This is from my post from March 17, 2011:
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Yes I have...
But first be informed that doing the change will cause your existing voucher-rolls to be not valid anymore....
You need to delete the old ones and create new once....
Be carefullllll - use it on you own risk....
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1) Goto: http://<your-m0n0wall-ip>/exec.php
2) Enter
/usr/local/bin/voucher -g 40 as Command and hit execute.
You'll get something similar to this:
$ /usr/local/bin/voucher -g 40
-----BEGIN RSA PRIVATE KEY-----
asdfghjkhgfdsfghjhgfdsdfghjhgfdfghjhgfdsfgh
-----END RSA PRIVATE KEY-----
-----BEGIN PUBLIC KEY-----
asdfghjkhgfdsfghjhgfdsdfghjhgfdfghjhgfdsfgh+lAgMBAAE=
-----END PUBLIC KEY-----
3) Copy paste both keys into the voucher configuration screen (best to save them first into a document as the key changes everytime you generate them).
Different key sizes can be specified of course.
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I got this from same post on this forum (Oktober 2007) and it worked for me very fine... I also removed the lower capital letters from the Captive Portal - Vouchers - Character set...
Now I have vouchers with lenght of 8....
BR
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let me know if you manage to do or need support...