Update secp256k1 subtree to upstream libsecp256k1 version 0.2.0

5 files changed, 130 insertions, 63 deletions

diff --git a/src/secp256k1/include/secp256k1.h b/src/secp256k1/include/secp256k1.h
index dddab346ae..826ab75850 100644
--- a/src/secp256k1/include/secp256k1.h
+++ b/src/secp256k1/include/secp256k1.h
@@ -7,7 +7,7 @@ extern "C" {
 
 #include <stddef.h>
 
-/* Unless explicitly stated all pointer arguments must not be NULL.
+/** Unless explicitly stated all pointer arguments must not be NULL.
  *
  * The following rules specify the order of arguments in API calls:
  *
@@ -24,15 +24,19 @@ extern "C" {
  * 5. Opaque data pointers follow the function pointer they are to be passed to.
  */
 
-/** Opaque data structure that holds context information (precomputed tables etc.).
+/** Opaque data structure that holds context information
  *
- *  The purpose of context structures is to cache large precomputed data tables
- *  that are expensive to construct, and also to maintain the randomization data
- *  for blinding.
+ *  The primary purpose of context objects is to store randomization data for
+ *  enhanced protection against side-channel leakage. This protection is only
+ *  effective if the context is randomized after its creation. See
+ *  secp256k1_context_create for creation of contexts and
+ *  secp256k1_context_randomize for randomization.
  *
- *  Do not create a new context object for each operation, as construction is
- *  far slower than all other API calls (~100 times slower than an ECDSA
- *  verification).
+ *  A secondary purpose of context objects is to store pointers to callback
+ *  functions that the library will call when certain error states arise. See
+ *  secp256k1_context_set_error_callback as well as
+ *  secp256k1_context_set_illegal_callback for details. Future library versions
+ *  may use context objects for additional purposes.
  *
  *  A constructed context can safely be used from multiple threads
  *  simultaneously, but API calls that take a non-const pointer to a context
@@ -45,7 +49,7 @@ extern "C" {
  */
 typedef struct secp256k1_context_struct secp256k1_context;
 
-/** Opaque data structure that holds rewriteable "scratch space"
+/** Opaque data structure that holds rewritable "scratch space"
  *
  *  The purpose of this structure is to replace dynamic memory allocations,
  *  because we target architectures where this may not be available. It is
@@ -130,7 +134,7 @@ typedef int (*secp256k1_nonce_function)(
 #  define SECP256K1_INLINE inline
 # endif
 
-/** When this header is used at build-time the SECP256K1_BUILD define needs to be set
+/*  When this header is used at build-time the SECP256K1_BUILD define needs to be set
  *  to correctly setup export attributes and nullness checks.  This is normally done
  *  by secp256k1.c but to guard against this header being included before secp256k1.c
  *  has had a chance to set the define (e.g. via test harnesses that just includes
@@ -159,9 +163,9 @@ typedef int (*secp256k1_nonce_function)(
 # endif
 #endif
 
-/**Warning attributes
-  * NONNULL is not used if SECP256K1_BUILD is set to avoid the compiler optimizing out
-  * some paranoid null checks. */
+/* Warning attributes
+ * NONNULL is not used if SECP256K1_BUILD is set to avoid the compiler optimizing out
+ * some paranoid null checks. */
 # if defined(__GNUC__) && SECP256K1_GNUC_PREREQ(3, 4)
 #  define SECP256K1_WARN_UNUSED_RESULT __attribute__ ((__warn_unused_result__))
 # else
@@ -173,7 +177,7 @@ typedef int (*secp256k1_nonce_function)(
 #  define SECP256K1_ARG_NONNULL(_x)
 # endif
 
-/** Attribute for marking functions, types, and variables as deprecated */
+/* Attribute for marking functions, types, and variables as deprecated */
 #if !defined(SECP256K1_BUILD) && defined(__has_attribute)
 # if __has_attribute(__deprecated__)
 #  define SECP256K1_DEPRECATED(_msg) __attribute__ ((__deprecated__(_msg)))
@@ -184,22 +188,26 @@ typedef int (*secp256k1_nonce_function)(
 # define SECP256K1_DEPRECATED(_msg)
 #endif
 
