Attributes of a session, between a source device and a verification device, for sending first verification data, such as a password and an account identifier, are determined. The verification device generates user device data based on an identifier, such as a mobile device number (MDN), for a user device associated with the account identifier. An identifier, such as an MDN, associated with the source device and an encryption key associated with the verification device are determined based on session attributes. Second verification data is generated based on the identifier associated with the source device. The second verification data is encrypted using the encryption key and forwarded to the verification device. The verification device decrypts the second verification data and compares the identifier for the user device to the identifier for the source device to determine whether the first verification data was sent from the user device.