Abstract Syntax Notation One

Abstract Syntax Notation One (ASN.1) is a mechanism for encoding data in a binary form. It 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 the following:

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 hold 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. That is handled by an encoding rule, and there are a number of different kinds of ASN.1 encoding rules. LDAP uses the Basic Encoding Rules encoding, but other types include Distinguished Encoding Rules (DER), Canonical Encoding Rules (CER), and Packed Encoding Rules (PER).