dcratomicswap: Add client cert auth

dcratomicswap uses dcrwallet's gRPC server, which will require clients
to authenticate themselves using TLS client certificates in the 1.6.0
release.

README.md

@@ -123,8 +123,8 @@ the swap.
 
 Different tools may require different flags to use them with the supported
 wallet.  For example, `btcatomicswap` includes flags for the RPC username and
-password while `dcratomicswap` does not.  Running a tool without any parameters
-will show the full usage help.
+password while `dcratomicswap` requires flags for TLS client authentication.
+Running a tool without any parameters will show the full usage help.
 
 All of the tools support the same six commands.  These commands are:
 
@@ -295,7 +295,7 @@ expires, but should be saved in case a refund is necessary.
 
 _Party B runs:_
 ```
-$ dcratomicswap --testnet participate TsfWDVTAcsLaHUhHnLLKkGnZuJz2vkmM6Vr 1.0 29c36b8dd380e0426bdc1d834e74a630bfd5d111
+$ dcratomicswap --testnet --clientcert=client.cert --clientkey=client.key participate TsfWDVTAcsLaHUhHnLLKkGnZuJz2vkmM6Vr 1.0 29c36b8dd380e0426bdc1d834e74a630bfd5d111
 Passphrase:
 
 Contract fee: 0.000251 DCR (0.00100400 DCR/kB)
@@ -346,7 +346,7 @@ reveals the secret to B, allowing B to withdraw from the Bitcoin contract.
 
 _Party A runs:_
 ```
-$ dcratomicswap --testnet redeem 63a61429c36b8dd380e0426bdc1d834e74a630bfd5d1118876a9149ee19833332a04d2be97b5c99c970191221c070c6704e6dabb59b17576a914b0ec0640c89cf803b8fdbd6e0183c354f71748c46888ac 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 3e0b064c97247732a3b345ce7b2a835d928623cb2871c26db4c2539a38e61a16
+$ dcratomicswap --testnet --clientcert=client.cert --clientkey=client.key redeem 63a61429c36b8dd380e0426bdc1d834e74a630bfd5d1118876a9149ee19833332a04d2be97b5c99c970191221c070c6704e6dabb59b17576a914b0ec0640c89cf803b8fdbd6e0183c354f71748c46888ac 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 3e0b064c97247732a3b345ce7b2a835d928623cb2871c26db4c2539a38e61a16
 Passphrase:
 
 Redeem fee: 0.000334 DCR (0.00100300 DCR/kB)

cmd/dcratomicswap/go.mod

@@ -1,5 +1,7 @@
 module github.com/decred/atomicswap/cmd/dcratomicswap
 
+go 1.15
+
 require (
 	github.com/decred/dcrd/chaincfg v1.3.0
 	github.com/decred/dcrd/chaincfg/chainhash v1.0.1

cmd/dcratomicswap/main.go

@@ -10,10 +10,13 @@ import (
 	"context"
 	"crypto/rand"
 	"crypto/sha256"
+	"crypto/tls"
+	"crypto/x509"
 	"encoding/hex"
 	"errors"
 	"flag"
 	"fmt"
+	"io/ioutil"
 	"net"
 	"os"
 	"path/filepath"
@@ -52,10 +55,12 @@ var (
 )
 
 var (
-	flagset     = flag.NewFlagSet("", flag.ExitOnError)
-	connectFlag = flagset.String("s", "localhost", "host[:port] of dcrwallet gRPC server")
-	certFlag    = flagset.String("c", filepath.Join(dcrutil.AppDataDir("dcrwallet", false), "rpc.cert"), "dcrwallet RPC certificate path")
-	testnetFlag = flagset.Bool("testnet", false, "use testnet network")
+	flagset        = flag.NewFlagSet("", flag.ExitOnError)
+	connectFlag    = flagset.String("s", "localhost", "host[:port] of dcrwallet gRPC server")
+	certFlag       = flagset.String("c", filepath.Join(dcrutil.AppDataDir("dcrwallet", false), "rpc.cert"), "dcrwallet RPC certificate path")
+	clientCertFlag = flagset.String("clientcert", "", "path to client authentication certificate")
+	clientKeyFlag  = flagset.String("clientkey", "", "path to client authentication key")
+	testnetFlag    = flagset.Bool("testnet", false, "use testnet network")
 )
 
 // There are two directions that the atomic swap can be performed, as the
@@ -346,15 +351,32 @@ func run() (err error, showUsage bool) {
 		return cmd.runOfflineCommand(), false
 	}
 
+	if *clientCertFlag == "" || *clientKeyFlag == "" {
+		return fmt.Errorf("-clientcert and -clientkey flags are required; see -h for usage"), false
+	}
+
 	connect, err := normalizeAddress(*connectFlag, walletPort(chainParams))
 	if err != nil {
 		return fmt.Errorf("wallet server address: %v", err), true
 	}
 
-	creds, err := credentials.NewClientTLSFromFile(*certFlag, "")
+	keypair, err := tls.LoadX509KeyPair(*clientCertFlag, *clientKeyFlag)
+	if err != nil {
+		return fmt.Errorf("open client keypair: %v", err), false
+	}
+	tc := &tls.Config{
+		Certificates: []tls.Certificate{keypair},
+		RootCAs:      x509.NewCertPool(),
+	}
+	serverCAs, err := ioutil.ReadFile(*certFlag)
 	if err != nil {
-		return fmt.Errorf("open certificate: %v", err), false
+		help := *certFlag == ""
+		return fmt.Errorf("cannot open server certificate: %v", err), help
+	}
+	if !tc.RootCAs.AppendCertsFromPEM(serverCAs) {
+		return fmt.Errorf("no certificates found in %q", *certFlag), false
 	}
+	creds := credentials.NewTLS(tc)
 	conn, err := grpc.Dial(connect, grpc.WithTransportCredentials(creds))
 	if err != nil {
 		return fmt.Errorf("grpc dial: %v", err), false