diff --git a/library/xtea.c b/library/xtea.c
index f8ab014..9404927 100644
--- a/library/xtea.c
+++ b/library/xtea.c
@@ -57,7 +57,7 @@
 /*
  * XTEA key schedule
  */
-void xtea_setup( xtea_context *ctx, unsigned char key[16] )
+void xtea_setup( xtea_context *ctx, const unsigned char key[16] )
 {
     int i;
 
@@ -72,13 +72,13 @@
 /*
  * XTEA encrypt function
  */
-int xtea_crypt_ecb( xtea_context *ctx, int mode, unsigned char input[8],
-                     unsigned char output[8])
+int xtea_crypt_ecb( xtea_context *ctx, int mode,
+                    const unsigned char input[8], unsigned char output[8])
 {
     uint32_t *k, v0, v1, i;
 
     k = ctx->k;
-    
+
     GET_UINT32_BE( v0, input, 0 );
     GET_UINT32_BE( v1, input, 4 );
 
@@ -114,27 +114,24 @@
 /*
  * XTEA-CBC buffer encryption/decryption
  */
-int xtea_crypt_cbc( xtea_context *ctx,
-                    int mode,
-                    size_t length,
-                    unsigned char iv[8],
-                    unsigned char *input,
+int xtea_crypt_cbc( xtea_context *ctx, int mode, size_t length,
+                    unsigned char iv[8], const unsigned char *input,
                     unsigned char *output)
 {
     int i;
     unsigned char temp[8];
 
-    if(length % 8)
+    if( length % 8 )
         return( POLARSSL_ERR_XTEA_INVALID_INPUT_LENGTH );
 
-    if( mode == XTEA_DECRYPT ) 
+    if( mode == XTEA_DECRYPT )
     {
         while( length > 0 )
         {
             memcpy( temp, input, 8 );
             xtea_crypt_ecb( ctx, mode, input, output );
 
-            for(i = 0; i < 8; i++) 
+            for(i = 0; i < 8; i++)
                 output[i] = (unsigned char)( output[i] ^ iv[i] );
 
             memcpy( iv, temp, 8 );
@@ -143,8 +140,8 @@
             output += 8;
             length -= 8;
         }
-    } 
-    else 
+    }
+    else
     {
         while( length > 0 )
         {
@@ -153,7 +150,7 @@
 
             xtea_crypt_ecb( ctx, mode, output, output );
             memcpy( iv, output, 8 );
-            
+
             input  += 8;
             output += 8;
             length -= 8;
@@ -225,7 +222,7 @@
 
         memcpy( buf, xtea_test_pt[i], 8 );
 
-        xtea_setup( &ctx, (unsigned char *) xtea_test_key[i] );
+        xtea_setup( &ctx, xtea_test_key[i] );
         xtea_crypt_ecb( &ctx, XTEA_ENCRYPT, buf, buf );
 
         if( memcmp( buf, xtea_test_ct[i], 8 ) != 0 )