Somitra Kumar Sanadhya Palash Sarkar New Collision Attacks against Up to 24-Step SHA-2. 91-103 2008 INDOCRYPT http://dx.doi.org/10.1007/978-3-540-89754-5_8 conf/indocrypt/2008 db/conf/indocrypt/indocrypt2008.html#SanadhyaS08