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Blowfish Encryption Algorithm

The Blowfish encryption algorithm, developed by Bruce Schneier, is a fast and effective block cipher in the public domain. This document outlines its design choices, requirements, and operational principles. It highlights historical weaknesses in other algorithms like DES, emphasizing Blowfish's large keyspace and scalability (32 to 448 bits). Through cryptanalysis, we explore its resilience against attacks. Future concerns include potential simplifications and adaptations such as Twofish and AES. This guide serves as a comprehensive introduction to Blowfish’s structure and relevance in modern cryptography.

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Blowfish Encryption Algorithm

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  1. Blowfish Encryption Algorithm Joshua Job Gordon Standart

  2. Introduction • Reason • Design Requirements and Decisions • Design Elements • Implementation • Cryptanalysis • Future Concerns

  3. Reason / Justification • DES Weaknesses • S-boxes • Too small • Not sufficiently random • Key management / complexity • Other Issues • Designed as general-purpose algorithm • In the public domain • C/C++, Java, C#, Visual Basic, Perl, Javascript • One of the fastest block ciphers in widespread use • Relatively large memory footprint. Generally not used for: • Small embedded systems • Early smartcards

  4. Design Requirements • Fast • Compact • Simple to code • Flat keyspace • Allow any random string (of required length) to be a possible key • Easily modifiable for different security levels

  5. Design Decisions • Manipulate data in large blocks • All operations use byte-sized blocks • Operations use 32-bit blocks where possible • Scalable Key (32 to 448 bits) • Simple operation that are efficient on microprocessors • XOR, Addition, Table lookup, etc • Employ Precomputable Subkeys • Variable number of iterations

  6. Design Decisions • If possible, have no weak keys • If not possible: • Unlikely to choose a weak key • Make weak keys explicitly known • No linear structures that reduce the complexity of exhaustive search • Use a design that is simple to understand • Facilitate analysis • Increase confidence in the algorithm • Feistel iterated block cipher

  7. Design Elements • 64-bit block cipher with variable length key • Large key-dependent S-boxes • More resistant to cryptanalysis • Key-dependent permutations • Diverse Mathematical Operations • Combine XOR and addition

  8. Implementation: Encryption Arrays: P – Number of rounds + 2 elements 4 S-boxes – 256 elements Wikipedia, http://en.wikipedia.org/wiki/Image:BlowfishDiagram.png

  9. Implementation: Function F(x) Addition is mod 232 Wikipedia, http://upload.wikimedia.org/wikipedia/en/8/81/BlowfishFFunction.png

  10. Implementation: Subkey and S-Box Generation • Fill arrays with hexadecimal digits of pi • Xor P array with the key • Repeat • Encrypt all zero string • Replace two elements of p array or subkeys

  11. Cryptanalysis • Differential Attack • After 4 rounds a differential attack is no better than a brute force attack • Weak Keys • S-box collisions

  12. Future Concerns • Simplifications • Fewer and Smaller S-boxes • Fewer Iterations • On-the-fly subkey calculation • Twofish • AES Finalist • 128-bit Block Size • More Operations

  13. Summary • Reason / Justification • Design Elements • Implementation • Cryptanalysis • Future Concerns

  14. References • Wikipedia (for illustrations) • http://en.wikipedia.org/wiki/Blowfish_cipher • Applied Cryptography • Bruce Schneier • John Wiley and Sons, Inc. 1996 • The Blowfish Paper • http://www.schneier.com/paper-blowfish-fse.html

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