Add explicit constant-time decoder helper

Author: eldryoth

CHANGELOG.md

@@ -76,6 +76,8 @@
   unwrapped `Profile`.
 - Added `ct::CtEngine::decoded_len()` so sensitive decode paths can size
   caller-owned buffers without switching to the diagnostic decoder.
+- Added `Engine::ct_decoder()` for explicit promotion to the matching
+  constant-time-oriented decoder without type annotations.
 
 ## 0.8.0 - 2026-05-16
 

README.md

@@ -248,15 +248,18 @@ assert!(MIME.validate(&encoded[..written]));
 ```
 
 An engine can also be promoted explicitly to an unwrapped profile when a common
-configuration path expects profile values:
+configuration path expects profile values, or to the matching
+constant-time-oriented decoder when sensitive decode policy is required:
 
 ```rust
 use base64_ng::STANDARD;
 
 let profile = STANDARD.profile();
+let ct_decoder = STANDARD.ct_decoder();
 
 assert!(profile.is_padded());
 assert!(!profile.is_wrapped());
+assert_eq!(ct_decoder.decoded_len(b"aGVsbG8=").unwrap(), 5);
 ```
 
 When wrapping policy comes from configuration, prefer checked construction:

portability/no_alloc_smoke/src/lib.rs

@@ -159,6 +159,9 @@ pub fn custom_profile_surfaces() -> bool {
     {
         return false;
     }
+    if STANDARD.ct_decoder().decoded_len(b"aGVsbG8=") != Ok(5) {
+        return false;
+    }
 
     let encoded = match SMOKE_NO_PAD.encode_buffer::<4>(&[0xff, 0xff, 0xff]) {
         Ok(encoded) => encoded,

src/lib.rs

@@ -3928,6 +3928,16 @@ where
         Profile::new(*self, None)
     }
 
+    /// Returns the matching constant-time-oriented decoder for this engine's
+    /// alphabet and padding policy.
+    ///
+    /// The returned decoder is still an explicit opt-in to the [`ct`] module's
+    /// slower, opaque-error, constant-time-oriented scalar path.
+    #[must_use]
+    pub const fn ct_decoder(&self) -> ct::CtEngine<A, PAD> {
+        ct::CtEngine::new()
+    }
+
     /// Returns the encoded length for this engine's padding policy.
     pub const fn encoded_len(&self, input_len: usize) -> Result<usize, EncodeError> {
         encoded_len(input_len, PAD)

tests/rfc4648.rs

@@ -254,6 +254,10 @@ fn named_profiles_expose_expected_policies() {
     assert_eq!(URL_SAFE_NO_PAD.profile().engine(), URL_SAFE_NO_PAD);
     assert!(!URL_SAFE_NO_PAD.profile().is_padded());
     assert!(!URL_SAFE_NO_PAD.profile().is_wrapped());
+    assert_eq!(STANDARD.ct_decoder(), ct::STANDARD);
+    assert_eq!(STANDARD_NO_PAD.ct_decoder(), ct::STANDARD_NO_PAD);
+    assert_eq!(URL_SAFE.ct_decoder(), ct::URL_SAFE);
+    assert_eq!(URL_SAFE_NO_PAD.ct_decoder(), ct::URL_SAFE_NO_PAD);
     assert!(standard_profile.is_padded());
     assert!(!standard_profile.is_wrapped());
     assert!(standard_profile.is_valid());