diff --git a/sys/crypto/3des.c b/sys/crypto/3des.c
index 09426a0169fbba2b72f837a55508f8af42615cf4..104495f99618dbeb7527fc99d1a6421c407268c8 100644
--- a/sys/crypto/3des.c
+++ b/sys/crypto/3des.c
@@ -350,7 +350,7 @@ int tripledes_decrypt(cipher_context_t *context, uint8_t *crypt, uint8_t *plain)
     return 1;
 }
 
-uint8_t tripledes_get_preferred_block_size()
+uint8_t tripledes_get_preferred_block_size(void)
 {
     return THREEDES_BLOCK_SIZE;
 }
diff --git a/sys/crypto/aes.c b/sys/crypto/aes.c
index ac65a0e3cefd2b0b3d10a29120dc9a1123463be7..38ff9800a79c592d93e8090ac748b2aa65ee6083 100644
--- a/sys/crypto/aes.c
+++ b/sys/crypto/aes.c
@@ -1457,7 +1457,7 @@ int aes_decrypt(cipher_context_t *context, uint8_t *cipherBlock,
     return 1;
 }
 
-uint8_t aes_get_preferred_block_size()
+uint8_t aes_get_preferred_block_size(void)
 {
     return AES_BLOCK_SIZE;
 }
diff --git a/sys/crypto/rc5.c b/sys/crypto/rc5.c
index aa7df7bbddee2537458f3964bcf3ebd182a28547..bec7c7a86008bb1dd78eae2d5dd6c1ae2219ed19 100644
--- a/sys/crypto/rc5.c
+++ b/sys/crypto/rc5.c
@@ -186,7 +186,7 @@ int rc5_setup_key(cipher_context_t *context, uint8_t *key, uint8_t keysize)
  *         cipher operates with multiple block sizes, this will pick one
  *         particular size (deterministically).
  */
-uint8_t rc5_get_preferred_block_size()
+uint8_t rc5_get_preferred_block_size(void)
 {
     return BLOCK_SIZE;
 }
diff --git a/sys/crypto/skipjack.c b/sys/crypto/skipjack.c
index 5bb57f50eafad6d93f1f251a70996e68167ae98f..5734de76615aeb19556852e0da8abbb99f87eeb3 100644
--- a/sys/crypto/skipjack.c
+++ b/sys/crypto/skipjack.c
@@ -345,7 +345,7 @@ int skipjack_setup_key(cipher_context_t *context, uint8_t *key, uint8_t keysize)
 }
 
 
-uint8_t skipjack_get_preferred_block_size()
+uint8_t skipjack_get_preferred_block_size(void)
 {
     return BLOCK_SIZE;
 }
diff --git a/sys/crypto/twofish.c b/sys/crypto/twofish.c
index d5120b8cb4d9717004c30dac3b5ad0e9a7cba6df..51b1bc0042c358e65521241f9076ec747faca582 100644
--- a/sys/crypto/twofish.c
+++ b/sys/crypto/twofish.c
@@ -753,7 +753,7 @@ int twofish_decrypt(cipher_context_t *context, uint8_t *in, uint8_t *out)
     return 1;
 }
 
-uint8_t twofish_get_preferred_block_size()
+uint8_t twofish_get_preferred_block_size(void)
 {
     return TWOFISH_BLOCK_SIZE;
 }