**Blowfish Cipher Tutorials - Herong's Tutorial Examples** - v2.03, by Herong Yang

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.

Table of Contents

Blowfish Cipher Encryption Algorithm

Key Schedule (Sub-Keys and S-Boxes) Algorithm

Efficient Form of the Blowfish Algorithm

►Blowfish Cipher Decryption Algorithm

Proof of Blowfish Cipher Algorithm

First 8336 Fractional Hex Digits of PI

piqpr8.c - Generating Hex Digits of PI

OpenSSL "enc -bf-ecb" for Blowfish/ECB Encryption

OpenSSL "enc -bf-cbc" for Blowfish/CBC Encryption

OpenSSL "enc -bf-cfb" for Blowfish/CFB Encryption

OpenSSL "enc -bf-ofb" for Blowfish/OFB Encryption