Doing a hash by hand/mathematically

I want to learn how to do a hash开发者_Python百科 by hand (like with paper and pencil). Is this feasible? Any pointers on where to learn about this would be appreciated.

That depends on the hash you want to do. You can do a really simple hash by hand pretty easily -- for example, one trivial one is to take the ASCII values of the string, and add them together, typically doing something like a left-rotate between characters. So, to hash the string "Hash", we'd start with the ASCII values of the letters (in hex): 48 61 73 68. We'll add those together, rotating our result left 4 bits (in a 16-bit word) between letters:

0048 + 0061 = 00A9
00A9 <<< 4 = 0A90
0A90 + 0073 = 0B03
B03 <<< 4 = B030
B030 + 68 = B098

Result: B098

Doing a cryptographic hash by hand would be a rather different story. It's certainly still possible, but would be extremely tedious, to put it mildly. A cryptographic hash is typically quite a bit more complex, and (more importantly) almost always has a lot of "rounds", meaning that you basically repeat a set of steps a number of times to get from the input to the output. Speaking from experience, just stepping through SHA-1 in a debugger to be sure you've implemented it correctly is a pain -- doing it all by hand would be pretty awful (but as I said, certainly possible anyway).

You can start by looking at

Hash function

I would suggest trying a CRC, since it seems to me to be the easiest to do by hand: https://en.wikipedia.org/wiki/CRC32#Computation .

You can do a smaller length than standard (it's usually 32 bit) to make things easier.