Blowfish Cipher Encryption Algorithm

Blowfish Cipher Tutorials - Herong's Tutorial Examples

∟Blowfish Cipher Encryption Algorithm

The Blowfish cipher algorithm is presented based on the algorithm designer Bruce Schneier's technical paper. Blowfish cipher is 16-round and 64-bit block cipher.

Blowfish algorithm was developed by Bruce Schneier in 1993. Here is the Blowfish encryption algorithm presented in Bruce's paper "Description of a New Variable-Length Key, 64-Bit Block Cipher (Blowfish)" at http://www.schneier.com/paper-blowfish-fse.html:

Input: 
   T: 64 bits of plaintext
   P{1), P(2), ..., P(18): 18 sub-keys, 32 bits each
   F(): Round function
   
Output:
   C: 64 bits of ciphertext

Algorithm - Blowfish Encryption:
   C = Blowfish(T), Blowfish round process defined as:
      (xL, xR) = T, dividing T into two 32-bit parts
      Loop on i from = 1 to 16
         xL = xL XOR P(i)
         xR = F(xL) XOR xR
         Swap xL and xR
      End of loop
      Swap xL and xR
      xR = xR XOR P(17)
      xL = xL XOR P(18)
      C = (xL, xR), combine xL, xR into 64-bit block

The round function F(A) is defined as:

Input: 
   A: 32-bit input data
   S1[], S2[], S3[], S4[]: 4 S-box lookup tables, 256 bits each
   
Output
   B: 32-bit output data

Algorithm
   (a, b, c, d) = A, dividing A into four 8-bit parts
   B = ( (S1[a] + S2[b] mod 2**32) XOR S3[c] ) + S[d] mod 2**32