IACR News item: 05 September 2014
Ahto Buldas, Risto Laanoja, Peeter Laud, Ahto Truu
ePrint ReportWe also show that the compression function of Shrimpton-Stam that uses non-compressing components is BPrA. The security proof for unbounded hash-tree schemes is very tight under the BPrA assumption. In order to have $2^s$-security against back-dating, the hash function must have $n=2s + 4$ output bits, assuming that the security of the hash function is close to the birthday barrier, i.e. that there are no structural weaknesses in the hash function itself. Note that the previous proofs that assume PrA gave the estimation $n=2s + 2 \\log_2 C + 2$, where $C$ is the maximum allowed size of the hash tree. For example, if $s=100$ ($2^{100}$-security) and $C=2^{50}$, the previous proofs require $n=302$ output bits, while the new proof requires $n=204$ output bits.
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