wdiff draft-schaad-smime-algorithm-attribute-00.txt draft-schaad-smime-algorithm-attribute-01.txt

Network Working Group J. Schaad
Internet-Draft Soaring Hawk Consulting
Intended status: Standards Track May 5, December 17, 2009
Expires: November 6, 2009 June 20, 2010

Signer Info Algorithm Protection Attribute
draft-schaad-smime-algorithm-attribute-00.txt
draft-schaad-smime-algorithm-attribute-01

Abstract

A new attribute is defined that allows for protection of the digest
and signature algorithm structures in an authenticated data or a
signer info structure. Using the attribute includes the algorithm
defintion information in the integrity protection process.

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Abstract

A new signed attribute is defined that allows for protection Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the
algorithm structures Trust Legal Provisions and are provided without warranty as
described in an authenticated data or a signer info
structure. By placing the information into a signed or authenticated
attribute its value is then covered by the validation process. BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Attribute Structure . . . . . . . . . . . . . . . . . . . . . 5
3. Verification Process . . . . . . . . . . . . . . . . . . . . . 7
3.1. Signed Data Verification Changes . . . . . . . . . . . . . 7
3.2. Authenticated Data Verification Changes . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Normative References . . . . . . . . . . . . . . . . . . . 9
5.2. Informational References . . . . . . . . . . . . . . . . . 9
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12 11

1. Introduction

In the current definition of [RFC3852] there are some fields that are
not protected in the process of doing either a signature validation
or an authentication validation. In this document a new signed or
authenticated attribute is defined which permits these fields to be
validated.

Taking the SignerInfo structure from CMS, lets look at each of the
fields for and discuss what is and is not protected by the signature.
The ASN.1 is included here for convience. (The analysis of
AuthenticedData is similar.)

SignerInfo ::= SEQUENCE {
version CMSVersion,
sid SignerIdentifier,
digestAlgorithm DigestAlgorithmIdentifier,
signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,
signatureAlgorithm SignatureAlgorithmIdentifier,
signature SignatureValue,
unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL }

version is not protected by the signature. Many implementations of
CMS today actually ignore the value of this field. If the
structure decodes then this is considered sufficient to continue
processing. Using most decoders on the market the value of this
field does not control how the decoding is actually processed.

sid can be protected by the use of either signing certificate
authenticated attribute. SigningCertificateV2 is defined in
[RFC5035]. SigningCertificate is defined in [RFC2634]. In
addition to allowing for the protection of the signer identifier,
the specific certificate is protected by including a hash of the
certificate to be used for validation.

digestAlgorithm the digest algorithm used has been implicitly
protected by the fact that CMS has only defined one digest
algorithm for each hash value length. (The algorithm RIPEM-160
was never standardized). If newer digest algorithms are defined
where there are multiple algorthms for a given hash length, or
where parameters are defined for a specific algorithm, this
implicit protection will no longer exist.

signedAttributes are directly protected by the signature when they
are present. The DER encoding of this value is what is actually
hashed for the signature computation.

signatureAlgorithm has been protected by implication in the past.
For RSA v 1.5 signatures, the fact that the RSA algorithm was
known from the public key and the digest algorithm used is
included in the formatted value over which the RSA computation is
done. For DSA signature, the fact that a DSA public key was
sufficient to know that SHA-1 was the digest algorithm as it was
the only acceptable digest algorithm. With the advent of newer
signature algorithms, especially those such as RSA-PSS which have
parameters, this implicit protection of the signature algorithm is
no longer possible.

signature is not directly protected by any other value unless a
counter signature is present. However this represents the
crytographically computed value to protected the rest of the
signature information.

unsignedAttrs is not protected by the signature value. It is also
explicitly designed not to be protected by the signature value.

As can be seen above, the digestAlgorithm and signatureAlgorithm
fields have been indirectly rather than explicity protected in the
past. With new algorithms that have been or are being defined this
will no longer be the case. This document defines and describes a
new attribute that will explicitly protect these fields along with
the macAlgorithm field of the AuthenticatedData structure.

1.1. Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in[RFC2119]. in [RFC2119].

2. Attribute Structure

The following defines the algorithm protection attribute:

The algorithm-protection attribute has the ASN.1 type
AlgorithmProtection:

aa-CMSAlgorithmProtection ATTRIBUTE ::= {
TYPE CMSAlgorithmProtection
IDENTIFIED BY { id-aa-CMSAlgorithmProtection }
}

The following object identifier identifies the algorithm-protection
attribute:

id-aa-CMSAlgorithmProtection OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) TBA }

The algorithm-protection attribute uses the following ASN.1 type:

CMSAlgorithmProtection ::= SEQUENCE {
digestAlgorithm DigestAlgorithmIdentifier,
signatureAlgorithm [1] SignatureAlgorithmIdentifier OPTIONAL,
macAlgorithm [2] MessageAuthenticationCodeAlgorithm OPTIONAL
}
(WITH COMPONENTS { signatureAlgorithm PRESENT,
macAlgorithm ABSENT } |
WITH COMPONENTS { signatureAlgorithm ABSENT,
macAlgorithm PRESENT })

The fields are defined as follows:

digestAlg contains a copy of the digest algorithm identifier and SignerInfo.digestAlgorithm field or
the AuthenticatedData.digestAlgorithm field including any
parameters associated with it.

signatureAlg contains a copy of the signature algorithm identifier
and any parameters associated with it. This field is only
populated if the attribute is placed in a SignerInfo structure.

authenticationAlg contains a copy of the message authentication code
and any parameters associated with it. This field is only
populated if the attribute is placed in an authenticated data
structure.

