Abstract Syntax Notation number One (ASN.1) is a notation used extensively by standards committees and other bodies for defining protocols for use in data communications. It resembles a programming language, but consists mostly of features for defining data structures. There are various data types, and syntax for building up complex data structures out of these elementary types. You might think of a protocol specification written in ASN.1 as resembling the Data Division of a COBOL program, although the actual syntax of the language is more like C.

You may well never see an example of ASN.1, since it is for use in defining protocols, and even protocols themselves tend to be things that the user does not see directly. Nevertheless, ASN.1 is very widely used, and a great many electronic devices in every day use - from air traffic control systems to washing machines - use data transmission protocols that were designed using ASN.1.

Many of the standards used in SSL that we have been discussing were designed in ASN.1, and consequently you will sometimes come across ASN.1 terminology when reading about SSL. This is especially true of the documentation for OpenSSL. Although as an end-user, or as a COBOL developer, you are unlikely to come directly in contact with these concepts, you will find the OpenSSL documentation easier to follow if we explain some of them.