Unicity distance is the ciphertext length at which key equivocation reaches zero and the key becomes uniquely determined under the source model.
Key equivocation is the conditional entropy remaining about a key after observing ciphertext symbols.
For ciphertext alphabet and source entropy , each symbol has redundancy , leading in Shannon's ideal model to .
If distinct keys transform every ciphertext into equally probable plaintexts through a probability-preserving source symmetry, key equivocation never vanishes and unicity distance is infinite.
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Unicity distance is a concept in cryptography that refers to the minimum amount of ciphertext required to ensure that a given ciphertext corresponds to exactly one possible plaintext. In other words, it is the length of ciphertext needed to guarantee that there is a unique plaintext that could produce that ciphertext using a particular encryption scheme. In contexts like symmetric encryption, the unicity distance is important for assessing the security of a cryptosystem.