digest: replace subtle with ctutils (#2301)

As discussed in #2275 and #2300, we are migrating from `subtle` to our
new `ctutils` library:

https://github.com/RustCrypto/utils/tree/master/ctutils

Author: tarcieri

.github/workflows/elliptic-curve.yml

@@ -102,6 +102,8 @@ jobs:
       MIRIFLAGS: "-Zmiri-symbolic-alignment-check -Zmiri-strict-provenance"
     steps:
       - uses: actions/checkout@v6
-      - uses: dtolnay/rust-toolchain@nightly
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          toolchain: nightly-2026-02-11 # pinned due to rust-lang/miri#4855
       - run: rustup component add miri && cargo miri setup
       - run: cargo miri test --all-features

Cargo.lock

@@ -17,9 +17,9 @@ common = { version = "0.2", package = "crypto-common" }
 
 # optional dependencies
 block-buffer = { version = "0.11", optional = true }
-subtle = { version = "2.4", default-features = false, optional = true }
 blobby = { version = "0.4", optional = true }
 const-oid = { version = "0.10", optional = true }
+ctutils = { version = "0.4", optional = true }
 zeroize = { version = "1.7", optional = true, default-features = false }
 
 [dev-dependencies]
@@ -28,7 +28,7 @@ sha2 = "0.11.0-rc.5"
 [features]
 default = ["block-api"]
 block-api = ["block-buffer"] # Enable block API traits
-mac = ["subtle"] # Enable MAC traits
+mac = ["dep:ctutils"] # Enable MAC traits
 rand_core = ["common/rand_core"] # Enable random key generation methods
 getrandom = ["common/getrandom", "rand_core"]
 oid = ["const-oid"]

digest/Cargo.toml

@@ -3,7 +3,7 @@ use common::{Output, OutputSizeUser, Reset};
 
 use common::typenum::Unsigned;
 use core::fmt;
-use subtle::ConstantTimeEq;
+use ctutils::CtEq;
 
 /// Marker trait for Message Authentication algorithms.
 pub trait MacMarker {}