rangeproof: add secp256k1_rangeproof_max_size function to estimate rangeproof size

include/secp256k1_rangeproof.h

@@ -10,6 +10,15 @@ extern "C" {
 
 #include <stdint.h>
 
+/** Length of a message that can be embedded into a maximally-sized rangeproof
+ *
+ * It is not be possible to fit a message of this size into a non-maximally-sized
+ * rangeproof, but it is guaranteed that any embeddable message can fit into an
+ * array of this size. This constant is intended to be used for memory allocations
+ * and sanity checks.
+ */
+#define SECP256K1_RANGEPROOF_MAX_MESSAGE_LEN 3968
+
 /** Opaque data structure that stores a Pedersen commitment
  *
  *  The exact representation of data inside is implementation defined and not
@@ -287,6 +296,33 @@ 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);
 
+/** Returns an upper bound on the size of a rangeproof with the given parameters
+ *
+ * An actual rangeproof may be smaller, for example if the actual value
+ * is less than both the provided `max_value` and 2^`min_bits`, or if
+ * the `exp` parameter to `secp256k1_rangeproof_sign` is set such that
+ * the proven range is compressed. In particular this function will always
+ * overestimate the size of single-value proofs. Also, if `min_value`
+ * is set to 0 in the proof, the result will usually, but not always,
+ * be 8 bytes smaller than if a nonzero value had been passed.
+ *
+ * The goal of this function is to provide a useful upper bound for
+ * memory allocation or fee estimation purposes, without requiring
+ * too many parameters be fixed in advance.
+ *
+ * To obtain the size of largest possible proof, set `max_value` to
+ * `UINT64_MAX` (and `min_bits` to any valid value such as 0).
+ *
+ *  In:       ctx: pointer to a context object
+ *      max_value: the maximum value that might be passed for `value` for the proof.
+ *       min_bits: the value that will be passed as `min_bits` for the proof.
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT size_t secp256k1_rangeproof_max_size(
+  const secp256k1_context* ctx,
+  uint64_t max_value,
+  int min_bits
+) SECP256K1_ARG_NONNULL(1);
+
 # ifdef __cplusplus
 }
 # endif

src/modules/rangeproof/main_impl.h

@@ -304,4 +304,16 @@ 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);
 }
 
+size_t secp256k1_rangeproof_max_size(const secp256k1_context* ctx, uint64_t max_value, int min_bits) {
+    const int val_mantissa = max_value > 0 ? 64 - secp256k1_clz64_var(max_value) : 1;
+    const int mantissa = min_bits > val_mantissa ? min_bits : val_mantissa;
+    const size_t rings = (mantissa + 1) / 2;
+    const size_t npubs = rings * 4 - 2 * (mantissa % 2);
+
+    VERIFY_CHECK(ctx != NULL);
+    (void) ctx;
+
+    return 10 + 32 * (npubs + rings - 1) + 32 + ((rings - 1 + 7) / 8);
+}
+
 #endif