-/** All flags' lower 8 bits indicate what they're for. Do not use directly. */
+/* All flags' lower 8 bits indicate what they're for. Do not use directly. */
 #define SECP256K1_FLAGS_TYPE_MASK ((1 << 8) - 1)
 #define SECP256K1_FLAGS_TYPE_CONTEXT (1 << 0)
 #define SECP256K1_FLAGS_TYPE_COMPRESSION (1 << 1)
-/** The higher bits contain the actual data. Do not use directly. */
+/* The higher bits contain the actual data. Do not use directly. */
 #define SECP256K1_FLAGS_BIT_CONTEXT_VERIFY (1 << 8)
 #define SECP256K1_FLAGS_BIT_CONTEXT_SIGN (1 << 9)
 #define SECP256K1_FLAGS_BIT_CONTEXT_DECLASSIFY (1 << 10)
 #define SECP256K1_FLAGS_BIT_COMPRESSION (1 << 8)
 
-/** Flags to pass to secp256k1_context_create, secp256k1_context_preallocated_size, and
+/** Context flags to pass to secp256k1_context_create, secp256k1_context_preallocated_size, and
  *  secp256k1_context_preallocated_create. */
+#define SECP256K1_CONTEXT_NONE (SECP256K1_FLAGS_TYPE_CONTEXT)
+
+/** Deprecated context flags. These flags are treated equivalent to SECP256K1_CONTEXT_NONE. */
 #define SECP256K1_CONTEXT_VERIFY (SECP256K1_FLAGS_TYPE_CONTEXT | SECP256K1_FLAGS_BIT_CONTEXT_VERIFY)
 #define SECP256K1_CONTEXT_SIGN (SECP256K1_FLAGS_TYPE_CONTEXT | SECP256K1_FLAGS_BIT_CONTEXT_SIGN)
+
+/* Testing flag. Do not use. */
 #define SECP256K1_CONTEXT_DECLASSIFY (SECP256K1_FLAGS_TYPE_CONTEXT | SECP256K1_FLAGS_BIT_CONTEXT_DECLASSIFY)
-#define SECP256K1_CONTEXT_NONE (SECP256K1_FLAGS_TYPE_CONTEXT)
 
 /** Flag to pass to secp256k1_ec_pubkey_serialize. */
 #define SECP256K1_EC_COMPRESSED (SECP256K1_FLAGS_TYPE_COMPRESSION | SECP256K1_FLAGS_BIT_COMPRESSION)
@@ -212,23 +220,66 @@ typedef int (*secp256k1_nonce_function)(
 #define SECP256K1_TAG_PUBKEY_HYBRID_EVEN 0x06
 #define SECP256K1_TAG_PUBKEY_HYBRID_ODD 0x07
 
-/** A simple secp256k1 context object with no precomputed tables. These are useful for
- *  type serialization/parsing functions which require a context object to maintain
- *  API consistency, but currently do not require expensive precomputations or dynamic
- *  allocations.
+/** A built-in constant secp256k1 context object with static storage duration, to be
+ *  used in conjunction with secp256k1_selftest.
+ *
+ *  This context object offers *only limited functionality* , i.e., it cannot be used
+ *  for API functions that perform computations involving secret keys, e.g., signing
+ *  and public key generation. If this restriction applies to a specific API function,
+ *  it is mentioned in its documentation. See secp256k1_context_create if you need a
+ *  full context object that supports all functionality offered by the library.
+ *
+ *  It is highly recommended to call secp256k1_selftest before using this context.
+ */
+SECP256K1_API extern const secp256k1_context *secp256k1_context_static;
+
+/** Deprecated alias for secp256k1_context_static. */
+SECP256K1_API extern const secp256k1_context *secp256k1_context_no_precomp
+SECP256K1_DEPRECATED("Use secp256k1_context_static instead");
+
+/** Perform basic self tests (to be used in conjunction with secp256k1_context_static)
+ *
+ *  This function performs self tests that detect some serious usage errors and
+ *  similar conditions, e.g., when the library is compiled for the wrong endianness.
+ *  This is a last resort measure to be used in production. The performed tests are
+ *  very rudimentary and are not intended as a replacement for running the test
+ *  binaries.
+ *
+ *  It is highly recommended to call this before using secp256k1_context_static.
+ *  It is not necessary to call this function before using a context created with
+ *  secp256k1_context_create (or secp256k1_context_preallocated_create), which will
+ *  take care of performing the self tests.
+ *
+ *  If the tests fail, this function will call the default error handler to abort the
+ *  program (see secp256k1_context_set_error_callback).
  */
-SECP256K1_API extern const secp256k1_context *secp256k1_context_no_precomp;
+SECP256K1_API void secp256k1_selftest(void);
+
 
