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...

Ciphers

Bilateral ciphers

Example: http://www.cabinetmagazine.org/issues/40/sherman.php

In this example, people on a photograph are forming a coded phrase by facing forward or sideways, using the code:

Sir Francis Bacon Bilateral code

code	meaning	code	meaning	code	meaning	code	meaning
aaaaa	A	aaaab	B	aaaba	C	aaabb	D
aabaa	E	aabab	F	aabba	G	aabbb	H
abaaa	I/J	abaab	K	ababa	L	ababb	M
abbaa	N	abbab	O	abbba	P	abbbb	Q
baaaa	R	baaab	S	baaba	T	baabb	U/V
babaa	W	babab	X	babba	Y	babbb	Z

This code was invented by Sir Francis Bacon. The power of that code is that a's and b's in a message can easily be hidden: he allowed the a’s and b’s in his system to designate the different forms of anything that can be divided into two classes, sorts, or types (which Bacon referred to as the a-form and the b-form). Examples of a/b-forms are: colors of flower, size of objects,

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.