate
which conveys an RSA public key, any combination of the following
values may be present: digitalSignature; nonRepudiation;
keyEncipherment; and dataEncipherment. If the keyUsage extension is
present in a CA certificate which conveys an RSA public key, any
combination of the following values may be present:
digitalSignature; nonRepudiation; keyEncipherment; dataEncipherment;
keyCertSign; and cRLSign. However, this specification RECOMMENDS
that if keyCertSign or cRLSign is present, both keyEncipherment and
dataEncipherment should not be present.
7.3.2 Diffie-Hellman Key Exchange Key
The Diffie-Hellman OID supported by this profile is defined by ANSI
X9.42 [X9.42].
dhpublicnumber OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) ansi-x942(10046) number-type(2) 1 }
The dhpublicnumber OID is intended to be used in the algorithm field
of a value of type AlgorithmIdentifier. The parameters field of that
type, which has the algorithm-specific syntax ANY DEFINED BY
algorithm, have the ASN.1 type DomainParameters for this algorithm.
DomainParameters ::= SEQUENCE {
p INTEGER, -- odd prime, p=jq +1
g INTEGER, -- generator, g
q INTEGER, -- factor of p-1
j INTEGER OPTIONAL, -- subgroup factor
validationParms ValidationParms OPTIONAL }
ValidationParms ::= SEQUENCE {
seed BIT STRING,
pgenCounter INTEGER }
The fields of type DomainParameters have the following meanings:
p identifies the prime p defining the Galois field;
g specifies the generator of the multiplicative subgroup of order
g;
q specifies the prime factor of p-1;
j optionally specifies the value that satisfies the equation
p=jq+1 to support the optional verification of group parameters;
seed optionally specifies the bit string parameter used as the
seed for the system parameter generation process; and
pgenCounter optionally specifies the integer value output as part
of the of the system parameter prime generation process.
If either of the parameter generation components (pgencounter or
seed) is provided, the other shall be present as well.
The Diffie-Hellman public key shall be ASN.1 encoded as an INTEGER;
this encoding shall be used as the contents (i.e., the value) of the
subjectPublicKey component (a BIT STRING) of the subjectPublicKeyInfo
data element.
DHPublicKey ::= INTEGER -- public key, y = g^x mod p
If the keyUsage extension is present in a certificate which conveys a
DH public key, the following values may be present: keyAgreement;
encipherOnly; and decipherOnly. At most one of encipherOnly and
decipherOnly shall be asserted in keyUsage extension.
7.3.3 DSA Signature Keys
The Digital Signature Algorithm (DSA) is also known as the Digital
Signature Standard (DSS). The DSA OID supported by this profile is
id-dsa ID ::= { iso(1) member-body(2) us(840) x9-57(10040)
x9cm(4) 1 }
The id-dsa algorithm syntax includes optional parameters. These
parameters are commonly referred to as p, q, and g. When omitted,
the parameters component shall be omitted entirely. That is, the
AlgorithmIdentifier shall be a SEQUENCE of one component - the OBJECT
IDENTIFIER id-dsa.
If the DSA algorithm parameters are present in the
subjectPublicKeyInfo AlgorithmIdentifier, the parameters are included
using the following ASN.1 structure:
Dss-Parms ::= SEQUENCE {
p INTEGER,
q INTEGER,
g INTEGER }
If the DSA algorithm parameters are absent from the
subjectPublicKeyInfo AlgorithmIdentifier and the CA signed the
subject certificate using DSA, then the certificate issuer's DSA
parameters apply to the subject's DSA key. If the DSA algorithm
parameters are absent from the subjectPublicKeyInfo
AlgorithmIdentifier and the CA signed the subject certificate using a
signature algorithm other than DSA, then the subject's DSA parameters
are distributed by other means. If the subjectPublicKeyInfo
AlgorithmIdentifier field omits the parameters component and the CA
signed the subject with a signature algorithm other than DSA, then
clients shall reject the certificate.
When signing, DSA algorithm generates two values. These values are
commonly referred to as r and s. To easily transfer these two values
as one signature, they are ASN.1 encoded using the following ASN.1
structure:
Dss-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER }
The encoded signature is conveyed as the value of the BIT STRING
signature in a Certificate or CertificateList.
The DSA public key shall be ASN.1 DER encoded as an INTEGER; this
encoding shall be used as the contents (i.e., the value) of the
subjectPublicKey component (a BIT STRING) of the SubjectPublicKeyInfo
data element.
DSAPublicKey ::= INTEGER -- public key, Y
If the keyUsage extension is present in an end entity certificate
which conveys a DSA public key, any combination of the following
values may be present: digitalSignature; and nonRepudiation.
