Fix ECDSA ASN.1 PublicKey serialization

Furthermore SecretKey can now be serialized/deserialized to/from a byte
vector.

core/src/identity/ecdsa.rs

@@ -22,9 +22,12 @@
 
 use super::error::DecodingError;
 use core::fmt;
-use p256::ecdsa::{
-    signature::{Signer, Verifier},
-    Signature, SigningKey, VerifyingKey,
+use p256::{
+    ecdsa::{
+        signature::{Signer, Verifier},
+        Signature, SigningKey, VerifyingKey,
+    },
+    EncodedPoint,
 };
 
 /// An ECDSA keypair.
@@ -93,6 +96,18 @@ impl SecretKey {
     pub fn sign(&self, msg: &[u8]) -> Vec<u8> {
         self.0.sign(msg).to_der().as_bytes().to_owned()
     }
+
+    /// Encode a secret key into a byte buffer.
+    pub fn to_bytes(&self) -> Vec<u8> {
+        self.0.to_bytes().to_vec()
+    }
+
+    /// Decode a secret key from a byte buffer.
+    pub fn from_bytes(buf: &[u8]) -> Result<Self, DecodingError> {
+        SigningKey::from_bytes(buf)
+            .map_err(|err| DecodingError::new("failed to parse ecdsa p256 secret key").source(err))
+            .map(SecretKey)
+    }
 }
 
 impl fmt::Debug for SecretKey {
@@ -115,16 +130,85 @@ impl PublicKey {
         self.0.verify(msg, &sig).is_ok()
     }
 
+    /// Decode a public key from a byte buffer without compression.
+    pub fn from_bytes(k: &[u8]) -> Result<PublicKey, DecodingError> {
+        let enc_pt = EncodedPoint::from_bytes(k).map_err(|_| {
+            DecodingError::new("failed to parse ecdsa p256 public key, bad point encoding")
+        })?;
+
+        VerifyingKey::from_encoded_point(&enc_pt)
+            .map_err(|err| DecodingError::new("failed to parse ecdsa p256 public key").source(err))
+            .map(PublicKey)
+    }
+
+    /// Encode a public key into a byte buffer without compression.
+    pub fn to_bytes(&self) -> Vec<u8> {
+        self.0.to_encoded_point(false).as_bytes().to_owned()
+    }
+
     /// Encode a public key into a DER encoded byte buffer as defined by SEC1 standard.
     pub fn encode_der(&self) -> Vec<u8> {
-        self.0.to_encoded_point(false).as_bytes().to_owned()
+        let buf = self.to_bytes();
+        Self::add_asn1_header(&buf)
     }
 
-    // Decode a public key from a DER encoded byte buffer as defined for SEC1 standard.
+    /// Decode a public key into a DER encoded byte buffer as defined by SEC1 standard.
     pub fn decode_der(k: &[u8]) -> Result<PublicKey, DecodingError> {
-        VerifyingKey::from_sec1_bytes(k)
-            .map_err(|_| DecodingError::new("failed to parse ecdsa p256 public key"))
-            .map(PublicKey)
+        let buf = Self::del_asn1_header(k).ok_or(DecodingError::new(
+            "failed to parse asn.1 encoded ecdsa p256 public key",
+        ))?;
+        Self::from_bytes(&buf)
+    }
+
+    // ecPublicKey (ANSI X9.62 public key type) OID: 1.2.840.10045.2.1
+    const EC_PUBLIC_KEY_OID: [u8; 9] = [0x06, 0x07, 0x2a, 0x86, 0x48, 0xce, 0x3d, 0x02, 0x01];
+    // secp256r1 OID: 1.2.840.10045.3.1.7
+    const SECP_256_R1_OID: [u8; 10] = [0x06, 0x08, 0x2A, 0x86, 0x48, 0xCE, 0x3D, 0x03, 0x01, 0x07];
+
+    // Add ASN1 header.
+    fn add_asn1_header(key_buf: &[u8]) -> Vec<u8> {
+        // ASN.1 struct type and length.
+        let mut asn1_buf = vec![
+            0x30,
+            0x00,
+            0x30,
+            (Self::EC_PUBLIC_KEY_OID.len() + Self::SECP_256_R1_OID.len()) as u8,
+        ];
+        // Append OIDs.
+        asn1_buf.extend_from_slice(&Self::EC_PUBLIC_KEY_OID);
+        asn1_buf.extend_from_slice(&Self::SECP_256_R1_OID);
+        // Append key bitstring type and length.
+        asn1_buf.extend_from_slice(&[0x03, (key_buf.len() + 1) as u8, 0x00]);
+        // Append key bitstring value.
+        asn1_buf.extend_from_slice(key_buf);
+        // Update overall length field.
+        asn1_buf[1] = (asn1_buf.len() - 2) as u8;
+
+        asn1_buf
+    }
+
+    // Check and remove ASN.1 header.
+    fn del_asn1_header(asn1_buf: &[u8]) -> Option<&[u8]> {
+        let oids_len = Self::EC_PUBLIC_KEY_OID.len() + Self::SECP_256_R1_OID.len();
+        let asn1_head = asn1_buf.get(..4)?;
+        let oids_buf = asn1_buf.get(4..4 + oids_len)?;
+        let bitstr_head = asn1_buf.get(4 + oids_len..4 + oids_len + 3)?;
+
+        // Sanity check
+        if asn1_head[0] != 0x30
+            || asn1_head[2] != 0x30
+            || asn1_head[3] as usize != oids_len
+            || &oids_buf[..Self::EC_PUBLIC_KEY_OID.len()] != &Self::EC_PUBLIC_KEY_OID
+            || &oids_buf[Self::EC_PUBLIC_KEY_OID.len()..] != &Self::SECP_256_R1_OID
+            || bitstr_head[0] != 0x03
+            || bitstr_head[2] != 0x00
+        {
+            return None;
+        }
+
+        let key_len = bitstr_head[1].checked_sub(1)? as usize;
+        let key_buf = asn1_buf.get(4 + oids_len + 3..4 + oids_len + 3 + key_len as usize)?;
+        Some(key_buf)
     }
 }
 
@@ -143,7 +227,7 @@ mod tests {
     use super::*;
 
     #[test]
-    fn ecdsa_signature() {
+    fn sign_verify() {
         let pair = Keypair::generate();
         let pk = pair.public();