diff --git a/include/sha512.h b/include/sha512.h
index 469a597..68b636d 100644
--- a/include/sha512.h
+++ b/include/sha512.h
@@ -1,6 +1,8 @@
 #ifndef SHA512_H
 #define SHA512_H
 
+#include <assert.h>
+
 #ifdef USE_OPENSSL
 	#include <openssl/sha.h>
 
@@ -17,8 +19,28 @@
 		unsigned char buf[128];
 	} sha512_context;
 #endif
-int _ed_sha512_init(sha512_context * md);
-int _ed_sha512_final(sha512_context * md, unsigned char *out);
-int _ed_sha512_update(sha512_context * md, const unsigned char *in, size_t inlen);
-int _ed_sha512(const unsigned char *message, size_t message_len, unsigned char *out);
+
+typedef struct sha512_functions_ {
+	int(*_ed_sha512_init)(sha512_context*);
+	int(*_ed_sha512_final)(sha512_context*, unsigned char *);
+	int(*_ed_sha512_update)(sha512_context*, const unsigned char *, size_t);
+} sha512_functions;
+extern sha512_functions _ed_sha512_functions;
+
+void _ed_sha512_validate() {
+	assert(_ed_sha512_functions._ed_sha512_init);
+	assert(_ed_sha512_functions._ed_sha512_final);
+	assert(_ed_sha512_functions._ed_sha512_update);
+}
+int _ed_sha512(const unsigned char *message, size_t message_len, unsigned char *out) {
+	_ed_sha512_validate();
+
+	int result = 1;
+	sha512_context ctx;
+	result &= _ed_sha512_functions._ed_sha512_init(&ctx);
+	result &= _ed_sha512_functions._ed_sha512_update(&ctx, message, message_len);
+	result &= _ed_sha512_functions._ed_sha512_final(&ctx, out);
+	return result;
+}
+
 #endif
diff --git a/src/add_scalar.c b/src/add_scalar.c
index 9263b7b..d5e8919 100644
--- a/src/add_scalar.c
+++ b/src/add_scalar.c
@@ -31,10 +31,11 @@ void ed25519_add_scalar(unsigned char *public_key, unsigned char *private_key, c
         sc_muladd(private_key, SC_1, n, private_key);
 
         // https://github.com/orlp/ed25519/issues/3
-        _ed_sha512_init(&hash);
-        _ed_sha512_update(&hash, private_key + 32, 32);
-        _ed_sha512_update(&hash, scalar, 32);
-        _ed_sha512_final(&hash, hashbuf);
+        _ed_sha512_validate();
+        _ed_sha512_functions._ed_sha512_init(&hash);
+        _ed_sha512_functions._ed_sha512_update(&hash, private_key + 32, 32);
+        _ed_sha512_functions._ed_sha512_update(&hash, scalar, 32);
+        _ed_sha512_functions._ed_sha512_final(&hash, hashbuf);
         for (i = 0; i < 32; ++i) {
             private_key[32 + i] = hashbuf[i];
         }
diff --git a/src/sha512.c b/src/sha512.c
index 7cb1a17..bdcb972 100644
--- a/src/sha512.c
+++ b/src/sha512.c
@@ -269,3 +269,9 @@ int _ed_sha512(const unsigned char *message, size_t message_len, unsigned char *
 	if ((ret = _ed_sha512_final(&ctx, out))) return ret;
 	return 0;
 }
+
+extern sha512_functions _ed_sha512_functions = {
+		_ed_sha512_init,
+		_ed_sha512_final,
+		_ed_sha512_update
+};
\ No newline at end of file
diff --git a/src/sha512_openssl.c b/src/sha512_openssl.c
index 27573c2..a3a9712 100644
--- a/src/sha512_openssl.c
+++ b/src/sha512_openssl.c
@@ -1,6 +1,7 @@
 #include <openssl/sha.h>
 #include "../include/sha512.h"
 
+
 int _ed_sha512_init(sha512_context * md) {
 	return SHA512_Init(md) != 1; /* Returns 0 on success */
 }
@@ -13,6 +14,8 @@ int _ed_sha512_update(sha512_context * md, const unsigned char *in, size_t inlen
 	return SHA512_Update(md, in, inlen) != 1; /* Returns 0 on success */
 }
 
-int _ed_sha512(const unsigned char *message, size_t message_len, unsigned char *out) {
-	return SHA512(message, message_len, out) != 0; /* Returns 0 on success */
-}
\ No newline at end of file
+extern sha512_functions _ed_sha512_functions = {
+		_ed_sha512_init,
+		_ed_sha512_final,
+		_ed_sha512_update
+};
\ No newline at end of file
diff --git a/src/sign.c b/src/sign.c
index 1a0c898..7b7e190 100644
--- a/src/sign.c
+++ b/src/sign.c
@@ -11,20 +11,21 @@ void ed25519_sign(unsigned char *signature, const unsigned char *message, size_t
     ge_p3 R;
 
 
-    _ed_sha512_init(&hash);
-    _ed_sha512_update(&hash, private_key + 32, 32);
-    _ed_sha512_update(&hash, message, message_len);
-    _ed_sha512_final(&hash, r);
+    _ed_sha512_validate();
+    _ed_sha512_functions._ed_sha512_init(&hash);
+    _ed_sha512_functions._ed_sha512_update(&hash, private_key + 32, 32);
+    _ed_sha512_functions._ed_sha512_update(&hash, message, message_len);
+    _ed_sha512_functions._ed_sha512_final(&hash, r);
 
     sc_reduce(r);
     ge_scalarmult_base(&R, r);
     ge_p3_tobytes(signature, &R);
 
-    _ed_sha512_init(&hash);
-    _ed_sha512_update(&hash, signature, 32);
-    _ed_sha512_update(&hash, public_key, 32);
-    _ed_sha512_update(&hash, message, message_len);
-    _ed_sha512_final(&hash, hram);
+    _ed_sha512_functions._ed_sha512_init(&hash);
+    _ed_sha512_functions._ed_sha512_update(&hash, signature, 32);
+    _ed_sha512_functions._ed_sha512_update(&hash, public_key, 32);
+    _ed_sha512_functions._ed_sha512_update(&hash, message, message_len);
+    _ed_sha512_functions._ed_sha512_final(&hash, hram);
 
     sc_reduce(hram);
     sc_muladd(signature + 32, hram, private_key, r);
diff --git a/src/verify.c b/src/verify.c
index b51f169..b010a28 100644
--- a/src/verify.c
+++ b/src/verify.c
@@ -59,11 +59,11 @@ int ed25519_verify(const unsigned char *signature, const unsigned char *message,
         return 0;
     }
 
-    _ed_sha512_init(&hash);
-    _ed_sha512_update(&hash, signature, 32);
-    _ed_sha512_update(&hash, public_key, 32);
-    _ed_sha512_update(&hash, message, message_len);
-    _ed_sha512_final(&hash, h);
+    _ed_sha512_functions._ed_sha512_init(&hash);
+    _ed_sha512_functions._ed_sha512_update(&hash, signature, 32);
+    _ed_sha512_functions._ed_sha512_update(&hash, public_key, 32);
+    _ed_sha512_functions._ed_sha512_update(&hash, message, message_len);
+    _ed_sha512_functions._ed_sha512_final(&hash, h);
     
     sc_reduce(h);
     ge_double_scalarmult_vartime(&R, h, &A, signature + 32);