  Digital signatures are frequently used in data transfer to prevent
impersonation, repudiation and message tampering. Currently used classical
digital signature schemes rely on public key encryption techniques, where the
complexity of so-called "one-way" mathematical functions is used to provide
security over sufficiently long timescales. No mathematical proofs are known
for the long-term security of such techniques. Quantum digital signatures offer
a means of sending a message which cannot be forged or repudiated, with
security verified by information-theoretical limits and quantum mechanics. Here
we demonstrate an experimental system which distributes quantum signatures from
one sender to two receivers and enables message sending ensured against forging
and repudiation. Additionally, we analyse the security of the system in some
typical scenarios. The system is based on the interference of phase encoded
coherent states of light and our implementation utilises polarisation
maintaining optical fibre and photons with a wavelength of 850 nm.
