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.
Abstract
In the original Enhanced Security Services for S/MIME document (RFC 2634), 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 a new attribute for this purpose.
In the original Enhanced Security Services (ESS) for S/MIME document [ESS], a structure for cryptographically linking the certificate to be used in validation with the signature was defined. This structure, called ESSCertID, identifies a certificate by its hash. The structure is hardwired to use a SHA-1 hash value. The recent attacks on SHA-1 require that we define a new attribute that allows for the use of different algorithms. This document performs that task.
This document defines the structure ESSCertIDv2 along with a new attribute SigningCertificateV2, which uses the updated structure. This document allows for the structure to have algorithm agility by including an algorithm identifier and defines a new signed attribute to use the new structure.
This document specifies the continued use of ESSCertID to ensure compatibility when SHA-1 is used for certificate disambiguation.
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].
The signing certificate attribute is designed to prevent simple substitution and re-issue attacks, and to allow for a restricted set of 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 SigningCertificateV2 allows for hash algorithm agility, while SigningCertificate forces the use of the SHA-1 hash algorithm. With the recent advances in the ability to create hash collisions for SHA-1, it is wise to move forward sooner rather than later.
When the SHA-1 hash function is used, the SigningCertificate attribute MUST be used. The SigningCertificateV2 attribute MUST be used if any algorithm other than SHA-1 is used and SHOULD NOT be used for SHA-1. Applications SHOULD recognize both attributes as long as they consider SHA-1 able to distinguish between two different certificates, (i.e., the possibility of a collision is sufficiently low). If both attributes exist in a single message, they are independently evaluated.
Four cases exist that need to be taken into account when using this attribute for correct processing:
1. Signature validates and the hashes match: This is the success case.
2. Signature validates and the hashes do not match: In this case, the certificate contained the correct public key, but the certificate containing the public key is not the one that the signer intended to be used. In this case the application should attempt a search for a different certificate with the same public key and for which the hashes match. If no such certificate can be found, this is a failure case.
3. Signature fails validation and the hashes match: In this case, it can be assumed that the signature has been modified in some fashion. This is a failure case.
4. Signature fails validation and the hashes do not match: In this case, it can be either that the signature has been modified, or that the wrong certificate has been used. Applications should attempt a search for a different certificate that matches the hash value in the attribute and use the new certificate to retry the signature validation.
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3. Insert New Section 5.4.1 'Signing Certificate Attribute Definition Version 2'
5.4.1 Signing Certificate Attribute Definition Version 2
The signing certificate attribute is designed to prevent the simple substitution and re-issue attacks, and to allow for a restricted set of certificates to be used in verifying a signature.
SigningCertificateV2 ::= SEQUENCE { certs SEQUENCE OF ESSCertIDv2, policies SEQUENCE OF PolicyInformation OPTIONAL }
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 ESSCertIDv2 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, subsequent certificates limit the set of certificates that are used during validation. Certificates can be either attribute certificates (limiting authorizations) or public key certificates (limiting path validation). The issuerSerial field (in the ESSCertIDv2 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 required for validation. If only the signing certificate is present in the sequence, there are no restrictions on the set of certificates used in validating the signature.
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policies 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 [RFC3280].
If present, the SigningCertificateV2 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 SigningCertificateV2 attribute. CMS defines the ASN.1 syntax for the signed attributes to include attrValues SET OF AttributeValue. A SigningCertificateV2 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.
4. Insert New Section 5.4.1.1 'Certificate Identification Version 2'
The best way to identify certificates is an often-discussed issue. The ESSCertIDv2 structure supplies two different fields that are used for this purpose.
The hash of the entire certificate allows for a verifier to check that the certificate used in the verification process was the same certificate the signer intended. Hashes are convenient in that they are frequently used by certificate stores as a method of indexing and retrieving certificates as well. The use of the hash is required by this structure since the detection of substituted certificates is based on the fact they would map to different hash values.
