Operation
IDEA operates on 64-bit blocks using a 128-bit key, and consists of a series of eight identical transformations (a round, see the illustration) and an output transformation (the half-round). The processes for encryption and decryption are similar. IDEA derives much of its security by interleaving operations from different groups — modular addition and multiplication, and bitwise eXclusive OR (XOR) — which are algebraically "incompatible" in some sense. In more detail, these operators, which all deal with 16-bit quantities, are:
- Bitwise eXclusive OR (denoted with a blue circled plus ⊕).
- Addition modulo 216 (denoted with a green boxed plus ⊞).
- Multiplication modulo 216+1, where the all-zero word (0x0000) is interpreted as 216 (denoted by a red circled dot ⊙).
After the eight rounds comes a final “half round”, the output transformation illustrated below:
Read more about this topic: International Data Encryption Algorithm
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