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5.3.2 What are the ITU-T (CCITT) Standards?

The International Telecommunications Union, ITU-T (formerly known as CCITT), is a multinational union that provides standards for telecommunication equipment and systems. ITU-T is responsible for standardization of elements such as the X.500 directory [CCI88b], X.509 certificates and Distinguished Names. Distinguished names are the standard form of naming. A distinguished name is comprised of one or more relative distinguished names, and each relative distinguished name is comprised of one or more attribute-value assertions. Each attribute-value assertion consists of an attribute identifier and its corresponding value information, for example, ``CountryName = US.''

Distinguished names were intended to identify entities in the X.500 directory tree. A relative distinguished name is the path from one node to a subordinate node. The entire distinguished name traverses a path from the root of the tree to an end node that represents a particular entity. A goal of the directory was to provide an infrastructure to uniquely name every communications entity everywhere (hence the ``distinguished'' in ``distinguished name''). As a result of the directory's goals, names in X.509 certificates are perhaps more complex than one might like (for example, compared to an e-mail address). Nevertheless, for business applications, distinguished names are worth the complexity, as they are closely coupled with legal name registration procedures; this is something simple names, such as e-mail addresses, do not offer.


ITU-T Recommendation X.400 [CCI88a], also known as the Message Handling System (MHS), is one of the two standard e-mail architectures used for providing e-mail services and interconnecting proprietary e-mail systems. The other is the Simple Mail Transfer Protocol (SMTP) used by the Internet. MHS allows e-mail and other store-and-forward message transferring such as Electronic business Data Interchange (EDI) and voice messaging. The MHS and Internet mail protocols are different but based on similar underlying architectural models. The noteworthy fact of MHS is that it has supported secure messaging since 1988 (though it has not been widely deployed in practice). The MHS message structure is similar to the MIME (see Question 5.1.1) message structure; it has both a header and a body. The body can be broken up into multiple parts, with each part being encoded differently. For example, one part of the body may be text, the next part a picture, and a third part encrypted information.


ITU-T Recommendation X.435 [CCI91] and its equivalent F.435 are X.400-based and designed to support EDI messaging. EDI needs more stringent security than typical e-mail because of its business nature: not only does an EDI message need protection against fraudulent or accidental modification in transit, but it also needs to be immune to repudiation after it has been sent and received.

In support of these security requirements, X.435 defines, in addition to normal EDI messages, a set of EDI ``notifications.'' Positive notification implies the recipient has received the document and accepts the responsibility for it, while negative notification means the recipient refused to accept the document due to a specified reason. For- warding notification means the document had been forwarded to another recipient. Together, these notifications form the basis for a system that can provide security controls comparable to those in the paper-based system that EDI replaces.


ITU-T Recommendation X.509 [CCI88c] specifies the authentication service for X.500 directories, as well as the widely adopted X.509 certificate syntax. The initial version of X.509 was published in 1988, version 2 was published in 1993, and version 3 was proposed in 1994 and published in 1995. Version 3 addresses some of the security concerns and limited flexibility that were issues in versions 1 and 2. Directory authentication in X.509 can be carried out using either secret-key techniques or public-key techniques. The latter is based on public-key certificates. The standard does not specify a particular cryptographic algorithm, although an informative annex of the standard describes the RSA algorithm (see Section 3.1).

An X.509 certificate consists of the following fields"> The X.509 standard is supported by a number of protocols, including PKCS (see Question 5.3.3) and SSL (see Question 5.1.2).

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