Password encryption for Ubuntu shadow

Today I've been in an interesting situation. I've had to change a password from an ubuntu linux manually.

In my shadow file, my password look something like:

manuel:$6$KdE/UHqBIiVg$/fVGwMTREK7fnoR2PdNCmkJ6QHkp8R/RMkpdpY4cE0Ghfqxx7cuIatBHr98C0zPsvSIi8NWr7miuTca1d9Vc6/:14576:0:99999:7:::

The password encrypted here is "asd". The encryption is "$6$KdE/UHqBIiVg$/fVGwMTREK7fnoR2PdNCmkJ6QHkp8R/RMkpdpY4cE0Ghfqxx7cuIatBHr98C0zPsvSIi8NWr7miuTca1d9Vc6/".

You can divide this string in 3 parts, each one separated by a "$" sign.

1. The ID of the encryption. - 6
2. The desired salt - KdE/UHqBIiVg
3. The encryption : /fVGwMTREK7fnoR2PdNCmkJ6QHkp8R/RMkpdpY4cE0Ghfqxx7cuIatBHr98C0zPsvSIi8NWr7miuTca1d9Vc6

The id's used in shadow are:

- 1 for MD5

- 5 for SHA-256

- 6 for SHA-512

In my case, I can generate my password with mkpasswd command:

mkpasswd -m sha-512 asd KdE/UHqBIiVg

P.S. Is a bug in mkpasswd. It has been fixed in ubuntu 9.10. Variable length salts are supported for SHA-512 password hashes, but mkpasswd didn't support them.

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=523387

http://changelogs.ubuntu.com/changelogs/pool/main/w/whois/whois_4.7.33/changelog