The keyword [class number] alone is the same as explicit tagging, except when the "module" in which the
ASN.1 type is defined has implicit tagging by default. ("Modules" are among the advanced features not
described in this note.)
For purposes of encoding, an implicitly tagged type is considered the same as the underlying type, except
that the tag is different. An explicitly tagged type is considered like a structured type with one component,
the underlying type. Implicit tags result in shorter encodings, but explicit tags may be necessary to avoid
ambiguity if the tag of the underlying type is indeterminate (e.g., the underlying type is CHOICE or ANY).
2.4 Other types
Other types in ASN.1 include the CHOICE and ANY types. The CHOICE type denotes a union of one or more
alternatives; the ANY type denotes an arbitrary value of an arbitrary type, where the arbitrary type is
possibly defined in the registration of an object identifier or integer value.
3. Basic Encoding Rules
The Basic Encoding Rules for ASN.1, abbreviated BER, give one or more ways to represent any ASN.1
value as an octet string. (There are certainly other ways to represent ASN.1 values, but BER is the
standard for interchanging such values in OSI.)
There are three methods to encode an ASN.1 value under BER, the choice of which depends on the type of
value and whether the length of the value is known. The three methods are primitive, definite-length
encoding; constructed, definite- length encoding; and constructed, indefinite-length encoding. Simple
non-string types employ the primitive, definite-length method; structured types employ either of the
constructed methods; and simple string types employ any of the methods, depending on whether the length
of the value is known. Types derived by implicit tagging employ the method of the underlying type and
types derived by explicit tagging employ the constructed methods.
In each method, the BER encoding has three or four parts:
Identifier octets. These identify the class and tag number of the ASN.1 value, and indicate
whether the method is primitive or constructed.
Length octets. For the definite-length methods, these give the number of contents octets. For
the constructed, indefinite-length method, these indicate that the length is indefinite.
Contents octets. For the primitive, definite-length method, these give a concrete
representation of the value. For the constructed methods, these give the concatenation of the
BER encodings of the components of the value.
End-of-contents octets. For the constructed, indefinite- length method, these denote the end of
the contents. For the other methods, these are absent.
The three methods of encoding are described in the following sections.
3.1 Primitive, definite-length method
This method applies to simple types and types derived from simple types by implicit tagging. It requires
that the length of the value be known in advance. The parts of the BER encoding are as follows:
Identifier octets. There are two forms: low tag number (for tag numbers between 0 and 30) and high tag
A Layman's Guide to a Subset of ASN.1, BER, and DER http://luca.ntop.org/Teaching/Appunti/asn1.html
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