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I can't understand how the "cryptographic functions" are to be selected RSA + OAEP which are used in OAEP. How to choose these "cryptographic functions"(G and H)?

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1 Answer 1

Since this picture is taken from wikipedia, I suggest reading the text beside that picture:

  • G and H are typically some cryptographic hash functions fixed by the protocol.

I think you're asking how OAEP and RSA actually are combined, and it goes like this:

  • Use OAEP (choose $r$, follow the instructions and you get $X$ and $Y$)
  • Concatenate $X$ and $Y$, interpret it as an integer (length has to be lower than the RSA keylength)
  • Use this number in RSA.

For decryption, reverse the process.

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Wikipaedia says "G and H are typically some cryptographic hash functions fixed by the protocol.". I don't understand it. –  Sahil Sareen Mar 14 at 12:36
    
I assume you know what cryptographic hash functions are (wikipedia), but there are quite a lot of them. And which one is chosen is up to the protocol designer. For example, RFC 3447 describes RSA-OAEP, but they didn't fix a hash function either but leave it as an optional parameter. That means in practice, you use e.g. RSA-OAEP with SHA-256. –  tylo Mar 14 at 13:24

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