What encryption algorithm should be used for the crypted password in the "auth:" attribute of a mntner object ?
Either MD5-PW or DES. Both are "one-way" algorithms; you can _guess_ the clear text password that was used to generate this password (if you have lots of time and many powerful computers), but you cannot reverse-engineer the clear text password from the crypted one; i.e. you cannot use an algorithm on the crypted password to find the clear text password.
Note: the level of security using clear text passwords is not high; you send your clear text password in an e-mail, which could be copied ("sniffed") without you knowing it. Also, a determined, malicious cracker may eventually guess the password.
More information is available in the RIPE Database Reference Manual