neilsagarwal / hpc Goto Github PK

In the Richard Schroeppel's page about HPC there are some citations about more than one KX Table in the cipher:

"Although indefinite size keys are allowed, there are in effect only 2^16384 distinct keys. (If all five tables are considered, then there are 81920 bits of key.)"

/\ https://web.archive.org/web/20211130025215if_/http://richard.schroeppel.name:8015/hpc/hpc-overview

"Each subcipher has a KX (key expansion) table, 256 words of 64-bits, pseudo-randomly generated from the key. All five tables may be computed when a key is setup, or the tables may be computed when needed. An application which only used a few blocksizes would need only a subset of the tables. The same algorithm is used for each KX table, changing only an initialization. The KX tables are firewalled: knowing a KX table won't help find the original key, or a KX table for a different subcipher."

/\ https://web.archive.org/web/20120722092329/http://richard.schroeppel.name:8015/hpc/

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It would be a good idea to implement it.

I found at this page (https://www.schneier.com/books/applied-cryptography-source), this file which is a C implementation of HPC: https://www.schneier.com/wp-content/uploads/2015/03/HPC-2.zip

It mixes up to 256bits of a key with the KX table.

    l_key[0][1] ^= io_swap(in_key[0]); l_key[0][0] ^= io_swap(in_key[1]);
    l_key[1][1] ^= io_swap(in_key[2]); l_key[1][0] ^= io_swap(in_key[3]);

    if(key_len > 128)
    {
        l_key[2][1] ^= io_swap(in_key[4]); l_key[2][0] ^= io_swap(in_key[5]);
    }

    if(key_len > 192)
    {
        l_key[3][1] ^= io_swap(in_key[6]); l_key[3][0] ^= io_swap(in_key[7]);
    }

After that, the 249th 64 bit word is processed in the algorithm:

    for(i = 0; i < 8; ++i)
    {
        s[i][0] = l_key[248 + i][0]; s[i][1] = l_key[248 + i][1];
    }

I'm really confused, the author says it's a variable key size cipher, as I read in this C implementation the maximum key size could be 248 words of 64 bits (15872 bit key), am I right?

What is the largest key size accepted by HPC? Do you know?

Traceback (most recent call last):
    ......
    in medium_encrypt
    s1 = m_add(ptxt[1], kx[blocksize+1]) & lmask
IndexError: list index out of range

The ptxt of the incoming function exists only ptxt[0], not ptxt[1]