7. Processing Rules for Internationalized Names Internationalized names may be encountered in numerous certificate and CRL fields and extensions, including distinguished names, internationalized domain names, electronic mail addresses, and Internationalized Resource Identifiers (IRIs). Storage, comparison, and presentation of such names require special care. Some characters may be encoded in multiple ways. The same names could be represented in multiple encodings (e.g., ASCII or UTF8). This section establishes conformance requirements for storage or comparison of each of these name forms. Informative guidance on presentation is provided for some of these name forms.
7.1. Internationalized Names in Distinguished Names Representation of internationalized names in distinguished names is covered in Sections 188.8.131.52, Issuer Name, and 184.108.40.206, Subject Name. Standard naming attributes, such as common name, employ the DirectoryString type, which supports internationalized names through a variety of language encodings. Conforming implementations MUST support UTF8String and PrintableString. RFC 3280 required only binary comparison of attribute values encoded in UTF8String, however, this specification requires a more comprehensive handling of comparison. Implementations may encounter certificates and CRLs with names encoded using TeletexString, BMPString, or UniversalString, but support for these is OPTIONAL. Conforming implementations MUST use the LDAP StringPrep profile (including insignificant space handling), as specified in [RFC4518], as the basis for comparison of distinguished name attributes encoded in either PrintableString or UTF8String. Conforming implementations MUST support name comparisons using caseIgnoreMatch. Support for attribute types that use other equality matching rules is optional. Before comparing names using the caseIgnoreMatch matching rule, conforming implementations MUST perform the six-step string preparation algorithm described in [RFC4518] for each attribute of type DirectoryString, with the following clarifications: * In step 2, Map, the mapping shall include case folding as specified in Appendix B.2 of [RFC3454]. * In step 6, Insignificant Character Removal, perform white space compression as specified in Section 2.6.1, Insignificant Space Handling, of [RFC4518]. When performing the string preparation algorithm, attributes MUST be treated as stored values. Comparisons of domainComponent attributes MUST be performed as specified in Section 7.3. Two naming attributes match if the attribute types are the same and the values of the attributes are an exact match after processing with the string preparation algorithm. Two relative distinguished names RDN1 and RDN2 match if they have the same number of naming attributes and for each naming attribute in RDN1 there is a matching naming attribute in RDN2. Two distinguished names DN1 and DN2 match if they have the same number of RDNs, for each RDN in DN1 there is a matching RDN in DN2, and the matching RDNs appear in the same order in both DNs. A distinguished name DN1 is within the subtree defined by the
distinguished name DN2 if DN1 contains at least as many RDNs as DN2, and DN1 and DN2 are a match when trailing RDNs in DN1 are ignored. 7.2. Internationalized Domain Names in GeneralName Internationalized Domain Names (IDNs) may be included in certificates and CRLs in the subjectAltName and issuerAltName extensions, name constraints extension, authority information access extension, subject information access extension, CRL distribution points extension, and issuing distribution point extension. Each of these extensions uses the GeneralName type; one choice in GeneralName is the dNSName field, which is defined as type IA5String. IA5String is limited to the set of ASCII characters. To accommodate internationalized domain names in the current structure, conforming implementations MUST convert internationalized domain names to the ASCII Compatible Encoding (ACE) format as specified in Section 4 of RFC 3490 before storage in the dNSName field. Specifically, conforming implementations MUST perform the conversion operation specified in Section 4 of RFC 3490, with the following clarifications: * in step 1, the domain name SHALL be considered a "stored string". That is, the AllowUnassigned flag SHALL NOT be set; * in step 3, set the flag called "UseSTD3ASCIIRules"; * in step 4, process each label with the "ToASCII" operation; and * in step 5, change all label separators to U+002E (full stop). When comparing DNS names for equality, conforming implementations MUST perform a case-insensitive exact match on the entire DNS name. When evaluating name constraints, conforming implementations MUST perform a case-insensitive exact match on a label-by-label basis. As noted in Section 220.127.116.11, any DNS name that may be constructed by adding labels to the left-hand side of the domain name given as the constraint is considered to fall within the indicated subtree. Implementations should convert IDNs to Unicode before display. Specifically, conforming implementations should perform the conversion operation specified in Section 4 of RFC 3490, with the following clarifications: * in step 1, the domain name SHALL be considered a "stored string". That is, the AllowUnassigned flag SHALL NOT be set; * in step 3, set the flag called "UseSTD3ASCIIRules";
* in step 4, process each label with the "ToUnicode" operation; and * skip step 5. Note: Implementations MUST allow for increased space requirements for IDNs. An IDN ACE label will begin with the four additional characters "xn--" and may require as many as five ASCII characters to specify a single international character. 7.3. Internationalized Domain Names in Distinguished Names 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. Conforming implementations shall perform a case-insensitive exact match when comparing domainComponent attributes in distinguished names, as described in Section 7.2. Implementations should convert ACE labels to Unicode before display. Specifically, conforming implementations should perform the "ToUnicode" conversion operation specified, as described in Section 7.2, on each ACE label before displaying the name. 7.4. Internationalized Resource Identifiers Internationalized Resource Identifiers (IRIs) are the internationalized complement to the Uniform Resource Identifier (URI). IRIs are sequences of characters from Unicode, while URIs are sequences of characters from the ASCII character set. [RFC3987] defines a mapping from IRIs to URIs. While IRIs are not encoded directly in any certificate fields or extensions, their mapped URIs may be included in certificates and CRLs. URIs may appear in the subjectAltName and issuerAltName extensions, name constraints extension, authority information access extension, subject information access extension, issuing distribution point extension, and CRL distribution points extension. Each of these extensions uses the GeneralName type; URIs are encoded in the uniformResourceIdentifier field in GeneralName, which is defined as type IA5String.
