Implementation of SHA256 in python3, final hash is too short

Question

I'm trying to write an implementation of SHA-256 in python 3. My version is supposed to take in a hexadecimal encoding and output the corresponding hash value. I've used https://en.wikipedia.org/wiki/SHA-2#Pseudocode as guide.

My function works well for most inputs but sometimes it gives an output that is only 63bits (instead of 64). My function uses 32bit binary strings.

I think I have found the problem, in the last step of the algorithm the binary addition

h4 := h4 + e (or another h-vector and corresponding letter)

yields a binary number that is too small. The last thing I do is to use hex() and I should get a string of 8 characters. In this example I only get 7.

out4 = hex(int(h4,2))[2:]

One problematic input is e5e5e5 It gives "10110101111110101011010101101100" for h4 and "01010001000011100101001001111111" for e so the addition gives "00000111000010010000011111101011" and out4 = 70907eb.

What should I do in these cases?

Are you doing this as a learning exercise? For production code you should never implement your own cryptographic primitives. Always always use a tried and true, widely-used, open-source library. — Chris
– Chris, Commented Nov 30, 2019 at 14:53
Not sure I fully understand your case. But maybe simply left-pad by zeros? For example, if you get 70907eb then you can pad it to be 070907eb. — Yaniv
– Yaniv, Commented Nov 30, 2019 at 14:54

ForceBru · Accepted Answer · 2019-11-30 14:51:54Z

1

I should get a string of 8 characters

Why do you think so? hex doesn't allow to specify the length of the output to begin with, so, for example, if the correct output is 8 bytes of zeros, hex will return 0x0 - the shortest representation possible.

I'm guessing the correct output should begin with zero, but hex is cutting it off. Use format strings to specify the length of output:

In [1]: f'{0:08x}'                                                             
Out[1]: '00000000'  # lowercase hexadecimal (x) digits that must fit into at least 8 characters, prefixed with zero (08) as needed

answered Nov 30, 2019 at 14:51

ForceBru

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3 Comments

xaxablyat Over a year ago

I think it should be so because I know that the total output is supposed to always be 64 characters long and I get it as a sum of 8 different vectors. I have tried experimenting a bit with format but I don't really understand it. My output vector is given like this out4 = hex(int(h4,2))[2:] Should I put the format like this out4 = hex(int(h4,2))[2:]f'{0:08x}' ?

ForceBru Over a year ago

@xaxablyat, no, please see this about string formatting and f-strings

xaxablyat Over a year ago

Thanks fo the help!

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