diff --git a/library/sha256.c b/library/sha256.c
index b9b3f09..2b4b7e1 100644
--- a/library/sha256.c
+++ b/library/sha256.c
@@ -483,7 +483,7 @@
 /*
  * FIPS-180-2 test vectors
  */
-static unsigned char sha256_test_buf[3][57] =
+static const unsigned char sha256_test_buf[3][57] =
 {
     { "abc" },
     { "abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq" },
@@ -533,7 +533,7 @@
 /*
  * RFC 4231 test vectors
  */
-static unsigned char sha256_hmac_test_key[7][26] =
+static const unsigned char sha256_hmac_test_key[7][26] =
 {
     { "\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B\x0B"
       "\x0B\x0B\x0B\x0B" },
@@ -553,7 +553,7 @@
     20, 4, 20, 25, 20, 131, 131
 };
 
-static unsigned char sha256_hmac_test_buf[7][153] =
+static const unsigned char sha256_hmac_test_buf[7][153] =
 {
     { "Hi There" },
     { "what do ya want for nothing?" },