SPRUJB6B November 2024 – May 2025 AM2612
The AES feedback mode block buffers the feedback parameters and contains all logic to implement the various feedback modes. Refer to [NIST-SP800-38A, Chapter 6] for details on the ECB, CBC, CTR, and CFB modes of operation.
The CTR (counter) mode of operation implements the "Standard Incrementing Function" as described in [NIST-SP800-38A, Appendix B] with m set to 16 or a multiple of 32:
| NIST-SP800-38A, Appendix B.1 | AES Engine |
|---|---|
| [x]m → [x+1 mod 2m]m | [x]32·n → [x+1 mod 232·n]32·n, with n Ɛ {½, 1, 2, 3, 4} |
The AES-XTS mode requires a polynomial multiplication for IV generation of the AES operation. This multiplication can be simplified once the first result is available due to the definition and usage of the block number within a unit. The input for the polynomial multiplication is not directly ‘j’, but αj, where α equals x2 in the GF(2^128) domain.
In addition the f8 encryption/decryption mode and f9 and (X)CBC-MAC authentication modes are available.