In Cryptology, OAEP ( Optimal Asymmetric Encryption Padding ) is a diagram of filling introduces in 1994 by Mihir Bellare and Phil Rogaway. It requires a source of risk like two functions of chopping.

Schematically, if one notes $M$ the message, $r$ a random quantity, and $G,\; H$ the two functions of chopping, one has

$\backslash \; mathrm\; \{OAEP\}\; (M)\; =\; \backslash \; left\; (M\; \backslash \; oplus\; G\; (R))\; ||\; (R\; \backslash \; oplus\; H\; (M\; \backslash \; oplus\; G\; (R))\; )\; \backslash \; right$,

where “ || ” indicates the concatenation and $\backslash \; oplus$ the operator “Or exclusive”.

To obtain $M$, it is enough to cut $\backslash \; mathrm\; \{OAEP\}\; (M)$ in two parts, $O\_1,\; O\_2$ where $O\_1$ with the same length as $M$, and to calculate:

$r=H\; (O\_1)\; \backslash \; oplus\; O\_2$,

there is then $M=\; O\_1\; \backslash \; oplus\; G\; (R)$.

It is generally a preliminary stage for a coding RSA, one speaks besides in this case about RSA-OAEP. One of the interest of RSA-OAEP is to be able to be proven sure in a idealized ideal model, that of random oracle. This algorithm is recommended by directive PKCS ( Public Key Cryptography Standard ) 1, version 2.1 (June 2002).

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