Network Working Group J. Schaad
Internet-Draft Soaring Hawk Consulting
Expires: September 22, 2006 March 21, 2006
ESS Update: Adding CertID Algorithm Agility
draft-ietf-smime-escertid-00.txt
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Copyright (C) The Internet Society (2006).
Abstract
In the original Enhanged Security Services for S/MIME draft, a
structure for cryptographically linking the certificate to be used in
validation with the signature was introduced, this structure was
hardwired to use SHA-1. This document allows for the structure to
have algorithm agility and defines new attributes to deal with the
updating.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Replace Section 5.4 Signing Certificate Attribute
Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Insert new section 5.4.1 . . . . . . . . . . . . . . . . . . . 5
4. Insert new section 5.4.1.1 . . . . . . . . . . . . . . . . . . 7
5. Insert new section 5.4.2 . . . . . . . . . . . . . . . . . . . 9
6. Renumber Section 5.4.1 Certificate Identification . . . . . . 11
7. Normative References . . . . . . . . . . . . . . . . . . . . . 11
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 18
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1. Introduction
In the original Enhanged Security Services (ESS) for S/MIME draft
[ESS], a structure for cryptographically linking the certificate to
be used in validation with the signature was defined. This
structure, called ESSCertID was hardwired to use a SHA-1 hash value.
The recent attacks on SHA-1 require that we change define a new
attribute which allows for the use of a different algorithm. This
document performs that task.
This document defines the structure ESSCertIDEx along with a new
attribute SigningCertificateEx which uses the updated structure.
This document allows for the structure to have algorithm agility and
defines new attributes to deal with the updating.
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].
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2. Replace Section 5.4 Signing Certificate Attribute Definitions
The signing certificate attribute is designed to prevent simple
substitution and re-issue attacks, and to allow for a restricted set
of authorization certificates to be used in verifying a signature.
Two different attributes exist for this due to a flaw in the original
design. The only substantial difference between the two attributes
is that SigningCertificateEx allows for hash algorithm agility, while
SigningCertificateEx forces the use of the SHA-1 hash algoirthm.
With the recent advances in the ability to create hash collisions for
SHA-1 it is deemed wise to move forward sooner rather than later.
The SigningCertificateEx attribute is now the perfered attribute to
be used. Applications SHOULD use the SigningCertificateEx attribute
even if they use SHA-1 as the hash algorithm. Applications SHOULD
recognize both attributes as long as they consider SHA-1 to be
sufficently stable.
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3. Insert new section 5.4.1
5.4.1 Signing Certificate Attribute Definition
The signing certificate attribute is designed to prevent the simple
substitution and re-issue attacks, and to allow for a restricted set
of authorization certificates to be used in verifying a signature.
The definition of SigningCertificateEx is
SigningCertificateEx ::= SEQUENCE {
certs SEQUENCE OF ESSCertIDEx,
policies SEQUENCE OF PolicyInformation OPTIONAL
}
id-aa-signingCertificateEx OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) id-aa(2) XX }
certs contains the list of certificates that are to be used in
validating the message. The first certificate identified in the
sequence of certificate identifiers MUST be the certificate used
to verify the signature. The encoding of the ESSCertIDEx for this
certificate SHOULD include the issuerSerial field. If other
constraints ensure that issuerAndSerialNumber will be present in
the SignerInfo, the issuerSerial field MAY be omitted. The
certificate identified is used during the signature verification
process. If the hash of the certificate does not match the
certificate used to verify the signature, the signature MUST be
considered invalid.
If more than one certificate is present, subsiquent certificates
limit the set of authorization certificates that are used during
signature validation. Authorization certificates can be either
attribute certificates or normal certificates. The issuerSerial
field (in the ESSCertIDEx structure) SHOULD be present for these
certificates, unless the client who is validating the signature is
expected to have easy access to all the certificates requred for
validation. If only the signing certificate is present in the
sequence, there are no restrictions on the set of authorization
certificates used in validating the signature.
contains a sequence of policy information terms that identify
those certificate policies that the signer asserts apply to the
certificate, and under which the certificate should be relied
upon. This value suggests a policy value to be used in the
relying party's certification path validation. The definition of
PolicyInformation can be found in [PKIXCERT].
