diff --git a/Makefile b/Makefile
index 1c093f89..a1ab3fdc 100644
--- a/Makefile
+++ b/Makefile
@@ -4,7 +4,7 @@ FLAGS_DEBUG:=-DVERIFY -ggdb3 -O1
 FLAGS_TEST:=-DVERIFY -ggdb3 -O2 -march=native
 
 SECP256K1_FILES := src/num.h src/field.h src/field_5x52.h src/group.h src/ecmult.h src/ecdsa.h \
-                   src/num.c src/field.c src/field_5x52.c src/group.c src/ecmult.c src/ecdsa.c
+                   src/impl/*.h
 
 JAVA_FILES := src/java/org_bitcoin_NativeSecp256k1.h src/java/org_bitcoin_NativeSecp256k1.c
 
@@ -19,33 +19,33 @@ default: all
 ifeq ($(CONF), gmpgmp)
 FLAGS_COMMON := $(FLAGS_COMMON) -DUSE_NUM_GMP -DUSE_FIELD_GMP
 LIBS := -lgmp
-SECP256K1_FILES := $(SECP256K1_FILES) src/num_gmp.h src/num_gmp.c src/field_gmp.c src/field_gmp.h
+SECP256K1_FILES := $(SECP256K1_FILES) src/num_gmp.h src/field_gmp.h
 else
 ifeq ($(CONF), gmp32)
 FLAGS_COMMON := $(FLAGS_COMMON) -DUSE_NUM_GMP -DUSE_FIELD_10X26
 LIBS := -lgmp
-SECP256K1_FILES := $(SECP256K1_FILES) src/num_gmp.h src/num_gmp.c src/field_10x26.c src/field_10x26.h
+SECP256K1_FILES := $(SECP256K1_FILES) src/num_gmp.h src/field_10x26.h
 else
 ifeq ($(CONF), openssl)
 FLAGS_COMMON := $(FLAGS_COMMON) -DUSE_NUM_OPENSSL -DUSE_FIELD_INV_BUILTIN
 LIBS := -lcrypto
-SECP256K1_FILES := $(SECP256K1_FILES) src/num_openssl.h src/num_openssl.c src/field_5x52_int128.c
+SECP256K1_FILES := $(SECP256K1_FILES) src/num_openssl.h src/field_5x52.h
 else
 ifeq ($(CONF), gmp)
 FLAGS_COMMON := $(FLAGS_COMMON) -DUSE_NUM_GMP
 LIBS := -lgmp
-SECP256K1_FILES := $(SECP256K1_FILES) src/num_gmp.h src/num_gmp.c src/field_5x52_int128.c
+SECP256K1_FILES := $(SECP256K1_FILES) src/num_gmp.h src/field_5x52.h
 else
 ifeq ($(CONF), gmpasm)
 FLAGS_COMMON := $(FLAGS_COMMON) -DUSE_NUM_GMP -DUSE_FIELD_5X52_ASM
 LIBS := -lgmp
 OBJS := $(OBJS) obj/field_5x52_asm.o
-SECP256K1_FILES := $(SECP256K1_FILES) src/num_gmp.h src/num_gmp.c src/field_5x52_asm.c
+SECP256K1_FILES := $(SECP256K1_FILES) src/num_gmp.h src/field_5x52.h
 
 obj/field_5x52_asm.o: src/field_5x52_asm.asm
 	yasm -f elf64 -o obj/field_5x52_asm.o src/field_5x52_asm.asm
 else
-SECP256K1_FILES := $(SECP256K1_FILES) src/field_5x52_int128.c
+error
 endif
 endif
 endif
diff --git a/src/bench.c b/src/bench.c
index 09a49b71..eeee710b 100644
--- a/src/bench.c
+++ b/src/bench.c
@@ -1,10 +1,10 @@
 #include <stdio.h>
 
