  We give the first composable security proof for continuous-variable quantum
key distribution with coherent states against collective attacks. Crucially, in
the limit of large blocks the secret key rate converges to the usual value
computed from the Holevo bound. Combining our proof with either the de Finetti
theorem or the Postselection technique then shows the security of the protocol
against general attacks, thereby confirming the long-standing conjecture that
Gaussian attacks are optimal asymptotically in the composable security
framework.
  We expect that our parameter estimation procedure, which does not rely on any
assumption, will find applications elsewhere, for instance for the reliable
quantification of continuous-variable entanglement in finite-size settings.
