Move external signer out of wallet module

This commit moves the ExternalSigner class and RPC methods out of the wallet module.

The enumeratesigners RPC can be used without a wallet since #21417.
With additional modifications external signers could be used without a wallet in general, e.g. via signrawtransaction.

The signerdisplayaddress RPC is ranamed to walletdisplayaddress because it requires wallet context.
A future displayaddress RPC call without wallet context could take a descriptor argument.

This commit fixes a rpc_help.py failure when configured with --disable-wallet.

1 files changed, 119 insertions, 0 deletions

diff --git a/src/external_signer.cpp b/src/external_signer.cpp
new file mode 100644
index 0000000000..b82dcc503d
--- /dev/null
+++ b/src/external_signer.cpp
@@ -0,0 +1,119 @@
+// Copyright (c) 2018-2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <chainparams.h>
+#include <core_io.h>
+#include <psbt.h>
+#include <util/strencodings.h>
+#include <util/system.h>
+#include <external_signer.h>
+
+ExternalSigner::ExternalSigner(const std::string& command, const std::string& fingerprint, std::string chain, std::string name): m_command(command), m_fingerprint(fingerprint), m_chain(chain), m_name(name) {}
+
+const std::string ExternalSigner::NetworkArg() const
+{
+    return " --chain " + m_chain;
+}
+
+#ifdef ENABLE_EXTERNAL_SIGNER
+
+bool ExternalSigner::Enumerate(const std::string& command, std::vector<ExternalSigner>& signers, std::string chain, bool ignore_errors)
+{
+    // Call <command> enumerate
+    const UniValue result = RunCommandParseJSON(command + " enumerate");
+    if (!result.isArray()) {
+        if (ignore_errors) return false;
+        throw ExternalSignerException(strprintf("'%s' received invalid response, expected array of signers", command));
+    }
+    for (UniValue signer : result.getValues()) {
+        // Check for error
+        const UniValue& error = find_value(signer, "error");
+        if (!error.isNull()) {
+            if (ignore_errors) return false;
+            if (!error.isStr()) {
+                throw ExternalSignerException(strprintf("'%s' error", command));
+            }
+            throw ExternalSignerException(strprintf("'%s' error: %s", command, error.getValStr()));
+        }
+        // Check if fingerprint is present
+        const UniValue& fingerprint = find_value(signer, "fingerprint");
+        if (fingerprint.isNull()) {
+            if (ignore_errors) return false;
+            throw ExternalSignerException(strprintf("'%s' received invalid response, missing signer fingerprint", command));
+        }
+        std::string fingerprintStr = fingerprint.get_str();
+        // Skip duplicate signer
+        bool duplicate = false;
+        for (ExternalSigner signer : signers) {
+            if (signer.m_fingerprint.compare(fingerprintStr) == 0) duplicate = true;
+        }
+        if (duplicate) break;
+        std::string name = "";
+        const UniValue& model_field = find_value(signer, "model");
+        if (model_field.isStr() && model_field.getValStr() != "") {
+            name += model_field.getValStr();
+        }
+        signers.push_back(ExternalSigner(command, fingerprintStr, chain, name));
+    }
+    return true;
+}
+
+UniValue ExternalSigner::DisplayAddress(const std::string& descriptor) const
+{
+    return RunCommandParseJSON(m_command + " --fingerprint \"" + m_fingerprint + "\"" + NetworkArg() + " displayaddress --desc \"" + descriptor + "\"");
+}
+
+UniValue ExternalSigner::GetDescriptors(int account)
+{
+    return RunCommandParseJSON(m_command + " --fingerprint \"" + m_fingerprint + "\"" + NetworkArg() + " getdescriptors --account " + strprintf("%d", account));
+}
+
+bool ExternalSigner::SignTransaction(PartiallySignedTransaction& psbtx, std::string& error)
+{
+    // Serialize the PSBT
+    CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
+    ssTx << psbtx;
+
+    // Check if signer fingerprint matches any input master key fingerprint
+    bool match = false;
+    for (unsigned int i = 0; i < psbtx.inputs.size(); ++i) {
+        const PSBTInput& input = psbtx.inputs[i];
+        for (auto entry : input.hd_keypaths) {
+            if (m_fingerprint == strprintf("%08x", ReadBE32(entry.second.fingerprint))) match = true;
+        }
+    }
+
+    if (!match) {
+        error = "Signer fingerprint " + m_fingerprint + " does not match any of the inputs:\n" + EncodeBase64(ssTx.str());
+        return false;
+    }
+
+    std::string command = m_command + " --stdin --fingerprint \"" + m_fingerprint + "\"" + NetworkArg();
+    std::string stdinStr = "signtx \"" + EncodeBase64(ssTx.str()) + "\"";
+
+    const UniValue signer_result = RunCommandParseJSON(command, stdinStr);
+
+    if (find_value(signer_result, "error").isStr()) {
+        error = find_value(signer_result, "error").get_str();
+        return false;
+    }
+
+    if (!find_value(signer_result, "psbt").isStr()) {
+        error = "Unexpected result from signer";
+        return false;
+    }
+
+    PartiallySignedTransaction signer_psbtx;
+    std::string signer_psbt_error;
+    if (!DecodeBase64PSBT(signer_psbtx, find_value(signer_result, "psbt").get_str(), signer_psbt_error)) {
+        error = strprintf("TX decode failed %s", signer_psbt_error);
+        return false;
+    }
+
+    psbtx = signer_psbtx;
+
+    return true;
+}
+
+#endif