To accommodate IRIs in the current structure, conforming implementations MUST map IRIs to URIs as specified in Section 3.1 of [RFC3987], with the following clarifications: * in step 1, generate a UCS character sequence from the original IRI format normalizing according to the NFC as specified in Variant b (normalization according to NFC); * perform step 2 using the output from step 1. Implementations MUST NOT convert the ireg-name component before performing step 2. Before URIs may be compared, conforming implementations MUST perform a combination of the syntax-based and scheme-based normalization techniques described in [RFC3987]. Specifically, conforming implementations MUST prepare URIs for comparison as follows: * Step 1: Where IRIs allow the usage of IDNs, those names MUST be converted to ASCII Compatible Encoding as specified in Section 7.2 above. * Step 2: The scheme and host are normalized to lowercase, as described in Section 18.104.22.168 of [RFC3987]. * Step 3: Perform percent-encoding normalization, as specified in Section 22.214.171.124 of [RFC3987]. * Step 4: Perform path segment normalization, as specified in Section 126.96.36.199 of [RFC3987]. * Step 5: If recognized, the implementation MUST perform scheme- based normalization as specified in Section 5.3.3 of [RFC3987]. Conforming implementations MUST recognize and perform scheme-based normalization for the following schemes: ldap, http, https, and ftp. If the scheme is not recognized, step 5 is omitted. When comparing URIs for equivalence, conforming implementations shall perform a case-sensitive exact match. Implementations should convert URIs to Unicode before display. Specifically, conforming implementations should perform the conversion operation specified in Section 3.2 of [RFC3987].
7.5. Internationalized Electronic Mail Addresses Electronic Mail addresses may be included in certificates and CRLs in the subjectAltName and issuerAltName extensions, name constraints extension, authority information access extension, subject information access extension, issuing distribution point extension, or CRL distribution points extension. Each of these extensions uses the GeneralName construct; GeneralName includes the rfc822Name choice, which is defined as type IA5String. To accommodate email addresses with internationalized domain names using the current structure, conforming implementations MUST convert the addresses into an ASCII representation. Where the host-part (the Domain of the Mailbox) contains an internationalized name, the domain name MUST be converted from an IDN to the ASCII Compatible Encoding (ACE) format as specified in Section 7.2. Two email addresses are considered to match if: 1) the local-part of each name is an exact match, AND 2) the host-part of each name matches using a case-insensitive ASCII comparison. Implementations should convert the host-part of internationalized email addresses specified in these extensions to Unicode before display. Specifically, conforming implementations should perform the conversion of the host-part of the Mailbox as described in Section 7.2. 8. Security Considerations The majority of this specification is devoted to the format and content of certificates and CRLs. Since certificates and CRLs are digitally signed, no additional integrity service is necessary. Neither certificates nor CRLs need be kept secret, and unrestricted and anonymous access to certificates and CRLs has no security implications. However, security factors outside the scope of this specification will affect the assurance provided to certificate users. This section highlights critical issues to be considered by implementers, administrators, and users. The procedures performed by CAs and RAs to validate the binding of the subject's identity to their public key greatly affect the assurance that ought to be placed in the certificate. Relying
parties might wish to review the CA's certification practice statement. This is particularly important when issuing certificates to other CAs. The use of a single key pair for both signature and other purposes is strongly discouraged. Use of separate key pairs for signature and key management provides several benefits to the users. The ramifications associated with loss or disclosure of a signature key are different from loss or disclosure of a key management key. Using separate key pairs permits a balanced and flexible response. Similarly, different validity periods or key lengths for each key pair may be appropriate in some application environments. Unfortunately, some legacy applications (e.g., Secure Sockets Layer (SSL)) use a single key pair for signature and key management. 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. Loss of a CA's private signing key may also be problematic. The CA would not be able to produce CRLs or perform normal key rollover. CAs SHOULD maintain secure backup for signing keys. The security of the key backup procedures is a critical factor in avoiding key compromise. The availability and freshness of revocation information affects the degree of assurance that ought to be placed in a certificate. While certificates expire naturally, events may occur during its natural lifetime that negate the binding between the subject and public key. If revocation information is untimely or unavailable, the assurance associated with the binding is clearly reduced. Relying parties might not be able to process every critical extension that can appear in a CRL. CAs SHOULD take extra care when making revocation information available only through CRLs that contain critical extensions, particularly if support for those extensions is not mandated by this profile. For example, if revocation information is supplied using a combination of delta CRLs and full CRLs, and the