-#include "num.c"
-#include "field.c"
-#include "group.c"
-#include "ecmult.c"
-#include "ecdsa.c"
+#include "impl/num.h"
+#include "impl/field.h"
+#include "impl/group.h"
+#include "impl/ecmult.h"
+#include "impl/ecdsa.h"
 
 int main() {
     secp256k1_num_start();
diff --git a/src/field_10x26.h b/src/field_10x26.h
index 11f6cf99..72c58290 100644
--- a/src/field_10x26.h
+++ b/src/field_10x26.h
@@ -1,5 +1,5 @@
-#ifndef _SECP256K1_FIELD_10x26_
-#define _SECP256K1_FIELD_10x26_
+#ifndef _SECP256K1_FIELD_REPR_
+#define _SECP256K1_FIELD_REPR_
 
 #include <stdint.h>
 
diff --git a/src/field_5x52.h b/src/field_5x52.h
index f6be2089..8ccad034 100644
--- a/src/field_5x52.h
+++ b/src/field_5x52.h
@@ -1,5 +1,5 @@
-#ifndef _SECP256K1_FIELD_5x52_
-#define _SECP256K1_FIELD_5x52_
+#ifndef _SECP256K1_FIELD_REPR_
+#define _SECP256K1_FIELD_REPR_
 
 #include <stdint.h>
 
diff --git a/src/field_gmp.h b/src/field_gmp.h
index 03b83aac..44a8c60c 100644
--- a/src/field_gmp.h
+++ b/src/field_gmp.h
@@ -1,5 +1,5 @@
-#ifndef _SECP256K1_FIELD_GMP_
-#define _SECP256K1_FIELD_GMP_
+#ifndef _SECP256K1_FIELD_REPR_
+#define _SECP256K1_FIELD_REPR_
 
 #include <gmp.h>
 
diff --git a/src/ecdsa.c b/src/impl/ecdsa.h
similarity index 96%
rename from src/ecdsa.c
rename to src/impl/ecdsa.h
index 879e01be..8589a225 100644
--- a/src/ecdsa.c
+++ b/src/impl/ecdsa.h
@@ -1,8 +1,11 @@
-#include "num.h"
-#include "field.h"
-#include "group.h"
-#include "ecmult.h"
-#include "ecdsa.h"
+#ifndef _SECP256K1_ECDSA_IMPL_H_
+#define _SECP256K1_ECDSA_IMPL_H_
+
+#include "../num.h"
+#include "../field.h"
+#include "../group.h"
+#include "../ecmult.h"
+#include "../ecdsa.h"
 
 void static secp256k1_ecdsa_sig_init(secp256k1_ecdsa_sig_t *r) {
     secp256k1_num_init(&r->r);
@@ -141,3 +144,5 @@ void static secp256k1_ecdsa_sig_set_rs(secp256k1_ecdsa_sig_t *sig, const secp256
     secp256k1_num_copy(&sig->r, r);
     secp256k1_num_copy(&sig->s, s);
 }
+
+#endif
diff --git a/src/ecmult.c b/src/impl/ecmult.h
similarity index 98%
rename from src/ecmult.c
rename to src/impl/ecmult.h
index b9301dc6..0261a277 100644
--- a/src/ecmult.c
+++ b/src/impl/ecmult.h
@@ -1,6 +1,9 @@
-#include "num.h"
-#include "group.h"
-#include "ecmult.h"
+#ifndef _SECP256K1_ECMULT_IMPL_H_
+#define _SECP256K1_ECMULT_IMPL_H_
+
+#include "../num.h"
+#include "../group.h"
+#include "../ecmult.h"
 