 /** Create a secp256k1 context object (in dynamically allocated memory).
  *
  *  This function uses malloc to allocate memory. It is guaranteed that malloc is
  *  called at most once for every call of this function. If you need to avoid dynamic
- *  memory allocation entirely, see the functions in secp256k1_preallocated.h.
+ *  memory allocation entirely, see secp256k1_context_static and the functions in
+ *  secp256k1_preallocated.h.
  *
  *  Returns: a newly created context object.
- *  In:      flags: which parts of the context to initialize.
+ *  In:      flags: Always set to SECP256K1_CONTEXT_NONE (see below).
+ *
+ *  The only valid non-deprecated flag in recent library versions is
+ *  SECP256K1_CONTEXT_NONE, which will create a context sufficient for all functionality
+ *  offered by the library. All other (deprecated) flags will be treated as equivalent
+ *  to the SECP256K1_CONTEXT_NONE flag. Though the flags parameter primarily exists for
+ *  historical reasons, future versions of the library may introduce new flags.
  *
- *  See also secp256k1_context_randomize.
+ *  If the context is intended to be used for API functions that perform computations
+ *  involving secret keys, e.g., signing and public key generation, then it is highly
+ *  recommended to call secp256k1_context_randomize on the context before calling
+ *  those API functions. This will provide enhanced protection against side-channel
+ *  leakage, see secp256k1_context_randomize for details.
+ *
+ *  Do not create a new context object for each operation, as construction and
+ *  randomization can take non-negligible time.
  */
 SECP256K1_API secp256k1_context* secp256k1_context_create(
     unsigned int flags
@@ -308,7 +359,10 @@ SECP256K1_API void secp256k1_context_set_illegal_callback(
 ) SECP256K1_ARG_NONNULL(1);
 
