Abstract Syntax Notation One (ASN.1) is a standard interface description language for defining data structures that can be serialized and deserialized in a cross-platform way. It is broadly used in telecommunications and computer networking, and especially in cryptography. Protocol developers define data structures in ASN.1 modules, which are generally a section of a broader standards document written in the ASN.1 language. The advantage is that the ASN.1 description of the data encoding is independent of a particular computer or programming language. Because ASN.1 is both human-readable and machine-readable, an ASN.1 compiler can compile modules into libraries of code, codecs, that decode or encode the data structures. Some ASN.1 compilers can produce code to encode or decode several encodings, e.g. packed, BER or XML. ASN.1 is a joint standard of the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) in ITU-T Study Group 17 and ISO/IEC, originally defined in 1984 as part of CCITT X.409:1984. In 1988, ASN.1 moved to its own standard, X.208, due to wide applicability. The substantially revised 1995 version is covered by the X.680 series. The latest revision of the X.680 series of recommendations is the 6.0 Edition, published in 2021. Language support ASN.1 is a data type declaration notation. It does not define how to manipulate a variable of such a type. Manipulation of variables is defined in other languages such as SDL (Specification and Description Language) for executable modeling or TTCN-3 (Testing and Test Control Notation) for conformance testing. Both these languages natively support ASN.1 declarations. It is possible to import an ASN.1 module and declare a variable of any of the ASN.1 types declared in the module. Applications ASN.1 is used to define a large number of protocols. Its most extensive uses continue to be telecommunications, cryptography, and biometrics. Encodings ASN.1 is closely associated with a set of encoding rules that specify how to represent a data structure as a series of bytes. The standard ASN.1 encoding rules include: Encoding Control Notation ASN.1 recommendations provide a number of predefined encoding rules. If none of the existing encoding rules are suitable, the Encoding Control Notation (ECN) provides a way for a user to define his or her own customized encoding rules. Relation to Privacy-Enhanced Mail (PEM) Encoding Privacy-Enhanced Mail (PEM) encoding is entirely unrelated to ASN.1 and its codecs, but encoded ASN.1 data, which is often binary, is often PEM-encoded so that it can be transmitted