Paper 2004/113 Original: eprint.iacr.org/2004/113 ↗

On the Security and Composability of the One Time Pad

Abstract

Recent experimental results in quantum cryptography have renewed the interest in information-theoretically secure ciphers. In April 2004, in Vienna a bank transfer was secured by means of a one time pad encryption, with the key material being derived from a quantum key exchange. However, in this experiment the integrity of the transmitted message remained unprotected. This can have severe consequences, if the bank transfer form itself contains no authentication mechanism and there is a known position where the amount of money or the recipient is specified. Through flipping bits at the corresponding positions in the ciphertext, the amount of transfered money or the recipient of the money can be changed. This concrete example illustrates the necessity for a thorough theoretical analysis of information-theoretically secure cryptographic techniques that are to be deployed in practice. In this work we show how to implement a statistically secure and composable system for message passing, that is, a channel with negligible failure rate secure against unbounded adversaries, using a one time pad based cryptosystem. We prove the security of our system in an asynchronous adversarially-controlled network using the framework put forward by Backes, Pfitzmann, and Waidner. The composition theorem offered by this framework enables the use of our scheme as a building block of more complex protocols as needed in practical applications.