OpenSSL "rsautl" Using OAEP Padding

Q

What is the OAEP padding schema used in OpenSSL "rsautl" command?

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A

OAEP (Optimal Asymmetric Encryption Padding), also called PKCS#1 2.0, is a padding standard specified in RFC3447 "PKCS #1: RSA Encryption, Version 1.5" proposed by RSA Laboratories in 1998.

OAEP padding can be illustrated by the diagram below:
OpenSSL rsautl OAEP Padding Algorithm

Here is the explanation of the above OAEP padding diagram given wikipedia.org:

In the diagram,
    n is the number of bits in the RSA modulus.
    k0 and k1 are integers fixed by the protocol.
    m is the plaintext message, an (n − k0 − k1 )-bit string
    G and H are typically some cryptographic hash functions fixed by the protocol.
    ⊕ is an xor operation.

To encode,
    messages are padded with k1 zeros to be n − k0 bits in length.
    r is a randomly generated k0-bit string
    G expands the k0 bits of r to n − k0 bits.
    X = m00..0 ⊕ G(r)
    H reduces the n − k0 bits of X to k0 bits.
    Y = r ⊕ H(X)
    The output is X || Y where X is shown in the diagram as the leftmost block 
    and Y as the rightmost block.

To decode,
    recover the random string as r = Y ⊕ H(X)
    recover the message as m00..0 = X ⊕ G(r)

Note that OAEP is not a simple padding by add some bytes before or after the clear text. OAEP will add bytes to the clear text and encode them to hide the clear text.

For more information on OAEP padding schema, read RFC2437.

 

OpenSSL "rsautl" Command for RSA Keys

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