Allow sign-to-contract commitments in schnorrsigs

.gitignore

@@ -1,9 +1,9 @@
 bench_inv
 bench_ecdh
 bench_ecmult
+bench_schnorrsig
 bench_sign
 bench_verify
-bench_schnorr_verify
 bench_recover
 bench_internal
 tests

Makefile.am

@@ -178,6 +178,10 @@ if ENABLE_MODULE_ECDH
 include src/modules/ecdh/Makefile.am.include
 endif
 
+if ENABLE_MODULE_SCHNORRSIG
+include src/modules/schnorrsig/Makefile.am.include
+endif
+
 if ENABLE_MODULE_RECOVERY
 include src/modules/recovery/Makefile.am.include
 endif

configure.ac

@@ -129,6 +129,11 @@ AC_ARG_ENABLE(module_ecdh,
     [enable_module_ecdh=$enableval],
     [enable_module_ecdh=no])
 
+AC_ARG_ENABLE(module_schnorrsig,
+    AS_HELP_STRING([--enable-module-schnorrsig],[enable schnorrsig module (experimental)]),
+    [enable_module_schnorrsig=$enableval],
+    [enable_module_schnorrsig=no])
+
 AC_ARG_ENABLE(module_recovery,
     AS_HELP_STRING([--enable-module-recovery],[enable ECDSA pubkey recovery module (default is no)]),
     [enable_module_recovery=$enableval],
@@ -431,6 +436,10 @@ if test x"$enable_module_ecdh" = x"yes"; then
   AC_DEFINE(ENABLE_MODULE_ECDH, 1, [Define this symbol to enable the ECDH module])
 fi
 
+if test x"$enable_module_schnorrsig" = x"yes"; then
+  AC_DEFINE(ENABLE_MODULE_SCHNORRSIG, 1, [Define this symbol to enable the schnorrsig module])
+fi
+
 if test x"$enable_module_recovery" = x"yes"; then
   AC_DEFINE(ENABLE_MODULE_RECOVERY, 1, [Define this symbol to enable the ECDSA pubkey recovery module])
 fi
@@ -458,11 +467,15 @@ if test x"$enable_experimental" = x"yes"; then
   AC_MSG_NOTICE([WARNING: experimental build])
   AC_MSG_NOTICE([Experimental features do not have stable APIs or properties, and may not be safe for production use.])
   AC_MSG_NOTICE([Building ECDH module: $enable_module_ecdh])
+  AC_MSG_NOTICE([Building schnorrsig module: $enable_module_schnorrsig])
   AC_MSG_NOTICE([******])
 else
   if test x"$enable_module_ecdh" = x"yes"; then
     AC_MSG_ERROR([ECDH module is experimental. Use --enable-experimental to allow.])
   fi
+  if test x"$enable_module_schnorrsig" = x"yes"; then
+    AC_MSG_ERROR([schnorrsig module is experimental. Use --enable-experimental to allow.])
+  fi
   if test x"$set_asm" = x"arm"; then
     AC_MSG_ERROR([ARM assembly optimization is experimental. Use --enable-experimental to allow.])
   fi
@@ -481,6 +494,7 @@ AM_CONDITIONAL([USE_EXHAUSTIVE_TESTS], [test x"$use_exhaustive_tests" != x"no"])
 AM_CONDITIONAL([USE_BENCHMARK], [test x"$use_benchmark" = x"yes"])
 AM_CONDITIONAL([USE_ECMULT_STATIC_PRECOMPUTATION], [test x"$set_precomp" = x"yes"])
 AM_CONDITIONAL([ENABLE_MODULE_ECDH], [test x"$enable_module_ecdh" = x"yes"])
+AM_CONDITIONAL([ENABLE_MODULE_SCHNORRSIG], [test x"$enable_module_schnorrsig" = x"yes"])
 AM_CONDITIONAL([ENABLE_MODULE_RECOVERY], [test x"$enable_module_recovery" = x"yes"])
 AM_CONDITIONAL([USE_JNI], [test x"$use_jni" == x"yes"])
 AM_CONDITIONAL([USE_EXTERNAL_ASM], [test x"$use_external_asm" = x"yes"])

include/secp256k1.h

@@ -80,9 +80,34 @@ typedef struct {
     unsigned char data[64];
 } secp256k1_ecdsa_signature;
 
