Blow Fish Encryption Algorithm

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Blow Fish Encryption Algorithm
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Blow Fish Encryption

• Blowfish is a symmetric block cipher that can be
  used as a drop-in replacement for DES or IDEA. It
  takes a variable-length key, from 32 bits to 448
  bits, making it ideal for both domestic and
  exportable use. Blowfish was designed in 1993 by
  Bruce Schneier as a fast, free alternative to
  existing encryption algorithms. It is also
  unpatented and license free and available free to
  all users.

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Blow Fish in Practice

• Blowfish is a fast block cipher, except when
  changing keys. Each new key requires
  pre-processing equivalent to encrypting about 4
  kilobytes of text, which is very slow compared to
  other block ciphers. This prevents its use in
  certain applications, but is not a problem in
  others. In one application, it is actually a
  benefit the password-hashing method used in
  OpenBSD uses an algorithm derived from Blowfish
  that makes use of the slow key schedule the idea
  is that the extra computational effort required
  gives protection against dictionary attacks.

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How Blow Fish Works

• Blowfish has 16 rounds. Each round consists of a
  key-dependent permutation, and a key- and
  data-dependent substitution. All operations are
  XORs and additions on 32-bit words. The only
  additional operations are four indexed array data
  lookups per round.
• Subkeys
• Blowfish uses a large number of subkeys. These
  keys must be precomputed before any data
  encryption or decryption. The P-array consists of
  18 32-bit subkeys P1, P2,..., P18. There are
  also four 32-bit S-boxes with 256 entries each
  S1,0, S1,1,..., S1,255 S2,0, S2,1,..,, S2,255
  S3,0, S3,1,..., S3,255 S4,0, S4,1,..,, S4,255.
• Encryption and Decryption
• Blowfish has 16 rounds. The input is a 64-bit
  data element, x. Divide x into two 32-bit halves
  xL, xR. Then, for i 1 to 16
• xL xL XOR PixR F(xL) XOR xRSwap xL and xR

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How Blow Fish Works Contd

• After the sixteenth round, swap xL and xR again
  to undo the last swap. Then, xR xR XOR P17 and
  xL xL XOR P18. Finally, recombine xL and xR to
  get the ciphertext.
• Function F looks like this Divide xL into four
  eight-bit quarters a, b, c, and d. Then, F(xL)
  ((S1,a S2,b mod 232) XOR S3,c) S4,d mod 232.
• Decryption is exactly the same as encryption,
  except that P1, P2,..., P18 are used in the
  reverse order.

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Blow Fish Hardware Requirements

• Blowfish has a memory footprint of just over 4
  kilobytes of RAM. This constraint is not a
  problem even for older desktop and laptop
  computers, though it does prevent use in the
  smallest embedded systems such as early
  smartcards.
• Blowfish was one of the first secure block
  ciphers not subject to any patents and therefore
  freely available for anyone to use. This benefit
  has contributed to its popularity in
  cryptographic software.

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References

• Front page photo
• http//actualdownload.com/pictures/icon/encryption
  -shield-57903.jpg
• http//www.schneier.com/blowfish.html
• Wikipedia
• http//en.wikipedia.org/wiki/Blowfish_(cipher)
• How Blow Fish Works
• http//www.schneier.com/paper-blowfish-oneyear.htm
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