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2.1 The ChaCha Quarter Round

2.1 The ChaCha Quarter Round​

The basic operation of the ChaCha algorithm is the quarter round. It operates on four 32-bit unsigned integers, denoted a, b, c, and d. The operation is as follows (in C-like notation):

a += b; d ^= a; d <<<= 16;
c += d; b ^= c; b <<<= 12;
a += b; d ^= a; d <<<= 8;
c += d; b ^= c; b <<<= 7;

Where "+" denotes integer addition modulo 2^32, "^" denotes a bitwise Exclusive OR (XOR), and "<<< n" denotes an n-bit left roll (towards the high bits).

For example, let's see the add, XOR, and roll operations from the fourth line with sample numbers:

a = 0x11111111
b = 0x01020304
c = 0x77777777
d = 0x01234567
c = c + d = 0x77777777 + 0x01234567 = 0x789abcde
b = b ^ c = 0x01020304 ^ 0x789abcde = 0x7998bfda
b = b <<< 7 = 0x7998bfda <<< 7 = 0xcc5fed3c

2.1.1 Test Vector for the ChaCha Quarter Round​

For a test vector, we will use the same numbers as in the example, adding something random for c.

a = 0x11111111
b = 0x01020304
c = 0x9b8d6f43
d = 0x01234567

After running a Quarter Round on these four numbers, we get these:

a = 0xea2a92f4
b = 0xcb1cf8ce
c = 0x4581472e
d = 0x5881c4bb
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