The XAES-256-GCM extended-nonce AEAD

About a 365 days ago I wrote that “I wish to reveal XAES-256-GCM/11, which has a range of good properties and only the anxious defect of not present.” Neatly, there could be now an XAES-256-GCM specification. (Needed to quit on the /11 share, but that used to be gleaming a performance optimization.)

XAES-256-GCM is an authenticated encryption with additional records (AEAD) algorithm with 256-bit keys and 192-bit nonces. It used to be designed with the next goals:

  1. supporting a nonce sizable ample to be staunch to generate randomly for a virtually unlimited selection of messages (2⁸⁰ messages with collision possibility 2⁻³²);
  2. fleshy, easy FIPS 140 compliance; and
  3. trivial implementation on high of in type cryptographic libraries.

The sizable nonce enables safer and more pleasant APIs that robotically read a contemporary nonce from the operating system’s CSPRNG for every message, with out burdening the user with any birthday certain calculations. Compliance and compatibility kind it on hand anywhere an AEAD could very successfully be wished, at the side of in settings the set different sizable-nonce AEADs will not be an option.

Love XChaCha20Poly1305, XAES-256-GCM is a long-nonce construction on high of AES-256-GCM. That is, it makes disclose of the most famous and the sizable nonce to compute a derived key for the underlying AEAD.

It’s easy ample to suit inline in this newsletter. Here we lunge. K and N are the input key and nonce, Kₓ and Nₓ are the derived AES-256-GCM key and nonce.

  1. L = AES-256ₖ(0¹²⁸)
  2. If MSB₁(L) = 0, then K1 = L << 1;

    Else K1 = (L << 1) ⊕ 0¹²⁰10000111
  3. M1 = 0x00 || 0x01 || X || 0x00 || N[: 12]
  4. M2 = 0x00 || 0x02 || X || 0x00 || N[: 12]
  5. Kₓ = AES-256ₖ(M1K1) || AES-256ₖ(M2K1)
  6. Nₓ = N[12:]

As it is possible you’ll maybe survey, it costs three AES-256ₖ calls per message, even supposing one could even be precomputed for a given key, and the opposite two can reuse its key agenda.

The Drag reference implementation suits in decrease than 100 lines of mostly boilerplate, at the side of the precomputation optimization, and only makes disclose of the long-established library’s crypto/cipher and crypto/aes.

Importantly, it is possible you’ll maybe maybe also describe XAES-256-GCM fully as regards to a ancient NIST SP 800-108r1 KDF and the long-established NIST AES-256-GCM AEAD (NIST SP 800-38D, FIPS 197).

Instantiate a counter-based mostly KDF (NIST SP 800-108r1, Share 4.1) with CMAC-AES256 (NIST SP 800-38B) and the input key as Kinfolk, the ASCII letter X (0x58) as Note, the first 96 bits of the input nonce as Context (as advised by NIST SP 800-108r1, Share 4, point 4), a counter (i) dimension of 16 bits, and omitting the not well-known L arena, and kind a 256-bit derived key. Utilize that derived key and the final 96 bits of the input nonce with AES-256-GCM.

Thanks to the preference of parameters, if we peel off the KDF and CMAC abstractions, the consequence is barely slower and more advanced than straightforwardly invoking AES-256 on a counter. In change, we ranking a vetted and compliant solution. The parameters are supported by the high-level OpenSSL API, too.

Why no more “/11”? Neatly, half the purpose of the disclose of AES-GCM is FIPS 140 compliance. (The opposite half being hardware acceleration.) If we mucked with the rounds amount the fabricate wouldn’t be compliant.

Indeed, if compliance just isn’t a purpose there are a amount of picks, from AES-GCM-SIV to up-to-the-minute AEAD constructions based totally on the AES core. The specification has an intensive Alternate alternatives share that compares every of them to XAES-256-GCM.

Furthermore incorporated in the specification are test vectors for the two most famous code paths (MSB₁(L) = 0 and 1), and gathered test vectors that compress 10 000 or 1 000 000 random iterations.

To sum up, XAES-256-GCM is designed to be a staunch, listless, compliant, and interoperable AEAD that can match high-level APIs, the kind we’d savor to be able to add to Drag. It’s designed to complement XChaCha20Poly1305 and AES-GCM-SIV as implementations of a hypothetical nonce-much less AEAD API. If other cryptography library maintainers savor it (or don’t), I would savor to hear about it, attributable to we will not be enormous fans of adding Drag-particular constructions to the long-established library.

By the system, I enjoy an exhilarating replace about my reliable starting up source maintainer effort coming in decrease than two weeks! Be obvious to subscribe to Maintainer Dispatches or to put collectively me on Bluesky at @filippo.abyssdomain.knowledgeable or on Mastodon at @filippo@abyssdomain.knowledgeable. (Or, survey you at GopherCon in Chicago!)

The image

Earlier this 365 days I ran in the Centopassi bike competition. It involves driving bigger than 1600km on mountain roads, via one hundred GPS coordinates you capture upfront from a prolonged listing, in three days and a half. It’s been amazing. It took me to corners of Italy I would enjoy never seen, and I had relatively loads of relaxing. This image is taken at our 100th assert, after a couple kilometers of unpaved hairpins on the aspect of the hill. The attain line used to be on the lake it is possible you’ll maybe survey in the distance. I was contented.

That’s my 2014 KTM Duke 690, a single-cylinder “bare” from earlier than KTM knew how to kind greater aspect road bikes. It’s weird and I indubitably savor it.

A black motorcycle with saddlebags and a race plate, parked on a dirt road overlooking a vast, scenic valley with green hills, a lake in the distance, and mountains under a bright blue sky with scattered white clouds.

My superior customers—Sigsum, Latacora, Interchain, Smallstep, Ava Labs, Teleport, SandboxAQ, Charm, and Tailscale—are funding all my work for the community and via our retainer contracts they ranking face time and unlimited ranking entry to to recommendation on Drag and cryptography.

Here are about a phrases from some of them!

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