The Norwegian company Encap has developed protocols enabling individuals to use their mobile phones as one-time password (OTP) generators. An initial analysis of the protocols reveals minor security flaws. System-level testing of an online bank utilizing Encap’s solution then shows that several attacks allow a malicious individual to turn his own mobile phone into an OTP generator for another individual’s bank account. Some of the suggested countermeasures to thwart the attacks are already incorporated in an updated version of the online banking system.