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If present, the SigningCertificateEx attribute MUST be a signed
attribute; it MUST NOT be an unsigned attribute. CMS defines
SignedAttributes as a SET OF Attribute. A SignerInfo MUST NOT
include multiple instances of the SigningCertificate attribute. CMS
defines the ASN.1 syntax for the signed attributes to include
attrValues SET OF AttributeValue. A SigningCertificate attribute
MUST include only a single instance of AttributeValue. There MUST
NOT be zero or multiple instances of AttributeValue present in the
attrValues SET OF AttributeValue.
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4. Insert new section 5.4.1.1
Insert the following text as a new section
5.4.1 Certificate Indentification
The best way to identify certificates is an often-discussed issue.
[PKIXCERT] has imposed a restriction for SignedData objects that the
issuer DN must be present in all signing certificates. The issuer/
serial number pair is therefore sufficient to identify the correct
signing certificate. This information is already present, as part of
the SignerInfo object, and duplication of this information would be
unfortunate. A hash of the entire certificate serves the same
function (allowing the receiver to verify that the same certificate
is being used as when the message was signed), is smaller, and
permits a detection of the simple substitution attacks.
Attribute certificates and additional public key certificates
containing authorization information do not have an issuer/serial
number pair represented anywhere in a SignerInfo object. When an
attribute certificate or an additional public key certificate is not
included in the SignedData object, it becomes much more difficult to
get the correct set of certificates based only on a hash of the
certificate. For this reason, these certificates SHOULD be
identified by the IssuerSerial object.
This document defines a certificate identifier as:
ESSCertIDEx ::= SEQUENCE {
certHash Hash,
hashAlg AlgorithmIdentifier DEFAULT {id-sha256},
issuerSerial IssuerSerial OPTIONAL
}
Hash ::= OCTET STRING
IssuerSerial ::= SEQUENCE {
issuer GeneralNames,
serialNumber CertificateSerialNumber
}
The fields of ESSCertIDEx are defined as follows:
certHash is computed over the entire DER encoded certificate
including the signature. The issuerSerial would normally be
present unless the value can be inferred from other information.
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hashAlg contains the identifier of the algorithm used in computing
certHash.
issuerSerial holds the identification of the certificate.
The fields of IssuerSerial are defined as follows:
issuer contains the issuer name of the certificate. For non-
attribute certificates, the issuer MUST contain only the issuer
name from the certificate encoded in the directoryName choice of
GeneralNames. For attribute certificates, the issuer MUST contain
the issuer name field from the attribute certificate.
serialNumber holds the serial number that uniquely identifies the
certificate.
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5. Insert new section 5.4.2
5.4.2 Signing Certificate Attribute Definition with SHA-1
The signing certificate attribute is designed to prevent the simple
substitution and re-issue attacks, and to allow for a restricted set
of authorization certificates to be used in verifying a signature.
The definition of SigningCertificate is
SigningCertificate ::= SEQUENCE {
certs SEQUENCE OF ESSCertID,
policies SEQUENCE OF PolicyInformation OPTIONAL
}
id-aa-signingCertificate OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) id-aa(2) 12 }
The first certificate identified in the sequence of certificate
identifiers MUST be the certificate used to verify the signature.
The encoding of the ESSCertID for this certificate SHOULD include the
issuerSerial field. If other constraints ensure that
issuerAndSerialNumber will be present in the SignerInfo, the
issuerSerial field MAY be omitted. The certificate identified is
used during the signature verification process. If the hash of the
certificate does not match the certificate used to verify the
signature, the signature MUST be considered invalid.
If more than one certificate is present in the sequence of
ESSCertIDs, the certificates after the first one limit the set of
authorization certificates that are used during signature validation.