 /** Set a callback function to be called when an internal consistency check
- *  fails. The default is crashing.
+ *  fails.
+ *
+ *  The default callback writes an error message to stderr and calls abort
+ *  to abort the program.
  *
  *  This can only trigger in case of a hardware failure, miscompilation,
  *  memory corruption, serious bug in the library, or other error would can
@@ -426,8 +480,8 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_cmp(
  *  encoding is invalid. R and S with value 0 are allowed in the encoding.
  *
  *  After the call, sig will always be initialized. If parsing failed or R or
- *  S are zero, the resulting sig value is guaranteed to fail validation for any
- *  message and public key.
+ *  S are zero, the resulting sig value is guaranteed to fail verification for
+ *  any message and public key.
  */
 SECP256K1_API int secp256k1_ecdsa_signature_parse_compact(
     const secp256k1_context* ctx,
@@ -447,7 +501,7 @@ SECP256K1_API int secp256k1_ecdsa_signature_parse_compact(
  *  encoded numbers are out of range.
  *
  *  After the call, sig will always be initialized. If parsing failed or the
- *  encoded numbers are out of range, signature validation with it is
+ *  encoded numbers are out of range, signature verification with it is
  *  guaranteed to fail for every message and public key.
  */
 SECP256K1_API int secp256k1_ecdsa_signature_parse_der(
@@ -494,7 +548,7 @@ SECP256K1_API int secp256k1_ecdsa_signature_serialize_compact(
  *
  *  Returns: 1: correct signature
  *           0: incorrect or unparseable signature
- *  Args:    ctx:       a secp256k1 context object, initialized for verification.
+ *  Args:    ctx:       a secp256k1 context object.
  *  In:      sig:       the signature being verified.
  *           msghash32: the 32-byte message hash being verified.
  *                      The verifier must make sure to apply a cryptographic
@@ -511,7 +565,7 @@ SECP256K1_API int secp256k1_ecdsa_signature_serialize_compact(
  *
  * If you need to accept ECDSA signatures from sources that do not obey this
  * rule, apply secp256k1_ecdsa_signature_normalize to the signature prior to
- * validation, but be aware that doing so results in malleable signatures.
+ * verification, but be aware that doing so results in malleable signatures.
  *
  * For details, see the comments for that function.
  */
@@ -582,7 +636,7 @@ SECP256K1_API extern const secp256k1_nonce_function secp256k1_nonce_function_def
  *
  *  Returns: 1: signature created
  *           0: the nonce generation function failed, or the secret key was invalid.
- *  Args:    ctx:       pointer to a context object, initialized for signing.
+ *  Args:    ctx:       pointer to a context object (not secp256k1_context_static).
  *  Out:     sig:       pointer to an array where the signature will be placed.
  *  In:      msghash32: the 32-byte message hash being signed.
  *           seckey:    pointer to a 32-byte secret key.
@@ -626,7 +680,7 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_seckey_verify(
  *
  *  Returns: 1: secret was valid, public key stores.
  *           0: secret was invalid, try again.
- *  Args:    ctx:    pointer to a context object, initialized for signing.
+ *  Args:    ctx:    pointer to a context object (not secp256k1_context_static).
  *  Out:     pubkey: pointer to the created public key.
  *  In:      seckey: pointer to a 32-byte secret key.
  */
@@ -705,7 +759,7 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_privkey_tweak_add(
  *  Returns: 0 if the arguments are invalid or the resulting public key would be
  *           invalid (only when the tweak is the negation of the corresponding
  *           secret key). 1 otherwise.
- *  Args:    ctx:   pointer to a context object initialized for validation.
+ *  Args:    ctx:   pointer to a context object.
  *  In/Out: pubkey: pointer to a public key object. pubkey will be set to an
  *                  invalid value if this function returns 0.
  *  In:    tweak32: pointer to a 32-byte tweak. If the tweak is invalid according to
@@ -750,7 +804,7 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_privkey_tweak_mul(
 /** Tweak a public key by multiplying it by a tweak value.
  *
  *  Returns: 0 if the arguments are invalid. 1 otherwise.
- *  Args:    ctx:   pointer to a context object initialized for validation.
+ *  Args:    ctx:   pointer to a context object.
  *  In/Out: pubkey: pointer to a public key object. pubkey will be set to an
  *                  invalid value if this function returns 0.
  *  In:    tweak32: pointer to a 32-byte tweak. If the tweak is invalid according to
@@ -764,30 +818,41 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_tweak_mul(
     const unsigned char *tweak32
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
 
