Where do I truncate the bits in SHA 256 hash algorithm?

Question

I'm implementing SHA 256 in PHP using strings. Where do you truncate in the algorithm?

There are a lot of places where there's binary addition happening so the no. of bits in the W(i) keep increasing, same with the compression function. So where do I truncate the values?

When I var_dump my array I get this:

array(64) { [0]=> string(32) "01100001001000000111010001100101" [1]=> string(32) "01110011011101001000000000000000" [2]=> string(32) "00000000000000000000000000000000" [3]=> string(32) "00000000000000000000000000000000" [4]=> string(32) "00000000000000000000000000000000" [5]=> string(32) "00000000000000000000000000000000" [6]=> string(32) "00000000000000000000000000000000" [7]=> string(32) "00000000000000000000000000000000" [8]=> string(32) "00000000000000000000000000000000" [9]=> string(32) "00000000000000000000000000000000" [10]=> string(32) "00000000000000000000000000000000" [11]=> string(32) "00000000000000000000000000000000" [12]=> string(32) "00000000000000000000000000000000" [13]=> string(32) "00000000000000000000000000000000" [14]=> string(32) "00000000000000000000000000000000" [15]=> string(32) "00000000000000000000000000110000" [16]=> string(32) "10001111101010001101101001000010" [17]=> string(32) "01110011100100101000000000000000" [18]=> string(32) "01110110010010101111110000010111" [19]=> string(32) "00010000000111001101001100011011" [20]=> string(32) "00100001100101001100011101010011" [21]=> string(32) "11110011111010101110110100111001" [22]=> string(32) "11111011010010111001000111111001" [23]=> string(32) "10111010101100000001011101110101" [24]=> string(33) "100101110100011111001000000101000" [25]=> string(32) "01111111110001011100001000100010" [26]=> string(33) "010000100100001011110010010001101" [27]=> string(32) "01111010110111110100100010111101" [28]=> string(33) "110010001011110100010000011100100" [29]=> string(33) "101001000101000110011011111011111" [30]=> string(33) "111000011011101011011101110010101" [31]=> string(34) "1111001011100001001101111011110110" [32]=> string(34) "1010010011000001000100101100000010" [33]=> string(35) "10101101001100111001111100111111010" [34]=> string(34) "1110100101001100001000011110000110" [35]=> string(36) "100101010100100001101110101111000011" [36]=> string(34) "1110001101001101001101110111011000" [37]=> string(36) "110101001100000100001110011000000101" [38]=> string(36) "100110010110100011010111011101100110" [39]=> string(36) "101110011100110000110001010010110000" [40]=> string(36) "100000101011010110001001000100011101" [41]=> string(37) "1001110001010110110001110110101011101" [42]=> string(37) "1000000101001011010111010000001100100" [43]=> string(37) "1001000011101111000001111110010100100" [44]=> string(38) "10010000101101110001100100010100010101" [45]=> string(37) "1011111001101101011111001000110010000" [46]=> string(39) "100101011110011011000000001001110010001" [47]=> string(37) "1001101011101110111000000100111010000" [48]=> string(39) "010001011100001101101000011010011100010" [49]=> string(38) "10011100010110000011111110010101101000" [50]=> string(39) "110011100111000010101100100101110011001" [51]=> string(38) "11001101111101010001000010110001100100" [52]=> string(40) "1001010000101100000100110100110000100011" [53]=> string(40) "1000010101011111001110000100101100111000" [54]=> string(40) "1111011111011011100000000001110000011101" [55]=> string(40) "0101110001001001110010001011110111010010" [56]=> string(40) "0100111101101110101111101011110000000011" [57]=> string(40) "1100101000101110000000101100000111111110" [58]=> string(40) "1101100110111011011010010001101100000100" [59]=> string(41) "10000100101011000111001100110101010010001" [60]=> string(41) "10111001010111100010000110001111101100110" [61]=> string(42) "110001101011011010100110111110101100101111" [62]=> string(42) "101001011000100100011001011101100100101111" [63]=> string(42) "101110011001100001001100001010110101001111" }

Do I truncate the values before pushing them to the array or after the array is done with? If I truncate before, corresponding iterations will take up truncated values and that changes their values.

Also

$tmp=$q4;
$tr1=substr($tmp,-32);

$tmp2=binary_add($s0,$maj);
$tr2=substr($tmp2,-32);

$H=$G;
$G=$F;
$F=$E;
$E=binary_add($tr1,$D);
$D=$C;
$C=$B;
$B=$A;
$A=binary_add($tr1,$tr2);

here too notice that I have truncated before adding the temp variables (and updating the var A to H) is this correct?

Answer 1

First off, you shouldn't be implementing SHA-256 yourself. You're likely to end up with something that's buggy and slow. You should use the implementation in your platform's library. Keep in mind the Rules of Crypto:

1. Never design your own crypto.
2. Never implement your own crypto.
3. Anyone can design crypto that they can't break themselves.

SHA-256 produces a 256-bit output. It looks like your dump contains the internal state of the algorithm, which is considerably more than 256 bits. If that's what you have at the end of your SHA-256 function, then you're missing a step. You shouldn't need to truncate anything to get the final output.