Unicity length

In the cryptology is called Unizitätslänge (also: unambiguity distance ; engl. Unicity distance , also: unicity point ) that length of a ciphertext , he must at least have in order for a by deciphering determined therefrom plain text can be recognized as a unique solution.

definition

The unicity length is a size proposed by Shannon in his work Communication Theory of Secrecy Systems , which corresponds to the length of a text that it must have at least so that it can be understood as a unique solution of a ciphertext. The text length here means the number of characters in the text, which are often letters from the Latin alphabet . The unity length then results from the quotient of the key length, i.e. the logarithm of the number of different possible keys of the encryption used , and the redundancy of the language of the plain text.

Examples

Typical values for the unicity length for some known methods are:

Caesar : 2 letters
Vigenère : 13 letters (for a keyword of length 10)
Playfair : 23 letters
Monoalphabetic substitution : 24 letters
Anagram : ∞ (infinite)
One-time pad : ∞ (infinite)

literature

Friedrich L. Bauer : Deciphered Secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, ISBN 3-540-67931-6 , p. 247 ff.
Cipher A. Deavours: Unicity Points in Cryptanalysis . Cryptologia , 1 (1), Jan. 1977, pp. 46-68.
Michael Miller: Symmetrical encryption methods. Design, development and cryptanalysis of classic and modern ciphers. Teubner, Stuttgart a. a. 2003, ISBN 3-519-02399-7 .
Claude E. Shannon : Communication Theory of Secrecy Systems . Bell System Technical Journal, Oct. 28, 1949, pp. 656-715. PDF; 0.6 MB

Individual evidence

^ Michael Miller: Symmetrical encryption methods. Design, development and cryptanalysis of classic and modern ciphers. Teubner, Stuttgart a. a. 2003, ISBN 3-519-02399-7 , p. 107.
^ Claude E. Shannon: Communication Theory of Secrecy Systems . Bell System Technical Journal, Oct. 28, 1949, pp. 656-715. PDF; 0.6 MB
↑ ^a ^b Cipher A. Deavours: Unicity Points in Cryptanalysis . Cryptologia , 1 (1), Jan. 1977, p. 49
↑ Cipher A. Deavours: Unicity Points in Cryptanalysis . Cryptologia , 1 (1), Jan. 1977, p. 54
↑ Friedrich L. Bauer: Deciphered secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, p. 105.

[1] Michael Miller: Symmetrical encryption methods. Design, development and cryptanalysis of classic and modern ciphers. Teubner, Stuttgart a. a. 2003, ISBN 3-519-02399-7 , p. 107.

[2] Claude E. Shannon: Communication Theory of Secrecy Systems . Bell System Technical Journal, Oct. 28, 1949, pp. 656-715. PDF; 0.6 MB

[Deavours-S49-3] Cipher A. Deavours: Unicity Points in Cryptanalysis . Cryptologia , 1 (1), Jan. 1977, p. 49

[4] Cipher A. Deavours: Unicity Points in Cryptanalysis . Cryptologia , 1 (1), Jan. 1977, p. 54

[5] Friedrich L. Bauer: Deciphered secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, p. 105.