txnbuild: SEP 10 challenge builder

txnbuild/transaction.go

@@ -11,8 +11,10 @@ package txnbuild
 
 import (
 	"bytes"
+	"crypto/rand"
 	"encoding/base64"
 	"fmt"
+	"time"
 
 	"github.com/stellar/go/keypair"
 	"github.com/stellar/go/network"
@@ -25,7 +27,8 @@ import (
 type Account interface {
 	GetAccountID() string
 	IncrementSequenceNumber() (xdr.SequenceNumber, error)
-	// To do: implement in v2.0.0: add GetSequenceNumber method
+	// Action needed in release: v2.0.0
+	// TODO: add GetSequenceNumber method
 	// GetSequenceNumber() (xdr.SequenceNumber, error)
 }
 
@@ -177,3 +180,78 @@ func (tx *Transaction) BuildSignEncode(keypairs ...*keypair.Full) (string, error
 
 	return txeBase64, err
 }
+
+// BuildChallengeTx is a factory method that creates a valid SEP 10 challenge, for use in web authentication.
+// "randomNonce" is a base64 encoded 64 byte long random string.
+// "timebound" is the number of seconds the transaction should be valid for, O means infinity.
+// More details on SEP 10: https://github.com/stellar/stellar-protocol/blob/master/ecosystem/sep-0010.md
+func BuildChallengeTx(serverSignerSecret, clientAccountID,
+	anchorName, network string, fee uint32, randomNonce string, timebound int64) (string, error) {
+	serverKP, err := keypair.Parse(serverSignerSecret)
+	if err != nil {
+		return "", err
+	}
+
+	randomNonceBytes, err := base64.StdEncoding.DecodeString(randomNonce)
+	if err != nil {
+		return "", errors.Wrap(err, "failed to decode random nonce")
+	}
+
+	if len(randomNonceBytes) != 64 {
+		return "", errors.New("64 byte long random nonce required")
+	}
+
+	// represent server signing account as SimpleAccount
+	sa := SimpleAccount{
+		AccountID: serverKP.Address(),
+		// Action needed in release: v2.0.0
+		// TODO: remove this and use "Sequence: 0" and build transaction with optional argument
+		//  (https://github.com/stellar/go/issues/1259)
+		Sequence: int64(-1),
+	}
+
+	// represent client account as SimpleAccount
+	ca := SimpleAccount{
+		AccountID: clientAccountID,
+	}
+
+	txTimebound := NewInfiniteTimeout()
+
+	if timebound > 0 {
+		txTimebound = NewTimebounds(time.Now().UTC().Unix(), time.Now().UTC().Unix()+timebound)
+	}
+
+	// Create a SEP 10 compatible response. See
+	// https://github.com/stellar/stellar-protocol/blob/master/ecosystem/sep-0010.md#response
+	tx := Transaction{
+		SourceAccount: &sa,
+		Operations: []Operation{
+			&ManageData{
+				SourceAccount: &ca,
+				Name:          anchorName + " auth",
+				Value:         randomNonceBytes,
+			},
+		},
+		Timebounds: txTimebound,
+		Network:    network,
+		BaseFee:    fee,
+	}
+
+	txeB64, err := tx.BuildSignEncode(serverKP.(*keypair.Full))
+	if err != nil {
+		return "", err
+	}
+	return txeB64, nil
+}
+
+// GenerateRandomString creates a base-64 encoded, cryptographically secure random string of `n` bytes.
+func GenerateRandomString(n int) (string, error) {
+	bytes := make([]byte, n)
+	_, err := rand.Read(bytes)
+
+	if err != nil {
+		return "", err
+	}
+
+	return base64.StdEncoding.EncodeToString(bytes), err
+}

txnbuild/transaction_test.go

@@ -708,3 +708,15 @@ func TestManageBuyOfferUpdateOffer(t *testing.T) {
 	expected := "AAAAACXK8doPx27P6IReQlRRuweSSUiUfjqgyswxiu3Sh2R+AAAAZAAAJWoAAAAKAAAAAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQAAAAAAAAAMAAAAAAAAAAFBQkNEAAAAACXK8doPx27P6IReQlRRuweSSUiUfjqgyswxiu3Sh2R+AAAAAB3NZQAAAAABAAAAMgAAAAAALJSWAAAAAAAAAAHSh2R+AAAAQK/sasTxgNqvkz3dGaDOyUgfa9UAAmUBmgiyaQU1dMlNNvTVH1D7PQKXkTooWmb6qK7Ee8vaTCFU6gGmShhA9wE="
 	assert.Equal(t, expected, received, "Base 64 XDR should match")
 }
+
+func TestBuildChallengeTx(t *testing.T) {
+	kp0 := newKeypair0()
+
+	// use GenerateRandomString(64) to get randomNonce like below
+	randomNonce := "faFEv1xTU4s58f9DVUXkHqKb6Vhj5EPSjPGV5bx0ceHK7/N6ftdlNk2FtqWx+XLVOmh2Q7W/6ZoKmd1uYZW24A=="
+	txeBase64, err := BuildChallengeTx(kp0.Seed(), kp0.Address(), "SDF", network.TestNetworkPassphrase, 500, randomNonce, 0)
+	assert.NoError(t, err)
+
+	expected := "AAAAAODcbeFyXKxmUWK1L6znNbKKIkPkHRJNbLktcKPqLnLFAAAB9AAAAAAAAAAAAAAAAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQAAAAEAAAAA4Nxt4XJcrGZRYrUvrOc1sooiQ+QdEk1suS1wo+oucsUAAAAKAAAACFNERiBhdXRoAAAAAQAAAEB9oUS/XFNTiznx/0NVReQeopvpWGPkQ9KM8ZXlvHRx4crv83p+12U2TYW2pbH5ctU6aHZDtb/pmgqZ3W5hlbbgAAAAAAAAAAHqLnLFAAAAQCzOl4lvDkW3aMs867Axz/2s09OqVZ6zjYbdyaaCutj63yHf098QhJWdFPv38ZGrUrfCitF2BsznwgUX0czATAM="
+	assert.Equal(t, expected, txeBase64, "Base 64 XDR should match")
+}