Live Journal about blowfish |

Blowfish is a block cipher that encrypts data in 8-byte blocks. The algorithm consists of two parts: a key-expansion part and a data-encryption part. Key expansion converts a variable-length key of at most 56 bytes (448 bits) into several subkey arrays totaling 4168 bytes. (Note: the description in this article differs slightly from the one in the April 1994 issue of Dr. Dobb’s Journal; there were typos in steps (5) and (6) of the subkey generation algorithm.)

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 Pi
xR = F(xL) XOR xR
Swap xL and xR

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.

Generating the Subkeys:

The subkeys are calculated using the Blowfish algorithm:

1. Initialize first the P-array and then the four S-boxes, in order, with a fixed string. This string consists of the hexadecimal digits of pi (less the initial 3): P1 = 0x243f6a88, P2 = 0x85a308d3, P3 = 0x13198a2e, P4 = 0×03707344, etc.

2. XOR P1 with the first 32 bits of the key, XOR P2 with the second 32-bits of the key, and so on for all bits of the key (possibly up to P14). Repeatedly cycle through the key bits until the entire P-array has been XORed with key bits. (For every short key, there is at least one equivalent longer key; for example, if A is a 64-bit key, then AA, AAA, etc., are equivalent keys.)

3. Encrypt the all-zero string with the Blowfish algorithm, using the subkeys described in steps (1) and (2).

4. Replace P1 and P2 with the output of step (3).

5. Encrypt the output of step (3) using the Blowfish algorithm with the modified subkeys.

6. Replace P3 and P4 with the output of step (5).

7. Continue the process, replacing all entries of the P array, and then all four S-boxes in order, with the output of the continuously changing Blowfish algorithm.

In total, 521 iterations are required to generate all required subkeys. Applications can store the subkeys rather than execute this derivation process multiple times.

C Code:

C code for Blowfish starts on page xx. This is improved and corrected code; the code in the April 1994 issue had some bugs and was less efficient than this code. The code is also available electronically; see “Availability,” page xx.