Unicity Distance
MD Mahmudur Rahman
2018831069
❏ What is Unicity Distance?
The Unicity Distance is a property of a certain cipher algorithm.In cryptography, unicity
distance is the length of an original ciphertext needed to break the cipher by reducing the
number of possible spurious keys to zero in a brute force attack. Every cipher system has a
"unicity distance".
❏ How is unicity distance calculated?
The unicity distance is the ratio of the number of bits required to express the key divided by
the redundancy of english in bits per character.
Formula of Unicity Distance :
UD= H(k) / D;
Where,
UD = unicity distance,
H(k) = entropy of the key space,
D = plaintext redundancy in bits per character.
❏ Where is unicity distance applied?
Unicity distance is a useful theoretical measure, but it doesn't say much about the security of
a block cipher when attacked by an adversary with real-world (limited) resources. Consider a
block cipher with a unicity distance of three ciphertext blocks. Although there is clearly
enough information for a computationally unbounded adversary to find the right key (simple
exhaustive search), this may be computationally infeasible in practice.
Ciphertexts greater than the unicity distance can be assumed to have only one meaningful
decryption. Ciphertexts shorter than the unicity distance may have multiple plausible
decryptions. Unicity distance is not a measure of how much ciphertext is required for
cryptanalysis, but how much ciphertext is required for there to be only one reasonable
solution for cryptanalysis.