Passphrase to WEP key

Short: How do you convert a pass phrase into a 64-bit hexadecimal WEP key?

Long: Recently I misplaced the pass phrase but still had the WEP key. I found a website on Google that converted pass phrases into hex WEP keys and plugged in a few likely phrases, and found the correct WEP key. Because it generated the same WEP key as the router originally did I realized there must be a standard conversion from ascii to hex WEP. Can anyone shed a little insight on the encryption process, or perhaps a link that explains it? Thank you

Edit: I realize to keep it secure it needs to be a one way conversion such as RSA, but then the same very large prime numbers would have to be used to generate the same key, leaving me still stumped.