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RFC 4055 - Additional Algorithms and Identifiers for RSA Cryptog


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Network Working Group                                          J. Schaad
Request for Comments: 4055                       Soaring Hawk Consulting
Updates: 3279                                                 B. Kaliski
Category: Standards Track                               RSA Laboratories
                                                              R. Housley
                                                          Vigil Security
                                                               June 2005

      Additional Algorithms and Identifiers for RSA Cryptography
       for use in the Internet X.509 Public Key Infrastructure
      Certificate and Certificate Revocation List (CRL) Profile

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document supplements RFC 3279.  It describes the conventions for
   using the RSA Probabilistic Signature Scheme (RSASSA-PSS) signature
   algorithm, the RSA Encryption Scheme - Optimal Asymmetric Encryption
   Padding (RSAES-OAEP) key transport algorithm and additional one-way
   hash functions with the Public-Key Cryptography Standards (PKCS) #1
   version 1.5 signature algorithm in the Internet X.509 Public Key
   Infrastructure (PKI).  Encoding formats, algorithm identifiers, and
   parameter formats are specified.

Table of Contents

   1. Introduction ....................................................2
      1.1. Terminology ................................................3
      1.2. RSA Public Keys ............................................3
   2. Common Functions ................................................5
      2.1. One-way Hash Functions .....................................5
      2.2. Mask Generation Functions ..................................6
   3. RSASSA-PSS Signature Algorithm ..................................7
      3.1. RSASSA-PSS Public Keys .....................................8
      3.2. RSASSA-PSS Signature Values ...............................10
      3.3. RSASSA-PSS Signature Parameter Validation .................10
   4. RSAES-OAEP Key Transport Algorithm .............................10
      4.1. RSAES-OAEP Public Keys ....................................11
   5. PKCS #1 Version 1.5 Signature Algorithm ........................13
   6. ASN.1 Module ...................................................14
   7. References .....................................................20
      7.1. Normative References ......................................20
      7.2. Informative References ....................................21
   8. Security Considerations ........................................21
   9. IANA Considerations ............................................24

1.  Introduction

   This document supplements RFC 3279 [PKALGS].  This document describes
   the conventions for using the RSASSA-PSS signature algorithm and the
   RSAES-OAEP key transport algorithm in the Internet X.509 Public Key
   Infrastructure (PKI) [PROFILE].  Both of these RSA-based algorithms
   are specified in [P1v2.1].  The algorithm identifiers and associated
   parameters for subject public keys that employ either of these
   algorithms, and the encoding format for RSASSA-PSS signatures are
   specified.  Also, the algorithm identifiers for using the SHA-224,
   SHA-256, SHA-384, and SHA-512 one-way hash functions with the PKCS #1
   version 1.5 signature algorithm [P1v1.5] are specified.

   This specification supplements RFC 3280 [PROFILE] which profiles the
   X.509 Certificates and Certificate Revocation Lists (CRLs) for use in
   the Internet.  This specification extends the list of algorithms
   discussed in RFC 3279 [PKALGS].  The X.509 Certificate and CRL
   definitions use ASN.1 [X.208-88], the Basic Encoding Rules (BER)
   [X.209-88], and the Distinguished Encoding Rules (DER) [X.509-88].

   This specification defines the contents of the signatureAlgorithm,
   signatureValue, signature, and subjectPublicKeyInfo fields within
   Internet X.509 Certificates and CRLs.  For each algorithm, the
   appropriate alternatives for the keyUsage certificate extension are
   provided.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [STDWORDS].

1.2.  RSA Public Keys

   RFC 3280 [PROFILE] specifies the profile for using X.509 Certificates
   in Internet applications.  When an RSA public key is used for
   RSASSA-PSS digital signatures or RSAES-OAEP key transport, the
   conventions specified in this section augment RFC 3280.

   Traditionally, the rsaEncryption object identifier is used to
   identify RSA public keys.  However, to implement all of the
   recommendations described in Security Considerations (Section 8), the
   certificate user needs to be able to determine the form of digital
   signature or key transport that the RSA private key owner associates
   with the public key.

   The rsaEncryption object identifier continues to identify the subject
   public key when the RSA private key owner does not wish to limit the
   use of the public key exclusively to either RSASSA-PSS or RSAES-OAEP.
   In this case, the rsaEncryption object identifier MUST be used in the
   algorithm field within the subject public key information, and the
   parameters field MUST contain NULL.

      rsaEncryption  OBJECT IDENTIFIER  ::=  { pkcs-1 1 }

   Further discussion of the conventions associated with use of the
   rsaEncryption object identifier can be found in RFC 3279 (see
   [PKALGS], Section 2.3.1).

   When the RSA private key owner wishes to limit the use of the public
   key exclusively to RSASSA-PSS, then the id-RSASSA-PSS object
   identifier MUST be used in the algorithm field within the subject
   public key information, and, if present, the parameters field MUST
   contain RSASSA-PSS-params.  The id-RSASSA-PSS object identifier value
   and the RSASSA-PSS-params syntax are fully described in Section 3.

   When the RSA private key owner wishes to limit the use of the public
   key exclusively to RSAES-OAEP, then the id-RSAES-OAEP object
   identifier MUST be used in the algorithm field within the subject
   public key information, and, if present, the parameters field MUST
   contain RSAES-OAEP-params.  The id-RSAES-OAEP object identifier value
   and the RSAES-OAEP-params syntax are fully described in Section 4.

   Note: It is not possible to restrict the use of a key to a set of
   algorithms (i.e., RSASSA-PSS and RSAES-OAEP).

   Regardless of the object identifier used, the RSA public key is
   encoded in the same manner in the subject public key information.
   The RSA public key MUST be encoded using the type RSAPublicKey type:

      RSAPublicKey  ::=  SEQUENCE  {
         modulus            INTEGER,    -- n
         publicExponent     INTEGER  }  -- e

   Here, the modulus is the modulus n, and publicExponent is the public
   exponent e.  The DER encoded RSAPublicKey is carried in the
   subjectPublicKey BIT STRING within the subject public key
   information.