delta CRLs are issued more frequently than the full CRLs, then relying parties that cannot handle the critical extensions related to delta CRL processing will not be able to obtain the most recent revocation information. Alternatively, if a full CRL is issued whenever a delta CRL is issued, then timely revocation information will be available to all relying parties. Similarly, implementations of the certification path validation mechanism described in Section 6 that omit revocation checking provide less assurance than those that support it. The certification path validation algorithm depends on the certain knowledge of the public keys (and other information) about one or more trusted CAs. The decision to trust a CA is an important decision as it ultimately determines the trust afforded a certificate. The authenticated distribution of trusted CA public keys (usually in the form of a "self-signed" certificate) is a security critical out-of-band process that is beyond the scope of this specification. In addition, where a key compromise or CA failure occurs for a trusted CA, the user will need to modify the information provided to the path validation routine. Selection of too many trusted CAs makes the trusted CA information difficult to maintain. On the other hand, selection of only one trusted CA could limit users to a closed community of users. The quality of implementations that process certificates also affects the degree of assurance provided. The path validation algorithm described in Section 6 relies upon the integrity of the trusted CA information, and especially the integrity of the public keys associated with the trusted CAs. By substituting public keys for which an attacker has the private key, an attacker could trick the user into accepting false certificates. The binding between a key and certificate subject cannot be stronger than the cryptographic module implementation and algorithms used to generate the signature. Short key lengths or weak hash algorithms will limit the utility of a certificate. CAs are encouraged to note advances in cryptology so they can employ strong cryptographic techniques. In addition, CAs SHOULD decline to issue certificates to CAs or end entities that generate weak signatures. Inconsistent application of name comparison rules can result in acceptance of invalid X.509 certification paths or rejection of valid ones. The X.500 series of specifications defines rules for comparing distinguished names that require comparison of strings without regard
to case, character set, multi-character white space substring, or leading and trailing white space. This specification relaxes these requirements, requiring support for binary comparison at a minimum. CAs MUST encode the distinguished name in the subject field of a CA certificate identically to the distinguished name in the issuer field in certificates issued by that CA. If CAs use different encodings, implementations might fail to recognize name chains for paths that include this certificate. As a consequence, valid paths could be rejected. In addition, name constraints for distinguished names MUST be stated identically to the encoding used in the subject field or subjectAltName extension. If not, then name constraints stated as excludedSubtrees will not match and invalid paths will be accepted and name constraints expressed as permittedSubtrees will not match and valid paths will be rejected. To avoid acceptance of invalid paths, CAs SHOULD state name constraints for distinguished names as permittedSubtrees wherever possible. In general, using the nameConstraints extension to constrain one name form (e.g., DNS names) offers no protection against use of other name forms (e.g., electronic mail addresses). While X.509 mandates that names be unambiguous, there is a risk that two unrelated authorities will issue certificates and/or CRLs under the same issuer name. As a means of reducing problems and security issues related to issuer name collisions, CA and CRL issuer names SHOULD be formed in a way that reduces the likelihood of name collisions. Implementers should take into account the possible existence of multiple unrelated CAs and CRL issuers with the same name. At a minimum, implementations validating CRLs MUST ensure that the certification path of a certificate and the CRL issuer certification path used to validate the certificate terminate at the same trust anchor. While the local-part of an electronic mail address is case sensitive [RFC2821], emailAddress attribute values are not case sensitive [RFC2985]. As a result, there is a risk that two different email addresses will be treated as the same address when the matching rule for the emailAddress attribute is used, if the email server exploits the case sensitivity of mailbox local-parts. Implementers should not include an email address in the emailAddress attribute if the email server that hosts the email address treats the local-part of email addresses as case sensitive. Implementers should be aware of risks involved if the CRL distribution points or authority information access extensions of
