diff --git a/library/md5.c b/library/md5.c
index a14a9cf..49f0674 100644
--- a/library/md5.c
+++ b/library/md5.c
@@ -443,7 +443,7 @@
 /*
  * RFC 1321 test vectors
  */
-static unsigned char md5_test_buf[7][81] =
+static const unsigned char md5_test_buf[7][81] =
 {
     { "" },
     { "a" },
@@ -481,7 +481,7 @@
 /*
  * RFC 2202 test vectors
  */
-static unsigned char md5_hmac_test_key[7][26] =
+static const unsigned char md5_hmac_test_key[7][26] =
 {
     { "\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B" },
     { "Jefe" },
@@ -498,7 +498,7 @@
     16, 4, 16, 25, 16, 80, 80
 };
 
-static unsigned char md5_hmac_test_buf[7][74] =
+static const unsigned char md5_hmac_test_buf[7][74] =
 {
     { "Hi There" },
     { "what do ya want for nothing?" },