INTERNET-DRAFT P. Yee Intended Status: Proposed Standard AKAYLA Updates: 5280 (if approved) June 26, 2012 Expires: December 28, 2012 Updates to the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." 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Yee Expires December 12, 2012 [Page 1] INTERNET-DRAFT RFC 5280 Clarifications June 12, 2012 Abstract This document updates RFC 5280, the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile. This document changes the set of acceptable encoding methods for the explicitText field of the user notice policy qualifier and clarifies the rules for converting internationalized domain name labels to ASCII. This document also provides some clarifications on the use of self-signed certificates, trust anchors, and some updated security considerations. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Update to RFC 5280, Section 3.2: Certification Paths and Trust . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Update to RFC 5280, Section 4.2.1.4: Certificate Policies . . 3 4. Update to RFC 5280, Section 6.2: Using the Path Validation Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5. Update to RFC 5280, Section 7.3: Internationalized Domain Names in Distinguished Names . . . . . . . . . . . . . . . . . 6 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 8.1. Normative References . . . . . . . . . . . . . . . . . . . 8 8.2. Informative References . . . . . . . . . . . . . . . . . . 8 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8 1. Introduction This document updates the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile [RFC5280]. This document makes a recommendation that self-signed certificates used to convey trust anchor data be marked as CA certificates, which is not always current practice. The acceptable and unacceptable encodings for the explicitText field of the user notice policy qualifier are updated to bring them in line with existing practice. The use of self-signed certificates as trust anchors in Section 6.2 is clarified. While it is optional to use additional information in these certificates in the path validation process, [RFC5937] is noted Yee Expires December 12, 2012 [Page 2] INTERNET-DRAFT RFC 5280 Clarifications June 12, 2012 as providing guidance in that regard. The Section 7.3 rules for ASCII encoding of Internationalized Domain Names (IDN) as Distinguished Names are aligned with the rules in Section 7.2 which govern IDN encoding as GeneralNames. In light of some observed attacks, the Security Considerations now give added depth to the consequences of CA key compromise. This section additionally notes that collision resistance is not a required property of one-way hash functions when used to generate key identifiers. 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 [RFC2119]. 2. Update to RFC 5280, Section 3.2: Certification Paths and Trust Add the following paragraph to the end of RFC 5280, Section 3.2: | In some cases, a self-signed certificate that does not contain a | BasicConstraints extension (and thus is implicitly an EE certificate) | is used to convey a public key. Such a certificate (i.e., a self- | signed certificate not marked as a CA certificate) is not compatible | with the path validation rules described in Section 6, or the | definitions in the paragraph above. This document RECOMMENDS that | self-signed certificates used to convey trust anchor data be marked | (see Section 4.2.1.9) as CA certificates, but acknowledges that this | convention is often not adopted in practice. Other uses of self- | signed EE certificates are outside the scope of this specification. 3. Update to RFC 5280, Section 4.2.1.4: Certificate Policies RFC 5280, Section 4.2.1.4, the tenth paragraph says: | An explicitText field includes the textual statement directly in | the certificate. The explicitText field is a string with a | maximum size of 200 characters. Conforming CAs SHOULD use the | UTF8String encoding for explicitText, but MAY use IA5String. | Conforming CAs MUST NOT encode explicitText as VisibleString or | BMPString. The explicitText string SHOULD NOT include any control | characters (e.g., U+0000 to U+001F and U+007F to U+009F). When | the UTF8String encoding is used, all character sequences SHOULD be | normalized according to Unicode normalization form C (NFC) [NFC]. This paragraph is replaced with: Yee Expires December 12, 2012 [Page 3] INTERNET-DRAFT RFC 5280 Clarifications June 12, 2012 | An explicitText field includes the textual statement directly in | the certificate. The explicitText field is a string with a | maximum size of 200 characters. Conforming CAs SHOULD use the | UTF8String encoding for explicitText, but MAY use VisibleString or | BMPString. Conforming CAs MUST NOT encode explicitText as | IA5String. The explicitText string SHOULD NOT include any control | characters (e.g., U+0000 to U+001F and U+007F to U+009F). When | the UTF8String or BMPString encoding is used, all character | sequences SHOULD be normalized according to Unicode normalization | form C (NFC) [NFC]. 4. Update to RFC 5280, Section 6.2: Using the Path Validation Algorithm RFC 5280, Section 6.2, the third paragraph says: | Where a CA distributes self-signed certificates to specify trust | anchor information, certificate extensions can be used to specify | recommended inputs to path validation. For example, a policy | constraints extension could be included in the self-signed | certificate to indicate that paths beginning with this trust anchor | should be trusted only for the specified policies. Similarly, a name | constraints extension could be included to indicate that paths | beginning with this trust anchor should be trusted only for the | specified name spaces. The path validation algorithm presented in | Section 6.1 does not assume that trust anchor information is provided | in self-signed certificates and does not specify processing rules for | additional information included in such certificates. Implementations | that use self-signed certificates to specify trust anchor information | are free to process or ignore such information. This paragraph is replaced with: | Where a CA distributes self-signed certificates to specify trust | anchor information, certificate extensions can be used to specify | recommended inputs to path validation. For example, a policy | constraints extension could be included in the self-signed | certificate to indicate that paths beginning with this trust anchor | should be trusted only for the specified policies. Similarly, a name | constraints extension could be included to indicate that paths | beginning with this trust anchor should be trusted only