Authorization certificates can be either attribute certificates or
normal certificates. The issuerSerial field (in the ESSCertID
structure) SHOULD be present for these certificates, unless the
client who is validating the signature is expected to have easy
access to all the certificates requred for validation. If only the
signing certificate is present in the sequence, there are no
restrictions on the set of authorization certificates used in
validating the signature.
The sequence of policy information terms identifies those certificate
policies that the signer asserts apply to the certificate, and under
which the certificate should be relied upon. This value suggests a
policy value to be used in the relying party's certification path
validation.
If present, the SigningCertificate attribute MUST be a signed
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attribute; it MUST NOT be an unsigned attribute. CMS defines
SignedAttributes as a SET OF Attribute. A SignerInfo MUST NOT
include multiple instances of the SigningCertificate attribute. CMS
defines the ASN.1 syntax for the signed attributes to include
attrValues SET OF AttributeValue. A SigningCertificate attribute
MUST include only a single instance of AttributeValue. There MUST
NOT be zero or multiple instances of AttributeValue present in the
attrValues SET OF AttributeValue.
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6. Renumber Section 5.4.1 Certificate Identification
Change the number on this section from 5.4.1 to 5.4.2.1
Change the title on this section to "Certificate Identification with
SHA-1".
7. Normative References
[ESS] Hoffman, P., "Enhanced Security Services for S/MIME",
RFC 2634, June 1999.
[PKIXCERT]
Housley, R., Ford, W., Polk, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP 14, March 1997.
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Appendix A. ASN.1 Module
ExtendedSecurityServices-2006
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) ess-2006(200) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
-- Cryptographic Message Syntax (CMS)
ContentType, IssuerAndSerialNumber, SubjectKeyIdentifier,
AlgorithmIdentifier
FROM CryptographicMessageSyntax { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1)}
-- PKIX Certificate and CRL Profile, Sec A.2 Implicitly Tagged Module,
-- 1988 Syntax
PolicyInformation, CertificateSerialNumber, GeneralNames FROM
PKIX1Implicit88 {iso(1)
identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7)id-mod(0) id-pkix1-implicit(19)};
-- Extended Security Services
-- The construct "SEQUENCE SIZE (1..MAX) OF" appears in several ASN.1
-- constructs in this module. A valid ASN.1 SEQUENCE can have zero or
-- more entries. The SIZE (1..MAX) construct constrains the SEQUENCE to
-- have at least one entry. MAX indicates the upper bound is unspecified.
-- Implementations are free to choose an upper bound that suits their
-- environment.
UTF8String ::= [UNIVERSAL 12] IMPLICIT OCTET STRING
-- The contents are formatted as described in [UTF8]
-- Section 2.7
ReceiptRequest ::= SEQUENCE {
signedContentIdentifier ContentIdentifier,
receiptsFrom ReceiptsFrom,
receiptsTo SEQUENCE SIZE (1..ub-receiptsTo) OF GeneralNames }
ub-receiptsTo INTEGER ::= 16
id-aa-receiptRequest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 1}
ContentIdentifier ::= OCTET STRING
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id-aa-contentIdentifier OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 7}
ReceiptsFrom ::= CHOICE {
allOrFirstTier [0] AllOrFirstTier,
-- formerly "allOrNone [0]AllOrNone"
receiptList [1] SEQUENCE OF GeneralNames }
AllOrFirstTier ::= INTEGER { -- Formerly AllOrNone
allReceipts (0),
firstTierRecipients (1) }
-- Section 2.8
Receipt ::= SEQUENCE {
version ESSVersion,
contentType ContentType,
signedContentIdentifier ContentIdentifier,
originatorSignatureValue OCTET STRING }
id-ct-receipt OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-ct(1) 1}
