From RFC 8452 Section 3 which defines POLYVAL for use in AES-GCM-SIV:
“POLYVAL, like GHASH (the authenticator in AES-GCM; …), operates in a binary field of size 2^128. The field is defined by the irreducible polynomial x^128 + x^127 + x^126 + x^121 + 1.”
By multiplying (in the finite field sense) a sequence of 128-bit blocks of
input data data by a field element
H, POLYVAL can be used to authenticate
the message sequence as powers (in the finite field sense) of
Rust 1.49 or higher.
In the future the minimum supported Rust version may be changed, but it be will be accompanied with a minor version bump.
This crate provides multiple backends including a portable pure Rust backend as well as ones based on CPU intrinsics.
As a baseline implementation, this crate provides a constant-time pure Rust implementation based on BearSSL, which is a straightforward and compact implementation which uses a clever but simple technique to avoid carry-spilling.
aarch64 targets including
aarch64-apple-darwin (Apple M1) and Linux
targets such as
support for using the
PMULL instructions in ARMv8’s Cryptography Extensions
is available when using the nightly compiler, and can be enabled using the
armv8 crate feature.
On Linux and macOS, when the
armv8 feature is enabled support for AES
intrinsics is autodetected at runtime. On other platforms the
target feature must be enabled via RUSTFLAGS.
By default this crate uses runtime detection on
in order to determine if
CLMUL is available, and if it is not, it will
fallback to using a constant-time software implementation.
For optimal performance, set
$ RUSTFLAGS="-Ctarget-cpu=sandybridge" cargo bench
POLYVAL can be thought of as the little endian equivalent of GHASH, which affords it a small performance advantage over GHASH when used on little endian architectures.
It has also been designed so it can also be used to compute GHASH and with it GMAC, the Message Authentication Code (MAC) used by AES-GCM.
From RFC 8452 Appendix A:
“GHASH and POLYVAL both operate in GF(2^128), although with different irreducible polynomials: POLYVAL works modulo x^128 + x^127 + x^126 + x^121 + 1 and GHASH works modulo x^128 + x^7 + x^2 + x + 1. Note that these irreducible polynomials are the ‘reverse’ of each other.”