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Basic Encoding Rules - Transfer Syntax for ASN.1
These are needed to disambiguate a stream of bytes
arriving over a network.
An ASN.1 object is encoded as a TAG, a LENGTH and a VALUE, (TLV
encoding) thus:
The TAG is (usually) encoded in a single octet, containing its
value (see earlier) in binary in the
low-order 5 bits and maybe some other information[3].
The LENGTH can be encoded in two possible ways,
definite (which usually only requires a single
octet) and indefinite (which requires, at least,
several more octets). Both are equally complicated!
The VALUE field is encoded using a specified method which is
appropriate to its type - for example, an
INTEGER is sent as its binary 2's complement
equivalent; an OCTET STRING is simply the
appropriate octets.
[3]The CLASS (see slide 4) of the object (most significant 2 bits),
and whether it is a simple or constructed (structured) data type (1
bit: 0 for simple, 1 for constructed).
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