corrupted certificates or CRLs contain links to malicious code. Implementers should always take the steps of validating the retrieved data to ensure that the data is properly formed. When certificates include a cRLDistributionPoints extension with an https URI or similar scheme, circular dependencies can be introduced. The relying party is forced to perform an additional path validation in order to obtain the CRL required to complete the initial path validation! Circular conditions can also be created with an https URI (or similar scheme) in the authorityInfoAccess or subjectInfoAccess extensions. At worst, this situation can create unresolvable dependencies. CAs SHOULD NOT include URIs that specify https, ldaps, or similar schemes in extensions. CAs that include an https URI in one of these extensions MUST ensure that the server's certificate can be validated without using the information that is pointed to by the URI. Relying parties that choose to validate the server's certificate when obtaining information pointed to by an https URI in the cRLDistributionPoints, authorityInfoAccess, or subjectInfoAccess extensions MUST be prepared for the possibility that this will result in unbounded recursion. Self-issued certificates provide CAs with one automated mechanism to indicate changes in the CA's operations. In particular, self-issued certificates may be used to implement a graceful change-over from one non-compromised CA key pair to the next. Detailed procedures for "CA key update" are specified in [RFC4210], where the CA protects its new public key using its previous private key and vice versa using two self-issued certificates. Conforming client implementations will process the self-issued certificate and determine whether certificates issued under the new key may be trusted. Self-issued certificates MAY be used to support other changes in CA operations, such as additions to the CA's policy set, using similar procedures. Some legacy implementations support names encoded in the ISO 8859-1 character set (Latin1String) [ISO8859] but tag them as TeletexString. TeletexString encodes a larger character set than ISO 8859-1, but it encodes some characters differently. The name comparison rules specified in Section 7.1 assume that TeletexStrings are encoded as described in the ASN.1 standard. When comparing names encoded using the Latin1String character set, false positives and negatives are possible. When strings are mapped from internal representations to visual representations, sometimes two different strings will have the same or similar visual representations. This can happen for many different reasons, including use of similar glyphs and use of
composed characters (such as e + ' equaling U+00E9, the Korean composed characters, and vowels above consonant clusters in certain languages). As a result of this situation, people doing visual comparisons between two different names may think they are the same when in fact they are not. Also, people may mistake one string for another. Issuers of certificates and relying parties both need to be aware of this situation. 9. IANA Considerations Extensions in certificates and CRLs are identified using object identifiers. The objects are defined in an arc delegated by IANA to the PKIX Working Group. No further action by IANA is necessary for this document or any anticipated updates. 10. Acknowledgments Warwick Ford participated with the authors in some of the design team meetings that directed development of this document. The design team's efforts were guided by contributions from Matt Crawford, Tom Gindin, Steve Hanna, Stephen Henson, Paul Hoffman, Takashi Ito, Denis Pinkas, and Wen-Cheng Wang. 11. References 11.1. Normative References [RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities", STD 13, RFC 1034, November 1987. [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. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key Infrastructure: Operational Protocols: FTP and HTTP", RFC 2585, May 1999.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [RFC2797] Myers, M., Liu, X., Schaad, J., and J. Weinstein, "Certificate Management Messages over CMS", RFC 2797, April 2000. [RFC2821] Klensin, J., Ed., "Simple Mail Transfer Protocol", RFC 2821, April 2001. [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ("stringprep")", RFC 3454, December 2002. [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello, "Internationalizing Domain Names in Applications (IDNA)", RFC 3490, March 2003. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. [RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource Identifiers (IRIs)", RFC 3987, January 2005. [RFC4516] Smith, M., Ed., and T. Howes, "Lightweight Directory Access Protocol (LDAP): Uniform Resource Locator", RFC 4516, June 2006. [RFC4518] Zeilenga, K., "Lightweight Directory Access Protocol (LDAP): Internationalized String Preparation", RFC 4518, June 2006. [RFC4523] Zeilenga, K., "Lightweight Directory Access Protocol (LDAP) Schema Definitions for X.509 Certificates", RFC 4523, June 2006. [RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan", BCP 122, RFC 4632, August 2006. [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.
