sign_tx: split out signing tx validation, add more checks

diff --git a/main/process/sign_tx.c b/main/process/sign_tx.c
index 26a51fd..2cb87eb 100644
--- a/main/process/sign_tx.c
+++ b/main/process/sign_tx.c
@@ -23,8 +23,8 @@
 
 #include "sign_utils.h"
 
-static struct wally_tx* params_txn(
-    jade_process_t* process, const CborValue* params, const network_t network_id, const bool for_liquid)
+static struct wally_tx* params_txn(jade_process_t* process, const CborValue* params, const network_t network_id,
+    const bool for_liquid, uint64_t* explicit_fee)
 {
     struct wally_tx* tx = NULL;
     const char* errmsg = NULL;
@@ -72,22 +72,7 @@ static struct wally_tx* params_txn(
         errmsg = "Unexpected number of inputs for transaction";
         goto fail;
     }
-
-    if (for_liquid) {
-        for (size_t i = 0; i < tx->num_outputs; ++i) {
-            bool exp_asset = tx->outputs[i].asset[0] == WALLY_TX_ASSET_CT_EXPLICIT_PREFIX;
-            bool exp_value = tx->outputs[i].value[0] == WALLY_TX_ASSET_CT_EXPLICIT_PREFIX;
-            if (exp_asset != exp_value) {
-                errmsg = "Output asset and value blinding inconsistent";
-                goto fail;
-            }
-        }
-    }
-
-    size_t is_elements = 0;
-    JADE_WALLY_VERIFY(wally_tx_is_elements(tx, &is_elements));
-    if (for_liquid != is_elements) {
-        errmsg = "Transaction is the wrong type for the current network";
+    if (!params_txn_validate(network_id, for_liquid, tx, explicit_fee, &errmsg)) {
         goto fail;
     }
     return tx;
@@ -458,7 +443,8 @@ static void sign_tx_impl(jade_process_t* process, const bool for_liquid)
     CHECK_NETWORK_CONSISTENT(process);
     const jade_msg_source_t source = process->ctx.source;
 
-    struct wally_tx* tx = params_txn(process, &params, network_id, for_liquid);
+    uint64_t explicit_fee = 0;
+    struct wally_tx* tx = params_txn(process, &params, network_id, for_liquid, &explicit_fee);
     if (!tx) {
         goto cleanup;
     }
@@ -497,7 +483,6 @@ static void sign_tx_impl(jade_process_t* process, const bool for_liquid)
     asset_summary_t *in_sums = NULL, *out_sums = NULL;
     size_t num_in_sums = 0, num_out_sums = 0;
     bool is_partial = false;
-    uint64_t explicit_fee = 0;
     TxType_t txtype = TXTYPE_SEND_PAYMENT;
     // Liquid: Get any data from the optional 'additional_info' section
     if (for_liquid
@@ -508,8 +493,8 @@ static void sign_tx_impl(jade_process_t* process, const bool for_liquid)
 
     // Liquid: Validate commitment, outputs and additional_info
     if (for_liquid
-        && !validate_elements_outputs(process, network_id, tx, txtype, commitments, output_info, in_sums, num_in_sums,
-            out_sums, num_out_sums, &explicit_fee)) {
+        && !validate_elements_outputs(
+            process, network_id, tx, txtype, commitments, output_info, in_sums, num_in_sums, out_sums, num_out_sums)) {
         goto cleanup;
     }
 
diff --git a/main/process/sign_utils.c b/main/process/sign_utils.c
index 763548a..1485a74 100644
--- a/main/process/sign_utils.c
+++ b/main/process/sign_utils.c
@@ -16,6 +16,75 @@
 static const char TX_TYPE_STR_SWAP[] = "swap";
 static const char TX_TYPE_STR_SEND_PAYMENT[] = "send_payment";
 
+bool params_txn_validate(const network_t network_id, const bool for_liquid, const struct wally_tx* const tx,
+    uint64_t* explicit_fee, const char** errmsg)
+{
+    JADE_ASSERT(tx);
+    JADE_ASSERT(explicit_fee);
+    JADE_INIT_OUT_PPTR(errmsg);
+
+    size_t is_elements = 0;
+    JADE_WALLY_VERIFY(wally_tx_is_elements(tx, &is_elements));
+    if (for_liquid != is_elements) {
+        *errmsg = "Transaction is the wrong type for the current network";
+        return false;
+    }
+
+    if (!for_liquid) {
+        return true; // Bitcoin: No further checks needed
+    }
+
+    // Liquid checks
+    uint8_t policy_asset[ASSET_TAG_LEN];
+    network_to_policy_asset(network_id, policy_asset, sizeof(policy_asset));
+    reverse_in_place(policy_asset, sizeof(policy_asset));
+
+    for (size_t i = 0; i < tx->num_outputs; ++i) {
+        const struct wally_tx_output* const txout = tx->outputs + i;
+        JADE_ASSERT(txout->asset && txout->asset_len == WALLY_TX_ASSET_CT_LEN);
+        JADE_ASSERT(txout->value && txout->value_len);
+        const uint8_t explicit_prefix = WALLY_TX_ASSET_CT_EXPLICIT_PREFIX;
+        const bool is_explicit_asset = txout->asset[0] == explicit_prefix;
+        const bool is_explicit_value = txout->value[0] == explicit_prefix;
+
+        if (is_explicit_asset != is_explicit_value) {
+            *errmsg = "Output asset and value blinding inconsistent";
+            return false;
+        }
+        if (is_explicit_value) {
+            JADE_ASSERT(txout->value_len == WALLY_TX_ASSET_CT_VALUE_UNBLIND_LEN);
+        } else {
+            JADE_ASSERT(txout->value_len == WALLY_TX_ASSET_CT_VALUE_LEN);
+        }
+
+        if (tx->outputs[i].script) {
+            continue; // Not a fee output, no further checks needed
+        }
+
+        // Fee output
+        if (!is_explicit_asset || !is_explicit_value) {
+            *errmsg = "Fee output (without script) cannot be blinded";
+            return false;
+        }
+        if (*explicit_fee) {
+            *errmsg = "Unexpected multiple fee outputs (without script)";
+            return false;
+        }
+        JADE_WALLY_VERIFY(wally_tx_confidential_value_to_satoshi(txout->value, txout->value_len, explicit_fee));
+        if (!*explicit_fee) {
+            *errmsg = "Fee output (without script) cannot be 0";
+            return false;
+        }
+        // Note we compare the asset id ignoring the initial explicit byte
+        if (memcmp(txout->asset + 1, policy_asset, sizeof(policy_asset))) {
+            *errmsg = "Unexpected fee output (without script) asset-id";
+            return false;
+        }
+    }
+
+    return true;
+}
+
 // Map a txtype string to an enum value
 #define TX_TYPE_STR_MATCH(typestr) ((len == sizeof(typestr) - 1 && !strncmp(type, typestr, sizeof(typestr) - 1)))
 static bool rpc_get_txtype(jade_process_t* process, CborValue* value, TxType_t* txtype)
@@ -549,15 +618,13 @@ static bool add_output_info(
 
