The Damm algorithm is a fantastic check digit algorithm that detects all single-digit errors and all adjacent transposition errors[1]. It detects all occurrences of altering one single digit and all occurrences of transposing two adjacent digits, the two most frequent transcription errors. The Damm algorithm also has the benefit that prepending leading base encoding zeros does not affect the check digit so you can use it on fixed size strings. All in all it is a great algorithm for checking if there have been any user errors when entering a number.
The only problem with the Damm algorithm is that all the current implementations are base10 (0-9) only. I wanted to be able to use it to check alpha-numerical strings (passwords) so I wrote a base32 implementation of the Damm algorithm in C/C++ and PHP. With the commonly confused characters (0/o and 1/l/i) avoided in the base32 encoding it makes a rather nice password checker.
[1] https://en.wikipedia.org/wiki/Damm_algorithm
The basic call of damm32 is:
./damm32 [base32 number]
This outputs the following when using the base32 number fdfsdfds
Base32 Number: fdfsdfds
Base32 Check Digit: 3
Error Check: 0
If the last digit is the expected Damm check digit the error check will be 1 else it will be 0.
Base32 Number: fdfsdfds3
Base32 Check Digit: 2
Error Check: 1
damm32 can be used in a bash one-liner to generate any length (x + the damn check digit) character password strings. You can make longer or shorter strings by changing the head -c switch value. If wanting to generating really long strings increase the openssl rand -base64 value to ~4x. Here is an example for generate a 10 digit string:
openssl rand -base64 50 | tr -dc '23456789abcdefghjkmnpqrstuvwxyz' | head -c9 | xargs damm32
damm32.c can be compiled with gcc (and most likely any other C compiler) using:
gcc damm32.c -o damm32
I have include the compiled MacOS X version (damm32).
I have ported the C version to PHP (damm32.php). It can be run using the php cli. The output is the same as the C version.
php damm32.php [base32 number]