Counter Mode is a way to define a pseudorandom keystream generator using
a block cipher. The keystream can be used for additive encryption, key
derivation, or any other application requiring pseudorandom data.
In ICM, the keystream is logically broken into segments. Each segment is
identified with a segment index, and the segments have equal lengths. This
segmentation makes ICM especially appropriate for securing packet-based
protocols.
This implementation adheres to the definition of the ICM keystream
generation function that allows for any symetric key block cipher algorithm
(initialisation parameter gnu.crypto.prng.icm.cipher.name
taken
to be an instance of
java.lang.String) to be used. If such a
parameter is not defined/included in the initialisation Map
,
then the "Rijndael" algorithm is used. Furthermore, if the initialisation
parameter gnu.crypto.cipher.block.size
(taken to be a instance
of
java.lang.Integer) is missing or undefined in the initialisation
Map
, then the cipher's default block size is used.
The practical limits and constraints of such generator are:
- The number of blocks in any segment MUST NOT exceed
256 ** BLOCK_INDEX_LENGTH
. The number of segments MUST NOT
exceed 256 ** SEGMENT_INDEX_LENGTH
. These restrictions ensure
the uniqueness of each block cipher input.
- Each segment contains
SEGMENT_LENGTH
octets; this value
MUST NOT exceed the value (256 ** BLOCK_INDEX_LENGTH)
BLOCK_LENGTH
.
- The sum of
SEGMENT_INDEX_LENGTH
and
BLOCK_INDEX_LENGTH
MUST NOT exceed BLOCK_LENGTH
/ 2
. This requirement protects the ICM keystream generator from
potentially failing to be pseudorandom.
NOTE: Rijndael is used as the default symmetric key block cipher
algorithm because, with its default block and key sizes, it is the AES. Yet
being Rijndael, the algorithm offers more versatile block and key sizes which
may prove to be useful for generating longer key streams.
References:
-
Integer Counter Mode, David A. McGrew.