   The intended application for the key MAY be indicated in the keyUsage
   certificate extension (see [PROFILE], Section 4.2.1.3).

   If the keyUsage extension is present in an end-entity certificate
   that conveys an RSA public key with the id-RSASSA-PSS object
   identifier, then the keyUsage extension MUST contain one or both of
   the following values:

      nonRepudiation; and
      digitalSignature.

   If the keyUsage extension is present in a certification authority
   certificate that conveys an RSA public key with the id-RSASSA-PSS
   object identifier, then the keyUsage extension MUST contain one or
   more of the following values:

      nonRepudiation;
      digitalSignature;
      keyCertSign; and
      cRLSign.

   When a certificate conveys an RSA public key with the id-RSASSA-PSS
   object identifier, the certificate user MUST only use the certified
   RSA public key for RSASSA-PSS operations, and, if RSASSA-PSS-params
   is present, the certificate user MUST perform those operations using
   the one-way hash function, mask generation function, and trailer
   field identified in the subject public key algorithm identifier
   parameters within the certificate.

   If the keyUsage extension is present in a certificate conveys an RSA
   public key with the id-RSAES-OAEP object identifier, then the
   keyUsage extension MUST contain only the following values:

      keyEncipherment; and
      dataEncipherment.

   However, both keyEncipherment and dataEncipherment SHOULD NOT be
   present.

   When a certificate that conveys an RSA public key with the
   id-RSAES-OAEP object identifier, the certificate user MUST only use
   the certified RSA public key for RSAES-OAEP operations, and, if
   RSAES-OAEP-params is present, the certificate user MUST perform those
   operations using the one-way hash function and mask generation
   function identified in the subject public key algorithm identifier
   parameters within the certificate.

2.  Common Functions

   The RSASSA-PSS signature and the RSAES-OAEP key transport algorithms
   make use of one-way hash functions and mask generation functions.

2.1.  One-way Hash Functions

   PKCS #1 version 2.1 [P1v2.1] supports four one-way hash functions for
   use with the RSASSA-PSS signature algorithm and the RSAES-OAEP key
   transport algorithm: SHA-1, SHA-256, SHA-384, and SHA-512 [SHA2].
   This document adds support for SHA-224 [SHA-224] with both the
   RSASSA-PSS and the RSAES-OAEP algorithms.  While support for
   additional one-way hash functions could be added in the future, no
   other one-way hash functions are supported by this specification.

   These one-way hash functions are identified by the following object
   identifiers:

      id-sha1  OBJECT IDENTIFIER  ::=  { iso(1)
                           identified-organization(3) oiw(14)
                           secsig(3) algorithms(2) 26 }
      id-sha224  OBJECT IDENTIFIER  ::=  {{ joint-iso-itu-t(2)
                           country(16) us(840) organization(1) gov(101)
                           csor(3) nistalgorithm(4) hashalgs(2) 4 }
      id-sha256  OBJECT IDENTIFIER  ::=  { joint-iso-itu-t(2)
                           country(16) us(840) organization(1) gov(101)
                           csor(3) nistalgorithm(4) hashalgs(2) 1 }
      id-sha384  OBJECT IDENTIFIER  ::=  { joint-iso-itu-t(2)
                           country(16) us(840) organization(1) gov(101)
                           csor(3) nistalgorithm(4) hashalgs(2) 2 }

      id-sha512  OBJECT IDENTIFIER  ::=  { joint-iso-itu-t(2)
                           country(16) us(840) organization(1) gov(101)
                           csor(3) nistalgorithm(4) hashalgs(2) 3 }

   There are two possible encodings for the AlgorithmIdentifier
   parameters field associated with these object identifiers.  The two
   alternatives arise from the loss of the OPTIONAL associated with the
   algorithm identifier parameters when the 1988 syntax for
   AlgorithmIdentifier was translated into the 1997 syntax.  Later the
   OPTIONAL was recovered via a defect report, but by then many people
   thought that algorithm parameters were mandatory.  Because of this
   history some implementations encode parameters as a NULL element
   while others omit them entirely.  The correct encoding is to omit the
   parameters field; however, when RSASSA-PSS and RSAES-OAEP were
   defined, it was done using the NULL parameters rather than absent
   parameters.

   All implementations MUST accept both NULL and absent parameters as
   legal and equivalent encodings.

   To be clear, the following algorithm identifiers are used when a NULL
   parameter MUST be present:

      sha1Identifier  AlgorithmIdentifier  ::=  { id-sha1, NULL }
      sha224Identifier  AlgorithmIdentifier  ::=  { id-sha224, NULL }
      sha256Identifier  AlgorithmIdentifier  ::=  { id-sha256, NULL }
      sha384Identifier  AlgorithmIdentifier  ::=  { id-sha384, NULL }
      sha512Identifier  AlgorithmIdentifier  ::=  { id-sha512, NULL }

2.2.  Mask Generation Functions

   One mask generation function is used with the RSASSA-PSS signature
   algorithm and the RSAES-OAEP key transport algorithm: MGF1 [P1v2.1].
   No other mask generation functions are supported by this
   specification.

   MGF1 is identified by the following object identifier:

      id-mgf1  OBJECT IDENTIFIER  ::=  { pkcs-1 8 }

   The parameters field associated with id-mgf1 MUST have a
   hashAlgorithm value which identifies the hash function being used
   with MGF1.  This value MUST be sha1Identifier, sha224Identifier,
   sha256Identifier, sha384Identifier, or sha512Identifier, as specified
   in Section 2.1.  Implementations MUST support the default value,
   sha1Identifier, and MAY support the other four values.

