A chosen-ciphertext attack (CCA) on the Blowfish Cipher involves an attacker choosing a ciphertext and decrypting it to reveal its plaintext. The attacker then analyzes this information to find patterns or weaknesses in the cipher’s algorithm, potentially revealing the secret key.
Blowfish is vulnerable to such attacks due to its structure. It uses 16 rounds of substitution and permutation operations, which are controlled by the key. If an attacker can predict the output of these operations for certain inputs, they can reverse-engineer the key.
This vulnerability arises from Blowfish’s use of static S-boxes, which do not change during encryption or decryption. Therefore, once an attacker has determined the values in these boxes, they can effectively break the cipher.
For instance, if an attacker chooses a ciphertext that results in a specific output after the first round of decryption, they can infer the value of one part of the key. Repeating this process with different ciphertexts allows them to gradually piece together the entire key.
