What is the correct way to unmarshal an encoded transaction for signing

Question

I am looking to unmarshal a pre-built transaction envelope for signing using the new txnbuild package in the Go SDK but I'm missing a step and can't find the correct way to unmarshal. This is what I have so far:

// Unmarshall the envelope
var decoded xdr.TransactionEnvelope
err = xdr.SafeUnmarshalBase64(env, &decoded) // env is the xdr string
check(err, "Unmarshal transaction envelope")
tx := // convert to txnbuild transaction... but how?

// Rebuild, sign, encode and prepare response
txe, err := tx.BuildSignEncode(signer.(*keypair.Full))
check(err, "Rebuild, sign and encode transaction")

I think I just need to convert the xdr.TransactionEnvelope to a txnbuild.Transaction for signing.

Answer 1

Support for this has now been added to the SDK in txnbuild 1.4. Here's an example that produces the same output as the previous lower-level workaround: https://play.golang.org/p/5uXzCEK_bgc

package main

import (
    "fmt"
    "github.com/stellar/go/txnbuild"
    "github.com/stellar/go/network"
)

func main() {
    // An existing Base 64 serialised transaction envelope
    txeB64 := "AAAAAPkNnnxDCzRheIvmmH7rVFxjEpX/LTXrynktSZf6AlRpAAAAZAAWgB0AAAABAAAAAAAAAAAAAAABAAAAAAAAAAoAAAAEdGVzdAAAAAEAAAAEdGVzdAAAAAAAAAAA"

    // Deserialise the provided transaction
    tx, err := txnbuild.TransactionFromXDR(txeB64)
    if err != nil {
        panic(err)
    }

    // Explicitly set the network where this transaction is to be valid
    tx.Network = network.TestNetworkPassphrase

    // Add a signature
    err = tx.SignWithKeyString("SC7HGDR2N3Z5D4OJ54PZLF6LSYSDNEKM7QGR6SBEPHLN63NZ2XSLUHVQ")
    if err != nil {
        panic(err)
    }

    // Serialise the transaction
    b64, err := tx.Base64()
    if err != nil {
        panic(err)
    }

    fmt.Println("Signed XDR is: ", b64)
}

Answer 2

There is no support yet for building a txnbuild.Transaction object from a xdr.TransactionEnvelope in the Go SDK. However, you can still sign the transaction by using this work around: https://play.golang.org/p/tr6y1Gim_4d

See code below as well

package main

import (
    "bytes"
    "encoding/base64"
    "fmt"

    "github.com/stellar/go/keypair"
    "github.com/stellar/go/network"
    "github.com/stellar/go/xdr"
)

func main() {

    var txEnv xdr.TransactionEnvelope
    err := xdr.SafeUnmarshalBase64("AAAAAPkNnnxDCzRheIvmmH7rVFxjEpX/LTXrynktSZf6AlRpAAAAZAAWgB0AAAABAAAAAAAAAAAAAAABAAAAAAAAAAoAAAAEdGVzdAAAAAEAAAAEdGVzdAAAAAAAAAAA",
        &txEnv)
    if err != nil {
        panic(err)
    }
    txHash, err := network.HashTransaction(&txEnv.Tx, network.TestNetworkPassphrase)
    if err != nil {
        panic(err)
    }
    kp := keypair.MustParse("SC7HGDR2N3Z5D4OJ54PZLF6LSYSDNEKM7QGR6SBEPHLN63NZ2XSLUHVQ")

    sig, err := kp.SignDecorated(txHash[:])
    if err != nil {
        panic(err)
    }
    // Append the signature to the envelope
    txEnv.Signatures = append(txEnv.Signatures, sig)

    // convert to base64 xdr
    var txBytes bytes.Buffer
    _, err = xdr.Marshal(&txBytes, txEnv)
    if err != nil {
        panic(err)
    }

    b64 := base64.StdEncoding.EncodeToString(txBytes.Bytes())
    fmt.Println("Signed XDR is: ", b64)
}