Add pkcs8 encoding function to secp256r1

Author: nhynes

src/crypto/secp256r1.sol

@@ -6,4 +6,27 @@ library Secp256r1 {
     function isScalar(uint256 scalar) internal pure returns (bool) {
         return scalar > 0 && scalar < ORDER;
     }
+
+    function encodeScalarPkcs8Der(uint256 scalar) internal pure returns (bytes memory) {
+        // PrivateKeyInfo ::= SEQUENCE {
+        //   version                   Version,
+        //   privateKeyAlgorithm       AlgorithmIdentifier,
+        //   privateKey                OCTET STRING,
+        //   attributes           [0]  IMPLICIT Attributes OPTIONAL }
+        //
+        // 30 81 41         ; SEQUENCE (0x41 bytes)
+        //    02 01 00       ; INTEGER (0)
+        //    30 13          ; SEQUENCE (0x13 bytes)
+        //       06 07       ; OBJECT IDENTIFIER (ecdsaWithSHA1)
+        //          2A 86 48 CE 3D 02 01
+        //       06 08       ; OBJECT IDENTIFIER (prime256v1)
+        //          2A 86 48 CE 3D 03 01 07
+        //    04 27          ; OCTET STRING (0x27 bytes)
+        //       30 25       ; SEQUENCE (0x25 bytes)
+        //          02 01 01 ; INTEGER (1)
+        //          04 20    ; OCTET STRING (32 bytes)
+        //             <32 byte private key>
+        return
+            bytes.concat(hex"308141020100301306072a8648ce3d020106082a8648ce3d030107042730250201010420", bytes32(scalar));
+    }
 }