tests: add constant time tests

diff --git a/src/ctime_tests.c b/src/ctime_tests.c
index 8a885ca..0d201d2 100644
--- a/src/ctime_tests.c
+++ b/src/ctime_tests.c
@@ -40,6 +40,10 @@
 #include "../include/secp256k1_ellswift.h"
 #endif
 
+#ifdef ENABLE_MODULE_SILENTPAYMENTS
+#include "../include/secp256k1_silentpayments.h"
+#endif
+
 #if defined(__GNUC__)
 # pragma GCC diagnostic push
 # pragma GCC diagnostic warning "-Wunused-function"
@@ -99,6 +103,27 @@ static void run_tests(secp256k1_context *ctx, unsigned char *key) {
     unsigned char ellswift[64];
     static const unsigned char prefix[64] = {'t', 'e', 's', 't'};
 #endif
+#ifdef ENABLE_MODULE_SILENTPAYMENTS
+    secp256k1_xonly_pubkey generated_output;
+    secp256k1_xonly_pubkey *generated_outputs[1];
+    secp256k1_silentpayments_recipient recipient;
+    secp256k1_silentpayments_label label;
+    const secp256k1_silentpayments_recipient *recipients[1];
+    unsigned char outpoint_smallest[36] = { 0 };
+    secp256k1_keypair sp_keypair;
+    const secp256k1_keypair *sp_keypairs[1];
+    const unsigned char *sp_seckeys[1];
+    secp256k1_silentpayments_found_output found_outputs[1];
+    secp256k1_silentpayments_found_output *found_outputs_ptrs[1];
+    uint32_t n_found_outputs;
+    const secp256k1_xonly_pubkey *tx_outputs[1];
+    secp256k1_silentpayments_prevouts_summary prevouts_summary;
+    unsigned char label_tweak[32] = { 0 };
+    secp256k1_xonly_pubkey sp_xonly_pubkey;
+    const secp256k1_xonly_pubkey *sp_xonly_pubkeys[1];
+    secp256k1_pubkey sp_pubkey;
+    const secp256k1_pubkey *sp_pubkeys[1];
+#endif
 
     for (i = 0; i < 32; i++) {
         msg[i] = i + 1;
@@ -268,6 +293,59 @@ static void run_tests(secp256k1_context *ctx, unsigned char *key) {
         CHECK(ret == 1);
     }
 
+#endif
+
+#ifdef ENABLE_MODULE_SILENTPAYMENTS
+    SECP256K1_CHECKMEM_DEFINE(key, 32);
+
+    generated_outputs[0] = &generated_output;
+
+    /* Initialize recipient */
+    CHECK(secp256k1_ec_pubkey_create(ctx, &recipient.scan_pubkey, key));
+    key[31] ^= 1;
+    CHECK(secp256k1_ec_pubkey_create(ctx, &recipient.spend_pubkey, key));
+    key[31] ^= (1 << 1);
+    recipient.index = 0;
+    recipients[0] = &recipient;
+
+    /* Set up secret keys */
+    SECP256K1_CHECKMEM_UNDEFINE(key, 32);
+    ret = secp256k1_keypair_create(ctx, &sp_keypair, key);
+    SECP256K1_CHECKMEM_DEFINE(&ret, sizeof(ret));
+    CHECK(ret);
+    key[31] ^= (1 << 2);
+    sp_keypairs[0] = &sp_keypair;
+    sp_seckeys[0] = key;
+
+    ret = secp256k1_silentpayments_sender_create_outputs(ctx, generated_outputs, recipients, 1, outpoint_smallest, sp_keypairs, 1, sp_seckeys, 1);
+    CHECK(ret == 1);
+
+    ret = secp256k1_silentpayments_recipient_label_create(ctx, &label, label_tweak, key, 0);
+    key[31] ^= (1 << 3);
+    SECP256K1_CHECKMEM_DEFINE(&ret, sizeof(ret));
+    CHECK(ret == 1);
+
+    CHECK(secp256k1_keypair_xonly_pub(ctx, &sp_xonly_pubkey, NULL, &sp_keypair));
+    SECP256K1_CHECKMEM_DEFINE(&sp_xonly_pubkey, sizeof(sp_xonly_pubkey));
+    sp_xonly_pubkeys[0] = &sp_xonly_pubkey;
+    ret = secp256k1_ec_pubkey_create(ctx, &sp_pubkey, sp_seckeys[0]);
+    SECP256K1_CHECKMEM_DEFINE(&ret, sizeof(ret));
+    CHECK(ret == 1);
+    SECP256K1_CHECKMEM_DEFINE(&sp_pubkey, sizeof(sp_pubkey));
+    sp_pubkeys[0] = &sp_pubkey;
+
+    ret = secp256k1_silentpayments_recipient_prevouts_summary_create(ctx, &prevouts_summary, outpoint_smallest, sp_xonly_pubkeys, 1, sp_pubkeys, 1);
+    CHECK(ret == 1);
+
+    tx_outputs[0] = generated_outputs[0];
+    found_outputs_ptrs[0] = &found_outputs[0];
+    n_found_outputs = 1;
+    SECP256K1_CHECKMEM_DEFINE(&recipient.spend_pubkey, sizeof(recipient.spend_pubkey));
+    /* It is sufficient to check _recipient_scan_outputs without a label lookup function, since the shared secret is created once (which is where the constant timeness matters)
+     * and then reused for the rest of the scanning logic.
+     */
+    CHECK(secp256k1_silentpayments_recipient_scan_outputs(ctx, found_outputs_ptrs, &n_found_outputs, tx_outputs, 1, key, &prevouts_summary, &recipient.spend_pubkey, NULL, NULL));
+
 #endif
 }