 bool validate_elements_outputs(jade_process_t* process, const network_t network_id, const struct wally_tx* tx,
     const TxType_t txtype, commitment_t* commitments, output_info_t* output_info, asset_summary_t* in_sums,
-    const size_t num_in_sums, asset_summary_t* out_sums, const size_t num_out_sums, uint64_t* fees)
+    const size_t num_in_sums, asset_summary_t* out_sums, const size_t num_out_sums)
 {
     JADE_ASSERT(tx);
     JADE_ASSERT(commitments);
     JADE_ASSERT(output_info);
-    JADE_ASSERT(fees);
 
     const char* errmsg = NULL;
-    *fees = 0;
 
     uint8_t policy_asset[ASSET_TAG_LEN];
     network_to_policy_asset(network_id, policy_asset, sizeof(policy_asset));
@@ -570,7 +637,6 @@ bool validate_elements_outputs(jade_process_t* process, const network_t network_
     // By default/in the basic 'send payment' case all outputs must have unconfidential/unblinded.
     const bool allow_blind_outputs = txtype == TXTYPE_SWAP; // swaps allow 'other wallets' blind outputs
 
-    // Save fees for the final confirmation screen
     for (size_t i = 0; i < tx->num_outputs; ++i) {
         // Gather the (unblinded) output info for user confirmation
         output_info_t* outinfo = output_info + i;
@@ -586,28 +652,6 @@ bool validate_elements_outputs(jade_process_t* process, const network_t network_
             }
         }
 
-        // Collect fees (ie. outputs with no script)
-        // NOTE: fees must be unconfidential, and must be denominated in the policy asset
-        if (!tx->outputs[i].script) {
-            if (outinfo->flags & OUTPUT_FLAG_CONFIDENTIAL) {
-                errmsg = "Fee output (without script) cannot be blinded";
-                goto done;
-            }
-            if (memcmp(outinfo->asset_id, policy_asset, sizeof(policy_asset))) {
-                errmsg = "Unexpected fee output (without script) asset-id";
-                goto done;
-            }
-            if (!outinfo->value) {
-                errmsg = "Fee output (without script) cannot be 0";
-                goto done;
-            }
-            if (*fees) {
-                errmsg = "Unexpected multiple fee outputs (without script)";
-                goto done;
-            }
-            *fees += outinfo->value;
-        }
-
         // If the output has been verified as belonging to this wallet, we can
         // use it to validate some part of any passed input- or output- summary.
         if (outinfo->flags & OUTPUT_FLAG_VALIDATED) {
diff --git a/main/process/sign_utils.h b/main/process/sign_utils.h
index d3162f5..48177ee 100644
--- a/main/process/sign_utils.h
+++ b/main/process/sign_utils.h
@@ -19,6 +19,9 @@ typedef struct _asset_summary {
     uint64_t validated_value;
 } asset_summary_t;
 
+bool params_txn_validate(network_t network_id, bool for_liquid, const struct wally_tx* const tx, uint64_t* explicit_fee,
+    const char** errmsg);
+
 bool params_trusted_commitments(
     jade_process_t* process, const CborValue* params, const struct wally_tx* tx, commitment_t** data);
 
@@ -35,7 +38,7 @@ bool asset_summary_validate(asset_summary_t* sums, size_t num_sums);
 
 bool validate_elements_outputs(jade_process_t* process, network_t network_id, const struct wally_tx* tx,
     TxType_t txtype, commitment_t* commitments, output_info_t* output_info, asset_summary_t* in_sums,
-    size_t num_in_sums, asset_summary_t* out_sums, size_t num_out_sums, uint64_t* fees);
+    size_t num_in_sums, asset_summary_t* out_sums, size_t num_out_sums);
 
 bool show_btc_fee_confirmation_activity(network_t network_id, const struct wally_tx* tx, const output_info_t* outinfo,
     script_flavour_t aggregate_inputs_scripts_flavour, uint64_t input_amount, uint64_t output_amount);