X.690

X.690 is an ITU-T standard specifying several ASN.1 encoding formats:

The Basic Encoding Rules were the original rules laid out by the ASN.1 standard for encoding abstract information into a concrete data stream. The rules, collectively referred to as a transfer syntax in ASN.1 parlance, specify the exact octet sequences which are used to encode a given data item. The syntax defines such elements as: the representations for basic data types, the structure of length information, and the means for defining complex or compound types based on more primitive types. The BER syntax, along with two subsets of BER (the Canonical Encoding Rules and the Distinguished Encoding Rules), are defined by the ITU-T's X.690 standards document, which is part of the ASN.1 document series.

The format for Basic Encoding Rules specifies a self-describing and self-delimiting format for encoding ASN.1 data structures. Each data element is encoded as a type identifier, a length description, the actual data elements, and, where necessary, an end-of-content marker. These types of encodings are commonly called type-length-value or TLV encodings. This format allows a receiver to decode the ASN.1 information from an incomplete stream, without requiring any pre-knowledge of the size, content, or semantic meaning of the data.

The encoding of data does generally consist of four components which appear in the following order:

The End-of-contents octets are optional and only used if the indefinite length form is used. The Contents octet may also be omitted if there is no content to encode like in the NULL type.

Data (especially members of sequences and sets and choices) can be tagged with a unique tag number (shown in ASN.1 within square brackets []) to distinguish that data from other members. Such tags can be implicit (where they are encoded as the TLV tag of the value instead of using the base type as the TLV tag) or explicit (where the tag is used in a constructed TLV that wraps the base type TLV). The default tagging style is explicit, unless implicit is set at ASN.1 module-level. Such tags have a default class of context-specific, but that can be overridden by using a class name in front of the tag.

The encoding of a choice value is the same as the encoding of a value of the chosen type. The encoding may be primitive or constructed, depending on the chosen type. The tag used in the identifier octets is the tag of the chosen type, as specified in the ASN.1 definition of the chosen type.

...
Wikipedia