Help with :crypto.hash

Questions / Help
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1

Hello there,
I’m trying to generate a checksum using :crypto.hash/2, I want to hash the first 8 characters only of my string and get the result as a decimal value. More precisely I’m trying to generate the checksum 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.

This is my code:

Integer.parse(:crypto.hash(:sha, String.slice("First Part" <> "\x1f" <> "Second Part", 0..8)), 16)

but it returns :error , any help with that?

Thanks in advance!

1 Like
2

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.

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3

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.

4

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).

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5

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

6

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.

2 Likes
7

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

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8

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
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