ESSVersion ::= INTEGER { v1(1) }
-- Section 2.9
ContentHints ::= SEQUENCE {
contentDescription UTF8String (SIZE (1..MAX)) OPTIONAL,
contentType ContentType }
id-aa-contentHint OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 4}
-- Section 2.10
MsgSigDigest ::= OCTET STRING
id-aa-msgSigDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 5}
-- Section 2.11
ContentReference ::= SEQUENCE {
contentType ContentType,
signedContentIdentifier ContentIdentifier,
originatorSignatureValue OCTET STRING }
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id-aa-contentReference OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 10 }
-- Section 3.2
ESSSecurityLabel ::= SET {
security-policy-identifier SecurityPolicyIdentifier,
security-classification SecurityClassification OPTIONAL,
privacy-mark ESSPrivacyMark OPTIONAL,
security-categories SecurityCategories OPTIONAL }
id-aa-securityLabel OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 2}
SecurityPolicyIdentifier ::= OBJECT IDENTIFIER
SecurityClassification ::= INTEGER {
unmarked (0),
unclassified (1),
restricted (2),
confidential (3),
secret (4),
top-secret (5) } (0..ub-integer-options)
ub-integer-options INTEGER ::= 256
ESSPrivacyMark ::= CHOICE {
pString PrintableString (SIZE (1..ub-privacy-mark-length)),
utf8String UTF8String (SIZE (1..MAX))
}
ub-privacy-mark-length INTEGER ::= 128
SecurityCategories ::= SET SIZE (1..ub-security-categories) OF
SecurityCategory
ub-security-categories INTEGER ::= 64
SecurityCategory ::= SEQUENCE {
type [0] OBJECT IDENTIFIER,
value [1] ANY DEFINED BY type -- defined by type
}
--Note: The aforementioned SecurityCategory syntax produces identical
--hex encodings as the following SecurityCategory syntax that is
--documented in the X.411 specification:
--
--SecurityCategory ::= SEQUENCE {
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-- type [0] SECURITY-CATEGORY,
-- value [1] ANY DEFINED BY type }
--
--SECURITY-CATEGORY MACRO ::=
--BEGIN
--TYPE NOTATION ::= type | empty
--VALUE NOTATION ::= value (VALUE OBJECT IDENTIFIER)
--END
-- Section 3.4
EquivalentLabels ::= SEQUENCE OF ESSSecurityLabel
id-aa-equivalentLabels OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 9}
-- Section 4.4
MLExpansionHistory ::= SEQUENCE
SIZE (1..ub-ml-expansion-history) OF MLData
id-aa-mlExpandHistory OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 3}
ub-ml-expansion-history INTEGER ::= 64
MLData ::= SEQUENCE {
mailListIdentifier EntityIdentifier,
expansionTime GeneralizedTime,
mlReceiptPolicy MLReceiptPolicy OPTIONAL }
EntityIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier SubjectKeyIdentifier }
MLReceiptPolicy ::= CHOICE {
none [0] NULL,
insteadOf [1] SEQUENCE SIZE (1..MAX) OF GeneralNames,
inAdditionTo [2] SEQUENCE SIZE (1..MAX) OF GeneralNames }
-- Section 5.4
SigningCertificate ::= SEQUENCE {
certs SEQUENCE OF ESSCertID,
policies SEQUENCE OF PolicyInformation OPTIONAL
}
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id-aa-signingCertificate OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) id-aa(2) 12 }
id-sha256 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2)
country(16) us(840) organization(1) gov(101)
csor(3) nistalgorithm(4) hashalgs(2) 1 }
ESSCertIDEx ::= SEQUENCE {
certHash Hash,
hashAlgorithm AlgorithmIdentifier DEFAULT {algorithm
id-sha256 parameters NULL}
issuerSerial IssuerSerial OPTIONAL
}
ESSCertID ::= SEQUENCE {
certHash Hash,
issuerSerial IssuerSerial OPTIONAL
}
Hash ::= OCTET STRING -- SHA1 hash of entire certificate
IssuerSerial ::= SEQUENCE {
issuer GeneralNames,
serialNumber CertificateSerialNumber
}
END -- of ExtendedSecurityServices-2006
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Author's Address
Jim Schaad
Soaring Hawk Consulting
PO Box 675
Gold Bar, WA 98251
Phone: (425) 785-1031
Email: jimsch@exmsft.com
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