Diffusion (cryptology)

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In cryptology, diffusion is one of the two central principles for obfuscating the structures of plain text in the course of encryption or hashing . The other of these principles is confusion . They go back to the American mathematician Claude Shannon .

Diffusion means that every bit that is input into a cryptographic function spreads over the entire processed data block and affects all output bits. If you change the input even slightly, each output bit should change with the probability .

A good method interlinks confusion and diffusion in several successive rounds so that when encrypting or hashing each information bit has spread over the entire data block after just a few rounds and is then processed by operations to create confusion (e.g. S-Box ) whereupon their results spread quickly, etc. If you intervene somewhere and only change one bit, a few rounds later the whole data block looks completely different. This property is also known as the avalanche effect .

Example of hash functions

The SHA-1 hash value of the sentence "Fischers Fritz fishes fresh fish." is

ac8ac8261cbfd50efcecf3b313faddf325ee1c75

Changing just one letter results in a completely different hash value: "Fischers Fr a tz fishes fresh fish." results

5caecace90a74375f5f3d2e4156750210c4a18b4

literature

  • Claude E. Shannon, " Communication Theory of Secrecy Systems ", Bell System Technical Journal, vol. 28-4, pages 656--715, 1949. ( PDF )