psbt: add psbt_get_input_signature_type to get the type of sig required for an input

As a first cut, this uses the pre-existing logic for differentiating
segwit and non-segwit inputs (the presence of a witness UTXO).

Author: jgriffiths

include/wally_psbt_members.h

@@ -60,6 +60,15 @@ WALLY_CORE_API int wally_psbt_get_input_unknown(const struct wally_psbt *psbt, s
 WALLY_CORE_API int wally_psbt_get_input_unknown_len(const struct wally_psbt *psbt, size_t index, size_t subindex, size_t *written);
 WALLY_CORE_API int wally_psbt_get_input_sighash(const struct wally_psbt *psbt, size_t index, size_t *written);
 
+/**
+ * Return the signature type of a PSBT input.
+ *
+ * :param psbt: The PSBT to get the signature type from.
+ * :param index: The zero-based index of the input to get the signature type from.
+ * :param value_out: Destination for the :ref:`tx-sig-type` of the input.
+ */
+WALLY_CORE_API int wally_psbt_get_input_signature_type(const struct wally_psbt *psbt, size_t index, uint32_t *value_out);
+
 /**
  * FIXED_SIZED_OUTPUT(len, bytes_out, WALLY_TXHASH_LEN)
  */

src/psbt.c

@@ -155,34 +155,32 @@ static const struct wally_tx_output *utxo_from_input(const struct wally_psbt *ps
     return NULL;
 }
 
-/* Try to determine if a PSBT input is taproot.
- * TODO: We could verify that the script and field checks are in sync
- * here, i.e. that an input with taproot fields has a taproot script,
- * and return an error otherwise.
- */
-static bool is_taproot_input(const struct wally_psbt *psbt,
-                             const struct wally_psbt_input *inp)
+struct wally_psbt_input *psbt_get_input_signature_type(const struct wally_psbt *psbt,
+                                                       size_t index, uint32_t *value_out)
 {
-    if (!inp)
-        return false;
-    else {
-        const struct wally_tx_output *utxo = utxo_from_input(psbt, inp);
-        if (utxo) {
-            /* Determine from the scriptpubkey if possible */
-            size_t script_type;
-            int ret = wally_scriptpubkey_get_type(utxo->script, utxo->script_len,
-                                                  &script_type);
-            if (ret == WALLY_OK)
-                return script_type == WALLY_SCRIPT_TYPE_P2TR;
-        }
-        /* No usable UTXO/script for this input, check for taproot fields */
-        return inp->taproot_leaf_hashes.num_items ||
-               inp->taproot_leaf_scripts.num_items ||
-               inp->taproot_leaf_signatures.num_items ||
-               wally_map_get_integer(&inp->psbt_fields, PSBT_IN_TAP_INTERNAL_KEY) ||
-               wally_map_get_integer(&inp->psbt_fields, PSBT_IN_TAP_MERKLE_ROOT) ||
-               wally_map_get_integer(&inp->psbt_fields, PSBT_IN_TAP_KEY_SIG);
+    struct wally_psbt_input *inp = psbt_get_input(psbt, index);
+    const struct wally_tx_output *utxo = utxo_from_input(psbt, inp);
+
+    if (value_out)
+        *value_out = 0;
+    if (!utxo || !value_out)
+        return NULL;
+
+    if (scriptpubkey_is_p2tr(utxo->script, utxo->script_len)) {
+        *value_out = WALLY_SIGTYPE_SW_V1;
+        return inp;
     }
+
+    /* Otherwise, follow core and use whether a witness utxo is present */
+    *value_out = inp->witness_utxo ? WALLY_SIGTYPE_SW_V0 : WALLY_SIGTYPE_PRE_SW;
+    return inp;
+}
+
+int wally_psbt_get_input_signature_type(const struct wally_psbt *psbt,
+                                        size_t index, uint32_t *value_out)
+{
+    struct wally_psbt_input *inp = psbt_get_input_signature_type(psbt, index, value_out);
+    return inp ? WALLY_OK : WALLY_EINVAL;
 }
 
 /* Set a struct member on a parent struct */
@@ -4579,12 +4577,10 @@ int wally_psbt_get_input_signature_hash(struct wally_psbt *psbt, size_t index,
                                         unsigned char *bytes_out, size_t len)
 {
     struct wally_map scripts, assets, values, *assets_p;
-    const struct wally_psbt_input *inp = psbt_get_input(psbt, index);
     size_t is_pset;
     uint32_t sighash, sighash_type;
-    const bool is_taproot = is_taproot_input(psbt, inp);
-    /* FIXME: Determine segwitness in a smarter way (e.g. prevout script */
-    const bool is_segwit = inp && inp->witness_utxo != NULL;
+    const struct wally_psbt_input *inp = psbt_get_input_signature_type(psbt, index, &sighash_type);
+    const bool is_taproot = sighash_type == WALLY_SIGTYPE_SW_V1;
     int ret;
 
     if (!tx || !inp || flags)
@@ -4608,11 +4604,7 @@ int wally_psbt_get_input_signature_hash(struct wally_psbt *psbt, size_t index,
         /* FIXME: Support script path spends */
         script = NULL;
         script_len = 0;
-        sighash_type = WALLY_SIGTYPE_SW_V1;
-    } else if (is_segwit)
-        sighash_type = WALLY_SIGTYPE_SW_V0;
-    else
-        sighash_type = WALLY_SIGTYPE_PRE_SW;
+    }
 
