Network Working Group J. Schaad

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.

Schaad, et al. Standards Track [Page 1]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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.

Schaad, et al. Standards Track [Page 2]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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.

Schaad, et al. Standards Track [Page 3]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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.

Schaad, et al. Standards Track [Page 4]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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 }

Schaad, et al. Standards Track [Page 5]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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.

Schaad, et al. Standards Track [Page 6]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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.

Schaad, et al. Standards Track [Page 7]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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

Schaad, et al. Standards Track [Page 8]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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}

Schaad, et al. Standards Track [Page 9]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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.

Schaad, et al. Standards Track [Page 10]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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.

Schaad, et al. Standards Track [Page 11]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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.

Schaad, et al. Standards Track [Page 12]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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.

Schaad, et al. Standards Track [Page 13]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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) } ;

Schaad, et al. Standards Track [Page 14]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

-- ============================
-- 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 }

Schaad, et al. Standards Track [Page 15]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

-- 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 }

Schaad, et al. Standards Track [Page 16]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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 }

Schaad, et al. Standards Track [Page 17]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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 }

Schaad, et al. Standards Track [Page 18]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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

Schaad, et al. Standards Track [Page 19]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

-- 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.

Schaad, et al. Standards Track [Page 20]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

[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.

Schaad, et al. Standards Track [Page 21]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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

Schaad, et al. Standards Track [Page 22]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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

Schaad, et al. Standards Track [Page 23]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

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: [email protected]

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

EMail: [email protected]

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

EMail: [email protected]

Schaad, et al. Standards Track [Page 24]

RFC 4055 Additional RSA Algorithms and Identifiers June 2005

Full Copyright Statement

Copyright (C) The Internet Society (2005).

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights 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; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
[email protected].

Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

Schaad, et al. Standards Track [Page 25]