Load X,Y public key into TPM with correct padding

big.Int.Bytes() will trim any leading zeros. But when passed to the TPM,
the X,Y coordinates of an ECC point have to be properly left-padded to
the ECC int size for that curve.

Ensure the integer is padded properly.

verification/tpm.go

@@ -197,6 +197,11 @@ func loadPubKey(t *testing.T, pubKey any, tpm io.ReadWriteCloser, alg tpm2.Algor
 	// Create a tpm2.Public structure from the parsed ECDSA public key
 	switch pubKey := pubKey.(type) {
 	case *ecdsa.PublicKey:
+		byteSize := pubKey.Params().BitSize / 8
+		x := make([]byte, byteSize)
+		y := make([]byte, byteSize)
+		x = pubKey.X.FillBytes(x)
+		y = pubKey.Y.FillBytes(y)
 		tpmPublic = tpm2.Public{
 			Type:       tpm2.AlgECC, // ECDSA key type
 			NameAlg:    alg,
@@ -208,8 +213,8 @@ func loadPubKey(t *testing.T, pubKey any, tpm io.ReadWriteCloser, alg tpm2.Algor
 				},
 				CurveID: ec,
 				Point: tpm2.ECPoint{
-					XRaw: new(big.Int).SetBytes(pubKey.X.Bytes()).Bytes(),
-					YRaw: new(big.Int).SetBytes(pubKey.Y.Bytes()).Bytes(),
+					XRaw: x,
+					YRaw: y,
 				},
 			},
 		}