  Alice communicates with words drawn uniformly amongst $\{\ket{j}\}_{j=1..n}$,
the canonical orthonormal basis. Sometimes however Alice interleaves quantum
decoys $\{\frac{\ket{j}+i\ket{k}}{\sqrt{2}}\}$ between her messages. Such
pairwise superpositions of possible words cannot be distinguished from the
message words. Thus as malevolent Eve observes the quantum channel, she runs
the risk of damaging the superpositions (by causing a collapse). At the
receiving end honest Bob, whom we assume is warned of the quantum decoys'
distribution, checks upon their integrity with a measurement. The present work
establishes, in the case of individual attacks, the tradeoff between Eve's
information gain (her chances, if a message word was sent, of guessing which)
and the disturbance she induces (Bob's chances, if a quantum decoy was sent, to
detect tampering). Besides secure channel protocols, quantum decoys seem a
powerful primitive for constructing n-dimensional quantum cryptographic
applications. Moreover the methods employed in this article should be of strong
interest to anyone concerned with the old but fundamental problem of how much
information may be gained about a system, versus how much this will disturb the
system, in quantum mechanics. Keywords: d-level systems cryptography
