emit: introduce CheckSig function

Which allows to create verification scripts without keys.PublicKey which is
convenient in some cases where we already have serialized key and don't want
to waste time decompressing it.

pkg/crypto/keys/publickey.go

@@ -12,7 +12,6 @@ import (
 
 	"github.com/btcsuite/btcd/btcec"
 	lru "github.com/hashicorp/golang-lru"
-	"github.com/nspcc-dev/neo-go/pkg/core/interop/interopnames"
 	"github.com/nspcc-dev/neo-go/pkg/crypto/hash"
 	"github.com/nspcc-dev/neo-go/pkg/encoding/address"
 	"github.com/nspcc-dev/neo-go/pkg/io"
@@ -319,8 +318,7 @@ func (p *PublicKey) GetVerificationScript() []byte {
 		buf.WriteB(0xAC) // CHECKSIG
 		return buf.Bytes()
 	}
-	emit.Bytes(buf.BinWriter, b)
-	emit.Syscall(buf.BinWriter, interopnames.SystemCryptoCheckSig)
+	emit.CheckSig(buf.BinWriter, b)
 
 	return buf.Bytes()
 }

pkg/vm/emit/emit.go

@@ -219,6 +219,14 @@ func AppCall(w *io.BinWriter, scriptHash util.Uint160, operation string, f callf
 	AppCallNoArgs(w, scriptHash, operation, f)
 }
 
+// CheckSig emits a single-key verification script using given []bytes as a key.
+// It does not check for key correctness, so you can get an invalid script if the
+// data passed is not really a public key.
+func CheckSig(w *io.BinWriter, key []byte) {
+	Bytes(w, key)
+	Syscall(w, interopnames.SystemCryptoCheckSig)
+}
+
 func isInstructionJmp(op opcode.Opcode) bool {
 	return opcode.JMP <= op && op <= opcode.CALLL || op == opcode.ENDTRYL
 }