crypto: reasonably complete the keypair absraction layer, and use it

utilities.go

@@ -2,6 +2,8 @@ package did
 
 import (
 	"fmt"
+
+	"github.com/INFURA/go-did/crypto"
 )
 
 // TryAllVerify tries to verify the signature with all the methods in the slice.
@@ -19,10 +21,10 @@ func TryAllVerify(methods []VerificationMethodSignature, data []byte, sig []byte
 // FindMatchingKeyAgreement tries to find a matching key agreement method for the given private key type.
 // It returns the shared key as well as the selected method.
 // If no matching method is found, it returns an error.
-func FindMatchingKeyAgreement(methods []VerificationMethodKeyAgreement, priv PrivateKey) ([]byte, VerificationMethodKeyAgreement, error) {
+func FindMatchingKeyAgreement(methods []VerificationMethodKeyAgreement, priv crypto.KeyExchangePrivateKey) ([]byte, VerificationMethodKeyAgreement, error) {
 	for _, method := range methods {
 		if method.PrivateKeyIsCompatible(priv) {
-			key, err := method.ECDH(priv)
+			key, err := method.KeyExchange(priv)
 			return key, method, err
 		}
 	}