psbt: avoid iterating private keys for input detection

We only need the pubkeys for the initial iteration where we identify inputs
belonging to the wallet. Avoid deriving private keys until we actually sign.

Author: jgriffiths

main/process/sign_psbt.c

@@ -636,7 +636,7 @@ int sign_psbt(const char* network, struct wally_psbt* psbt, const char** errmsg)
     }
     JADE_ASSERT(tx->num_inputs == psbt->num_inputs && tx->num_outputs == psbt->num_outputs);
 
-    key_iter iter; // Holds any private key in use
+    key_iter iter; // Holds any public/private key in use
     SENSITIVE_PUSH(&iter, sizeof(iter));
     int retval = 0;
 
@@ -669,7 +669,7 @@ int sign_psbt(const char* network, struct wally_psbt* psbt, const char** errmsg)
         input_amount += utxo->satoshi;
 
         // If we are signing this input, look at the script type, sighash, multisigs etc.
-        if (key_iter_input_begin(psbt, index, &iter)) {
+        if (key_iter_input_begin_public(psbt, index, &iter)) {
             // Found our key - we are signing this input
             JADE_LOGD("Key %u belongs to this signer, so we will need to sign input %u", iter.key_index, index);
             signing_inputs[index] = true;

main/utils/psbt.c

@@ -59,6 +59,11 @@ bool key_iter_input_begin(const struct wally_psbt* psbt, const size_t index, key
     return key_iter_init(psbt, index, /* is_input */ true, /* is_private */ true, iter);
 }
 
+bool key_iter_input_begin_public(const struct wally_psbt* psbt, const size_t index, key_iter* iter)
+{
+    return key_iter_init(psbt, index, /* is_input */ true, /* is_private */ false, iter);
+}
+
 bool key_iter_output_begin_public(const struct wally_psbt* psbt, const size_t index, key_iter* iter)
 {
     return key_iter_init(psbt, index, /* is_input */ false, /* is_private */ false, iter);

main/utils/psbt.h

@@ -27,6 +27,9 @@ typedef struct key_iter_t {
 // Initialize a key iterator for the `index`th PSBT input
 bool key_iter_input_begin(const struct wally_psbt* psbt, size_t index, key_iter* iter);
 
+// Initialize a public key iterator for the `index`th PSBT input
+bool key_iter_input_begin_public(const struct wally_psbt* psbt, size_t index, key_iter* iter);
+
 // Initialize a public key iterator for the `index`th PSBT output
 bool key_iter_output_begin_public(const struct wally_psbt* psbt, size_t index, key_iter* iter);