Skip to main content

NOTICE: Due to a lapse in annual appropriations, most of this website is not being updated. Learn more.

Form submissions will still be accepted but will not receive responses at this time. Sections of this site for programs using non-appropriated funds (such as NVLAP) or those that are excepted from the shutdown (such as CHIPS and NVD) will continue to be updated.

U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

NIST Launches Development of Cryptographic Accordions

NIST proposes to develop three general-purpose cryptographic accordions that are variants of the HCTR2 technique. These will be specified in future SP 800-197x publications. Submit comments on this proposal through August 6, 2025.

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 [at] nist.gov (subject: Comments%20on%20Accordion%20Development) (ciphermodes[at]nist[dot]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.

Released June 6, 2025
Was this page helpful?