O'Reilly logo

Computer Security and Cryptography by Alan G. Konheim

Stay ahead with the world's most comprehensive technology and business learning platform.

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more.

Start Free Trial

No credit card required

2.3 CRIBBING

The Oxford Dictionary of English Etymology gives to steal and to pilfer as definitions of the Shakespearian verb to crib. The term cribbing in cryptography refers to the process of inferring key and plaintext from ciphertext based on partial knowledge of the plaintext. A crib is a word or phrase w = (w0, w1, …, wM−1) known (or assumed) to appear in the plaintext. Partial knowledge of the plaintext is a reasonable assumption:

  • Letters usually contain stereotyped beginnings and/or endings: Dear …, Sincerely yours, Att:, Senator …;
  • Message transmitted over a network have special formats; and
  • Files are often highly structured, records divided into fields containing data with known characteristics.

When the crib w = (w0, w1, …, wM − l) occurs in the plaintext x, certain strings of letters derived from w will also occur in the ciphertext y = TN, τ(x).

If NM, then w determines N subcribs, which are all the maximal length strings image ≡ {S0, S1, …, Sn−1} formed by the letters in w, which are pairwise-separated by exactly N positions.

image

where si will denote the length of Si

The cryptanalysis of columnar transposition by cribbing is based on the following result.

Proposition 2.1: If xy = TN, τ (x), then

2.1a Pairs of letters (xt, xt+N) in the plaintext separated by N places ...

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, interactive tutorials, and more.

Start Free Trial

No credit card required