ASn1

ASN.1 stands for Abstract Syntax Notation One, a formal language for describing data structures for cross-platform communication. It separates the description of data (the abstract syntax) from its encoding, enabling interoperable exchange across different hardware and software environments. ASN.1 defines a typed value system, constraints, and a set of encoding rules to convert values to and from byte streams.

The ASN.1 family was developed by ITU-T and ISO/IEC, with core specifications published as ITU-T X.680 and X.690 and ISO/IEC 8824/8825. The standard supports modular definitions, parameterized types, and a rich set of base and constructed types.

A typical ASN.1 module defines a set of named types, such as BOOLEAN, INTEGER, OCTET STRING, NULL, OBJECT IDENTIFIER, SEQUENCE, SEQUENCE OF, SET, SET OF, and CHOICE. Types may be constrained (value ranges, sizes) and can be tagged for explicit or implicit tagging. Modules can import definitions from other modules.

To exchange information, there are encoding rules that translate the abstract syntax into a byte representation. Basic Encoding Rules (BER) were the original method, with Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER) providing more strict encodings for unambiguity, used especially in X.509 certificates. Packed Encoding Rules (PER) offer a more compact encoding for bandwidth-constrained environments.

ASN.1 is widely used in protocols and systems such as digital certificates, X.509, email security (S/MIME), SNMP, LDAP, and various telecommunication standards.

Because it is a description language rather than a communication protocol, ASN.1 is often paired with portable encoding rules in software libraries and tools used to generate, parse, and validate data.