Overview:

Classical ciphers presented interesting and important ideas to the development of cryptology.

• The concepts of substitution and transposition remain crucial in the field.
• The emergence of polyalphabetic substitution was a major improvement.

<aside> 💡 However, as we have studied, all of the schemes have security vulnerabilities, especially with the advent of high-computing power and reduced cost of memory.

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We will present two concepts that cryptologists developed to improve the security of cryptographic systems, namely:

1- Repetition

2- Composition

Repetition

Main idea:

Instead of running the plaintext through a single cycle of encryption, apply the encryption scheme multiple times, such that the output of the first cycle is the input of the second cycle, and so forth.

<aside> 💡 Cryptologists were hopeful that this would make breaking the cipher more difficult.

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Analysis:

Studying various encryption schemes, we find that applying repetition may:

1- Increase the security of the system, or

2- Make no change to the security of the system, or

3- Decrease the security of the system

Improved Security:

An example of schemes that are improved by repetition is Block Rotate Cipher.

<aside> 💡 If we encrypt using the key (b1,r1) and then by the key (b2,r2), provided that b1 ≠ b2, this makes brute-force analysis more difficult.

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