How to truncate a SHA1 hashed string into a 32 bit string

Question

I want to create a 32 bit string that I can use as encryption key. This string/key should be derived from a plain text string, e.g.:

'I am a string'

My approach would first be to hash it:

hashed_string = Digest::SHA1.hexdigest('I am a string') # => 'bd82fb0e81ee9f15f5929e0564093bc9f8015f1d'

And then to use just the first 32 characters:

hashed_string[0..31] # => 'bd82fb0e81ee9f15f5929e0564093bc9'

However, I feel there must be a better approach, and I'm not sure if I risk the possibility of 2 input strings yielding similar keys.

What would be a better approach? I have seen this post that touches on truncation, but can't find an answer that appeals to me there.

Answer 1

If you want a string with 32 bits out of your (weak) password :

Digest::SHA1.digest('I am a string').unpack('B32').first
#=> "10111101100000101111101100001110"

The same amount of information can also be displayed with 8 hexadecimal digits :

Digest::SHA1.hexdigest('I am a string')[0,8]
#=> "bd82fb0e"

or 4 ascii chars :

Digest::SHA1.digest('I am a string')[0,4]
#=> "\xBD\x82\xFB\x0E"