What is the right way to verify an off-chain transaction with golang?

Question

When you receive a transaction in the http request, you are getting a string with the XRD base64 form of the client signed transaction:

{"transaction": "AAAAAGjeCRajN67nRkVtYO+lpxax9gvitX9FxhZYGXQvs16hAAAAZAAAAAAAAAAAAAAAAQAAAABbcutKAAAAAFty7HYAAAAAAAAAAQAAAAEAAAAAVIx5odtAgqQ+dp4m4QfWntHbOq0hxRCLGI+2Sm0P6EMAAAAKAAAAC01vYml1cyBhdXRoAAAAAAEAAABAG01TL/Ha0YGVrAF6t0UEKP/0Q/NDUymciQBA/CXzYMVlEx2KcHrq3MkpQ9+9sCbCiOYa7wCtusa1tHKygvZRSwAAAAAAAAABL7NeoQAAAEC9v5jdxReIxoCcCXw90dVsIpXwHXkSHUUthCs98D/zpd6TNPvcMgUsQd6cDHzjNk+/00P8M5bHP4rIpFTm7MwN"}

If you are using JS, you can use the following to verify that the transaction was signed by the server PK:

const tx = new Transaction(req.body.transaction);
const op = tx.operations[0];
const { signatures } = tx;
const hash = tx.hash();

// Source account is ours
if (tx.source != SERVER_KEY_PAIR.publicKey()) {
  return res.json({ error: "Invalid source account." });
}

// Challenge transaction was generated by us
if (
  !signatures.some(signature =>
    SERVER_KEY_PAIR.verify(hash, signature.signature())
  )
) {
  return res.json({ error: "Server signature is missing or invalid." });
}

I'm using Golang, but I think I'm using the wrong types and methods because the verification is failing.

var txXDR xdr.TransactionEnvelope
err = xdr.SafeUnmarshalBase64(txeBase64, &txXDR)
if err != nil {
    return fmt.Errorf("Error Unmarhsaling TX: %s", err)
}

The problem using that Transaction Envelope is that I do not see in the Go-SDK the equivalent of

const hash = tx.hash();
SERVER_KEY_PAIR.verify(hash, signat`ure.signature())

when using that XDR Tx.

Can anyone point me to the right direction?

Answer 1

play.golang.org

package main

import (
  "fmt"

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

func main() {
  var env xdr.TransactionEnvelope
  err := xdr.SafeUnmarshalBase64("AAAAAE//tVvL/xn3WM2MincerNCe6rekrjZD+f1ACDuU5AxkAAAAZAAAJAEAAuElAAAAAAAAAAAAAAABAAAAAAAAAAMAAAABSFQAAAAAAACbIx6CN2rDLn2KMYzJ36wouCgJ9qAN7GEXXvDJ3frLagAAAAFCVEMAAAAAAJsjHoI3asMufYoxjMnfrCi4KAn2oA3sYRde8Mnd+stqAAAAAAAAAAAAAACpAAWPcAAAAAABmk29AAAAAAAAAAGU5AxkAAAAQFmbZIHErm5yTe9UnrVoRoTtH5aA98Uw1m+H4ZXJs/Y6EIhCGb4e32wijsxMF12HQ73NxkJXE8+fgTSx2x0VIgA=", &env)
  if err != nil {
    panic(err)
  }
  txHash, err := network.HashTransaction(&env.Tx, network.TestNetworkPassphrase)
  if err != nil {
    panic(err)
  }
  kp := keypair.MustParse("GBH77NK3ZP7RT52YZWGIU5Y6VTIJ52VXUSXDMQ7Z7VAAQO4U4QGGIROV")

  err = kp.Verify(txHash[:], env.Signatures[0].Signature)
  fmt.Println("Will be non-nil when sig is invalid:", err)
}