Cryprography

This page just gathers miscellaneous remarks I collected on Cryptography in general.

No effort is made to tidy things up. This will be done as soon as the page gets some volume...

Stream Cipher

Security Properties

• Stream cipher building block must be invertible, otherwise it is easy to create collisions.

Hash Functions

Security Attacks

• Man-in-the-Middle pre-image attacks.

Principle is to generate a message m = m₁||m₂, such that H(m)=h. If H(m)=g(F(IV,m₁),m₂), the MITM attacks consists in generate random m₁, m₂ until one get G^-1(h,m₂) = F(IV,m₁). Power of the attack relies on the fact that probability of finding a collision is inv. prop. to sqrt of the state size.

'Countermeasures' - prevent attacker to exploit symmmetry properties between round so that he can't discard part of the state, or control part of the state. Make attacker to use too much memory.