A cryptographic accordion is a tweakable block cipher mode that is itself a cipher on variable-length input. NIST proposes to develop three general-purpose accordions:
- Acc128 to support typical usage (birthday bounds) with the Advanced Encryption Standard (AES)
- Acc256 to support typical usage with a 256-bit block cipher (possibly Rijndael-256)
- BBBAcc to support extended usage (beyond-birthday-bound) with AES
In particular, NIST proposes to develop variants of the HCTR2 technique for these accordions.
NIST invites public comments through August 6, 2025. Please submit them to ciphermodes@nist.gov with the subject line “Comments on Accordion Development.” Comments received in response to this request will be posted on the publication page for a future NIST Special Publication (SP) 800-197A. Submitters’ names and affiliations (when provided) will be included, though contact information will be removed.