ssh-encoding: adds a DigestWriter (#268)

This allows to write to the digest directly from the serializer.

ssh-encoding/Cargo.toml

@@ -20,6 +20,7 @@ base64ct = { version = "1.4", optional = true }
 bytes = { version = "1", optional = true, default-features = false }
 pem-rfc7468 = { version = "1.0.0-rc.1", optional = true }
 sha2 = { version = "=0.11.0-pre.4", optional = true, default-features = false }
+digest = { version = "=0.11.0-pre.9", default-features = false }
 
 [dev-dependencies]
 hex-literal = "0.4.1"

ssh-encoding/src/lib.rs

@@ -48,7 +48,7 @@ pub use crate::{
     error::{Error, Result},
     label::{Label, LabelError},
     reader::Reader,
-    writer::Writer,
+    writer::{DigestWriter, Writer},
 };
 
 #[cfg(feature = "base64")]

ssh-encoding/src/writer.rs

@@ -8,8 +8,7 @@ use alloc::vec::Vec;
 #[cfg(feature = "bytes")]
 use bytes::{BufMut, BytesMut};
 
-#[cfg(feature = "sha2")]
-use sha2::{Digest, Sha256, Sha512};
+use digest::Digest;
 
 /// Writer trait which encodes the SSH binary format to various output
 /// encodings.
@@ -34,18 +33,18 @@ impl Writer for BytesMut {
     }
 }
 
-#[cfg(feature = "sha2")]
-impl Writer for Sha256 {
-    fn write(&mut self, bytes: &[u8]) -> Result<()> {
-        self.update(bytes);
-        Ok(())
-    }
-}
+/// Wrapper for digests.
+///
+/// This allows to update digests from the serializer directly.
+#[derive(Debug)]
+pub struct DigestWriter<'d, D>(pub &'d mut D);
 
-#[cfg(feature = "sha2")]
-impl Writer for Sha512 {
+impl<D> Writer for DigestWriter<'_, D>
+where
+    D: Digest,
+{
     fn write(&mut self, bytes: &[u8]) -> Result<()> {
-        self.update(bytes);
+        self.0.update(bytes);
         Ok(())
     }
 }

ssh-key/src/fingerprint.rs

@@ -10,7 +10,7 @@ use core::{
 };
 use encoding::{
     base64::{Base64Unpadded, Encoding},
-    Encode,
+    DigestWriter, Encode,
 };
 use sha2::{Digest, Sha256, Sha512};
 
@@ -63,12 +63,16 @@ impl Fingerprint {
         match algorithm {
             HashAlg::Sha256 => {
                 let mut digest = Sha256::new();
-                public_key.encode(&mut digest).expect(FINGERPRINT_ERR_MSG);
+                public_key
+                    .encode(&mut DigestWriter(&mut digest))
+                    .expect(FINGERPRINT_ERR_MSG);
                 Self::Sha256(digest.finalize().into())
             }
             HashAlg::Sha512 => {
                 let mut digest = Sha512::new();
-                public_key.encode(&mut digest).expect(FINGERPRINT_ERR_MSG);
+                public_key
+                    .encode(&mut DigestWriter(&mut digest))
+                    .expect(FINGERPRINT_ERR_MSG);
                 Self::Sha512(digest.finalize().into())
             }
         }