  The security issues facing quantum key distribution (QKD) are explained,
herein focusing on those issues that are cryptographic and information
theoretic in nature and not those based on physics. The problem of security
criteria is addressed. It is demonstrated that an attacker's success
probabilities are the fundamental criteria of security that any theoretic
security criterion must relate to in order to have operational significance.
The errors committed in the prevalent interpretation of the trace distance
criterion are analyzed. The security proofs of QKD protocols are discussed and
assessed in regard to three main features: their validity, completeness, and
adequacy of the achieved numerical security level. Problems are identified in
all these features. It appears that the QKD security situation is quite
different from the common perception that a QKD-generated key is nearly
perfectly secure. Built into our discussion is a simple but complete
quantitative description of the information theoretic security of classical key
distribution that is also applicable to the quantum situation. In the
appendices, we provide a brief outline of the history of some major QKD
security proofs, a rather unfavorable comparison of current QKD proven security
with that of conventional symmetric key ciphers, and a list of objections and
answers concerning some major points of the paper.
