  We analyse the distribution of secure keys using quantum cryptography based
on the continuous variable degree of freedom of entangled photon pairs. We
derive the information capacity of a scheme based on the spatial entanglement
of photons from a realistic source, and show that the standard measures of
security known for quadrature-based continuous variable quantum cryptography
(CV-QKD) are inadequate. A specific simple eavesdropping attack is analysed to
illuminate how secret information may be distilled well beyond the bounds of
the usual CV-QKD measures.