+/** Data structure that holds a sign-to-contract ("s2c") context. Sign-to-contract
+ *  allows a signer to commit to some data as part of a signature. If the nonce
+ *  function supports sign-to-contract, after creating it with
+ *  secp256k1_s2c_commit_context_create the context can be given to a signing
+ *  algorithm via the nonce data argument.
+ *
+ *  This structure is not opaque, but it is strongly discouraged to read or write to
+ *  it directly. Use the secp256k1_s2c_commit_* instead to access a sign-to-contract
+ *  context.
+ *
+ *  The exact representation of data inside is implementation defined and not
+ *  guaranteed to be portable between different platforms or versions. It is however
+ *  guaranteed to be 136 bytes in size, and can be safely copied/moved.
+ */
+typedef struct {
+    /* magic is set during initialization. It allows functions casting to
+     * s2c_commit_contexts from a void pointer to check if they actually got an
+     * s2c_commit_context and if it has been initialized. */
+    unsigned char magic[8];
+    unsigned char data[32];
+    unsigned char data_hash[32];
+    secp256k1_pubkey original_pubnonce;
+} secp256k1_s2c_commit_context;
+
 /** A pointer to a function to deterministically generate a nonce.
  *
  * Returns: 1 if a nonce was successfully generated. 0 will cause signing to fail.
+ * Args:    ctx:       an existing context object
  * Out:     nonce32:   pointer to a 32-byte array to be filled by the function.
  * In:      msg32:     the 32-byte message hash being verified (will not be NULL)
  *          key32:     pointer to a 32-byte secret key (will not be NULL)
@@ -97,6 +122,7 @@ typedef struct {
  * the message, the algorithm, the key and the attempt.
  */
 typedef int (*secp256k1_nonce_function)(
+    const secp256k1_context *ctx,
     unsigned char *nonce32,
     const unsigned char *msg32,
     const unsigned char *key32,
@@ -469,6 +495,36 @@ SECP256K1_API int secp256k1_ecdsa_signature_normalize(
     const secp256k1_ecdsa_signature *sigin
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(3);
 
+/** Creates a sign-to-contract context.
+ *
+ *  Returns: 1 if the context was created successfully, 0 otherwise
+ *  Args:    ctx: a secp256k1 context object
+ *  Out: s2c_ctx: pointer to an s2c context to initialize (cannot be NULL)
+ *  In:   data32: the 32-byte data to commit to (cannot be NULL)
+ */
+SECP256K1_API int secp256k1_s2c_commit_context_create(
+    secp256k1_context *ctx,
+    secp256k1_s2c_commit_context *s2c_ctx,
+    const unsigned char *data32
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
+
+/** Gets the original nonce of a sign-to-contract commitment from an s2c_ctx after
+ *  signing. The original nonce is the signature nonce minus the s2c commitment
+ *  tweak. Together with the committed data this is the opening of the commitment.
+ *
+ *  Returns: 1 if getting the original nonce was successful, 0 otherwise
+ *  Args:           ctx: a secp256k1 context object
+ *  Out: original_nonce: pointer to a pubkey object where the original nonce will be
+ *                       placed  (cannot be NULL)
+ *  In:         s2c_ctx: pointer to an s2c context to get the original nonce from
+ *                       after signing. (cannot be NULL)
+ */
+SECP256K1_API int secp256k1_s2c_commit_get_original_nonce(
+    secp256k1_context *ctx,
+    secp256k1_pubkey *original_nonce,
+    const secp256k1_s2c_commit_context *s2c_ctx
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
+
 /** An implementation of RFC6979 (using HMAC-SHA256) as nonce generation function.
  * If a data pointer is passed, it is assumed to be a pointer to 32 bytes of
  * extra entropy.