 // optimal for 128-bit and 256-bit exponents.
 #define WINDOW_A 5
@@ -226,3 +229,5 @@ void static secp256k1_ecmult(secp256k1_gej_t *r, const secp256k1_gej_t *a, const
     secp256k1_num_free(&ng_1);
     secp256k1_num_free(&ng_128);
 }
+
+#endif
diff --git a/src/field.c b/src/impl/field.h
similarity index 97%
rename from src/field.c
rename to src/impl/field.h
index 26c721b3..13274e1b 100644
--- a/src/field.c
+++ b/src/impl/field.h
@@ -1,10 +1,13 @@
+#ifndef _SECP256K1_FIELD_IMPL_H_
+#define _SECP256K1_FIELD_IMPL_H_
+
 #ifdef USE_FIELD_GMP
-#include "field_gmp.c"
+#include "field_gmp.h"
 #else
 #ifdef USE_FIELD_10X26
-#include "field_10x26.c"
+#include "field_10x26.h"
 #else
-#include "field_5x52.c"
+#include "field_5x52.h"
 #endif
 #endif
 
@@ -158,3 +161,5 @@ void static secp256k1_fe_stop(void) {
         secp256k1_fe_consts = NULL;
     }
 }
+
+#endif
diff --git a/src/field_10x26.c b/src/impl/field_10x26.h
similarity index 99%
rename from src/field_10x26.c
rename to src/impl/field_10x26.h
index 9f9edb9d..d1767b55 100644
--- a/src/field_10x26.c
+++ b/src/impl/field_10x26.h
@@ -1,8 +1,11 @@
+#ifndef _SECP256K1_FIELD_REPR_IMPL_H_
+#define _SECP256K1_FIELD_REPR_IMPL_H_
+
 #include <stdio.h>
 #include <assert.h>
 #include <string.h>
-#include "num.h"
-#include "field.h"
+#include "../num.h"
+#include "../field.h"
 
 void static secp256k1_fe_normalize(secp256k1_fe_t *r) {
 //    fog("normalize in: ", r);
@@ -473,3 +476,5 @@ void static secp256k1_fe_sqr(secp256k1_fe_t *r, const secp256k1_fe_t *a) {
 #endif
     secp256k1_fe_sqr_inner(a->n, r->n);
 }
+
+#endif
diff --git a/src/field_5x52.c b/src/impl/field_5x52.h
similarity index 96%
rename from src/field_5x52.c
rename to src/impl/field_5x52.h
index 0d53352f..e11d1e20 100644
--- a/src/field_5x52.c
+++ b/src/impl/field_5x52.h
@@ -1,12 +1,15 @@
+#ifndef _SECP256K1_FIELD_REPR_IMPL_H_
+#define _SECP256K1_FIELD_REPR_IMPL_H_
+
 #include <assert.h>
 #include <string.h>
-#include "num.h"
-#include "field.h"
+#include "../num.h"
+#include "../field.h"
 
 #ifdef USE_FIELD_5X52_ASM
-#include "field_5x52_asm.c"
+#include "field_5x52_asm.h"
 #else
-#include "field_5x52_int128.c"
+#include "field_5x52_int128.h"
 #endif
 
 /** Implements arithmetic modulo FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFE FFFFFC2F,
@@ -180,3 +183,5 @@ void static secp256k1_fe_sqr(secp256k1_fe_t *r, const secp256k1_fe_t *a) {
 #endif
     secp256k1_fe_sqr_inner(a->n, r->n);
 }
+
+#endif
diff --git a/src/field_5x52_asm.c b/src/impl/field_5x52_asm.h
similarity index 67%
rename from src/field_5x52_asm.c
rename to src/impl/field_5x52_asm.h
index 1082c634..32cd55d0 100644
--- a/src/field_5x52_asm.c
+++ b/src/impl/field_5x52_asm.h
@@ -1,2 +1,7 @@
+#ifndef _SECP256K1_FIELD_INNER5X52_IMPL_H_
+#define _SECP256K1_FIELD_INNER5X52_IMPL_H_
+
 void __attribute__ ((sysv_abi)) secp256k1_fe_mul_inner(const uint64_t *a, const uint64_t *b, uint64_t *r);
 void __attribute__ ((sysv_abi)) secp256k1_fe_sqr_inner(const uint64_t *a, uint64_t *r);
+
+#endif
diff --git a/src/field_5x52_int128.c b/src/impl/field_5x52_int128.h
similarity index 97%
rename from src/field_5x52_int128.c
rename to src/impl/field_5x52_int128.h
index 6ad3fa12..8f187def 100644
--- a/src/field_5x52_int128.c
+++ b/src/impl/field_5x52_int128.h
@@ -1,4 +1,7 @@
-#include "field.h"
+#ifndef _SECP256K1_FIELD_INNER5X52_IMPL_H_
+#define _SECP256K1_FIELD_INNER5X52_IMPL_H_
+
+#include <stdint.h>
 