-/** Updates the context randomization to protect against side-channel leakage.
- *  Returns: 1: randomization successfully updated or nothing to randomize
+/** Randomizes the context to provide enhanced protection against side-channel leakage.
+ *
+ *  Returns: 1: randomization successful (or called on copy of secp256k1_context_static)
  *           0: error
  *  Args:    ctx:       pointer to a context object.
  *  In:      seed32:    pointer to a 32-byte random seed (NULL resets to initial state)
  *
- * While secp256k1 code is written to be constant-time no matter what secret
- * values are, it's possible that a future compiler may output code which isn't,
+ * While secp256k1 code is written and tested to be constant-time no matter what
+ * secret values are, it is possible that a compiler may output code which is not,
  * and also that the CPU may not emit the same radio frequencies or draw the same
- * amount power for all values.
- *
- * This function provides a seed which is combined into the blinding value: that
- * blinding value is added before each multiplication (and removed afterwards) so
- * that it does not affect function results, but shields against attacks which
- * rely on any input-dependent behaviour.
- *
- * This function has currently an effect only on contexts initialized for signing
- * because randomization is currently used only for signing. However, this is not
- * guaranteed and may change in the future. It is safe to call this function on
- * contexts not initialized for signing; then it will have no effect and return 1.
- *
- * You should call this after secp256k1_context_create or
- * secp256k1_context_clone (and secp256k1_context_preallocated_create or
- * secp256k1_context_clone, resp.), and you may call this repeatedly afterwards.
+ * amount of power for all values. Randomization of the context shields against
+ * side-channel observations which aim to exploit secret-dependent behaviour in
+ * certain computations which involve secret keys.
+ *
+ * It is highly recommended to call this function on contexts returned from
+ * secp256k1_context_create or secp256k1_context_clone (or from the corresponding
+ * functions in secp256k1_preallocated.h) before using these contexts to call API
+ * functions that perform computations involving secret keys, e.g., signing and
+ * public key generation. It is possible to call this function more than once on
+ * the same context, and doing so before every few computations involving secret
+ * keys is recommended as a defense-in-depth measure.
+ *
+ * Currently, the random seed is mainly used for blinding multiplications of a
+ * secret scalar with the elliptic curve base point. Multiplications of this
+ * kind are performed by exactly those API functions which are documented to
+ * require a context that is not the secp256k1_context_static. As a rule of thumb,
+ * these are all functions which take a secret key (or a keypair) as an input.
+ * A notable exception to that rule is the ECDH module, which relies on a different
+ * kind of elliptic curve point multiplication and thus does not benefit from
+ * enhanced protection against side-channel leakage currently.
+ *
+ * It is safe call this function on a copy of secp256k1_context_static in writable
+ * memory (e.g., obtained via secp256k1_context_clone). In that case, this
+ * function is guaranteed to return 1, but the call will have no effect because
+ * the static context (or a copy thereof) is not meant to be randomized.
  */
 SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_context_randomize(
     secp256k1_context* ctx,
diff --git a/src/secp256k1/include/secp256k1_extrakeys.h b/src/secp256k1/include/secp256k1_extrakeys.h
index 09cbeaaa80..3591bc0012 100644
--- a/src/secp256k1/include/secp256k1_extrakeys.h
+++ b/src/secp256k1/include/secp256k1_extrakeys.h
@@ -108,7 +108,7 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_xonly_pubkey_from_pubke
  *           invalid (only when the tweak is the negation of the corresponding
  *           secret key). 1 otherwise.
  *
- *  Args:           ctx: pointer to a context object initialized for verification.
+ *  Args:           ctx: pointer to a context object.
  *  Out:  output_pubkey: pointer to a public key to store the result. Will be set
  *                       to an invalid value if this function returns 0.
  *  In: internal_pubkey: pointer to an x-only pubkey to apply the tweak to.
@@ -137,7 +137,7 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_xonly_pubkey_tweak_add(
  *
  *  Returns: 0 if the arguments are invalid or the tweaked pubkey is not the
  *           result of tweaking the internal_pubkey with tweak32. 1 otherwise.
