wallet: cold wallet transaction signing

This change adds the ability to create a new unsigned transaction
from a watch only wallet, and save it to a file. This file can
then be moved to another computer/VM where a cold wallet may load
it, sign it, and save it. That cold wallet does not need to have
a blockchain nor daemon. The signed transaction file can then be
moved back to the watch only wallet, which can load it and send
it to the daemon.

Two new simplewallet commands to use it:
sign_transfer (on the cold wallet)
submit_transfer (on the watch only wallet)

The transfer command used on a watch only wallet now writes an
unsigned transaction set in a file called 'unsigned_monero_tx'
instead of submitting the tx to the daemon as a normal wallet does.
The signed tx file is called 'signed_monero_tx'.

2 files changed, 26 insertions, 5 deletions

diff --git a/src/cryptonote_core/cryptonote_format_utils.cpp b/src/cryptonote_core/cryptonote_format_utils.cpp
index 7e0c75ccf..64f8eb924 100644
--- a/src/cryptonote_core/cryptonote_format_utils.cpp
+++ b/src/cryptonote_core/cryptonote_format_utils.cpp
@@ -472,10 +472,7 @@ namespace cryptonote
   bool construct_tx_and_get_tx_key(const account_keys& sender_account_keys, const std::vector<tx_source_entry>& sources, const std::vector<tx_destination_entry>& destinations, std::vector<uint8_t> extra, transaction& tx, uint64_t unlock_time, crypto::secret_key &tx_key, bool rct)
   {
     std::vector<rct::key> amount_keys;
-    tx.vin.clear();
-    tx.vout.clear();
-    tx.signatures.clear();
-    tx.rct_signatures.type = rct::RCTTypeNull;
+    tx.set_null();
     amount_keys.clear();
 
     tx.version = rct ? 2 : 1;
@@ -615,6 +612,14 @@ namespace cryptonote
       return false;
     }
 
+    // check for watch only wallet
+    bool zero_secret_key = true;
+    for (size_t i = 0; i < sizeof(sender_account_keys.m_spend_secret_key); ++i)
+      zero_secret_key &= (sender_account_keys.m_spend_secret_key.data[i] == 0);
+    if (zero_secret_key)
+    {
+      LOG_PRINT_L1("Null secret key, skipping signatures");
+    }
 
     if (tx.version == 1)
     {
@@ -641,7 +646,8 @@ namespace cryptonote
         tx.signatures.push_back(std::vector<crypto::signature>());
         std::vector<crypto::signature>& sigs = tx.signatures.back();
         sigs.resize(src_entr.outputs.size());
-        crypto::generate_ring_signature(tx_prefix_hash, boost::get<txin_to_key>(tx.vin[i]).k_image, keys_ptrs, in_contexts[i].in_ephemeral.sec, src_entr.real_output, sigs.data());
+        if (!zero_secret_key)
+          crypto::generate_ring_signature(tx_prefix_hash, boost::get<txin_to_key>(tx.vin[i]).k_image, keys_ptrs, in_contexts[i].in_ephemeral.sec, src_entr.real_output, sigs.data());
         ss_ring_s << "signatures:" << ENDL;
         std::for_each(sigs.begin(), sigs.end(), [&](const crypto::signature& s){ss_ring_s << s << ENDL;});
         ss_ring_s << "prefix_hash:" << tx_prefix_hash << ENDL << "in_ephemeral_key: " << in_contexts[i].in_ephemeral.sec << ENDL << "real_output: " << src_entr.real_output;
diff --git a/src/cryptonote_core/cryptonote_format_utils.h b/src/cryptonote_core/cryptonote_format_utils.h
index a42296994..24db8008e 100644
--- a/src/cryptonote_core/cryptonote_format_utils.h
+++ b/src/cryptonote_core/cryptonote_format_utils.h
@@ -62,6 +62,16 @@ namespace cryptonote
     rct::key mask;                      //ringct amount mask
 
     void push_output(uint64_t idx, const crypto::public_key &k, uint64_t amount) { outputs.push_back(std::make_pair(idx, rct::ctkey({rct::pk2rct(k), rct::zeroCommit(amount)}))); }
+
+    BEGIN_SERIALIZE_OBJECT()
+      FIELD(outputs)
+      VARINT_FIELD(real_output)
+      FIELD(real_out_tx_key)
+      VARINT_FIELD(real_output_in_tx_index)
+      VARINT_FIELD(amount)
+      FIELD(rct)
+      FIELD(mask)
+    END_SERIALIZE()
   };
 
   struct tx_destination_entry
@@ -71,6 +81,11 @@ namespace cryptonote
 
     tx_destination_entry() : amount(0), addr(AUTO_VAL_INIT(addr)) { }
     tx_destination_entry(uint64_t a, const account_public_address &ad) : amount(a), addr(ad) { }
+
+    BEGIN_SERIALIZE_OBJECT()
+      VARINT_FIELD(amount)
+      FIELD(addr)
+    END_SERIALIZE()
   };
 
   //---------------------------------------------------------------