diff --git a/README.md b/README.md
index 3ee2915..fcfb6a7 100644
--- a/README.md
+++ b/README.md
@@ -43,6 +43,7 @@ cd pixiewps*/
 cd src/
 make
 ```
+Optionally, you can run make OPENSSL=1 to use faster OpenSSL SHA256 functions.
 
 **Install**
 
diff --git a/src/Makefile b/src/Makefile
index d3f728f..04f1754 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -1,12 +1,20 @@
-CFLAGS ?= -std=c99 -O3
+CFLAGS = -std=c99 -O3
+
+LIBS   = -lpthread
+ifeq ($(OPENSSL),1)
+LIBS += -lcrypto
+CFLAGS += -DUSE_OPENSSL
+else
+CRYPTO = mbedtls/sha256.c mbedtls/md.c mbedtls/md_wrap.c
+endif
+
+PREFIX = /usr
+BINDIR = $(PREFIX)/bin
 
 TARGET = pixiewps
-CRYPTO = mbedtls/sha256.c mbedtls/md.c mbedtls/md_wrap.c
 SOURCE = $(TARGET).c $(CRYPTO)
-LIBS   = -lpthread
 
-PREFIX ?= /usr
-BINDIR = $(PREFIX)/bin
+-include config.mak
 
 all:
 	$(CC) $(CFLAGS) $(CPPFLAGS) -o $(TARGET) $(SOURCE) $(LIBS) $(LDFLAGS)
diff --git a/src/config.h b/src/config.h
index 41ba711..f73845f 100644
--- a/src/config.h
+++ b/src/config.h
@@ -25,9 +25,9 @@
 #include "mbedtls/md_internal.h"
 #include "mbedtls/sha256.h"
 
-#define sha256(i, l, d) mbedtls_sha256(i, l, d, 0)
+#define sha256(i, l, d) sha256_full(i, l, d)
 #define hmac_sha256(k, l, i, n, o) \
-	mbedtls_md_hmac(mbedtls_md_info_from_type(MBEDTLS_MD_SHA256), k, l, i, n, o)
+	hmac_sha256_full(k, l, i, n, o)
 
 #define u8  uint8_t
 #define u16	uint16_t
diff --git a/src/crypto/hmac_sha256.c b/src/crypto/hmac_sha256.c
new file mode 100644
index 0000000..481d1b8
--- /dev/null
+++ b/src/crypto/hmac_sha256.c
@@ -0,0 +1,119 @@
+/* public domain hmac_sha256 implementation written by rofl0r for pixiewps */
+
+#include <stdint.h>
+#include <string.h>
+#ifdef USE_OPENSSL
+#include <openssl/sha.h>
+#else
+#include "../mbedtls/sha256.h"
+#define SHA256_CTX mbedtls_sha256_context
+#define SHA256_Init(x) do { mbedtls_sha256_init(x); mbedtls_sha256_starts(x, 0); } while(0)
+#define SHA256_Update(x, y, z) mbedtls_sha256_update(x, y, z)
+#define SHA256_Final(y, x) mbedtls_sha256_finish(x, y)
+#endif
+
+#define PAD_SIZE  64
+#define HASH_SIZE 32
+
+static void sha256_full(const uint8_t *input, size_t ilen, uint8_t *output)
+{
+	SHA256_CTX ctx;
+	SHA256_Init(&ctx);
+	SHA256_Update(&ctx, input, ilen);
+	SHA256_Final(output, &ctx);
+}
+
+static void hmac_sha256_full(const uint8_t *key, size_t keylen,
+	const uint8_t *input, size_t ilen, uint8_t *output)
+{
+	size_t i;
+	uint8_t opad[PAD_SIZE], ipad[PAD_SIZE], hash[HASH_SIZE];
+	SHA256_CTX ctx;
+
+	memset(ipad, 0x36, PAD_SIZE);
+	memset(opad, 0x5C, PAD_SIZE);
+
+	if (keylen > PAD_SIZE) {
+
+		SHA256_Init(&ctx);
+		SHA256_Update(&ctx, key, keylen);
+		SHA256_Final(hash, &ctx);
+
+		for (i = 0; i < HASH_SIZE; i++) {
+			ipad[i] ^= hash[i];
+			opad[i] ^= hash[i];
+		}
+
+	} else for (i = 0; i < keylen; i++) {
+		ipad[i] ^= key[i];
+		opad[i] ^= key[i];
+	}
+
+	SHA256_Init(&ctx);
+	SHA256_Update(&ctx, ipad, PAD_SIZE);
+	SHA256_Update(&ctx, input, ilen);
+	SHA256_Final(hash, &ctx);
+
+	SHA256_Init(&ctx);
+	SHA256_Update(&ctx, opad, PAD_SIZE);
+	SHA256_Update(&ctx, hash, sizeof hash);
+
+	SHA256_Final(output, &ctx);
+}
+
+struct hmac_ctx {
+	SHA256_CTX ictx;
+	SHA256_CTX octx;
+};
+
+static void hmac_sha256_init(struct hmac_ctx *hctx, const uint8_t *key, size_t keylen)
+{
+	size_t i;
+	uint8_t opad[PAD_SIZE], ipad[PAD_SIZE], hash[HASH_SIZE];
+	SHA256_CTX ctx;
+
+	memset(ipad, 0x36, PAD_SIZE);
+	memset(opad, 0x5C, PAD_SIZE);
+
+	if (keylen > PAD_SIZE) {
+
+		SHA256_Init(&ctx);
+		SHA256_Update(&ctx, key, keylen);
+		SHA256_Final(hash, &ctx);
+
+		for (i = 0; i<HASH_SIZE; i++) {
+			ipad[i] ^= hash[i];
+			opad[i] ^= hash[i];
+		}
+
+	} else for (i = 0; i < keylen; i++) {
+		ipad[i] ^= key[i];
+		opad[i] ^= key[i];
