diff --git a/Makefile.am b/Makefile.am index 48ee2fc..e3fdf4d 100644 --- a/Makefile.am +++ b/Makefile.am @@ -69,7 +69,7 @@ noinst_HEADERS += contrib/lax_der_parsing.h noinst_HEADERS += contrib/lax_der_parsing.c noinst_HEADERS += contrib/lax_der_privatekey_parsing.h noinst_HEADERS += contrib/lax_der_privatekey_parsing.c -noinst_HEADERS += examples/random.h +noinst_HEADERS += examples/examples_util.h PRECOMPUTED_LIB = libsecp256k1_precomputed.la noinst_LTLIBRARIES = $(PRECOMPUTED_LIB) diff --git a/examples/ecdh.c b/examples/ecdh.c index 027d52f..4b7b7d6 100644 --- a/examples/ecdh.c +++ b/examples/ecdh.c @@ -14,8 +14,7 @@ #include #include -#include "random.h" - +#include "examples_util.h" int main(void) { unsigned char seckey1[32]; @@ -112,12 +111,12 @@ int main(void) { * example through "out of bounds" array access (see Heartbleed), Or the OS * swapping them to disk. Hence, we overwrite the secret key buffer with zeros. * - * TODO: Prevent these writes from being optimized out, as any good compiler + * Here we are preventing these writes from being optimized out, as any good compiler * will remove any writes that aren't used. */ - memset(seckey1, 0, sizeof(seckey1)); - memset(seckey2, 0, sizeof(seckey2)); - memset(shared_secret1, 0, sizeof(shared_secret1)); - memset(shared_secret2, 0, sizeof(shared_secret2)); + secure_erase(seckey1, sizeof(seckey1)); + secure_erase(seckey2, sizeof(seckey2)); + secure_erase(shared_secret1, sizeof(shared_secret1)); + secure_erase(shared_secret2, sizeof(shared_secret2)); return 0; } diff --git a/examples/ecdsa.c b/examples/ecdsa.c index 01088e3..d1d2b0e 100644 --- a/examples/ecdsa.c +++ b/examples/ecdsa.c @@ -13,9 +13,7 @@ #include -#include "random.h" - - +#include "examples_util.h" int main(void) { /* Instead of signing the message directly, we must sign a 32-byte hash. @@ -133,9 +131,9 @@ int main(void) { * example through "out of bounds" array access (see Heartbleed), Or the OS * swapping them to disk. Hence, we overwrite the secret key buffer with zeros. * - * TODO: Prevent these writes from being optimized out, as any good compiler + * Here we are preventing these writes from being optimized out, as any good compiler * will remove any writes that aren't used. */ - memset(seckey, 0, sizeof(seckey)); + secure_erase(seckey, sizeof(seckey)); return 0; } diff --git a/examples/random.h b/examples/examples_util.h similarity index 69% rename from examples/random.h rename to examples/examples_util.h index 439226f..a52b1fa 100644 --- a/examples/random.h +++ b/examples/examples_util.h @@ -71,3 +71,32 @@ static void print_hex(unsigned char* data, size_t size) { } printf("\n"); } + +#if defined(_MSC_VER) +// For SecureZeroMemory +#include +#endif +/* Cleanses memory to prevent leaking sensitive info. Won't be optimized out. */ +static SECP256K1_INLINE void secure_erase(void *ptr, size_t len) { +#if defined(_MSC_VER) + /* SecureZeroMemory is guaranteed not to be optimized out by MSVC. */ + SecureZeroMemory(ptr, len); +#elif defined(__GNUC__) + /* We use a memory barrier that scares the compiler away from optimizing out the memset. + * + * Quoting Adam Langley in commit ad1907fe73334d6c696c8539646c21b11178f20f + * in BoringSSL (ISC License): + * As best as we can tell, this is sufficient to break any optimisations that + * might try to eliminate "superfluous" memsets. + * This method used in memzero_explicit() the Linux kernel, too. Its advantage is that it is + * pretty efficient, because the compiler can still implement the memset() efficently, + * just not remove it entirely. See "Dead Store Elimination (Still) Considered Harmful" by + * Yang et al. (USENIX Security 2017) for more background. + */ + memset(ptr, 0, len); + __asm__ __volatile__("" : : "r"(ptr) : "memory"); +#else + void *(*volatile const volatile_memset)(void *, int, size_t) = memset; + volatile_memset(ptr, 0, len); +#endif +} diff --git a/examples/schnorr.c b/examples/schnorr.c index 535b59a..4c0dd1c 100644 --- a/examples/schnorr.c +++ b/examples/schnorr.c @@ -15,7 +15,7 @@ #include #include -#include "random.h" +#include "examples_util.h" int main(void) { unsigned char msg[12] = "Hello World!"; @@ -149,9 +149,8 @@ int main(void) { * example through "out of bounds" array access (see Heartbleed), Or the OS * swapping them to disk. Hence, we overwrite the secret key buffer with zeros. * - * TODO: Prevent these writes from being optimized out, as any good compiler + * Here we are preventing these writes from being optimized out, as any good compiler * will remove any writes that aren't used. */ - memset(seckey, 0, sizeof(seckey)); - + secure_erase(seckey, sizeof(seckey)); return 0; }