 void static inline secp256k1_fe_mul_inner(const uint64_t *a, const uint64_t *b, uint64_t *r) {
     __int128 c = (__int128)a[0] * b[0];
@@ -94,3 +97,5 @@ void static inline secp256k1_fe_sqr_inner(const uint64_t *a, uint64_t *r) {
     r[1] = t1 + c;
 
 }
+
+#endif
diff --git a/src/field_gmp.c b/src/impl/field_gmp.h
similarity index 96%
rename from src/field_gmp.c
rename to src/impl/field_gmp.h
index a34667b7..387cb3af 100644
--- a/src/field_gmp.c
+++ b/src/impl/field_gmp.h
@@ -1,8 +1,11 @@
+#ifndef _SECP256K1_FIELD_REPR_IMPL_H_
+#define _SECP256K1_FIELD_REPR_IMPL_H_
+
 #include <stdio.h>
 #include <assert.h>
 #include <string.h>
-#include "num.h"
-#include "field.h"
+#include "../num.h"
+#include "../field.h"
 
 void static secp256k1_fe_normalize(secp256k1_fe_t *r) {
 #if (GMP_NUMB_BITS >= 40)
@@ -119,3 +122,5 @@ void static secp256k1_fe_sqr(secp256k1_fe_t *r, const secp256k1_fe_t *a) {
     mpn_sqr(tmp, ac.n, FIELD_LIMBS);
     secp256k1_fe_reduce(r, tmp);
 }
+
+#endif
diff --git a/src/group.c b/src/impl/group.h
similarity index 99%
rename from src/group.c
rename to src/impl/group.h
index d0cf9c77..6c038695 100644
--- a/src/group.c
+++ b/src/impl/group.h
@@ -1,8 +1,11 @@
+#ifndef _SECP256K1_GROUP_IMPL_H_
+#define _SECP256K1_GROUP_IMPL_H_
+
 #include <string.h>
 
-#include "num.h"
-#include "field.h"
-#include "group.h"
+#include "../num.h"
+#include "../field.h"
+#include "../group.h"
 
 void static secp256k1_ge_set_infinity(secp256k1_ge_t *r) {
     r->infinity = 1;
@@ -370,3 +373,5 @@ void static secp256k1_ge_stop(void) {
         secp256k1_ge_consts = NULL;
     }
 }
+
+#endif
diff --git a/src/impl/num.h b/src/impl/num.h
new file mode 100644
index 00000000..f27f0d1f
--- /dev/null
+++ b/src/impl/num.h
@@ -0,0 +1,14 @@
+#ifndef _SECP256K1_NUM_IMPL_H_
+#define _SECP256K1_NUM_IMPL_H_
+
+#include "../num.h"
+
+#if defined(USE_NUM_GMP)
+#include "num_gmp.h"
+#elif defined(USE_NUM_OPENSSL)
+#include "num_openssl.h"
+#else
+#error "Please select num implementation"
+#endif
+
+#endif
diff --git a/src/num_gmp.c b/src/impl/num_gmp.h
similarity index 98%
rename from src/num_gmp.c
rename to src/impl/num_gmp.h
index 4c3f0a4a..f3599ddc 100644
--- a/src/num_gmp.c
+++ b/src/impl/num_gmp.h
@@ -1,3 +1,6 @@
+#ifndef _SECP256K1_NUM_REPR_IMPL_H_
+#define _SECP256K1_NUM_REPR_IMPL_H_
+
 #include <assert.h>
 #include <string.h>
 #include <stdlib.h>
@@ -153,3 +156,5 @@ void static secp256k1_num_negate(secp256k1_num_t *r) {
 void static secp256k1_num_set_rand(secp256k1_num_t *r, const secp256k1_num_t *a) {
     mpz_urandomm(r->bn, secp256k1_num_state.rng, a->bn);
 }
+
+#endif
diff --git a/src/num_openssl.c b/src/impl/num_openssl.h
similarity index 97%
rename from src/num_openssl.c
rename to src/impl/num_openssl.h
index 5b88d98b..93b26d7d 100644
--- a/src/num_openssl.c
+++ b/src/impl/num_openssl.h
@@ -1,10 +1,13 @@
+#ifndef _SECP256K1_NUM_REPR_IMPL_H_
+#define _SECP256K1_NUM_REPR_IMPL_H_
+
 #include <assert.h>
 #include <string.h>
 #include <stdlib.h>
 #include <openssl/bn.h>
 #include <openssl/crypto.h>
 
