By default the passwords must be supplied in clear-text, and are encrypted by chpasswd. Also the password age will be updated, if present.
By default, passwords are encrypted by PAM, but (even if not recommended) you can select a different encryption method with the -e, -m, or -c options.
Except when PAM is used to encrypt the passwords,chpasswd first updates all the passwords in memory, and then commits all the changes to disk if no errors occured for any user.
When PAM is used to encrypt the passwords (and update the passwords in the system database) then if a password cannot be updated chpasswd continues updating the passwords of the next users, and will return an error code on exit.
The options which apply to the chpasswd command are:
-c, --crypt-method METHOD
The available methods are DES, MD5, NONE, and SHA256 or SHA512 if your libc support these methods.
By default, PAM is used to encrypt the passwords.
-R, --root CHROOT_DIR
-s, --sha-rounds ROUNDS
The value 0 means that the system will choose the default number of rounds for the crypt method (5000).
A minimal value of 1000 and a maximal value of 999,999,999 will be enforced.
You can only use this option with the SHA256 or SHA512 crypt method.
By default, the number of rounds is defined by the SHA_CRYPT_MIN_ROUNDS and SHA_CRYPT_MAX_ROUNDS variables in /etc/login.defs.
The following configuration variables in /etc/login.defs change the behavior of this tool:
SHA_CRYPT_MIN_ROUNDS (number), SHA_CRYPT_MAX_ROUNDS (number)
With a lot of rounds, it is more difficult to brute forcing the password. But note also that more CPU resources will be needed to authenticate users.
If not specified, the libc will choose the default number of rounds (5000).
The values must be inside the 1000-999,999,999 range.
If only one of the SHA_CRYPT_MIN_ROUNDS or SHA_CRYPT_MAX_ROUNDS values is set, then this value will be used.
If SHA_CRYPT_MIN_ROUNDS > SHA_CRYPT_MAX_ROUNDS, the highest value will be used.
Note: This only affect the generation of group passwords. The generation of user passwords is done by PAM and subject to the PAM configuration. It is recommended to set this variable consistently with the PAM configuration.