rpc, evm: secure tx signing (#20)

* rpc, evm: secure signing

* evm, ante: test signer

* tests

tests/signer.go

@@ -0,0 +1,52 @@
+package tests
+
+import (
+	"fmt"
+
+	"github.com/cosmos/cosmos-sdk/crypto/keyring"
+	cryptotypes "github.com/cosmos/cosmos-sdk/crypto/types"
+	sdk "github.com/cosmos/cosmos-sdk/types"
+
+	"github.com/cosmos/ethermint/crypto/ethsecp256k1"
+)
+
+var _ keyring.Signer = &Signer{}
+
+// Signer defines a type that is used on testing for signing MsgEthereumTx
+type Signer struct {
+	privKey cryptotypes.PrivKey
+}
+
+func NewSigner(sk cryptotypes.PrivKey) keyring.Signer {
+	return &Signer{
+		privKey: sk,
+	}
+}
+
+// Sign signs the message using the underlying private key
+func (s Signer) Sign(_ string, msg []byte) ([]byte, cryptotypes.PubKey, error) {
+	if s.privKey.Type() != ethsecp256k1.KeyType {
+		return nil, nil, fmt.Errorf(
+			"invalid private key type for signing ethereum tx; expected %s, got %s",
+			ethsecp256k1.KeyType,
+			s.privKey.Type(),
+		)
+	}
+
+	sig, err := s.privKey.Sign(msg)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return sig, s.privKey.PubKey(), nil
+}
+
+// SignByAddress sign byte messages with a user key providing the address.
+func (s Signer) SignByAddress(address sdk.Address, msg []byte) ([]byte, cryptotypes.PubKey, error) {
+	signer := sdk.AccAddress(s.privKey.PubKey().Address())
+	if !signer.Equals(address) {
+		return nil, nil, fmt.Errorf("address mismatch: signer %s ≠ given address %s", signer, address)
+	}
+
+	return s.Sign("", msg)
+}