  Digital signatures guarantee the authenticity and transferability of
messages, and are widely used in modern communication. The security of
currently used classical digital signature schemes, however, relies on
computational assumptions. In contrast, quantum digital signature (QDS) schemes
offer information-theoretic security guaranteed by the laws of quantum
mechanics. We present two QDS protocols which have the same experimental
requirements as quantum key distribution, which is already commercially
available. We also present the first security proof for any QDS scheme against
coherent forging attacks.
