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**2006/223**

**Title:**

What Hashes Make RSA-OAEP Secure?

**Authors:**
Daniel R. L. Brown

**Abstract:**

Firstly, we demonstrate a pathological hash function choice that makes RSA-OAEP insecure. This shows that at least some security property is necessary for the hash functions used in RSA-OAEP. Nevertheless, we conjecture that only some very minimal security properties of the hash functions are actually necessary for the security of RSA-OAEP. Secondly, we consider certain types of reductions that could be used to prove the OW-CPA (i.e., the bare minimum) security of RSA-OAEP. We apply metareductions that show if such reductions existed, then RSA-OAEP would be OW-CCA2 insecure, or even worse, that the RSA problem would solvable. Therefore, it seems unlikely that such reductions could exist. Indeed, no such reductions proving the OW-CCA2 security of RSA-OAEP exist.

**ePrint:**
https://eprint.iacr.org/2006/223

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