diff --git a/include/secp256k1.h b/include/secp256k1.h index 7b48e565f686d..112e0360142fc 100644 --- a/include/secp256k1.h +++ b/include/secp256k1.h @@ -26,11 +26,17 @@ extern "C" { /** Opaque data structure that holds context information * - * The purpose of context structures is to store the randomization data for - * blinding, see secp256k1_context_randomize. - * - * Do not create a new context object for each operation, as construction is - * far slower than all other API calls. + * 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. + * + * 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 @@ -43,7 +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 @@ -265,6 +271,15 @@ SECP256K1_API void secp256k1_selftest(void); * 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. + * + * 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 @@ -344,7 +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 @@ -800,30 +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,