   The following algorithm identifiers have been assigned for each of
   these alternatives:

      mgf1SHA1Identifier  AlgorithmIdentifier  ::=
                           { id-mgf1, sha1Identifier }
      mgf1SHA224Identifier  AlgorithmIdentifier  ::=
                           { id-mgf1, sha224Identifier }
      mgf1SHA256Identifier  AlgorithmIdentifier  ::=
                           { id-mgf1, sha256Identifier }
      mgf1SHA384Identifier  AlgorithmIdentifier  ::=
                           { id-mgf1, sha384Identifier }
      mgf1SHA512Identifier  AlgorithmIdentifier  ::=
                           { id-mgf1, sha512Identifier }

3.  RSASSA-PSS Signature Algorithm

   This section describes the conventions for using the RSASSA-PSS
   signature algorithm with the Internet X.509 Certificate and CRL
   profile [PROFILE].  The RSASSA-PSS signature algorithm is specified
   in PKCS #1 version 2.1 [P1v2.1].  The five one-way hash functions
   discussed in Section 2.1 and the one mask generation function
   discussed in Section 2.2 can be used with RSASSA-PSS.

   CAs that issue certificates with the id-RSASSA-PSS algorithm
   identifier SHOULD require the presence of parameters in the
   publicKeyAlgorithms field if the cA boolean flag is set in the basic
   constraints certificate extension.  CAs MAY require that the
   parameters be present in the publicKeyAlgorithms field for end-entity
   certificates.

   CAs that use the RSASSA-PSS algorithm for signing certificates SHOULD
   include RSASSA-PSS-params in the subjectPublicKeyInfo algorithm
   parameters in their own certificates.  CAs that use the RSASSA-PSS
   algorithm for signing certificates or CRLs MUST include RSASSA-PSS-
   params in the signatureAlgorithm parameters in the TBSCertificate or
   TBSCertList structures.

   Entities that validate RSASSA-PSS signatures MUST support SHA-1.
   They MAY also support any other one-way hash functions in Section
   2.1.

   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 decryption) 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
   signatureValue field.  Section 3.2 specifies the format of RSASSA-PSS
   signature values.

3.1.  RSASSA-PSS Public Keys

   When RSASSA-PSS is used in an AlgorithmIdentifier, the parameters
   MUST employ the RSASSA-PSS-params syntax.  The parameters may be
   either absent or present when used as subject public key information.
   The parameters MUST be present when used in the algorithm identifier
   associated with a signature value.

   When signing, it is RECOMMENDED that the parameters, except for
   possibly saltLength, remain fixed for all usages of a given RSA key
   pair.

      id-RSASSA-PSS  OBJECT IDENTIFIER  ::=  { pkcs-1 10 }

      RSASSA-PSS-params  ::=  SEQUENCE  {
         hashAlgorithm      [0] HashAlgorithm DEFAULT
                                   sha1Identifier,
         maskGenAlgorithm   [1] MaskGenAlgorithm DEFAULT
                                   mgf1SHA1Identifier,
         saltLength         [2] INTEGER DEFAULT 20,
         trailerField       [3] INTEGER DEFAULT 1  }

   The fields of type RSASSA-PSS-params have the following meanings:

      hashAlgorithm

         The hashAlgorithm field identifies the hash function.  It MUST
         be one of the algorithm identifiers listed in Section 2.1, and
         the default hash function is SHA-1.  Implementations MUST
         support SHA-1 and MAY support any of the other one-way hash
         functions listed in Section 2.1.  Implementations that perform
         signature generation MUST omit the hashAlgorithm field when
         SHA-1 is used, indicating that the default algorithm was used.
         Implementations that perform signature validation MUST
         recognize both the sha1Identifier algorithm identifier and an
         absent hashAlgorithm field as an indication that SHA-1 was
         used.

      maskGenAlgorithm

         The maskGenAlgorithm field identifies the mask generation
         function.  The default mask generation function is MGF1 with
         SHA-1.  For MGF1, it is strongly RECOMMENDED that the
         underlying hash function be the same as the one identified by
         hashAlgorithm.  Implementations MUST support MGF1.  MGF1
         requires a one-way hash function that is identified in the
         parameters field of the MGF1 algorithm identifier.
         Implementations MUST support SHA-1 and MAY support any of the

         other one-way hash functions listed in section Section 2.1.
         The MGF1 algorithm identifier is comprised of the id-mgf1
         object identifier and a parameter that contains the algorithm
         identifier of the one-way hash function employed with MGF1.
         The SHA-1 algorithm identifier is comprised of the id-sha1
         object identifier and an (optional) parameter of NULL.
         Implementations that perform signature generation MUST omit the
         maskGenAlgorithm field when MGF1 with SHA-1 is used, indicating
         that the default algorithm was used.

         Although mfg1SHA1Identifier is defined as the default value for
         this field, implementations MUST accept both the default value
         encoding (i.e., an absent field) and mfg1SHA1Identifier to be
         explicitly present in the encoding.

      saltLength

         The saltLength field is the octet length of the salt.  For a
         given hashAlgorithm, the recommended value of saltLength is the
         number of octets in the hash value.  Unlike the other fields of
         type RSASSA-PSS-params, saltLength does not need to be fixed
         for a given RSA key pair; a different value could be used for
         each RSASSA-PSS signature generated.

      trailerField

         The trailerField field is an integer.  It provides
         compatibility with IEEE Std 1363a-2004 [P1363A].  The value
         MUST be 1, which represents the trailer field with hexadecimal
         value 0xBC.  Other trailer fields, including the trailer field
         composed of HashID concatenated with 0xCC that is specified in
         IEEE Std 1363a, are not supported.  Implementations that
         perform signature generation MUST omit the trailerField field,
         indicating that the default trailer field value was used.
         Implementations that perform signature validation MUST
         recognize both a present trailerField field with value 1 and an
         absent trailerField field.

   If the default values of the hashAlgorithm, maskGenAlgorithm, and
   trailerField fields of RSASSA-PSS-params are used, then the algorithm
   identifier will have the following value:

      rSASSA-PSS-Default-Identifier  AlgorithmIdentifier  ::=  {
                           id-RSASSA-PSS, rSASSA-PSS-Default-Params }

      rSASSA-PSS-Default-Params RSASSA-PSS-Params ::= {
                           sha1Identifier, mgf1SHA1Identifier, 20, 1}

3.2.  RSASSA-PSS Signature Values

   The output of the RSASSA-PSS signature algorithm is an octet string,
   which has the same length in octets as the RSA modulus n.