Exactly one of signatureAlgorithm and macAlgorithm SHALL be present.

An algorithm protection MUST have a single attribute value, even
though the syntax is defined as a SET OF AttributeValue. There MUST
not be zero or multiple instances of AttributeValue present.

The authentication protection attribute MUST be a signed attribute or
an authenticated attribute; it MUST NOT be an unsigned attribute, an
unauthenticated attribute or an unprotected attribute.

The SignedAttributes and AuthAttributes syntax are each defined as a
SET of Attributes. The SignedAttributes in a signerInfo MUST include
only one instance of the algorithm protection attribute. Similarly,
the AuthAttributes in an AuthenticatedData MUST include only one
instance of the algorithm protection attribute.

3. Verification Process

The exact verification process depends on the structure being dealt
with.

When doing comparisions of the fields, a field whos value is a
default value and one which is explicitly provided MUST compare as
equivalent. It is not required that a field which is absent in one
case and present in another case be compared as equivelent. (This
mean that an algorithm identifier with absent parameters and one with
NULL parameters need not compare as equivalent.)

3.1. Signed Data Verification Changes

If a CMS validator supports this attribute, the following additional
verification steps MUST be performed:

1. The SignerInfo.digestAlgorithm field MUST be compared to the
digestAlgorithm field in the attribute. If the fields are not same
(modulo encoding) then signature validation MUST fail.

2. The SignerInfo.signatureAlgorithm field MUST be compared to the
signatureAlgorithm field in the attribute. If the fields are not the
same (modulo encoding) then the signature validation MUST fail.

3.2. Authenticated Data Verification Changes

If a CMS validator supports this attribute, the following additional
verification steps MUST be performed:

1. The AuthenticatedData.digestAlgorithm field MUST be compared to
the digestAlgorithm field in the attribute. If the fields are not
same (modulo encoding) then signature validation MUST fail.

2. The AuthenticatedData.macAlgorithm field MUST be compared to the
macAlgorithm field in the attribute. If the fields are not the same
(modulo encoding) then the signature validation MUST fail.

4. Security Considerations

This document is designed to address the security issue of algorithm
substitutions of the algorithms used by the validator. At this time
there is no known method to exploit this type of attack. If the
attack could be successful, then either a weaker algorithm could be
substituted for a stronger algorithm. algorithm or the parameters can be
modified by an attacker to change the state of the hashing algorithm
used. (One example would be changing the initial parmeter value for
[I-D.schaad-smime-hash-experiment]

The attributes defined in this document are to be placed in locations
that are protected by the signature. This attribute does not provide
any additional security if placed in an un-signed or un-authenticated
location.

5. References

5.1. Normative References

[RFC2634] Hoffman, P., "Enhanced Security Services for S/MIME",
RFC 2634, June 1999.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP 14, March 1997.

[RFC5280] Cooper, D., Santesson, S., Farrel, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.

[RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 3852, July 2004.

[RFC5035] Schaad, J., "Enhanced Security Services (ESS) Update:
Adding CertID Algorithm Agility", RFC 5035, August 2007.

[I-D.ietf-pkix-new-asn1]
Hoffman, P. and J. Schaad, "New ASN.1 Modules for PKIX",
draft-ietf-pkix-new-asn1-07 (work in progress),
August 2009.

5.2. Informational References

[I-D.schaad-smime-hash-experiment]
Schaad, J., "Experiment: Hash functions with parameters in
CMS and S/MIME", draft-schaad-smime-hash-experiment-00
(work in progress), May 2009.

Appendix A. ASN.1 Module

CMSAlgorithmAttribute
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) TBD 989 }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
-- Cryptographic Message Syntax (CMS) [RFC 3852]
ContentType, IssuerAndSerialNumber, SubjectKeyIdentifier [RFC3852]
DigestAlgorithmIdentifier, MessageAuthenticationCodeAlgorithm,
SignatureAlgorithmIdentifier
FROM CryptographicMessageSyntax2004 CryptographicMessageSyntax-2009
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2004(24)}
-- PKIX Certificate and CRL Profile, Section A.1 Explicity Tagged Module
-- 1988 Syntax [RFC 5280]
AlgorithmIdentifier, CertificateSerialNumber
FROM PKIX1Explicit88 { iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0) pkix1-explicit(18) id-mod-cms-2004-02(41) }

-- Common PKIX Certificate and CRL Profile, Sec A.2 Implicitly Tagged Module,
-- 1988 Syntax [RFC 5280]
PolicyInformation, CertificateSerialNumber, GeneralNames structures [I-D.ietf-pkix-new-asn1]
ATTRIBUTE
FROM PKIX1Implicit88 PKIX-CommonTypes-2009
{iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7)id-mod(0) id-pkix1-implicit(19)}; pkix(7) id-mod(0) id-mod-pkixCommon-02(57)};

--
-- The CMS Algorithm Protection attribute is a Signed Attribute or
-- an Authenticated Attribute.
--
-- Add this attribute to SignedAttributesSet in [CMS] [RFC3852]
-- Add this attribute to AuthAttriuteSet in [CMS] [RFC3852]
--

aa-CMSAlgorithmProtection ATTRIBUTE ::= {
TYPE CMSAlgorithmProtection
IDENTIFIED BY { id-aa-CMSAlgorithmProtection }
}

id-aa-CMSAlgorithmProtection OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) TBA 989 }

Author's Address

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

Email: jimsch@exmsft.com ietf@augustcellars.com