If the keyUsage extension is present in an CA certificate which
conveys a DSA public key, any combination of the following values may
be present: digitalSignature; nonRepudiation; keyCertSign; and
cRLSign.
8 References
[FIPS 180-1] Federal Information Processing Standards Publication
(FIPS PUB) 180-1, Secure Hash Standard, 17 April 1995.
[Supersedes FIPS PUB 180 dated 11 May 1993.]
[FIPS 186] Federal Information Processing Standards Publication
(FIPS PUB) 186, Digital Signature Standard, 18 May
1994.
[RC95] Rogier, N. and Chauvaud, P., "The compression function
of MD2 is not collision free," Presented at Selected
Areas in Cryptography '95, May 1995.
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC791,
September 1981.
[RFC822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC822, August 1982.
[RFC1034] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC1034, November 1987.
[RFC1319] Kaliski, B., "The MD2 Message-Digest Algorithm," RFC
1319, April 1992.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm," RFC
1321, April 1992.
[RFC1422] Kent, S., "Privacy Enhancement for Internet Electronic
Mail: Part II: Certificate-Based Key Management," RFC
1422, February 1993.
[RFC1423] Balenson, D., "Privacy Enhancement for Internet
Electronic Mail: Part III: Algorithms, Modes, and
Identifiers," RFC1423, February 1993.
[RFC1519] Fuller, V., Li, T., Yu, J. and K. Varadhan. "Classless
Inter-Domain Routing (CIDR): an Address Assignment and
Aggregation Strategy", RFC1519, September 1993.
[RFC1738] Berners-Lee, T., Masinter L., and M. McCahill.
"Uniform Resource Locators (URL)", RFC1738, December
1994.
[RFC1778] Howes, T., Kille S., Yeong, W. and C. Robbins. "The
String Representation of Standard Attribute Syntaxes,"
RFC1778, March 1995.
[RFC1883] Deering, S. and R. Hinden. "Internet Protocol, Version
6 (IPv6) Specification", RFC1883, December 1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.
[RFC2247] Kille, S., Wahl, M., Grimstad, A., Huber, R. and S.
Sataluri. "Using Domains in LDAP/X.500 Distinguished
Names", RFC2247, January 1998.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", RFC2277, January 1998.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC2279, January 1998.
[RFC2313] Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", RFC
2313, March 1998.
[SDN.701] SDN.701, "Message Security Protocol 4.0", Revision A
1997-02-06.
[X.208] CCITT Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1), 1988.
[X.501] ITU-T Recommendation X.501: Information Technology -
Open Systems Interconnection - The Directory: Models,
1993.
[X.509] ITU-T Recommendation X.509 (1997 E): Information
Technology - Open Systems Interconnection - The
Directory: Authentication Framework, June 1997.
[X.520] ITU-T Recommendation X.520: Information Technology -
Open Systems Interconnection - The Directory: Selected
Attribute Types, 1993.
[X9.42] ANSI X9.42-199x, Public Key Cryptography for The
Financial Services Industry: Agreement of Symmetric
Algorithm Keys Using Diffie-Hellman (Working Draft),
December 1997.
[X9.55] ANSI X9.55-1995, Public Key Cryptography For The
Financial Services Industry: Extensions To Public Key
Certificates And Certificate Revocation Lists, 8
December, 1995.
[X9.57] ANSI X9.57-199x, Public Key Cryptography For The
Financial Services Industry: Certificate Management
(Working Draft), 21 June, 1996.
9 Intellectual Property Rights
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
10 Security Considerations
The majority of this specification is devoted to the format and
content of certificates and CRLs. Since certificates and CRLs are
digitally signed, no additional integrity service is necessary.
Neither certificates nor CRLs need be kept secret, and unrestricted
and anonymous access to certificates and CRLs has no security
implications.
However, security factors outside the scope of this specification
will affect the assurance provided to certificate users. This
section highlights critical issues that should be considered by
implementors, administrators, and users.
The procedures performed by CAs and RAs to validate the binding of
the subject's identity of their public key greatly affect the
assurance that should be placed in the certificate. Relying parties
may wish to review the CA's certificate practice statement. This may
be particularly important when issuing certificates to other CAs.
The use of a single key pair for both signature and other purposes is
strongly discouraged. Use of separate key pairs for signature and key
management provides several benefits to the users. The ramifications
associated with loss or disclosure of a signature key are different
from loss or disclosure of a key management key. Using separate key
pairs permits a balanced and flexible response. Similarly, different
validity periods or key lengths for each key pair may be appropriate
in some application environments. Unfortunately, some legacy
applications (e.g., SSL) use a single key pair for signature and key
management.
The protection afforded private keys is a critical factor in