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2.2 A Quarter Round on the ChaCha State

2.2 A Quarter Round on the ChaCha State​

The ChaCha state does not have four integer numbers: it has 16. So the quarter-round operation works on only four of them -- hence the name. Each quarter round operates on four predetermined numbers in the ChaCha state. We will denote by QUARTERROUND(x, y, z, w) a quarter-round operation on the numbers at indices x, y, z, and w of the ChaCha state when viewed as a vector. For example, if we apply QUARTERROUND(1, 5, 9, 13) to a state, this means running the quarter-round operation on the elements marked with an asterisk, while leaving the others alone:

   0  *a   2   3
   4  *b   6   7
   8  *c  10  11
  12  *d  14  15

Note that this run of quarter round is part of what is called a "column round".

2.2.1 Test Vector for the Quarter Round on the ChaCha State​

For a test vector, we will use a ChaCha state that was generated randomly:

Sample ChaCha State

    879531e0  c5ecf37d  516461b1  c9a62f8a
    44c20ef3  3390af7f  d9fc690b  2a5f714c
    53372767  b00a5631  974c541a  359e9963
    5c971061  3d631689  2098d9d6  91dbd320

We will apply the QUARTERROUND(2, 7, 8, 13) operation to this state. For obvious reasons, this one is part of what is called a "diagonal round":

After applying QUARTERROUND(2, 7, 8, 13)

    879531e0  c5ecf37d *bdb886dc  c9a62f8a
    44c20ef3  3390af7f  d9fc690b *cfacafd2
   *e46bea80  b00a5631  974c541a  359e9963
    5c971061 *ccc07c79  2098d9d6  91dbd320

Note that only the numbers in positions 2, 7, 8, and 13 changed.

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