[X.690] ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002, Information technology - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER). 11.2. Informative References [ISO8859] ISO/IEC 8859-1:1998. Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1. [ISO10646] ISO/IEC 10646:2003. Information technology -- Universal Multiple-Octet Coded Character Set (UCS). [NFC] Davis, M. and M. Duerst, "Unicode Standard Annex #15: Unicode Normalization Forms", October 2006, <http://www.unicode.org/reports/tr15/>. [RFC1422] Kent, S., "Privacy Enhancement for Internet Electronic Mail: Part II: Certificate-Based Key Management", RFC 1422, February 1993. [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and Languages", BCP 18, RFC 2277, January 1998. [RFC2459] Housley, R., Ford, W., Polk, W., and D. Solo, "Internet X.509 Public Key Infrastructure Certificate and CRL Profile", RFC 2459, January 1999. [RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams, "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP", RFC 2560, June 1999. [RFC2985] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object Classes and Attribute Types Version 2.0", RFC 2985, November 2000. [RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato, "Internet X.509 Public Key Infrastructure Time-Stamp Protocol (TSP)", RFC 3161, August 2001. [RFC3279] 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.
[RFC3280] 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. [RFC4055] Schaad, J., Kaliski, B., and R. Housley, "Additional Algorithms and Identifiers for RSA Cryptography for use in the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 4055, June 2005. [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, July 2005. [RFC4210] Adams, C., Farrell, S., Kause, T., and T. Mononen, "Internet X.509 Public Key Infrastructure Certificate Management Protocol (CMP)", RFC 4210, September 2005. [RFC4325] Santesson, S. and R. Housley, "Internet X.509 Public Key Infrastructure Authority Information Access Certificate Revocation List (CRL) Extension", RFC 4325, December 2005. [RFC4491] Leontiev, S., Ed., and D. Shefanovski, Ed., "Using the GOST R 34.10-94, GOST R 34.10-2001, and GOST R 34.11-94 Algorithms with the Internet X.509 Public Key Infrastructure Certificate and CRL Profile", RFC 4491, May 2006. [RFC4510] Zeilenga, K., Ed., "Lightweight Directory Access Protocol (LDAP): Technical Specification Road Map", RFC 4510, June 2006. [RFC4512] Zeilenga, K., Ed., "Lightweight Directory Access Protocol (LDAP): Directory Information Models", RFC 4512, June 2006. [RFC4514] Zeilenga, K., Ed., "Lightweight Directory Access Protocol (LDAP): String Representation of Distinguished Names", RFC 4514, June 2006. [RFC4519] Sciberras, A., Ed., "Lightweight Directory Access Protocol (LDAP): Schema for User Applications", RFC 4519, June 2006.
[RFC4630] Housley, R. and S. Santesson, "Update to DirectoryString Processing in the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 4630, August 2006. [X.500] ITU-T Recommendation X.500 (2005) | ISO/IEC 9594-1:2005, Information technology - Open Systems Interconnection - The Directory: Overview of concepts, models and services. [X.501] ITU-T Recommendation X.501 (2005) | ISO/IEC 9594-2:2005, Information technology - Open Systems Interconnection - The Directory: Models. [X.509] ITU-T Recommendation X.509 (2005) | ISO/IEC 9594-8:2005, Information technology - Open Systems Interconnection - The Directory: Public-key and attribute certificate frameworks. [X.520] ITU-T Recommendation X.520 (2005) | ISO/IEC 9594-6:2005, Information technology - Open Systems Interconnection - The Directory: Selected attribute types. [X.660] ITU-T Recommendation X.660 (2004) | ISO/IEC 9834-1:2005, Information technology - Open Systems Interconnection - Procedures for the operation of OSI Registration Authorities: General procedures, and top arcs of the ASN.1 Object Identifier tree. [X.683] ITU-T Recommendation X.683 (2002) | ISO/IEC 8824-4:2002, Information technology - Abstract Syntax Notation One (ASN.1): Parameterization of ASN.1 specifications. [X9.55] ANSI X9.55-1997, Public Key Cryptography for the Financial Services Industry: Extensions to Public Key Certificates and Certificate Revocation Lists, January 1997.