Help with :crypto.hash

Ok, what exactly you are trying to achieve. Give us an example input and example output of what you want, maybe then it will be easier to understand what you want.

I’m trying to generate a checksum for the first 8 characters of a string using SHA1, in Javascript it would be something like that parseInt(sha1(str).substr(0, 8), 16); for a string “Hi?Hi.” it should be 174888039. Note the character “\x1f” between ? and H.

Your main problem is thinking that 2fd4e1c67a2d28fced849ee1bb76e7391b93eb12 is SHA-1 output, it isn’t. Just check out what value does the crypto:hash/2 produce and work from there (hint, you can use pattern matching to generate integer from binary).

Hmmmm. As far as I understand (I might be wrong tho ) from :crypto.hash/2 documentation this is the message digest of SHA-1. what am I missing?

The digest is return as a binary representation.

iex(18)> :crypto.hash(:sha, "abc123")
<<99, 103, 196, 141, 209, 147, 213, 110, 167, 176, 186, 173, 37, 177, 148, 85,
  229, 41, 245, 238>>

So this binary representation must be converted to the integer you want. You are trying to use Integer.parse(..,16) which requires a hex representation.

So one option could be to convert the binary to hex first and then parse to integer. Or perhaps it is possible to pattern-match out the integer directly from the binary hash as hauleth is hinting at.

For example:

<<y::integer-size(160)>> = yourhash

I don’t know if that gives the same result as:

yourhash |> Base.encode(16) |> Integer.parse(16)

but if it does it saves a few functions and are generally cleaner.

it’s Base.encode16().
Thank you so much!

You can avoid the conversion to Hex:

iex(1)> :crypto.hash(:sha, "Hi?\x1fHi.") |> binary_part(0, 4) |> :crypto.bytes_to_integer()
174888039

Or:

iex(1)> <<int :: integer-size(32), _ :: binary>> = :crypto.hash(:sha, "Hi?\x1fHi.")
iex(2)> int
174888039