+	}
+
+	SHA256_Init(&hctx->ictx);
+	SHA256_Update(&hctx->ictx, ipad, PAD_SIZE);
+
+	SHA256_Init(&hctx->octx);
+	SHA256_Update(&hctx->octx, opad, PAD_SIZE);
+}
+
+
+static void hmac_sha256_yield(const struct hmac_ctx *hctx,
+	const uint8_t *input, size_t ilen, uint8_t *output)
+{
+	SHA256_CTX ctx;
+	uint8_t hash[HASH_SIZE];
+
+	memcpy(&ctx, &hctx->ictx, sizeof(ctx));
+
+	SHA256_Update(&ctx, input, ilen);
+	SHA256_Final(hash, &ctx);
+
+
+	memcpy(&ctx, &hctx->octx, sizeof(ctx));
+
+	SHA256_Update(&ctx, hash, sizeof hash);
+	SHA256_Final(output, &ctx);
+}
+
diff --git a/src/pixiewps.c b/src/pixiewps.c
index 25eed09..304fb46 100644
--- a/src/pixiewps.c
+++ b/src/pixiewps.c
@@ -38,6 +38,7 @@
 #include "pixiewps.h"
 #include "crypto/crypto_internal-modexp.c"
 #include "crypto/aes-cbc.c"
+#include "crypto/hmac_sha256.c"
 #include "utils.h"
 #include "wps.h"
 #include "version.h"
@@ -1482,17 +1483,17 @@ static int int_pow(int a, int exp)
 }
 
 /* return non-zero if pin half is correct, zero otherwise */
-static int check_pin_half(const char pinhalf[4], uint8_t *psk, const uint8_t *es, struct global *wps, const uint8_t *ehash)
+static int check_pin_half(const struct hmac_ctx *hctx, const char pinhalf[4], uint8_t *psk, const uint8_t *es, struct global *wps, const uint8_t *ehash)
 {
 	uint8_t buffer[WPS_SECRET_NONCE_LEN + WPS_PSK_LEN + WPS_PKEY_LEN * 2];
 	uint8_t result[WPS_HASH_LEN];
 
-	hmac_sha256(wps->authkey, WPS_AUTHKEY_LEN, (unsigned char *)pinhalf, 4, psk);
+	hmac_sha256_yield(hctx, pinhalf, 4, psk);
 	memcpy(buffer, es, WPS_SECRET_NONCE_LEN);
 	memcpy(buffer + WPS_SECRET_NONCE_LEN, psk, WPS_PSK_LEN);
 	memcpy(buffer + WPS_SECRET_NONCE_LEN + WPS_PSK_LEN, wps->pke, WPS_PKEY_LEN);
 	memcpy(buffer + WPS_SECRET_NONCE_LEN + WPS_PSK_LEN + WPS_PKEY_LEN, wps->pkr, WPS_PKEY_LEN);
-	hmac_sha256(wps->authkey, WPS_AUTHKEY_LEN, buffer, sizeof buffer, result);
+	hmac_sha256_yield(hctx, buffer, sizeof buffer, result);
 
 	return !memcmp(result, ehash, WPS_HASH_LEN);
 }
@@ -1525,10 +1526,12 @@ static int crack_first_half(struct global *wps, char *pin, const uint8_t *es1_ov
 
 	unsigned first_half;
 	uint8_t psk[WPS_HASH_LEN];
+	struct hmac_ctx hc;
+	hmac_sha256_init(&hc, wps->authkey, WPS_AUTHKEY_LEN);
 
 	for (first_half = 0; first_half < 10000; first_half++) {
 		uint_to_char_array(first_half, 4, pin);
-		if (check_pin_half(pin, psk, es1, wps, wps->e_hash1)) {
+		if (check_pin_half(&hc, pin, psk, es1, wps, wps->e_hash1)) {
 			pin[4] = 0; /* make sure pin string is zero-terminated */
 			memcpy(wps->psk1, psk, sizeof psk);
 			return 1;
@@ -1549,12 +1552,15 @@ static int crack_second_half(struct global *wps, char *pin)
 	unsigned second_half, first_half = atoi(pin);
 	char *s_pin = pin + strlen(pin);
 	uint8_t psk[WPS_HASH_LEN];
+	struct hmac_ctx hc;
+	hmac_sha256_init(&hc, wps->authkey, WPS_AUTHKEY_LEN);
+
 
 	for (second_half = 0; second_half < 1000; second_half++) {
 		unsigned int checksum_digit = wps_pin_checksum(first_half * 1000 + second_half);
 		unsigned int c_second_half = second_half * 10 + checksum_digit;
 		uint_to_char_array(c_second_half, 4, s_pin);
-		if (check_pin_half(s_pin, psk, wps->e_s2, wps, wps->e_hash2)) {
+		if (check_pin_half(&hc, s_pin, psk, wps->e_s2, wps, wps->e_hash2)) {
 			memcpy(wps->psk2, psk, sizeof psk);
 			pin[8] = 0;
 			return 1;
@@ -1569,7 +1575,7 @@ static int crack_second_half(struct global *wps, char *pin)
 		}
 
 		uint_to_char_array(second_half, 4, s_pin);
-		if (check_pin_half(s_pin, psk, wps->e_s2, wps, wps->e_hash2)) {
+		if (check_pin_half(&hc, s_pin, psk, wps->e_s2, wps, wps->e_hash2)) {
 			memcpy(wps->psk2, psk, sizeof psk);
 			pin[8] = 0; /* make sure pin string is zero-terminated */
 			return 1;