Don Coppersmith Markus Jakobsson Almost Optimal Hash Sequence Traversal. 102-119 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2357&spage=102 2002 conf/fc/2002 Financial Cryptography db/conf/fc/fc2002.html#CoppersmithJ02