   Signature values in CMS [CMS] are represented as octet strings, and
   the output is used directly.  However, signature values in
   certificates and CRLs [PROFILE] are represented as bit strings, and
   conversion is needed.

   To convert a signature value to a bit string, the most significant
   bit of the first octet of the signature value SHALL become the first
   bit of the bit string, and so on through the least significant bit of
   the last octet of the signature value, which SHALL become the last
   bit of the bit string.

3.3.  RSASSA-PSS Signature Parameter Validation

   Three possible parameter validation scenarios exist for RSASSA-PSS
   signature values.

   1.  The key is identified by the rsaEncryption algorithm identifier.
       In this case no parameter validation is needed.

   2.  The key is identified by the id-RSASSA-PSS signature algorithm
       identifier, but the parameters field is absent.  In this case no
       parameter validation is needed.

   3.  The key is identified by the id-RSASSA-PSS signature algorithm
       identifier and the parameters are present.  In this case all
       parameters in the signature structure algorithm identifier MUST
       match the parameters in the key structure algorithm identifier
       except the saltLength field.  The saltLength field in the
       signature parameters MUST be greater or equal to that in the key
       parameters field.

4.  RSAES-OAEP Key Transport Algorithm

   This section describes the conventions for using the RSAES-OAEP key
   transport algorithm with the Internet X.509 Certificate and CRL
   profile [PROFILE].  RSAES-OAEP is specified in PKCS #1 version 2.1
   [P1v2.1].  The five one-way hash functions discussed in Section 2.1
   and the one mask generation function discussed in Section 2.2 can be
   used with RSAES-OAEP.  Conforming CAs and applications MUST support
   RSAES-OAEP key transport algorithm using SHA-1.  The other four one-
   way hash functions MAY also be supported.

   CAs that issue certificates with the id-RSAES-OAEP algorithm
   identifier SHOULD require the presence of parameters in the
   publicKeyAlgorithms field for all certificates.  Entities that use a
   certificate with a publicKeyAlgorithm value of id-RSA-OAEP where the
   parameters are absent SHOULD use the default set of parameters for
   RSAES-OAEP-params.  Entities that use a certificate with a
   publicKeyAlgorithm value of rsaEncryption SHOULD use the default set
   of parameters for RSAES-OAEP-params.

4.1.  RSAES-OAEP Public Keys

   When id-RSAES-OAEP is used in an AlgorithmIdentifier, the parameters
   MUST employ the RSAES-OAEP-params syntax.  The parameters may be
   either absent or present when used as subject public key information.
   The parameters MUST be present when used in the algorithm identifier
   associated with an encrypted value.

      id-RSAES-OAEP  OBJECT IDENTIFIER  ::=  { pkcs-1 7 }

      RSAES-OAEP-params  ::=  SEQUENCE  {
         hashFunc          [0] AlgorithmIdentifier DEFAULT
                                  sha1Identifier,
         maskGenFunc       [1] AlgorithmIdentifier DEFAULT
                                  mgf1SHA1Identifier,
         pSourceFunc       [2] AlgorithmIdentifier DEFAULT
                                  pSpecifiedEmptyIdentifier  }

      pSpecifiedEmptyIdentifier  AlgorithmIdentifier  ::=
                           { id-pSpecified, nullOctetString }

      nullOctetString  OCTET STRING (SIZE (0))  ::=  { ''H }

   The fields of type RSAES-OAEP-params have the following meanings:

      hashFunc

         The hashFunc field identifies the one-way hash function.  It
         MUST be one of the algorithm identifiers listed in Section 2.1,
         and the default hash function is SHA-1.  Implementations MUST
         support SHA-1 and MAY support other one-way hash functions
         listed in Section 2.1.  Implementations that perform encryption
         MUST omit the hashFunc field when SHA-1 is used, indicating
         that the default algorithm was used.  Implementations that
         perform decryption MUST recognize both the sha1Identifier
         algorithm identifier and an absent hashFunc field as an
         indication that SHA-1 was used.

      maskGenFunc

         The maskGenFunc field identifies the mask generation function.
         The default mask generation function is MGF1 with SHA-1.  For
         MGF1, it is strongly RECOMMENDED that the underlying hash
         function be the same as the one identified by hashFunc.
         Implementations MUST support MGF1.  MGF1 requires a one-way
         hash function that is identified in the parameter field of the
         MGF1 algorithm identifier.  Implementations MUST support SHA-1
         and MAY support any of the other one-way hash functions listed
         in Section 2.1.  The MGF1 algorithm identifier is comprised of
         the id-mgf1 object identifier and a parameter that contains the
         algorithm identifier of the one-way hash function employed with
         MGF1.  The SHA-1 algorithm identifier is comprised of the id-
         sha1 object identifier and an (optional) parameter of NULL.
         Implementations that perform encryption MUST omit the
         maskGenFunc field when MGF1 with SHA-1 is used, indicating that
         the default algorithm was used.

         Although mfg1SHA1Identifier is defined as the default value for
         this field, implementations MUST accept both the default value
         encoding (i.e., an absent field) and the mfg1SHA1Identifier to
         be explicitly present in the encoding.

      pSourceFunc

         The pSourceFunc field identifies the source (and possibly the
         value) of the encoding parameters, commonly called P.
         Implementations MUST represent P by an algorithm identifier,
         id-pSpecified, indicating that P is explicitly provided as an
         OCTET STRING in the parameters.  The default value for P is an
         empty string.  In this case, pHash in EME-OAEP contains the
         hash of a zero length string.  Implementations MUST support a
         zero length P value.  Implementations that perform encryption
         MUST omit the pSourceFunc field when a zero length P value is
         used, indicating that the default value was used.
         Implementations that perform decryption MUST recognize both the
         id-pSpecified object identifier and an absent pSourceFunc field
         as an indication that a zero length P value was used.
         Implementations that perform decryption MUST support a zero
         length P value and MAY support other values.  Compliant
         implementations MUST NOT use any value other than id-pSpecified
         for pSourceFunc.

