[CFRG] Random-access authenticated encryption (raAE) draft

[Nick Sullivan <[e-mail]>]

Hi all,

Writing as an author, not a chair. I've been working on a draft I want to
share with the group. This is not a call for adoption, just sharing the
early draft in case there's interest. Comments and reviews welcome.

*The **problem*. Applications dealing with large encrypted objects
(encrypted backups, encrypted archives, object stores) need to read any
part of the object without decrypting the whole thing. They also need to
verify that each part read belongs to the current object at its claimed
position, without processing the entire object. Mutable objects, like
encrypted file formats, encrypted disk images, and object stores that
accept partial updates, also need in-place modification of any part without
re-encrypting the whole. The positional and inclusion proofs should also
remain valid after each modification. Take an encrypted database file or an
encrypted disk image: any block is read or written by index, and a reader
wants to know that the block it reads really belongs at the offset it came
from in the current image, without re-hashing every block on every write.
No proven security notion in the literature covers all of this under one
primitive. As such, each application ends up rolling its own segmented AE
layer, mostly without security proofs for the rewrite and whole-object
cases.

Prior work covers pieces of the problem. CHAIN (Hoang, Reyhanitabar,
Rogaway, Vizár 2015) is sequential. STREAM (same paper) supports
random-access decryption of individual segments and encodes a final-segment
bit for truncation detection, but has no whole-object snapshot and no
in-place rewrite. Its deployed relatives (Tink Streaming AEAD, OpenPGP v2
SEIPD) are write-once streaming formats and don't expose rewrite at all.
The v2 SEIPD design is itself the outcome of Efail (Poddebniak et al.,
USENIX 2018), where v1's whole-message integrity failed and applications
acted on unauthenticated plaintext. FLOE (Fábrega, Len, Ristenpart, Rubin;
ePrint 2025/2275) formalizes random-access AEAD security cleanly and gives
a proven construction. This work builds on that formal foundation.

I've posted draft-sullivan-cfrg-raae-02, "Random-Access Authenticated
Encryption." It covers the primitive, the security notions, the prior
constructions in the space, and a concrete family of instantiations with
test vectors called SEAL (Segmented Encryption and Authentication Layer),
as well as a security analysis (companion paper forthcoming) and
write/rewrite budgets.

The draft has two layers. raAE is the abstract primitive: the base
interface and security notions come from Fábrega et al., which this
document extends. SEAL is one concrete construction of raAE, parameterized
by an AEAD, a KDF, and an epoch length. It has two profiles (immutable and
mutable) and an optional snapshot authenticator that binds the whole
segment set and updates cheaply on rewrite. A protocol that would
previously have signed the whole encrypted object can sign the snapshot
authenticator instead and get the same authentication scope without
touching the segments. The snapshot authenticator is also not a single
construction, different authenticators fit different consuming-protocol
contexts. For example, settings that supply only a single CEKs and owner
per object versus settings with a shared CEK (as in MLS) necessitate
different authenticators.

Freshness against whole-object rollback, storage transactions, and key
management are explicitly out of scope. Those belong in a consuming
protocol.

The shortest path to a concrete, testable thing is to read
SEAL-simple(AES-256-GCM, HKDF-SHA-256) in Appendix E
<https://www.ietf.org/archive/id/draft-sullivan-cfrg-raae-02.html#name-seal-simple-implementation->,
which is the degenerate case for SEAL with no rewrites or snapshot
authentication and fixed legacy AEAD/KDF.

Reviews and comments on the framing, the security notions, the
construction, or the scope are all useful.

Draft: https://www.ietf.org/archive/id/draft-sullivan-cfrg-raae-02.html

Best,
Nick