diff --git a/include/polarssl/sha2.h b/include/polarssl/sha2.h
index 5e2d907..c963ca1 100644
--- a/include/polarssl/sha2.h
+++ b/include/polarssl/sha2.h
@@ -27,6 +27,8 @@
 #ifndef POLARSSL_SHA2_H
 #define POLARSSL_SHA2_H
 
+#include <string.h>
+
 /**
  * \brief          SHA-256 context structure
  */
@@ -61,7 +63,7 @@
  * \param input    buffer holding the  data
  * \param ilen     length of the input data
  */
-void sha2_update( sha2_context *ctx, const unsigned char *input, int ilen );
+void sha2_update( sha2_context *ctx, const unsigned char *input, size_t ilen );
 
 /**
  * \brief          SHA-256 final digest
@@ -79,7 +81,7 @@
  * \param output   SHA-224/256 checksum result
  * \param is224    0 = use SHA256, 1 = use SHA224
  */
-void sha2( const unsigned char *input, int ilen,
+void sha2( const unsigned char *input, size_t ilen,
            unsigned char output[32], int is224 );
 
 /**
@@ -102,7 +104,7 @@
  * \param keylen   length of the HMAC key
  * \param is224    0 = use SHA256, 1 = use SHA224
  */
-void sha2_hmac_starts( sha2_context *ctx, const unsigned char *key, int keylen,
+void sha2_hmac_starts( sha2_context *ctx, const unsigned char *key, size_t keylen,
                        int is224 );
 
 /**
@@ -112,7 +114,7 @@
  * \param input    buffer holding the  data
  * \param ilen     length of the input data
  */
-void sha2_hmac_update( sha2_context *ctx, const unsigned char *input, int ilen );
+void sha2_hmac_update( sha2_context *ctx, const unsigned char *input, size_t ilen );
 
 /**
  * \brief          SHA-256 HMAC final digest
@@ -139,8 +141,8 @@
  * \param output   HMAC-SHA-224/256 result
  * \param is224    0 = use SHA256, 1 = use SHA224
  */
-void sha2_hmac( const unsigned char *key, int keylen,
-                const unsigned char *input, int ilen,
+void sha2_hmac( const unsigned char *key, size_t keylen,
+                const unsigned char *input, size_t ilen,
                 unsigned char output[32], int is224 );
 
 /**