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How does IDEA Encryption work?
IDEA is a block cipher and it operates on 64bit plaintext and 128 bit key. IDEA is reversible like DES that is, the equivalent algorithm can be used for encryption and decryption. IDEA needs both diffusion and confusion for encryption.
The 64-bit plaintext is divided into four portions of 16bit plaintext (P1 to P4). These are input to the first round. There are eight such rounds. The key includes 128 bits. In each round, six sub-keys are produced from the original key, each of these sub-key includes 16 bits.
For the first round it can have key K1 to K6, for second round it can have keys K7 to K12 and finally the last round. The final step includes an output transformation, which needs four subkeys (K49 to K52).
The final output is the output created by the output transformation step. The blocks C1to C4 are linked to form the final output.
Rounds − There are eight round in IDEA. Each round contains a sequence of operations on the four data blocks, utilizing six keys. The Add * and Multiply * in the following step of each round are not easy addition and multiplication but they are addition module 216 i.e., 65536 and Multiplication Module 216 + 1 i. e. , 65537.
Sub-key generation for a round −
In the first round bit position 1-96 of the key are utilized. Bits 97-128 remain unutilized. They are given to round 2.
In round second bits 97-128 are first used, thus circular left shift of 25 bit appears and new bits from 26-89 are used. Bits position 90-128 and 1-25 remains unused.
In round third unused bit 90-128 and 1-25 are first used again, circular leftshift of 25 bit appears and bit position 51-82 are used. Bit position 83-128 and 1-50 remains unused.
In the fourth round bit 83-128 and 1-50 are used.
In the fifth round circular left-shift of 25 bit appears and bit position 76-128 and 1-43 are used. Bit position 44-75 remains unused.
In the sixth round unused bit of fifth round is used and left circular shift of 25 bit occur thus making bit position 37-100 unused.
In the seventh round unused bit of sixth round i.e., 37-100 are first used and circular left shift of 25 bit appear bit position 126-128 and 1-29 are used. Bit 30-125 remain unused.
Unused bit position 30-125 from round seven is used and key is disabled.
Output Transformation − It is a one-time operation. It takes place at the end of the 8th round. Therefore 64-bit value is divided into four sub-blocks (say R1 to R4) and four subkeys are used here.
IDEA Decryption − The decryption procedure is the equivalent as the encryption process. There are some alteration in the generation and design of subkeys. The decryption subkeys are opposite of the encryption subkeys.
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