   If the default values of the hashFunc, maskGenFunc, and pSourceFunc
   fields of RSAES-OAEP-params are used, then the algorithm identifier
   will have the following value:

      rSAES-OAEP-Default-Identifier  AlgorithmIdentifier  ::=
                            { id-RSAES-OAEP,
                              rSAES-OAEP-Default-Params }

      rSAES-OAEP-Default-Params RSASSA-OAEP-params ::=
                               { sha1Identifier,
                                 mgf1SHA1Identifier,
                                 pSpecifiedEmptyIdentifier  }

5.  PKCS #1 Version 1.5 Signature Algorithm

   RFC 2313 [P1v1.5] specifies the PKCS #1 Version 1.5 signature
   algorithm.  This specification is also included in PKCS #1 Version
   2.1 [P1v2.1].  RFC 3279 [PKALGS] specifies the use of the PKCS #1
   Version 1.5 signature algorithm with the MD2, MD5, and the SHA-1
   one-way hash functions.  This section specifies the algorithm
   identifiers for using the SHA-224, SHA-256, SHA-384, and SHA-512
   one-way hash functions with the PKCS #1 version 1.5 signature
   algorithm.

   The RSASSA-PSS signature algorithm is preferred over the PKCS #1
   Version 1.5 signature algorithm.  Although no attacks are known
   against PKCS #1 Version 1.5 signature algorithm, in the interest of
   increased robustness, RSASSA-PSS signature algorithm is recommended
   for eventual adoption, especially by new applications.  This section
   is included for compatibility with existing applications, and while
   still appropriate for new applications, a gradual transition to the
   RSASSA-PSS signature algorithm is encouraged.

   The PKCS #1 Version 1.5 signature algorithm with these one-way hash
   functions and the RSA cryptosystem is implemented using the padding
   and encoding conventions described in RFC 2313 [P1v1.5].

   The message digest is computed using the SHA-224, SHA-256, SHA-384,
   or SHA-512 one-way hash function.

   The PKCS #1 version 1.5 signature algorithm, as specified in RFC
   2313, includes a data encoding step.  In this step, the message
   digest and the object identifier for the one-way hash function used
   to compute the message digest are combined.  When performing the data
   encoding step, the id-sha224, id-sha256, id-sha384, and id-sha512
   object identifiers (see Section 2.1) MUST be used to specify the
   SHA-224, SHA-256, SHA-384, and SHA-512 one-way hash functions,
   respectively.

   The object identifier used to identify the PKCS #1 version 1.5
   signature algorithm with SHA-224 is:

      sha224WithRSAEncryption  OBJECT IDENTIFIER  ::=  { pkcs-1 14 }

   The object identifier used to identify the PKCS #1 version 1.5
   signature algorithm with SHA-256 is:

      sha256WithRSAEncryption  OBJECT IDENTIFIER  ::=  { pkcs-1 11 }

   The object identifier used to identify the PKCS #1 version 1.5
   signature algorithm with SHA-384 is:

      sha384WithRSAEncryption  OBJECT IDENTIFIER  ::=  { pkcs-1 12 }

   The object identifier used to identify the PKCS #1 version 1.5
   signature algorithm with SHA-512 is:

      sha512WithRSAEncryption  OBJECT IDENTIFIER  ::=  { pkcs-1 13 }

   When any of these four object identifiers appears within an
   AlgorithmIdentifier, the parameters MUST be NULL.  Implementations
   MUST accept the parameters being absent as well as present.

   The RSA signature generation process and the encoding of the result
   are described in detail in RFC 2313 [P1v1.5].

6.  ASN.1 Module

   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) }

      DEFINITIONS EXPLICIT TAGS ::= BEGIN

      -- EXPORTS All;

      IMPORTS

        AlgorithmIdentifier
            FROM PKIX1Explicit88 -- Found in [PROFILE]
            { iso(1) identified-organization(3) dod(6) internet(1)
              security(5) mechanisms(5) pkix(7) id-mod(0)
              id-pkix1-explicit(18) } ;

      -- ============================
      --   Basic object identifiers
      -- ============================

      pkcs-1  OBJECT IDENTIFIER  ::=  { iso(1) member-body(2)
                              us(840) rsadsi(113549) pkcs(1) 1 }

      -- When rsaEncryption is used in an AlgorithmIdentifier the
      -- parameters MUST be present and MUST be NULL.

      rsaEncryption  OBJECT IDENTIFIER  ::=  { pkcs-1 1 }

      -- When id-RSAES-OAEP is used in an AlgorithmIdentifier,
      -- and the parameters field is present, it MUST be
      -- RSAES-OAEP-params

      id-RSAES-OAEP  OBJECT IDENTIFIER  ::=  { pkcs-1 7 }

      -- When id-pSpecified is used in an AlgorithmIdentifier the
      -- parameters MUST be an OCTET STRING.

      id-pSpecified  OBJECT IDENTIFIER  ::=  { pkcs-1 9 }

      -- When id-RSASSA-PSS is used in an AlgorithmIdentifier, and the
      -- parameters field is present, it MUST be RSASSA-PSS-params.

      id-RSASSA-PSS  OBJECT IDENTIFIER  ::=  { pkcs-1 10 }

      -- When id-mgf1 is used in an AlgorithmIdentifier the parameters
      -- MUST be present and MUST be a HashAlgorithm.

      id-mgf1  OBJECT IDENTIFIER  ::=  { pkcs-1 8 }

      -- When the following OIDs are used in an AlgorithmIdentifier, the
      -- parameters MUST be present and MUST be NULL.

