## Cryptology ePrint Archive: Report 2010/631

**Black-box property of Cryptographic Hash Functions**

*Michal Rjaško*

**Abstract: **We define a new black-box property for cryptographic hash function families $H:\{0,1\}^K\times\{0,1\}^*\rightarrow\{0,1\}^y$ which guarantees that for a randomly chosen hash function $H_K$ from the family, everything ``non-trivial'' we are able to compute having access to the key $K$, we can compute only with oracle access to $H_K$. If a hash function family is pseudo-random and has the black-box property then a randomly chosen hash function $H_K$ from the family is resistant to all non-trivial types of attack. We also show that the HMAC domain extension transform is Prf-BB preserving, i.e. if a compression function $f$ is pseudo-random and has black-box property (Prf-BB for short) then $\HMAC^f$ is Prf-BB. On the other hand we show that the Merkle-Damg\aa rd construction is not Prf-BB preserving. Finally we show that every pseudo-random oracle preserving domain extension transform is Prf-BB preserving and vice-versa. Hence, Prf-BB seems to be an all-in-one property for cryptographic hash function families, which guarantees their ``total'' security.

**Category / Keywords: **cryptographic hash function, pseudo-random oracle, black-box

**Date: **received 11 Dec 2010

**Contact author: **rjasko at dcs fmph uniba sk

**Available format(s): **PDF | BibTeX Citation

**Version: **20101213:190543 (All versions of this report)

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