Blowfish Cipher Decryption Algorithm

Blowfish Cipher Tutorials - Herong's Tutorial Examples

∟Blowfish Cipher Decryption Algorithm

The Blowfish cipher decryption algorithm is presented. It is identical to the encryption algorithm, except that Sub-keys, P1, P2, ..., P18, are applied in reverse order.

The decryption algorithm of a block cipher is usually identical to encryption algorithm step by step in reverse order. But for the Blowfish cipher, the encryption algorithm is so well designed, that the decryption algorithm is identical to the encryption algorithm step by step in the same order, only with the sub-keys applied in the reverse order.

Here is the Blowfish decryption algorithm with the 16-round loop expanded inline:

Input: 
   CC: 64 bits of ciphertext
   P1, P2, ..., P18: 18 sub-keys
   F(): Round function
   
Output: 
   TT: 64 bits of plaintext

Algorithm - Blowfish Decryption:
   (LL, RR) = CC, dividing CC into two 32-bit parts
   LL = LL XOR P18
   RR = RR XOR F(LL) XOR P17
   LL = LL XOR F(RR) XOR P16
   RR = RR XOR F(LL) XOR P15
   LL = LL XOR F(RR) XOR P14
   RR = RR XOR F(LL) XOR P13
   LL = LL XOR F(RR) XOR P12
   RR = RR XOR F(LL) XOR P11
   LL = LL XOR F(RR) XOR P10
   RR = RR XOR F(LL) XOR P9
   LL = LL XOR F(RR) XOR P8
   RR = RR XOR F(LL) XOR P7
   LL = LL XOR F(RR) XOR P6
   RR = RR XOR F(LL) XOR P5
   LL = LL XOR F(RR) XOR P4
   RR = RR XOR F(LL) XOR P3
   LL = LL XOR F(RR) XOR P2
   RR = RR XOR P1
   TT = (RR, LL), combine RR and LL into 64-bit block

As you can see, the decryption algorithm is indeed identical to the encryption algorithm, except Sub-keys, P1, P2, ..., P18, are applied in reverse order.