      sha224WithRSAEncryption  OBJECT IDENTIFIER  ::=  { pkcs-1 14 }

      sha256WithRSAEncryption  OBJECT IDENTIFIER  ::=  { pkcs-1 11 }

      sha384WithRSAEncryption  OBJECT IDENTIFIER  ::=  { pkcs-1 12 }

      sha512WithRSAEncryption  OBJECT IDENTIFIER  ::=  { pkcs-1 13 }

      -- When the following OIDs are used in an AlgorithmIdentifier the
      -- parameters SHOULD be absent, but if the parameters are present,
      -- they MUST be NULL.

      id-sha1  OBJECT IDENTIFIER  ::=  { iso(1)
                           identified-organization(3) oiw(14)
                           secsig(3) algorithms(2) 26 }

      id-sha224  OBJECT IDENTIFIER  ::=  { joint-iso-itu-t(2)
                           country(16) us(840) organization(1) gov(101)
                           csor(3) nistalgorithm(4) hashalgs(2) 4 }

      id-sha256  OBJECT IDENTIFIER  ::=  { joint-iso-itu-t(2)
                           country(16) us(840) organization(1) gov(101)
                           csor(3) nistalgorithm(4) hashalgs(2) 1 }

      id-sha384  OBJECT IDENTIFIER  ::=  { joint-iso-itu-t(2)
                           country(16) us(840) organization(1) gov(101)
                           csor(3) nistalgorithm(4) hashalgs(2) 2 }

      id-sha512  OBJECT IDENTIFIER  ::=  { joint-iso-itu-t(2)
                           country(16) us(840) organization(1) gov(101)
                           csor(3) nistalgorithm(4) hashalgs(2) 3 }

      -- =============
      --   Constants
      -- =============

      nullOctetString  OCTET STRING (SIZE (0))  ::=  ''H

      nullParameters NULL  ::=  NULL

      -- =========================
      --   Algorithm Identifiers
      -- =========================

      sha1Identifier  AlgorithmIdentifier  ::=  {
                              algorithm id-sha1,
                              parameters nullParameters  }

      sha224Identifier    AlgorithmIdentifier  ::=  {
                              algorithm id-sha224,
                              parameters nullParameters  }

      sha256Identifier    AlgorithmIdentifier  ::=  {
                              algorithm id-sha256,
                              parameters nullParameters  }

      sha384Identifier    AlgorithmIdentifier  ::=  {
                              algorithm id-sha384,
                              parameters nullParameters  }

      sha512Identifier    AlgorithmIdentifier  ::=  {
                              algorithm id-sha512,
                              parameters nullParameters  }

      mgf1SHA1Identifier  AlgorithmIdentifier  ::=  {
                              algorithm id-mgf1,
                              parameters sha1Identifier }

      mgf1SHA224Identifier  AlgorithmIdentifier  ::=  {
                              algorithm id-mgf1,
                              parameters sha224Identifier }

      mgf1SHA256Identifier  AlgorithmIdentifier  ::=  {
                              algorithm id-mgf1,
                              parameters sha256Identifier }

      mgf1SHA384Identifier  AlgorithmIdentifier  ::=  {
                              algorithm id-mgf1,
                              parameters sha384Identifier }

      mgf1SHA512Identifier  AlgorithmIdentifier  ::=  {
                              algorithm id-mgf1,
                              parameters sha512Identifier }

      pSpecifiedEmptyIdentifier  AlgorithmIdentifier  ::=  {
                              algorithm id-pSpecified,
                              parameters nullOctetString }

      rSASSA-PSS-Default-Params RSASSA-PSS-params ::=  {
                              hashAlgorithm sha1Identifier,
                              maskGenAlgorithm mgf1SHA1Identifier,
                              saltLength 20,
                              trailerField 1  }

      rSASSA-PSS-Default-Identifier  AlgorithmIdentifier  ::=  {
                              algorithm id-RSASSA-PSS,
                              parameters rSASSA-PSS-Default-Params }

      rSASSA-PSS-SHA224-Identifier  AlgorithmIdentifier  ::=  {
                              algorithm id-RSASSA-PSS,
                              parameters rSASSA-PSS-SHA224-Params }

      rSASSA-PSS-SHA224-Params RSASSA-PSS-params ::= {
                              hashAlgorithm sha224Identifier,
                              maskGenAlgorithm mgf1SHA224Identifier,
                              saltLength 20,
                              trailerField 1  }

      rSASSA-PSS-SHA256-Identifier  AlgorithmIdentifier  ::=  {
                              algorithm id-RSASSA-PSS,
                              parameters rSASSA-PSS-SHA256-Params }

      rSASSA-PSS-SHA256-Params RSASSA-PSS-params ::=  {
                              hashAlgorithm sha256Identifier,
                              maskGenAlgorithm mgf1SHA256Identifier,
                              saltLength 20,
                              trailerField 1  }

      rSASSA-PSS-SHA384-Identifier  AlgorithmIdentifier  ::=  {
                              algorithm id-RSASSA-PSS,
                              parameters rSASSA-PSS-SHA384-Params }

      rSASSA-PSS-SHA384-Params RSASSA-PSS-params ::= {
                              hashAlgorithm sha384Identifier,
                              maskGenAlgorithm mgf1SHA384Identifier,
                              saltLength 20,
                              trailerField 1  }

      rSASSA-PSS-SHA512-Identifier  AlgorithmIdentifier  ::=  {
                              algorithm id-RSASSA-PSS,
                              parameters rSSASSA-PSS-SHA512-params }

      rSSASSA-PSS-SHA512-params RSASSA-PSS-params ::= {
                              hashAlgorithm sha512Identifier,
                              maskGenAlgorithm mgf1SHA512Identifier,
                              saltLength 20,
                              trailerField 1  }

      rSAES-OAEP-Default-Params RSAES-OAEP-params ::=  {
                              hashFunc sha1Identifier,
                              maskGenFunc mgf1SHA1Identifier,
                              pSourceFunc pSpecifiedEmptyIdentifier  }

