Overview#

Abstract Syntax Notation One (ASN.1) was defined in X.208 is a mechanism for encoding data in a binary form.

Abstract Syntax Notation One uses a TLV structure, in which each element has a type, length, and value. The type component is a data type that indicates what kind of information is stored in the element and indicates how the value should be encoded. The length component specifies the number of bytes in the value, and the value is the actual data held by the element.

Examples of ASN.1 elements include:

• null -- null elements do not hold any value. They are generally used as placeholders when an element is required but no value is needed.
• Octet String -- Octet string elements hold a set of zero or more octets (bytes) of data. It can be used for holding string or binary data.
• Boolean -- Boolean elements hold values that represent either "true" or "false".
• Integer -- Integer elements hold values that represent integer values.
• Enumerated -- Enumerated elements hold values that represent integer values where each value has a specific meaning.
• Sequence -- Sequence elements are containers that zero or more other ASN.1 elements in a manner where the order of the elements is significant.
• Set -- Set elements are containers that hold zero or more other ASN.1 elements in a manner where the order of the elements is not significant.

Note that ASN.1 is a general framework for binary encoding, but doesn't actually define how the data should be encoded. There are a number of different kinds of ASN.1 encoding rules:

• Basic Encoding Rules (BER)
• Distinguished Encoding Rules (DER)
• Canonical Encoding Rules (CER)
• Packed Encoding Rules (PER)

LDAP uses the Basic Encoding Rules (BER) encoding.

The book ASN.1 Complete (PDF) by Professor John Larmouth, which is an excellent reference on the Abstract Syntax Notation One.

Abstract Syntax Notation One is now a joint standard of the  International Telecommunication Union (ITU-T and ISO/IEC, originally defined in 1984 as part of CCITT X.409. In 1988, ASN.1 moved to its own standard, X.208, due to wide applicability. The substantially revised 1995 version X.680 series of recommendations is the 5.0 Edition, published in 2015.

More Information#

There might be more information for this subject on one of the following:

• ASN.1
• Generic String Encoding Rules
• Management Information Base
• Octet
• X.680

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• [#1] - ASN.1 Complete (PDF) - based on information obtained 2014-12-20
• [#2] - ASN.1 - based on information obtained 2021-12-06