for the | specified name spaces. The path validation algorithm presented | in Section 6.1 does not assume that trust anchor information is | provided in self-signed certificates and does not specify processing | rules for additional information included in such certificates. | However, [RFC5937] provides an example of how additional information | included in self-signed certificates may be used to initialize the | path validation inputs. Implementations that use self-signed | certificates to specify trust anchor information are free to make Yee Expires December 12, 2012 [Page 4] INTERNET-DRAFT RFC 5280 Clarifications June 12, 2012 | use of any additional information that is included in the | certificates, or to ignore such information. Yee Expires December 12, 2012 [Page 5] INTERNET-DRAFT RFC 5280 Clarifications June 12, 2012 5. Update to RFC 5280, Section 7.3: Internationalized Domain Names in Distinguished Names RFC 5280, Section 7.3, the first paragraph says: | Domain Names may also be represented as distinguished names using | domain components in the subject field, the issuer field, the | subjectAltName extension, or the issuerAltName extension. As with | the dNSName in the GeneralName type, the value of this attribute is | defined as an IA5String. Each domainComponent attribute represents a | single label. To represent a label from an IDN in the distinguished | name, the implementation MUST perform the "ToASCII" label conversion | specified in Section 4.1 of RFC 3490. The label SHALL be considered | a "stored string". That is, the AllowUnassigned flag SHALL NOT be | set. This paragraph is replaced with: | Domain Names may also be represented as distinguished names using | domain components in the subject field, the issuer field, the | subjectAltName extension, or the issuerAltName extension. As with | the dNSName in the GeneralName type, the value of this attribute is | defined as an IA5String. Each domainComponent attribute represents a | single label. To represent a label from an IDN in the distinguished | name, the implementation MUST perform the "ToASCII" label conversion | specified in Section 4.1 of RFC 3490 with the UseSTD3ASCIIRules flag | set. The label SHALL be considered a "stored string". That is, the | AllowUnassigned flag SHALL NOT be set. The conversion process is the | same as is performed in step 4 in Section 7.2. Yee Expires December 12, 2012 [Page 6] INTERNET-DRAFT RFC 5280 Clarifications June 12, 2012 6. Security Considerations This document modifies the Security Considerations section of RFC 5280 as follows. The fifth paragraph of the Security Considerations section of RFC 5280 says: | The protection afforded private keys is a critical security factor. | On a small scale, failure of users to protect their private keys will | permit an attacker to masquerade as them or decrypt their personal | information. On a larger scale, compromise of a CA's private signing | key may have a catastrophic effect. If an attacker obtains the | private key unnoticed, the attacker may issue bogus certificates and | CRLs. Existence of bogus certificates and CRLs will undermine | confidence in the system. If such a compromise is detected, all | certificates issued to the compromised CA MUST be revoked, preventing | services between its users and users of other CAs. Rebuilding after | such a compromise will be problematic, so CAs are advised to | implement a combination of strong technical measures (e.g., tamper- | resistant cryptographic modules) and appropriate management | procedures (e.g., separation of duties) to avoid such an incident. This paragraph is replaced with: | The protection afforded private keys is a critical security factor. | On a small scale, failure of users to protect their private keys will | permit an attacker to masquerade as them or decrypt their personal | information. On a larger scale, compromise of a CA's private signing | key may have a catastrophic effect. | | If an attacker obtains the private key of a CA unnoticed, the | attacker may issue bogus certificates and CRLs. Even if an attacker | is unable to obtain a copy of a CA's private key, the attacker may be | able to issue bogus certificates and CRLs by making unauthorized use | of the CA's workstation or of an RA's workstation. Such an attack | may be the result of an attacker obtaining unauthorized access to the | workstation, either locally or remotely, or may be the result of | inappropriate activity by an insider. Existence of bogus | certificates and CRLs will undermine confidence in the system. Among | many other possible attacks, the attacker may issue bogus | certificates that have the same subject names as legitimate | certificates in order impersonate legitimate certificate subjects. | This could include bogus CA certificates in which the subject names | in the bogus certificates match the names under which legitimate CAs | issue certificates and CRLs. This would allow the attacker to issue | bogus certificates and CRLs that have the same issuer names, and | possibly the same serial numbers, as certificates and CRLs issued by | legitimate CAs. Yee Expires December 12, 2012 [Page 7] INTERNET-DRAFT RFC 5280 Clarifications June 12, 2012 The following text is added to the end of the Security Considerations section of 5280: One-way hash functions are commonly used to generate key identifier values (AKI and SKI), e.g., as described in Sections 4.1.1 and 4.1.2. However, none of the security properties of such functions are required for this context. 7. IANA Considerations This document has no actions for IANA. 8. References 8.1. Normative References [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts - Application and Support", STD 3, RFC 1123, October 1989. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC5280] Cooper, D., Santesson, S., Farrell, 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. 8.2. Informative References [RFC5937] Ashmore, S. and C. Wallace, "Using Trust Anchor Constraints during Certification Path Processing", RFC 5937, August 2010. [X.680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002, Information Technology - Abstract Syntax Notation One (ASN.1): Specification of basic notation. [NFC] Davis, M. and M. Duerst, "Unicode Standard Annex #15: Unicode Normalization Forms", October 2006, . 9. Acknowledgements David Cooper is acknowledged for his fine work in editing versions 00 through 04 of this document. Author's Address Yee Expires December 12, 2012 [Page 8] INTERNET-DRAFT RFC 5280 Clarifications June 12, 2012 Peter E. Yee AKAYLA 7150 Moorland Drive Clarksville, MD 21029 USA EMail: peter@akayla.com Yee Expires December 12, 2012 [Page 9]