The issuer/serial number pair is the method of identification of certificates used in [RFC3280]. That document imposes a restriction for certificates that the issuer distinguished name must be present. The issuer/serial number pair would therefore normally be sufficient to identify the correct signing certificate. (This assumes the same issuer name is not reused from the set of trust anchors.) The issuer/serial number pair can be stored in the sid field of the SignerInfo object. However, the sid field is not covered by the signature. In the cases where the issuer/serial number pair is not used in the sid or the issuer/serial number pair needs to be signed, it SHOULD be placed in the issuerSerial field of the ESSCertIDv2 structure.
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Attribute certificates and additional public key certificates containing 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:
hashAlgorithm contains the identifier of the algorithm used in computing certHash.
certHash is computed over the entire DER-encoded certificate (including the signature) using the SHA-1 algorithm.
issuerSerial holds the identification of the certificate. The issuerSerial would normally be present unless the value can be inferred from other information (e.g., the sid field of the SignerInfo object).
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.
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serialNumber holds the serial number that uniquely identifies the certificate for the issuer.
5. Insert New Section 5.4.2 'Signing Certificate Attribute Definition Version 1'
(Note: This section does not present new material. This section contains the original contents of Section 5.4 in [ESS], which are retained with minor changes in this specification to achieve backwards compatibility.)
Insert the following text as a new section.
5.4.2 Signing Certificate Attribute Definition Version 1
The signing certificate attribute is designed to prevent the simple substitution and re-issue attacks, and to allow for a restricted set of certificates to be used in verifying a signature.
The definition of SigningCertificate is
SigningCertificate ::= SEQUENCE { certs SEQUENCE OF ESSCertID, policies SEQUENCE OF PolicyInformation OPTIONAL }
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 certificates that are used during validation. Certificates can be either attribute certificates (limiting authorizations) or public key certificates (limiting path validation). 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
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easy access to all the certificates required for validation. If only the signing certificate is present in the sequence, there are no restrictions on the set of 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 attribute; it MUST NOT be an unsigned attribute. Cryptographic Message Syntax (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.
6. Insert New Section 5.4.2.1 'Certificate Identification Version 1'
(Note: This section does not present new material. This section contains the original contents of Section 5.4 in [ESS], which are retained with minor changes in this specification to achieve backwards compatibility.)
certHash is computed over the entire DER-encoded certificate (including the signature).
issuerSerial holds the identification of the certificate. This field would normally be present unless the value can be inferred from other information (e.g., the sid field of the SignerInfo object).
This document is designed to address the security issue of a substituted certificate used by the validator. If a different certificate is used by the validator than the signer, the validator may not get the correct result. An example of this would be that the original certificate was revoked and a new certificate with the same public key was issued for a different individual. Since the issuer/ serial number field is not protected, the attacker could replace this and point to the new certificate and validation would be successful.
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 unsigned or un-authenticated location.
The attributes defined in this document permit a signer to select a hash algorithm to identify a certificate. A poorly selected hash algorithm may provide inadequate protection against certificate substitution or result in denial of service for this protection. By employing the attributes defined in this specification with the same hash algorithm used for message signing, the sender can ensure that these attributes provide commensurate security.
Since recipients must support the hash algorithm to verify the signature, selecting the same hash algorithm also increases the likelihood that the hash algorithm is supported in the context of certificate identification. Note that an unsupported hash algorithm for certificate identification does not preclude validating the message but does deny the message recipient protection against certificate substitution.
To ensure that legacy implementations are provided protection against certificate substitution, clients are permitted to include both ESScertID and ESScertIDv2 in the same message. Since these
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attributes are generated and evaluated independently, the contents could conceivably be in conflict. Specifically, where a signer has multiple certificates containing the same public key, the two attributes could specify different signing certificates. The result of signature processing may vary depending on which certificate is used to validate the signature.
Recipients that attempt to evaluate both attributes may choose to reject such a message.
[ESS] 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.
[RFC3280] 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.
[RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852, July 2004.
[UTF8] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003.
-- 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.
SecurityCategories ::= SET SIZE (1..ub-security-categories) OF SecurityCategory
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ub-security-categories INTEGER ::= 64
SecurityCategory ::= SEQUENCE { type [0] OBJECT IDENTIFIER, value [1] ANY 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 {
-- 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
Jim Schaad Soaring Hawk Consulting PO Box 675 Gold Bar, WA 98251
EMail: jimsch@exmsft.com
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