The impact of message losses and retransmissions on quantum cryptographic protocols

Quantum cryptography promises information theoretic security of several cryptographic primitives. Most of the proposed security proofs work in an ideal communication scenario where all messages are correctly delivered after the first attempt. However, real networks are subject to message losses and require retransmissions to cope with them. While the latter is hardly ever a problem in classical cryptography, with quantum communication copy-and-retransmit could be impossible due to the famous no-cloning theorem. In this work, we analyze some quantum cryptoschemes such as public-key encryption, authentication and quantum money, assuming that quantum messages may be lost as they travel through the communication medium. Although all these schemes are theoretically secure, we show that this degree of realism renders some protocols insecure or impractical, while others are completely unaffected. When possible, we provide mitigations such as teleportation or protocol modifications to circumvent these challenges.