Format-Preserving Encryption via Rotating Block Ciphers

The patent applies parameter changes by modifying the encryption process to preserve format parameters. The Feistel cipher is configured with specific parameters (number of rounds, key schedule) that ensure the ciphertext maintains the same length, character set, and format structure as the plaintext, while still providing strong encryption. This resolves the contradiction by changing the encryption parameters to simultaneously achieve security and format compatibility.

The patent introduces an intermediary mapping layer between the plaintext and ciphertext. The FPE scheme uses intermediate representations (such as mapping characters to numerical values, applying encryption, then mapping back) that preserve the format while providing encryption. This intermediary process allows traditional encryption algorithms to be applied without breaking format constraints.

The patent applies preliminary action by pre-configuring the Feistel cipher with the exact domain size matching the target format. Instead of using cycle walking with unpredictable iterations, the system is set up in advance with the correct number of rounds and key schedule parameters to ensure a single pass through the cipher produces the desired format. This eliminates the need for repeated applications and unpredictable iteration counts.

The patent makes the encryption process dynamic by adapting the Feistel cipher structure to the specific format requirements. The number of rounds, key derivation, and substitution boxes are dynamically configured based on the input format characteristics, allowing the system to efficiently handle different formats without requiring cycle walking or repeated encryption passes.