-#include "num.h"
+#include "../num.h"
 
 void static secp256k1_num_start() {
 }
@@ -148,3 +151,5 @@ void static secp256k1_num_negate(secp256k1_num_t *r) {
 void static secp256k1_num_set_rand(secp256k1_num_t *r, const secp256k1_num_t *a) {
     BN_pseudo_rand_range(&r->bn, &a->bn);
 }
+
+#endif
diff --git a/src/num.c b/src/num.c
deleted file mode 100644
index bcc1c018..00000000
--- a/src/num.c
+++ /dev/null
@@ -1,7 +0,0 @@
-#if defined(USE_NUM_GMP)
-#include "num_gmp.c"
-#elif defined(USE_NUM_OPENSSL)
-#include "num_openssl.c"
-#else
-#error "Please select num implementation"
-#endif
diff --git a/src/num_gmp.h b/src/num_gmp.h
index d908e001..f1c910d1 100644
--- a/src/num_gmp.h
+++ b/src/num_gmp.h
@@ -1,5 +1,5 @@
-#ifndef _SECP256K1_NUM_GMP_
-#define _SECP256K1_NUM_GMP_
+#ifndef _SECP256K1_NUM_REPR_
+#define _SECP256K1_NUM_REPR_
 
 #include <gmp.h>
 
diff --git a/src/num_openssl.h b/src/num_openssl.h
index 22c502a6..de3f1a47 100644
--- a/src/num_openssl.h
+++ b/src/num_openssl.h
@@ -1,5 +1,5 @@
-#ifndef _SECP256K1_NUM_OPENSSL_
-#define _SECP256K1_NUM_OPENSSL_
+#ifndef _SECP256K1_NUM_REPR_
+#define _SECP256K1_NUM_REPR_
 
 #include <openssl/bn.h>
 
diff --git a/src/secp256k1.c b/src/secp256k1.c
index 8b93100c..aecdad98 100644
--- a/src/secp256k1.c
+++ b/src/secp256k1.c
@@ -1,8 +1,8 @@
-#include "num.c"
-#include "field.c"
-#include "group.c"
-#include "ecmult.c"
-#include "ecdsa.c"
+#include "impl/num.h"
+#include "impl/field.h"
+#include "impl/group.h"
+#include "impl/ecmult.h"
+#include "impl/ecdsa.h"
 
 void secp256k1_start(void) {
     secp256k1_num_start();
diff --git a/src/tests.c b/src/tests.c
index f0c61905..1d5b48cd 100644
--- a/src/tests.c
+++ b/src/tests.c
@@ -1,10 +1,10 @@
 #include <assert.h>
 
-#include "num.c"
-#include "field.c"
-#include "group.c"
-#include "ecmult.c"
-#include "ecdsa.c"
+#include "impl/num.h"
+#include "impl/field.h"
+#include "impl/group.h"
+#include "impl/ecmult.h"
+#include "impl/ecdsa.h"
 
 // #define COUNT 2
 #define COUNT 100