As I study SHA-256, I wonder how the values inside the algorithm were determined.

Take a look, for example, at how the 16-64th Words are generated. (in Java language)

    for (int j = 16; j < 64; ++j) {
        int s0 = Integer.rotateRight(words[j - 15], 7) ^
                Integer.rotateRight(words[j - 15], 18) ^
                (words[j - 15] >>> 3);

        int s1 = Integer.rotateRight(words[j - 2], 17) ^
                Integer.rotateRight(words[j - 2], 19) ^
                (words[j - 2] >>> 10);

        words[j] = words[j - 16] + s0 + words[j - 7] + s1;

Just why were the values such as 15, 7, 18, 3, 2, 17, 19, 10, 16, and 7 chosen? Are these just arbitrary values that were randomly chosen for no reason? Or were these each chosen so that the algorithm is as difficult to reverse as possible?

    registers[7] = registers[6];
    registers[6] = registers[5];
    registers[5] = registers[4];
    registers[4] = registers[3] + temp1;
    registers[3] = registers[2];
    registers[2] = registers[1];
    registers[1] = registers[0];
    registers[0] = temp1 + temp2;

Also, in the main iteration loop (snippet shown above), why is the temp1 added to register[3]? Is that also just arbitrary or was it chosen for a particular reason?

Edit: To rephrase the question without code and with equations, I'll put it another way. In the equations to determine $W_t$ (for 16 <= t <= 63), you use the equation $$W_t = \sigma_1 (W_{t-2})+W_{t-7}+\sigma_0 (W_{t-15})+W_{t-16}$$ Where $$\sigma_0(x) = {ROTR}^{7}(x) \oplus {ROTR}^{18}(x) \oplus {SHR}^3(x)$$ $$\sigma_1(x) = {ROTR}^{17}(x) \oplus {ROTR}^{19}(x) \oplus {SHR}^{10}(x)$$ (according to FIPS PUB 180-4)

So, my question is, why were the arbitrary numbers in these equations chosen? The displacement of the word index by 2, 7, 15, and 16 appears to be random, but is there a reason to it? For the definition of $\sigma_0$ and $\sigma_1$, why were the arbitrary numbers 7, 18, 3, 17, 19, and 10 chosen?

The same question could also be asked about the main iteration loop, where $\Sigma_0$ and $\Sigma_1$ are used, which have just as arbitrary values in them.

As for the other question, it asks, why is it that $e = d + T_1$ in the loop from t=0 to t=63. It could have been placed in any of the other 6 working variables. Was it randomly chosen that $d$ would be the variable that has $T_1$ added to it?


In $W_t = \sigma_1 (W_{t-2})+W_{t-7}+\sigma_0 (W_{t-15})+W_{t-16}$

  • The $16$ is the number of 32-bit words in a SHA-256 block. It is here so that, when we consider $W$ as an array of $16$ rather than $64$ words (as most hardware and many software implementations do), the equation become adding to $W_{t\bmod 16}$ a feedback term $\sigma_1 (W_{t-2\bmod 16})+W_{t-7\bmod 16}+\sigma_0 (W_{t-15\bmod 16})$.
  • The $-2$, $-7$, $-15$ controls how far back in the formerly produced values of $W$ the feedback terms are taken from.
    • The $-15$ is as far back as it can go, given the $-16$. That maximizes the memory involved in the feedback.
    • The $-2$ indexes into a recently produced word (second only to what $-1$ would give), which promotes fast diffusion, and splits short-term feedback into two flows (even and odd indexes); also $-1$ could be a bottleneck to parallelization.
    • The $-7$ indexes somewhere else near the middle of the 16-word array, is odd so as to mix said two flows faster that $-15$ would ultimately do, and is slightly biased towards recent production, perhaps to promote faster diffusion.
      Note: That later terms does not go thru a $\sigma$ function, contrary to the other two; that alternates XOR and addition (modulo the wordsize) in the feedback loop.

The shifts values are more difficult to justify. The pattern gives good diffusion, much better than a haphazard choice would do. But many other choices would be possible, and I can't pinpoint a selection process such that the choice made would be optimal.

  • $\begingroup$ Selecting as accepted answer, but if anyone else wants, feel free to make another answer if you feel it better answers the question. $\endgroup$ – Synchronic Oct 8 '19 at 20:05

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