Brand-New Hash FunctionBrand-New Hash FunctionBrand-New Hash FunctionBrand-New Hash Function

BeeMA. Satoh

SCIS2006

SHA-1 Broken!

Prof. Xiaoyun WangProf. Xiaoyun Wang

SCIS2006

Essential Characteristics of Hash Function

Hash Function– Algorithm that compresses an arbitrarily size of message into a fixed size

hash value. It is also called a message digest function.

– Used for authentication, error detection, and pseudo-random number generators

One-Wayness– It is easy to generate the hash value from a given message,

but it is computationally infeasible to generate a message producing a given hash value.

Collision Resistance– It is computationally infeasible to generate a message pair that

has the same hash value.

SCIS2006

Birthday Attack

In a group of 40 people, a pair who have the same birthday can be found with 89% probability

When a hash value size is n bits, a collision can be found in 2n/2 messages with 50% probability

Birthday Paradox

SCIS2006

ExampleOne-wayness is also satisfied

Collision Resistance againstBirthdayAttack

Birthday

Brand-New Hash FunctionBrand-New Hash Function

SCIS2006

Bye-Bye My LoveBye-Bye My Love