Add Sec1 EC key support

Signed-off-by: Vladimir Pouzanov <farcaller@gmail.com>

src/ec/suite_b.rs

@@ -160,8 +160,17 @@ pub(crate) fn key_pair_from_pkcs8(
     cpu_features: cpu::Features,
 ) -> Result<ec::KeyPair, error::KeyRejected> {
     let (ec_private_key, _) = pkcs8::unwrap_key(template, pkcs8::Version::V1Only, input)?;
+    key_pair_from_der(curve, template, ec_private_key, cpu_features)
+}
+
+pub(crate) fn key_pair_from_der(
+    curve: &'static ec::Curve,
+    template: &pkcs8::Template,
+    input: untrusted::Input,
+    cpu_features: cpu::Features,
+) -> Result<ec::KeyPair, error::KeyRejected> {
     let (private_key, public_key) =
-        ec_private_key.read_all(error::KeyRejected::invalid_encoding(), |input| {
+        input.read_all(error::KeyRejected::invalid_encoding(), |input| {
             // https://tools.ietf.org/html/rfc5915#section-3
             der::nested(
                 input,

src/ec/suite_b/ecdsa/signing.rs

@@ -116,6 +116,30 @@ impl EcdsaKeyPair {
         Self::new(alg, key_pair, &rng)
     }
 
+    /// Constructs an ECDSA key pair by parsing an unencrypted Sec1 v1
+    /// id-ecPublicKey `ECPrivateKey` key.
+    ///
+    /// The input must be in Sec1 v1 format. It must contain the public key in
+    /// the `ECPrivateKey` structure; `from_der()` will verify that the public
+    /// key and the private key are consistent with each other. The algorithm
+    /// identifier must identify the curve by name; it must not use an
+    /// "explicit" encoding of the curve. The `parameters` field of the
+    /// `ECPrivateKey`, if present, must be the same named curve that is in the
+    /// algorithm identifier in the PKCS#8 header.
+    pub fn from_der(
+        alg: &'static EcdsaSigningAlgorithm,
+        der: &[u8],
+    ) -> Result<Self, error::KeyRejected> {
+        let key_pair = ec::suite_b::key_pair_from_der(
+            alg.curve,
+            alg.pkcs8_template,
+            untrusted::Input::from(der),
+            cpu::features(),
+        )?;
+        let rng = rand::SystemRandom::new(); // TODO: make this a parameter.
+        Self::new(alg, key_pair, &rng)
+    }
+
     /// Constructs an ECDSA key pair from the private key and public key bytes
     ///
     /// The private key must encoded as a big-endian fixed-length integer. For