How to use HMAC SHA256?

Question

As per the various docs that I have read for using HMAC SHA256, I have understood that:

H (K XOR opad, H (K XOR ipad, text)) where H in my case is SHA256.

But, SHA256 input has only one parameter i.e a Message. Whereas H(K,text) has two inputs. So how to calculate H(k,text)?

Should I first encode text with k and then use H(encoded_text), where encoded_text will be used as a message?

Thank You

Answer 1

• H() is your cryptographic hash function, in this case SHA256() but could also be MD5 or whatever;
• K is your predifined key
• Text is the message to be authenticated
• opad be the outer padding (0x5c5c5c…5c5c, one-block-long hexadecimal constant)
• ipad be the inner padding (0x363636…3636, one-block-long hexadecimal constant)
• Then HMAC(K,m) is mathematically defined by

HMAC(K,m) = H((K ⊕ opad) ∥ H((K ⊕ ipad) ∥ m)).

• blocksized is determined by your hash function (MD5 would be 64 bytes)
• o_key_pad = [opad * blocksize] ⊕ key
• i_key_pad = [ipad * blocksize] ⊕ key

Your result would be:

H(o_key_pad || H(i_key_pad || TEXT))

You can find a good read here: http://timdinh.nl/index.php/hmac/

With also the following pseudocode which almost looks like mine :

function hmac (key, message)
    opad = [0x5c * blocksize] // Where blocksize is that of the underlying hash function
    ipad = [0x36 * blocksize]

    if (length(key) > blocksize) then
        key = hash(key) // Where 'hash' is the underlying hash function
    end if

    for i from 0 to length(key) - 1 step 1
        ipad[i] = ipad[i] XOR key[i]
        opad[i] = opad[i] XOR key[i]
    end for

    return hash(opad || hash(ipad || message)) // Where || is concatenation
end function