- *  Args:            ctx: pointer to a context object initialized for verification.
+ *  Args:            ctx: pointer to a context object.
  *  In: tweaked_pubkey32: pointer to a serialized xonly_pubkey.
  *     tweaked_pk_parity: the parity of the tweaked pubkey (whose serialization
  *                        is passed in as tweaked_pubkey32). This must match the
@@ -159,7 +159,7 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_xonly_pubkey_tweak_add_
  *
  *  Returns: 1: secret was valid, keypair is ready to use
  *           0: secret was invalid, try again with a different secret
- *  Args:    ctx: pointer to a context object, initialized for signing.
+ *  Args:    ctx: pointer to a context object (not secp256k1_context_static).
  *  Out: keypair: pointer to the created keypair.
  *  In:   seckey: pointer to a 32-byte secret key.
  */
@@ -228,7 +228,7 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_keypair_xonly_pub(
  *           invalid (only when the tweak is the negation of the keypair's
  *           secret key). 1 otherwise.
  *
- *  Args:       ctx: pointer to a context object initialized for verification.
+ *  Args:       ctx: pointer to a context object.
  *  In/Out: keypair: pointer to a keypair to apply the tweak to. Will be set to
  *                   an invalid value if this function returns 0.
  *  In:     tweak32: pointer to a 32-byte tweak. If the tweak is invalid according
diff --git a/src/secp256k1/include/secp256k1_preallocated.h b/src/secp256k1/include/secp256k1_preallocated.h
index d2d9014f02..ed846f75f9 100644
--- a/src/secp256k1/include/secp256k1_preallocated.h
+++ b/src/secp256k1/include/secp256k1_preallocated.h
@@ -58,6 +58,8 @@ SECP256K1_API size_t secp256k1_context_preallocated_size(
  *                     bytes, as detailed above.
  *           flags:    which parts of the context to initialize.
  *
+ *  See secp256k1_context_create (in secp256k1.h) for further details.
+ *
  *  See also secp256k1_context_randomize (in secp256k1.h)
  *  and secp256k1_context_preallocated_destroy.
  */
diff --git a/src/secp256k1/include/secp256k1_recovery.h b/src/secp256k1/include/secp256k1_recovery.h
index 0e2847db96..824c604025 100644
--- a/src/secp256k1/include/secp256k1_recovery.h
+++ b/src/secp256k1/include/secp256k1_recovery.h
@@ -72,7 +72,7 @@ SECP256K1_API int secp256k1_ecdsa_recoverable_signature_serialize_compact(
  *
  *  Returns: 1: signature created
  *           0: the nonce generation function failed, or the secret key was invalid.
- *  Args:    ctx:       pointer to a context object, initialized for signing.
+ *  Args:    ctx:       pointer to a context object (not secp256k1_context_static).
  *  Out:     sig:       pointer to an array where the signature will be placed.
  *  In:      msghash32: the 32-byte message hash being signed.
  *           seckey:    pointer to a 32-byte secret key.
@@ -94,7 +94,7 @@ SECP256K1_API int secp256k1_ecdsa_sign_recoverable(
  *
  *  Returns: 1: public key successfully recovered (which guarantees a correct signature).
  *           0: otherwise.
- *  Args:    ctx:       pointer to a context object, initialized for verification.
+ *  Args:    ctx:       pointer to a context object.
  *  Out:     pubkey:    pointer to the recovered public key.
  *  In:      sig:       pointer to initialized signature that supports pubkey recovery.
  *           msghash32: the 32-byte message hash assumed to be signed.
diff --git a/src/secp256k1/include/secp256k1_schnorrsig.h b/src/secp256k1/include/secp256k1_schnorrsig.h
index 5fedcb07b0..e579e1b1d8 100644
--- a/src/secp256k1/include/secp256k1_schnorrsig.h
+++ b/src/secp256k1/include/secp256k1_schnorrsig.h
@@ -106,7 +106,7 @@ typedef struct {
  *  signatures from being valid in multiple contexts by accident.
  *
  *  Returns 1 on success, 0 on failure.
- *  Args:    ctx: pointer to a context object, initialized for signing.
+ *  Args:    ctx: pointer to a context object (not secp256k1_context_static).
  *  Out:   sig64: pointer to a 64-byte array to store the serialized signature.
  *  In:    msg32: the 32-byte message being signed.
  *       keypair: pointer to an initialized keypair.
@@ -161,7 +161,7 @@ SECP256K1_API int secp256k1_schnorrsig_sign_custom(
  *
  *  Returns: 1: correct signature
  *           0: incorrect signature
- *  Args:    ctx: a secp256k1 context object, initialized for verification.
+ *  Args:    ctx: a secp256k1 context object.
  *  In:    sig64: pointer to the 64-byte signature to verify.
  *           msg: the message being verified. Can only be NULL if msglen is 0.
  *        msglen: length of the message