Problem 44964. Optimal Asymmetric Encryption Padding of message for RSA Cryptography

Created by David Hill
Appears in Advanced Cryptography Algorithms and Mathematics

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EME-OAEP encoding of input message using a Feistel network with SHA-1 hashing and mask generation function (MGF1) (see problem 44963/below link, section 9.1.1.1 (figure 1 shows the encoding process)). Inputs will include message (M), label (L), a random seed (seed) array of octets of length 20 (needed such that outputs can be compared), and output length (k). Output (EM) array will be an array of octets of length, k. Hash length will always be 20 from SHA-1 (160-20 bytes). The character length of input message will never exceed k-42.
For example:
M = 'I like to swim.';%input
L = 'abcdefghijklmnopqrstuvwxyz';%input
seed = [147,11,108,119,5,16,236,136,93,93,38,38,89,86,200,124,118,33,226,172];%input
k = 100;%input
EM = [0,167,202,74,98,63,234,235,44,82,53,39,70,214,204,79,45,80,35,147,55,32,35,39,45,...
87,180,129,63,219,223,165,72,19,131,160,131,131,107,231,157,238,213,10,237,132,55,...
35,182,202,151,21,82,130,121,35,14,35,75,255,236,72,45,210,0,243,57,43,137,79,9,...
217,83,81,227,171,156,105,168,121,224,201,41,98,84,79,3,247,109,100,214,227,120,...
148,12,50,108,248,32,94];%output
See: https://www.foo.be/docs/opensst/ref/pkcs/pkcs-1/pkcs-1v2-1d1.pdf

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14 Solutions
3 Solvers

Last Solution submitted on Mar 28, 2025

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Problem Comments

3 Comments

3 Comments

Rafael S.T. Vieira on 24 Oct 2020

Hi, David, is the problem description right? Seems that section 9.1.1 has the EME-OAEP-encode/decode pair, while 7.1.1 is another function that uses EME-OAEP, but it is not it. Moreover, how do we apply the Feistel network inside the MGF1 (disjoint sets or with intersection: 1,1,2,2,3,3,..., n n or 2 3 4 .. .n 1)? And how many rounds? 2 or more? Should the Feistel network replace the MGF1 function? Could you, please, explain?

David Hill on 25 Oct 2020

Yes, changed description to the proper section 9.1.1.1. The steps show how the MGF1 function is used. Look at figure 1. There are not multiple rounds.

Rafael S.T. Vieira on 26 Oct 2020

Thanks, David. I've struggled for a while with this problem but I've finally figured it out. First, although we don't use the encryption function from 7.1.1, we must create an output EM of length k-1 and apply a pad of 0 as the first character of EM (It was strange to see all hashes starting with 0). Next, I was confused with the Feistel Network, because my MGF from the previous problem wasn't working (an accepted solution), but I realized that you were probably using a hex input, instead of char (which is weird since the MGF function requires an Octect String as input), but anyway that solved it. :)