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I'm really confused about the computation about SHA 256.

$$W_t = begincases
M_t^(i) & 0 leq t leq 15 \
&\
sigma_1^256(W_t-2) + W_t-7 + sigma_0^256(W_t-15) + W_t-16 & 16 leq t leq 63
endcases$$

The $W_t$ variable: What's the value of $W_t$? How do I get that value?

I'm still confused because of the explanation using the English language. I'm a beginner guys, any help from you will be a plus point for me about hashing.

It is in SHA-256 message schedule (NIST-FIPS 180-4);

The message $M$ with length $l$ is first padded as the usual way;

• append 1 to the end of the message,


• then, add $k$ zero bits such that $$l+1+k equiv 448 mod 512$$
  


• finally, add the length of the message in 64-bit. Now, the total padded length is divisible by 512.

After padding, the padded message parsed into $M^1,ldots,M^N$ where each has size 512-bit.

The sub index $t$ represents 32-bits in 512 bits. Thus, The $M_t^(i)$ is the $t$-th 32-bit in the $M^(i)$ for $0 leq t leq 15$

The $W_t$ is defined as your equation;

$$W_t = begincases
M_t^(i) & 0 leq t leq 15 \
&\
sigma_1^256(W_t-2) + W_t-7 + sigma_0^256(W_t-15) + W_t-16 & 16 leq t leq 63
endcases$$

Where
$$sigma_1^256(x) = operatornameROTR^17(x) oplus operatornameROTR^19(x) oplus operatornameSHR^10(x)$$
$$sigma_0^256(x) = operatornameROTR^7(x) oplus operatornameROTR^18(x) oplus operatornameSHR^3(x)$$

$sigma_1^256(x)$ and $sigma_0^256(x)$ operate on 32 bits and produce 32 bits.

Note: the 256 above the $sigma$ represents the 256 in SHA-256. Similarly, there is $sigma_0^512(x)$ and $sigma_1^512(x)$ for SHA-512.