rangeproof: add method to verify single-value proofs

include/secp256k1_rangeproof.h

@@ -286,6 +286,29 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_rangeproof_info(
   size_t plen
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5);
 
+/** Verify a rangeproof with a single-value range. Useful as a "proof of value"
+ *  of a Pedersen commitment. Such proofs can be created with `secp256k1_rangeproof_sign`
+ *  by passing an `exp` parameter of -1 and the target value as both `value` and `min_value`.
+ *  (In this case `min_bits` is ignored and may take any value, but for clarity it's best
+ *  to pass zero.)
+ *  Returns 1: Proof was valid and proved the given value
+ *          0: Otherwise
+ *  In:   ctx: pointer to a context object
+ *        proof: pointer to character array with the proof.
+ *        plen: length of proof in bytes.
+ *        value: value being claimed for the Pedersen commitment
+ *        commit: the Pedersen commitment whose value is being proven
+ *        gen: additional generator 'h'
+ */
+SECP256K1_API int secp256k1_rangeproof_verify_value(
+  const secp256k1_context* ctx,
+  const unsigned char* proof,
+  size_t plen,
+  uint64_t value,
+  const secp256k1_pedersen_commitment* commit,
+  const secp256k1_generator* gen
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(5) SECP256K1_ARG_NONNULL(6);
+
 # ifdef __cplusplus
 }
 # endif

src/modules/rangeproof/main_impl.h

@@ -304,4 +304,97 @@ int secp256k1_rangeproof_sign(const secp256k1_context* ctx, unsigned char *proof
      proof, plen, min_value, &commitp, blind, nonce, exp, min_bits, value, message, msg_len, extra_commit, extra_commit_len, &genp);
 }
 
+int secp256k1_rangeproof_verify_value(const secp256k1_context* ctx, const unsigned char* proof, size_t plen, uint64_t value, const secp256k1_pedersen_commitment* commit, const secp256k1_generator* gen) {
+    secp256k1_ge commitp;
+    secp256k1_ge genp;
+    secp256k1_gej tmpj;
+    secp256k1_gej xj;
+    secp256k1_ge rp;
+    secp256k1_scalar es;
+    secp256k1_scalar ss;
+    secp256k1_sha256 sha2;
+    unsigned char tmpch[33];
+    unsigned char pp_comm[32];
+    size_t offset;
+    size_t sz;
+    int overflow;
+
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(proof != NULL);
+    ARG_CHECK(commit != NULL);
+    ARG_CHECK(gen != NULL);
+
+    if (plen != 73 && plen != 65) {
+        return 0;
+    }
+    /* 0x80 must be unset for any rangeproof; 0x40 indicates "has nonzero range"
+     * so must also be unset for single-value proofs */
+    if ((proof[0] & 0xC0) != 0x00) {
+        return 0;
+    }
+
+    secp256k1_pedersen_commitment_load(&commitp, commit);
+    secp256k1_generator_load(&genp, gen);
+    /* Verify that value in the header is what we expect; 0x20 is "has nonzero min-value" */
+    if ((proof[0] & 0x20) == 0x00) {
+        if (value != 0) {
+            return 0;
+        }
+        offset = 1;
+    } else {
+        uint64_t claimed = 0;
+        /* Iterate from 0 to 8, setting `offset` to 9 as a side-effect */
+        for (offset = 1; offset < 9; offset++) {
+            claimed = (claimed << 8) | proof[offset];
+        }
+        if (value != claimed) {
+            return 0;
+        }
+    }
+    /* Subtract value from commitment; store modified commitment in xj */
+    secp256k1_pedersen_ecmult_small(&tmpj, value, &genp);
+    secp256k1_gej_neg(&tmpj, &tmpj);
+    secp256k1_gej_add_ge_var(&xj, &tmpj, &commitp, NULL);
+
+    /* Now we just have a Schnorr signature in (e, s) form. The verification
+     * equation is e == H(sG - eX || proof params) */
+
+    /* 1. Compute slow/overwrought commitment to proof params */
+    secp256k1_sha256_initialize(&sha2);
+    secp256k1_rangeproof_serialize_point(tmpch, &commitp);
+    secp256k1_sha256_write(&sha2, tmpch, 33);
+    secp256k1_rangeproof_serialize_point(tmpch, &genp);
+    secp256k1_sha256_write(&sha2, tmpch, 33);
+    secp256k1_sha256_write(&sha2, proof, offset); /* lol we commit to one extra byte here */
+    secp256k1_sha256_finalize(&sha2, pp_comm);
+
+    /* ... feed this into our hash */
+    secp256k1_borromean_hash(tmpch, pp_comm, 32, &proof[offset], 32, 0, 0);
+    secp256k1_scalar_set_b32(&es, tmpch, &overflow);
+    if (overflow || secp256k1_scalar_is_zero(&es)) {
+        return 0;
+    }
+
+    /* 1. Compute R = sG - eX */
+    secp256k1_scalar_set_b32(&ss, &proof[offset + 32], &overflow);
+    if (overflow || secp256k1_scalar_is_zero(&ss)) {
+        return 0;
+    }
+    secp256k1_ecmult(&tmpj, &xj, &es, &ss);
+    if (secp256k1_gej_is_infinity(&tmpj)) {
+        return 0;
+    }
+    secp256k1_ge_set_gej(&rp, &tmpj);
+    secp256k1_eckey_pubkey_serialize(&rp, tmpch, &sz, 1);
+
+    /* 2. Compute e = H(R || proof params) */
+    secp256k1_sha256_initialize(&sha2);
+    secp256k1_sha256_write(&sha2, tmpch, sz);
+    secp256k1_sha256_write(&sha2, pp_comm, sizeof(pp_comm));
+    secp256k1_sha256_finalize(&sha2, tmpch);
+
+    /* 3. Check computed e against original e */
+    return !memcmp(tmpch, &proof[offset], 32);
+}
+
 #endif

src/modules/rangeproof/tests_impl.h

@@ -919,6 +919,8 @@ void test_rangeproof_fixed_vectors(void) {
     CHECK(min_value == UINT64_MAX);
     CHECK(max_value == UINT64_MAX);
     CHECK(m_len == 0);
+
+    CHECK(secp256k1_rangeproof_verify_value(ctx, vector_3, sizeof(vector_3), UINT64_MAX, &pc, secp256k1_generator_h));
 }
 }