     ret = get_signing_data(psbt, &scripts, assets_p, &values);
     if (ret == WALLY_OK)
@@ -4640,9 +4632,9 @@ int wally_psbt_sign_input_bip32(struct wally_psbt *psbt,
     unsigned char sig[EC_SIGNATURE_LEN + 1], der[EC_SIGNATURE_DER_MAX_LEN + 1];
     unsigned char signing_key[EC_PRIVATE_KEY_LEN];
     size_t sig_len = EC_SIGNATURE_LEN, der_len, pubkey_idx;
-    uint32_t sighash;
-    struct wally_psbt_input *inp = psbt_get_input(psbt, index);
-    const bool is_taproot = is_taproot_input(psbt, inp);
+    uint32_t sighash, sighash_type;
+    struct wally_psbt_input *inp = psbt_get_input_signature_type(psbt, index, &sighash_type);
+    const bool is_taproot = sighash_type == WALLY_SIGTYPE_SW_V1;
     int ret;
 
     if (!inp || !hdkey || hdkey->priv_key[0] != BIP32_FLAG_KEY_PRIVATE ||

src/swig_java/swig.i

@@ -809,6 +809,7 @@ static jobjectArray create_jstringArray(JNIEnv *jenv, char **p, size_t len) {
 %returns_size_t(wally_psbt_get_input_required_locktime);
 %returns_size_t(wally_psbt_get_input_scriptcode);
 %returns_size_t(wally_psbt_get_input_scriptcode_len);
+%returns_size_t(wally_psbt_get_input_signature_type);
 %returns_size_t(wally_psbt_get_input_signing_script);
 %returns_size_t(wally_psbt_get_input_signing_script_len);
 %returns_size_t(wally_psbt_get_input_sequence);

src/test/util.py

@@ -859,6 +859,7 @@ class wally_psbt(Structure):
     ('wally_psbt_get_input_sighash', c_int, [POINTER(wally_psbt), c_size_t, c_size_t_p]),
     ('wally_psbt_get_input_signature', c_int, [POINTER(wally_psbt), c_size_t, c_size_t, c_void_p, c_size_t, c_size_t_p]),
     ('wally_psbt_get_input_signature_len', c_int, [POINTER(wally_psbt), c_size_t, c_size_t, c_size_t_p]),
+    ('wally_psbt_get_input_signature_type', c_int, [POINTER(wally_psbt), c_size_t, c_uint32_p]),
     ('wally_psbt_get_input_signatures_size', c_int, [POINTER(wally_psbt), c_size_t, c_size_t_p]),
     ('wally_psbt_get_input_taproot_internal_key', c_int, [POINTER(wally_psbt), c_size_t, c_void_p, c_size_t, c_size_t_p]),
     ('wally_psbt_get_input_taproot_internal_key_len', c_int, [POINTER(wally_psbt), c_size_t, c_size_t_p]),

src/wasm_package/src/const.js

@@ -223,7 +223,7 @@ export const WALLY_SIGHASH_NONE = 0x02;
 export const WALLY_SIGHASH_RANGEPROOF = 0x40 ; /* Liquid/Elements only */
 export const WALLY_SIGHASH_SINGLE = 0x03;
 export const WALLY_SIGHASH_TR_IN_MASK = 0xc0; /* Taproot mask for determining input hash type */
-export const WALLY_SIGTYPE_MASK = 0xf; /* Mask for signature hash in signature hash flags */
+export const WALLY_SIGTYPE_MASK = 0xf; /* Mask for signature type in in flags */
 export const WALLY_SIGTYPE_PRE_SW = 0x1; /* Pre-segwit signature hash */
 export const WALLY_SIGTYPE_SW_V0 = 0x2; /* Segwit v0 signature hash */
 export const WALLY_SIGTYPE_SW_V1 = 0x3; /* Segwit v1 (taproot) signature hash */

src/wasm_package/src/functions.js

@@ -333,6 +333,7 @@ export function psbt_get_input_sequence(psbt: Ref_wally_psbt, index: number): nu
 export function psbt_get_input_sighash(psbt: Ref_wally_psbt, index: number): number;
 export function psbt_get_input_signature_hash(psbt: Ref_wally_psbt, index: number, tx: Ref_wally_tx, script: Buffer|Uint8Array, flags: number): Buffer;
 export function psbt_get_input_signature_len(psbt: Ref_wally_psbt, index: number, subindex: number): number;
+export function psbt_get_input_signature_type(psbt: Ref_wally_psbt, index: number): number;
 export function psbt_get_input_signatures_size(psbt: Ref_wally_psbt, index: number): number;
 export function psbt_get_input_signing_script_len(psbt: Ref_wally_psbt, index: number): number;
 export function psbt_get_input_taproot_internal_key_len(psbt: Ref_wally_psbt, index: number): number;

src/wasm_package/src/index.d.ts

@@ -247,6 +247,7 @@ EXPORTED_FUNCTIONS="['_malloc','_free','_bip32_key_free' \
 ,'_wally_psbt_get_input_signature' \
 ,'_wally_psbt_get_input_signature_hash' \
 ,'_wally_psbt_get_input_signature_len' \
+,'_wally_psbt_get_input_signature_type' \
 ,'_wally_psbt_get_input_signatures_size' \
 ,'_wally_psbt_get_input_signing_script' \
 ,'_wally_psbt_get_input_signing_script_len' \