### OAEP Reconsidered

Victor Shoup

##### Abstract

The OAEP encryption scheme was introduced by Bellare and Rogaway at Eurocrypt '94. It converts any trapdoor permutation scheme into a public-key encryption scheme. OAEP is widely believed to provide resistance against adaptive chosen ciphertext attack. The main justification for this belief is a supposed proof of security in the random oracle model, assuming the underlying trapdoor permutation scheme is one way. This paper shows conclusively that this justification is invalid. First, it observes that there appears to be a non-trivial gap in the OAEP security proof. Second, it proves that this gap cannot be filled, in the sense that there can be no standard "black box" security reduction for OAEP. This is done by proving that there exists an oracle relative to which the general OAEP scheme is insecure. The paper also presents a new scheme OAEP+, along with a complete proof of security in the random oracle model. OAEP+ is essentially just as efficient as OAEP, and even has a tighter security reduction. It should be stressed that these results do not imply that a particular instantiation of OAEP, such as RSA-OAEP, is insecure. They simply undermine the original justification for its security. In fact, it turns out -- essentially by accident, rather than by design -- that RSA-OAEP is secure in the random oracle model; however, this fact relies on special algebraic properties of the RSA function, and not on the security of the general OAEP scheme.

Available format(s)
Category
Public-key cryptography
Publication info
Published elsewhere. Unknown where it was published
Contact author(s)
sho @ zurich ibm com
History
2001-02-13: last of 10 revisions
See all versions
Short URL
https://ia.cr/2000/060

CC BY

BibTeX

@misc{cryptoeprint:2000/060,
author = {Victor Shoup},
title = {OAEP Reconsidered},
howpublished = {Cryptology ePrint Archive, Paper 2000/060},
year = {2000},
note = {\url{https://eprint.iacr.org/2000/060}},
url = {https://eprint.iacr.org/2000/060}
}

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