      rSAES-OAEP-Default-Identifier  AlgorithmIdentifier  ::=  {
                              algorithm id-RSAES-OAEP,
                              parameters rSAES-OAEP-Default-Params }

      rSAES-OAEP-SHA224-Identifier  AlgorithmIdentifier  ::=  {
                              algorithm id-RSAES-OAEP,
                              parameters rSAES-OAEP-SHA224-Params }

      rSAES-OAEP-SHA224-Params RSAES-OAEP-params ::=   {
                              hashFunc sha224Identifier,
                              maskGenFunc mgf1SHA224Identifier,
                              pSourceFunc pSpecifiedEmptyIdentifier  }

      rSAES-OAEP-SHA256-Identifier  AlgorithmIdentifier  ::=  {
                              algorithm id-RSAES-OAEP,
                              parameters rSAES-OAEP-SHA256-Params }

      rSAES-OAEP-SHA256-Params RSAES-OAEP-params ::=  {
                              hashFunc sha256Identifier,
                              maskGenFunc mgf1SHA256Identifier,
                              pSourceFunc pSpecifiedEmptyIdentifier  }

      rSAES-OAEP-SHA384-Identifier  AlgorithmIdentifier  ::=  {
                              algorithm id-RSAES-OAEP,
                              parameters rSAES-OAEP-SHA384-Params }

      rSAES-OAEP-SHA384-Params RSAES-OAEP-params ::=  {
                              hashFunc sha384Identifier,
                              maskGenFunc mgf1SHA384Identifier,
                              pSourceFunc pSpecifiedEmptyIdentifier  }

      rSAES-OAEP-SHA512-Identifier  AlgorithmIdentifier  ::=  {
                              algorithm id-RSAES-OAEP,
                              parameters rSAES-OAEP-SHA512-Params }

      rSAES-OAEP-SHA512-Params RSAES-OAEP-params ::=  {
                              hashFunc sha512Identifier,
                              maskGenFunc mgf1SHA512Identifier,
                              pSourceFunc pSpecifiedEmptyIdentifier  }

      -- ===================
      --   Main structures
      -- ===================

      -- Used in SubjectPublicKeyInfo of X.509 Certificate.

      RSAPublicKey  ::=  SEQUENCE  {
         modulus           INTEGER,    -- n
         publicExponent    INTEGER  }  -- e

      -- AlgorithmIdentifier parameters for id-RSASSA-PSS.
      -- Note that the tags in this Sequence are explicit.

      RSASSA-PSS-params  ::=  SEQUENCE  {
         hashAlgorithm     [0] HashAlgorithm DEFAULT
                                  sha1Identifier,
         maskGenAlgorithm  [1] MaskGenAlgorithm DEFAULT
                                  mgf1SHA1Identifier,
         saltLength        [2] INTEGER DEFAULT 20,
         trailerField      [3] INTEGER DEFAULT 1  }

      HashAlgorithm  ::=  AlgorithmIdentifier

      MaskGenAlgorithm  ::=  AlgorithmIdentifier

      -- AlgorithmIdentifier parameters for id-RSAES-OAEP.
      -- Note that the tags in this Sequence are explicit.

      RSAES-OAEP-params  ::=  SEQUENCE  {
         hashFunc          [0] AlgorithmIdentifier DEFAULT
                                  sha1Identifier,
         maskGenFunc       [1] AlgorithmIdentifier DEFAULT
                                  mgf1SHA1Identifier,
         pSourceFunc       [2] AlgorithmIdentifier DEFAULT
                                  pSpecifiedEmptyIdentifier  }

      END

7.  References

7.1.  Normative References

   [P1v1.5]       Kaliski, B., "PKCS #1: RSA Encryption Version 1.5",
                  RFC 2313, March 1998.

   [P1v2.1]       Jonsson, J. and B. Kaliski, "PKCS #1: RSA Cryptography
                  Specifications Version 2.1", RFC 3447, February 2003.

   [PROFILE]      Housley, R., Polk, W., Ford, W., and D. Solo,
                  "Internet X.509 Public Key Infrastructure Certificate
                  and Certificate Revocation List (CRL) Profile", RFC
                  3280, April 2002.

   [SHA2]         National Institute of Standards and Technology (NIST),
                  FIPS 180-2: Secure Hash Standard, 1 August 2002.

   [SHA224]       Housley, R., "A 224-bit One-way Hash Function: SHA-
                  224", RFC 3874, September 2004.

   [STDWORDS]     Bradner, S., "Key Words for Use in RFCs to Indicate
                  Requirement Levels", RFC 2119, March 1997.

   [X.208-88]     CCITT Recommendation X.208: Specification of Abstract
                  Syntax Notation One (ASN.1), 1988.

   [X.209-88]     CCITT Recommendation X.209: Specification of Basic
                  Encoding Rules for Abstract Syntax Notation One
                  (ASN.1), 1988.

   [X.509-88]     CCITT Recommendation X.509: The Directory -
                  Authentication Framework, 1988.

7.2.  Informative References

   [CMS]          Housley, R., "Cryptographic Message Syntax (CMS)", RFC
                  3852, July 2004.

   [GUIDE]        National Institute of Standards and Technology, Second
                  Draft: "Key Management Guideline, Part 1: General
                  Guidance."  June 2002.
                  [http://csrc.nist.gov/encryption/kms/guideline-1.pdf]

   [P1363A]       IEEE Std 1363a-2004, Standard Specifications for
                  Public Key Cryptography - Amendment 1: Additional
                  Techniques, 2004.

   [PKALGS]       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.

   [RANDOM]       Eastlake 3rd, D., Crocker, S., and J. Schiller,
                  "Randomness Recommendations for Security", RFC 1750,
                  December 1994.

   [SHA-1-ATTACK] Wang, X., Yin, Y.L., and H. Yu, "Finding Collisions in
                  the Full SHA1", to appear, CRYPTO 2005.  Preprint
                  available at
                  http://theory.csail.mit.edu/~yiqun/shanote.pdf.

