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OTPW-GEN(1) FreeBSD General Commands Manual OTPW-GEN(1)

otpw-gen - one-time password generator

otpw-gen [ options ]

OTPW is a one-time password authentication system. It can be plugged into any application that needs to authenticate users interactively. One-time password authentication is a valuable protection against password eavesdropping, especially for logins from untrusted terminals.

Before you can use OTPW to log into your system, two preparation steps are necessary. Firstly, your system administrator has to enable it. (This is usually done by configuring your login software (e.g., sshd) to use OTPW via the Pluggable Authentication Module (PAM) configuration files in /etc/pam.d/.)

Secondly, you need to generate a list of one-time passwords and print it out. This can be done by calling

otpw-gen | lpr

or something like

otpw-gen -h 70 -s 2 | a2ps -1B -L 70 --borders no

if more control over the layout is desired.

You will be asked for a prefix password, which you need to memorize. It has to be entered immediately before the one-time password. The prefix password reduces the risk that anyone who finds or steals your password printout can use that alone to impersonate you.

Each one-time password will be printed behind a three digit password number. Such a number will appear in the password prompt when OTPW has been activated:

Password 026:

When you see this prompt, enter the memorized prefix password, followed immediately by the one-time password identified by the number. Any spaces within a password have only been inserted to improve legibility and do not have to be copied. OTPW will ignore the difference between the easily confused characters 0O and Il1 in passwords.

In some situations, for example if multiple logins occur simultaneously for the same user, OTPW defends itself against the possibility of various attacks by asking for three random passwords simultaneously.

Password 047/192/210:

You then have to enter the prefix password, followed immediately by the three requested one-time passwords. This fall-back mode is activated by the existence of the lock file ~/.otpw.lock. If it was left over by some malfunction, it can safely be deleted manually using option -l.

Call otpw-gen again when you have used up about half of the printed one-time passwords or when you have lost your password sheet. This will disable all remaining passwords on the previous sheet.

-h number
Specify the total number of lines per page to be sent to standard output. This number minus four header lines determines the number of rows of passwords on each page. The maximum number of passwords that can be printed is 1000. (Minimum: 5, default: 60)
-w number
Specify the maximum width of lines to be sent to standard output. This parameter determines together with the password length the number of columns in the printed password matrix. (Minimum: 64, default: 79)
-s number
Specify the number of form-feed separated pages to be sent to standard output. (Default: 1)
-e number
Specify the minimum entropy of each one-time password in bits. The length of each password will be chosen automatically, such that there are at least two to the power of the specified number possible passwords. A value below 30 might make the passwords vulnerable to a brute-force guessing attack. If the attacker might have read access to the ~/.otpw file, the value should be at least 48. Paranoid users might prefer long high-security passwords with at least 60 bits of entropy. (Default: 48)
Generate passwords by transforming a random bit string into a sequence of letters and digits, using a form of base-64 encoding (6 bits per character). (Default)
Generate passwords by transforming a random bit string into a sequence of English four-letter words, each chosen from a fixed list of 2048 words (2.75 bits per character).
Generate passwords by transforming a random bit string into a sequence of lowercase letters and digits (5 bits per character). These are easier to communicate by voice (e.g., using the NATO alphabet).
-f filename
Specify a file to be used instead of ~/.otpw for storing the hash values of the generated one-time passwords.
Suppress the addition of a header and footer line to each output page. This reduces the minimum value for option -h to 1.
Instead of generating each password randomly, generate a random master key and then derive each password from that in a deterministic way. The master key will be printed to standard error. It can later be used with option -k to recreate another copy of the same one-time password list. (Each password is generated from the output of a secure hash function applied to the master key and the challenge string.)
-E number
Specify the minimum entropy of the master key in bits. (It contains in addition four bits redundancy for error checking.)
-P number
Choose the text format in which the master key will be displayed. The supported values are the same as with option -p.
Ask for a master key, as it was generated by option -m, and then recreate the same password list from that. With this option, only a password list will be generated; the hash values in ~/.otpw remain unmodified.
Output a suggestion for a random password, then exit. The length and type of password can be selected with options -e and -p.
Remove any lock file left by previous authentication attempts, then exit.

If the otpw-gen binary, owned by some system pseudo user (e.g., “otpw”), has the SETUID bit set, then the password hash file will be owned by and stored in the home directory of that pseudo user (e.g., “/var/lib/otpw”), using the user's name instead of “.otpw”. This way, the hash files are out of reach from the users, and cannot be manipulated by tools other than otpw-gen, which can help to enforce policies about how passwords are generated. Storing the password hash files outside the user's home directory can also be useful where the home directory may not yet be accessible during login.

The OTPW package, which includes the otpw-gen progam, has been developed by Markus Kuhn. The most recent version is available from <>.

pam(8), pam_otpw(8)

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