What Is ASN.1 (Abstract Syntax Notation One)

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∟What Is ASN.1 (Abstract Syntax Notation One)

This section introduces ASN.1 (Abstract Syntax Notation One), which is a language that allows you to define your own constructed data types.

What Is the ASN.1 (Abstract Syntax Notation One)? ASN.1 is a language that provides data types and systaxes for you to define your own constructed data types. It also provides encoding/decoding rules to serialize/unserialize values of constructed data types.

ASN.1 was specified in "X.208 - Specification of Abstract Syntax Notation One (ASN.1)" published by ISO and ITU-T jointly in 1988, and revided in "X.680 - Abstract Syntax Notation One (ASN.1): Specification of basic notation" in 1995 at itu.int/rec/T-REC-X.680.

Commonly used primitive data types provided in ASN.1 are listed below. Each of them is assigned with a tag ID to be used in the encoding/decoding process.

BOOLEAN - Contains TRUE or FALSE.
INTEGER- Contains an integer value.
BIT STRING - Contains a bit sequences of arbitrary length.
OCTET STRING - Contains a byte sequences of arbitrary length.
NULL - Contains no value.
OBJECT IDENTIFIER - Contains an OID (Object IDentifier) value.
REAL - Contains a real number.
ENUMERATED - Contains a named integer value.
PrintableString - Contains a sequence of a printable characters except "@", "&", and "*".
IA5String - Contains a sequence of ASCII characters.
UTF8String - Contains a sequence of UTF-8 encoded Unicode characters.
TIME - Contains a time value defined by the ISO 8601 standard.
DURATION - Contains a time interval.

With those primitive types, you can define constructed types using the following notation syntaxes:

SEQUENCE Constructed Type - Defines a new constructed type, which contains an ordered list of values of any types.

type-name ::= SEQUENCE {
  member-id modifier type constraints,
  member-id modifier type constraints,
  ...,
  member-id modifier type constraints
}

where:
  type-name - Specifies the name of the new type
  member-id - Specifies a unique identifier of the given member value
  modifier - Optional. Specifies a tagging (type tag change) option
  type - Specifies the type of the give member value
  constraints - Optional. Adds constraints for the given member value

For example, the following notation defines 2 constructed types, Square and Point, as SEQUENCE types:

Square ::= SEQUENCE {
  size INTEGER,
  location Point
}

Point ::= SEQUENCE {
  x INTEGER,
  y INTEGER
}

SET Constructed Type Defines a new constructed type, which contains an unordered list of values of any types.

type-name ::= SET {
  member-id modifier type constraints,
  member-id modifier type constraints,
  ...,
  member-id modifier type constraints
}

where:
  type-name - Specifies the name of the new type
  member-id - Specifies a unique identifier of the given member value
  modifier - Optional. Specifies a tagging (type tag change) option
  type - Specifies the type of the give member value
  constraints - Optional. Adds constraints for the given member value

For example, the following notation redefines Square as a SET type, because the order of its members is not important:

Square ::= SET {
  size INTEGER,
  location Point
}

CHOICE Constructed Type Defines a new "virtual" type, which contains a set of values of any types. When a CHOICE value is specified, its type and value will be mapped to the selected member type and value.

type-name ::= CHOICE {
  member-id modifier type constraints,
  member-id modifier type constraints,
  ...,
  member-id modifier type constraints
}

where:
  type-name - Specifies the name of the new type
  member-id - Specifies a unique identifier of the given member value
  modifier - Optional. Specifies a tagging (type tag change) option
  type - Specifies the type of the give member value
  constraints - Optional. Adds constraints for the given member value