spki: adds a der::Writer adapter for Digest

spki/Cargo.toml

@@ -21,6 +21,7 @@ der = { version = "0.8.0-rc.0", features = ["oid"] }
 # Optional dependencies
 arbitrary = { version = "1.4", features = ["derive"], optional = true }
 base64ct = { version = "1", optional = true, default-features = false }
+digest = { version = "0.11.0-pre.10", optional = true, default-features = false }
 sha2 = { version = "=0.11.0-pre.5", optional = true, default-features = false }
 
 [dev-dependencies]
@@ -33,7 +34,7 @@ std = ["der/std", "alloc"]
 
 arbitrary = ["std", "dep:arbitrary", "der/arbitrary"]
 base64 = ["dep:base64ct"]
-fingerprint = ["sha2"]
+fingerprint = ["digest", "sha2"]
 pem = ["alloc", "der/pem"]
 
 [package.metadata.docs.rs]

spki/src/digest.rs

@@ -0,0 +1,15 @@
+use der::{Result, Writer};
+use digest::Digest;
+
+/// Adapter object to write to a digest backend
+pub struct DigestWriter<'d, D>(pub &'d mut D);
+
+impl<D> Writer for DigestWriter<'_, D>
+where
+    D: Digest,
+{
+    fn write(&mut self, slice: &[u8]) -> Result<()> {
+        self.0.update(slice);
+        Ok(())
+    }
+}

spki/src/lib.rs

@@ -43,8 +43,8 @@ mod error;
 mod spki;
 mod traits;
 
-#[cfg(feature = "fingerprint")]
-mod fingerprint;
+#[cfg(feature = "digest")]
+mod digest;
 
 pub use crate::{
     algorithm::{AlgorithmIdentifier, AlgorithmIdentifierRef, AlgorithmIdentifierWithOid},
@@ -67,5 +67,18 @@ pub use {
     der::Document,
 };
 
+#[cfg(feature = "digest")]
+pub use crate::digest::DigestWriter;
+
+/// Size of a SHA-256 SPKI fingerprint in bytes.
+#[cfg(feature = "fingerprint")]
+pub(crate) const SIZE: usize = 32;
+
+/// Raw bytes of a SPKI fingerprint i.e. SHA-256 digest of
+/// `SubjectPublicKeyInfo`'s DER encoding.
+///
+/// See [RFC7469 § 2.1.1] for more information.
+///
+/// [RFC7469 § 2.1.1]: https://datatracker.ietf.org/doc/html/rfc7469#section-2.1.1
 #[cfg(feature = "fingerprint")]
-pub use crate::fingerprint::FingerprintBytes;
+pub type FingerprintBytes = [u8; SIZE];

spki/src/spki.rs

@@ -15,7 +15,11 @@ use der::{
 };
 
 #[cfg(feature = "fingerprint")]
-use crate::{FingerprintBytes, fingerprint};
+use {
+    crate::{DigestWriter, FingerprintBytes},
+    digest::Digest,
+    sha2::Sha256,
+};
 
 #[cfg(feature = "pem")]
 use der::pem::PemLabel;
@@ -80,9 +84,9 @@ where
     /// [RFC7469 § 2.1.1]: https://datatracker.ietf.org/doc/html/rfc7469#section-2.1.1
     #[cfg(feature = "fingerprint")]
     pub fn fingerprint_bytes(&self) -> Result<FingerprintBytes> {
-        let mut builder = fingerprint::Builder::new();
-        self.encode(&mut builder)?;
-        Ok(builder.finish())
+        let mut hash = Sha256::new();
+        self.encode(&mut DigestWriter(&mut hash))?;
+        Ok(hash.finalize().into())
     }
 }