8.  Security Considerations

   This specification supplements RFC 3280 [PROFILE].  The Security
   Considerations section of that document applies to this specification
   as well.

   Implementations must protect the RSA private key.  Compromising the
   RSA private key may result in the disclosure of all messages
   protected with that key.

   The generation of RSA public/private key pairs relies on a random
   numbers.  Using inadequate pseudo-random number generators (PRNGs) to
   generate cryptographic keys can result in little or no security.  An
   attacker may find it much easier to reproduce the PRNG environment
   that produced the keys and search the resulting small set of
   possibilities, than to brute force search the whole key space.  The
   generation of quality random numbers is difficult and RFC 1750
   [RANDOM] offers important guidance in this area.

   Generally, good cryptographic practice employs a given RSA key pair
   in only one scheme.  This practice avoids the risk that vulnerability
   in one scheme may compromise the security of the other, and may be
   essential to maintain provable security.  While PKCS #1 Version 1.5
   [P1v1.5] has been employed for both key transport and digital
   signature without any known bad interactions, such a combined use of
   an RSA key pair is not recommended in the future.  Therefore, an RSA
   key pair used for RSASSA-PSS signature generation should not be used
   for other purposes.  For similar reasons, one RSA key pair should
   always be used with the same RSASSA-PSS parameters (except possibly
   for the salt length).  Likewise, an RSA key pair used for RSAES-OAEP
   key transport should not be used for other purposes.  For similar
   reasons, one RSA key pair should always be used with the same RSAES-
   OAEP parameters.

   This specification requires implementations to support the SHA-1
   one-way hash function for interoperability, but support for other
   one-way hash functions is permitted.  Wang et al. [SHA-1-ATTACK] have
   recently discovered a collision attack against SHA-1 with complexity
   2^69.  This attack, which can produce two new messages with the same
   hash value, is the first attack on SHA-1 faster than the generic
   attack with complexity 2^80, where 80 is one-half the bit length of
   the hash value.

   In general, when a one-way hash function is used with a digital
   signature scheme, a collision attack is easily translated into a
   signature forgery.  Therefore, using SHA-1 in a digital signature
   scheme provides a security level of no more than 69 bits if the
   attacker can persuade the signer to sign a message resulting from a
   collision attack.  If the attacker can't persuade the signer to sign
   such a message, however, then SHA-1 still provides a security level
   of at least 80 bits since the best (known) inversion attack (which
   produces a new message with a previous hash value) is the generic
   attack with complexity 2^160.  If a greater level of security is
   desired, then a secure one-way hash function with a longer hash value

   is needed.  SHA-256, SHA-384, and SHA-512 are reasonable choices
   [SHA2], although their security needs to be reconfirmed in light of
   the SHA-1 results.

   The metrics for choosing a one-way hash function for use in digital
   signatures do not directly apply to the RSAES-OAEP key transport
   algorithm, since a collision attack on the one-way hash function does
   not directly translate into an attack on the key transport algorithm,
   unless the encoding parameters P vary (in which case a collision of
   the hash value for different encoding parameters might be exploited).

   Nevertheless, for consistency with the practice for digital signature
   schemes, and in case the encoding parameters P is not the empty
   string, it is recommended that the same rule of thumb be applied to
   selecting a one-way hash function for use with RSAES-OAEP.  That is,
   the one-way hash function should be selected so that the bit length
   of the hash value is at least twice as long as the desired security
   level in bits.

   The key size selected impacts the strength achieved when implementing
   cryptographic services.  Thus, selecting appropriate key sizes is
   critical to implementing appropriate security.  A 1024-bit RSA public
   key is considered to provide a security level of about 80 bits.  In
   [GUIDE], the National Institute of Standards and Technology (NIST)
   suggests that a security level of 80 bits is adequate for the
   protection of sensitive information until 2015.  This recommendation
   is likely to be revised based on recent advances, and is expected to
   be more conservative, suggesting that a security level of 80 bits is
   adequate protection of sensitive information until 2010.  If a
   security level greater than 80 bits is needed, then a longer RSA
   public key and a secure one-way hash function with a longer hash
   value are needed.  SHA-224, SHA-256, SHA-384, and SHA-512 are
   reasonable choices for such a one-way hash function, modulo the
   reconfirmation noted above.  For this reason, the algorithm
   identifiers for these one-way hash functions are included in the
   ASN.1 module in Section 6.

   Current implementations MUST support 1024-bit RSA public key sizes.
   Before the end of 2007, implementations SHOULD support RSA public key
   sizes of at least 2048 bits and SHOULD support SHA-256.  This
   requirement is intended to allow adequate time for users to deploy
   the stronger digital signature capability by 2010.

   When using RSASSA-PSS, the same one-way hash function should be
   employed for the hashAlgorithm and the maskGenAlgorithm, but it is
   not required.  When using RSAES-OAEP, the same one-way hash function

   should be employed for the hashFunc and the maskGenFunc, but it is
   not required.  In each case, using the same one-way hash function
   helps with security analysis and reduces implementation complexity.

9.  IANA Considerations

   Within the certificates and CRLs, algorithms are identified by object
   identifiers.  All object identifiers used in this document were
   assigned in Public-Key Cryptography Standards (PKCS) documents or by
   the National Institute of Standards and Technology (NIST).  No
   further action by the IANA is necessary for this document or any
   anticipated updates.

Authors' Addresses

   Russell Housley
   Vigil Security, LLC
   918 Spring Knoll Drive
   Herndon, VA 20170
   USA

   EMail: housley@vigilsec.com

   Burt Kaliski
   RSA Laboratories
   174 Middlesex Turnpike
   Bedford, MA 01730
   USA

   EMail: bkaliski@rsasecurity.com

   Jim Schaad
   Soaring Hawk Consulting
   PO Box 675
   Gold Bar, WA 98251
   USA

   EMail: jimsch@exmsft.com

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