ssh-key: support for undersized ECDSA/P-521 keys (#351)

Fixes #350

Author: zaczkows

ssh-key/src/private/ecdsa.rs

@@ -39,7 +39,8 @@ impl<const SIZE: usize> Decode for EcdsaPrivateKey<SIZE> {
 
     fn decode(reader: &mut impl Reader) -> Result<Self> {
         reader.read_prefixed(|reader| {
-            if reader.remaining_len() == SIZE.checked_add(1).ok_or(encoding::Error::Length)? {
+            let len = reader.remaining_len();
+            if len == SIZE.checked_add(1).ok_or(encoding::Error::Length)? {
                 // Strip leading zero
                 // TODO(tarcieri): make sure leading zero was necessary
                 if u8::decode(reader)? != 0 {
@@ -48,7 +49,17 @@ impl<const SIZE: usize> Decode for EcdsaPrivateKey<SIZE> {
             }
 
             let mut bytes = [0u8; SIZE];
-            reader.read(&mut bytes)?;
+            if SIZE == 66 {
+                // https://stackoverflow.com/questions/50002149/why-p-521-public-key-x-y-some-time-is-65-bytes-some-time-is-66-bytes
+                // although lower keys than 64 are vanishingly possible, but lets stop here
+                if len > 63 {
+                    reader.read(&mut bytes[..core::cmp::min(len, SIZE)])?;
+                } else {
+                    return Err(encoding::Error::Length.into());
+                }
+            } else {
+                reader.read(&mut bytes)?;
+            }
             Ok(Self { bytes })
         })
     }

ssh-key/tests/private_key.rs

@@ -713,3 +713,31 @@ fn encoding_integration_test(private_key: PrivateKey) {
     // Ensure ssh-keygen successfully parsed our public key
     assert_eq!(&public_key, private_key.public_key());
 }
+
+#[cfg(all(feature = "alloc", feature = "p521"))]
+#[test]
+fn paramiko_ecdsa_key() {
+    let key_data = r#"-----BEGIN OPENSSH PRIVATE KEY-----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+-----END OPENSSH PRIVATE KEY-----"#;
+
+    let key = PrivateKey::from_openssh(key_data);
+    assert!(key.is_ok());
+    let key = key.unwrap();
+    assert!(!key.is_encrypted());
+    assert_eq!(
+        key.algorithm(),
+        Algorithm::Ecdsa {
+            curve: ssh_key::EcdsaCurve::NistP521
+        }
+    );
+}