Linux /etc/shadow encrypted password field
From Brandonhutchinson.com
Description of the encrypted password field in /etc/shadow on Red Hat Linux systems:
- Contains only ! or * -- account is locked; user is unable to login
- Contains only !! -- no password has been set; user is unable to login
- Begins with !! -- password is locked by prefixing it with an invalid string (!), and can be unlocked using passwd -u
- Begins with $1$ -- password is encrypted using the MD5 hash algorithm
- All others -- password is encrypted using the crypt library function
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