Final answer:
The algorithm similar to Blowfish that operates on 128-bit blocks is Twofish. Twofish is the successor to Blowfish, designed by the same individual, Bruce Schneier, and was a finalist in the AES competition.
Step-by-step explanation:
The algorithm that is similar to Blowfish but operates on 128-bit blocks is Twofish. Twofish was actually designed by Bruce Schneier, who also designed Blowfish, as a successor to the earlier algorithm. It was one of the finalists in the competition to select the Advanced Encryption Standard (AES), but in the end, AES was selected as the new standard for encrypting electronic data.
Twofish is known for its flexibility; it can be implemented with key sizes that are multiples of 64 bits, anywhere from 128 to 256 bits. While AES also operates on 128-bit blocks and is widely adopted around the world for robust security, it is not derived from Blowfish but rather from the algorithm known as Rijndael.
In contrast, IDEA and CCITT refer to different cryptographic methods and standards that are not directly related to Blowfish in the same way that Twofish is. Therefore, the correct answer to which algorithm is similar to Blowfish but works on 128-bit blocks is Twofish.