include/secp256k1_schnorrsig.h

@@ -0,0 +1,141 @@
+#ifndef SECP256K1_SCHNORRSIG_H
+#define SECP256K1_SCHNORRSIG_H
+
+/** This module implements a variant of Schnorr signatures compliant with
+ * BIP-schnorr
+ * (https://github.com/sipa/bips/blob/bip-schnorr/bip-schnorr.mediawiki).
+ */
+
+/** Opaque data structure that holds a parsed Schnorr signature.
+ *
+ *  The exact representation of data inside is implementation defined and not
+ *  guaranteed to be portable between different platforms or versions. It is
+ *  however guaranteed to be 64 bytes in size, and can be safely copied/moved.
+ *  If you need to convert to a format suitable for storage, transmission, or
+ *  comparison, use the `secp256k1_schnorrsig_serialize` and
+ *  `secp256k1_schnorrsig_parse` functions.
+ */
+typedef struct {
+    unsigned char data[64];
+} secp256k1_schnorrsig;
+
+/** Serialize a Schnorr signature.
+ *
+ *  Returns: 1
+ *  Args:    ctx: a secp256k1 context object
+ *  Out:   out64: pointer to a 64-byte array to store the serialized signature
+ *  In:      sig: pointer to the signature
+ *
+ *  See secp256k1_schnorrsig_parse for details about the encoding.
+ */
+SECP256K1_API int secp256k1_schnorrsig_serialize(
+    const secp256k1_context* ctx,
+    unsigned char *out64,
+    const secp256k1_schnorrsig* sig
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
+
+/** Parse a Schnorr signature.
+ *
+ *  Returns: 1 when the signature could be parsed, 0 otherwise.
+ *  Args:    ctx: a secp256k1 context object
+ *  Out:     sig: pointer to a signature object
+ *  In:     in64: pointer to the 64-byte signature to be parsed
+ *
+ * The signature is serialized in the form R||s, where R is a 32-byte public
+ * key (x-coordinate only; the y-coordinate is considered to be the unique
+ * y-coordinate satisfying the curve equation that is a quadratic residue)
+ * and s is a 32-byte big-endian scalar.
+ *
+ * After the call, sig will always be initialized. If parsing failed or the
+ * encoded numbers are out of range, signature validation with it is
+ * guaranteed to fail for every message and public key.
+ */
+SECP256K1_API int secp256k1_schnorrsig_parse(
+    const secp256k1_context* ctx,
+    secp256k1_schnorrsig* sig,
+    const unsigned char *in64
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
+
+/** Create a Schnorr signature.
+ *
+ * Returns 1 on success, 0 on failure.
+ *  Args:    ctx: pointer to a context object, initialized for signing (cannot be NULL)
+ *  Out:     sig: pointer to the returned signature (cannot be NULL)
+ *       nonce_is_negated: a pointer to an integer indicates if signing algorithm negated the
+ *                nonce (can be NULL)
+ *  In:    msg32: the 32-byte message hash being signed (cannot be NULL)
+ *        seckey: pointer to a 32-byte secret key (cannot be NULL)
+ *       noncefp: pointer to a nonce generation function. If NULL,
+ *                secp256k1_nonce_function_bipschnorr is used
+ *         ndata: pointer to arbitrary data used by the nonce generation function. If non-NULL must
+ *                be a pointer to an s2c_context object when using the default nonce function
+ *                secp256k1_nonce_function_bipschnorr. s2c_context must be initialized with
+ *                secp256k1_s2c_commit_context_create. (can be NULL)
+ */
+SECP256K1_API int secp256k1_schnorrsig_sign(
+    const secp256k1_context* ctx,
+    secp256k1_schnorrsig *sig,
+    int *nonce_is_negated,
+    const unsigned char *msg32,
+    const unsigned char *seckey,
+    secp256k1_nonce_function noncefp,
+    void *ndata
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5);
+
+/** Verify a Schnorr signature.
+ *
+ *  Returns: 1: correct signature
+ *           0: incorrect or unparseable signature
+ *  Args:    ctx: a secp256k1 context object, initialized for verification.
+ *  In:      sig: the signature being verified (cannot be NULL)
+ *         msg32: the 32-byte message hash being verified (cannot be NULL)
+ *        pubkey: pointer to a public key to verify with (cannot be NULL)
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_schnorrsig_verify(
+    const secp256k1_context* ctx,
+    const secp256k1_schnorrsig *sig,
+    const unsigned char *msg32,
+    const secp256k1_pubkey *pubkey
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
+
+/** Verifies a set of Schnorr signatures.
+ *
+ * Returns 1 if all succeeded, 0 otherwise. In particular, returns 1 if n_sigs is 0.
+ *
+ *  Args:    ctx: a secp256k1 context object, initialized for verification.
+ *       scratch: scratch space used for the multiexponentiation
+ *  In:      sig: array of signatures, or NULL if there are no signatures
+ *         msg32: array of messages, or NULL if there are no signatures
+ *            pk: array of public keys, or NULL if there are no signatures
+ *        n_sigs: number of signatures in above arrays. Must be smaller than
+ *                2^31 and smaller than half the maximum size_t value. Must be 0
+ *                if above arrays are NULL.
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_schnorrsig_verify_batch(
+    const secp256k1_context* ctx,
+    secp256k1_scratch_space *scratch,
+    const secp256k1_schnorrsig *const *sig,
+    const unsigned char *const *msg32,
+    const secp256k1_pubkey *const *pk,
+    size_t n_sigs
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2);
+
+/** Verify a sign-to-contract commitment.
+ *
+ *  Returns: 1: the signature contains a commitment to data32
+ *           0: incorrect opening
+ *  Args:    ctx: a secp256k1 context object, initialized for verification.
+ *  In:      sig: the signature containing the sign-to-contract commitment (cannot be NULL)
+ *        data32: the 32-byte data that was committed to (cannot be NULL)
+ *       original_nonce: pointer to the original_nonce created when signing (cannot be NULL)
+ * negated_nonce: integer indicating if signing algorithm negated the nonce
+ */
+SECP256K1_API int secp256k1_schnorrsig_verify_s2c_commit(
+    const secp256k1_context* ctx,
+    const secp256k1_schnorrsig *sig,
+    const unsigned char *data32,
+    const secp256k1_pubkey *original_nonce,
+    int negated_nonce
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
+
+#endif