High Quality Content by WIKIPEDIA articles! A one-time password (OTP) is a password that is only valid for a single login session or transaction. OTPs avoid a number of shortcomings that are associated with traditional (static) passwords. The most important shortcoming that is addressed by OTPs is that, in contrast to static passwords, they are not vulnerable to replay attacks. This means that, if a potential intruder manages to record an OTP that was already used to log into a service or to conduct a transaction, he will not be able to abuse it since it will be no longer valid. On the downside, OTPs cannot be memorized by human beings. Therefore they require additional technology in order to work.