How are the hashes in /etc/shadow generated

hashshadow

I was reading the Wikipedia article on the shadow file and it mentioned the format of the lines is like this:

$id$salt$hashed

So, my question is, how does it work?

I tried to calcualte the hash for my own account to which I had the password so I used this command:

sha512sum
{salt}+{my_clear_text_password}

But the output is different from the hash I see for myself. So how does it work? What am I missing?

Best Answer

A couple of things to think about (you'll have to read the sources in the Linux coreutils and glibc2 to confirm)

  • The output of sha512sum appears to be printable hex notation whereas the output stored in the shadow file appears to be base64 so they will be different.

  • I think that the sha512sum in the shadow file has been passed through the hash function more than once ( #define ROUNDS_DEFAULT 5000 ) whereas the sha512sum just passes the 'file' through the hash once.

  • There may be padding added by one or both commands to align the data it may be different.

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