diff --git a/sys/crypto/modes/ctr.c b/sys/crypto/modes/ctr.c
index 22c6da37a4..bf60adc6ab 100644
--- a/sys/crypto/modes/ctr.c
+++ b/sys/crypto/modes/ctr.c
@@ -21,7 +21,7 @@
 #include "crypto/helper.h"
 #include "crypto/modes/ctr.h"
 
-int cipher_encrypt_ctr(cipher_t *cipher, uint8_t nonce_counter[16],
+int cipher_encrypt_ctr(const cipher_t *cipher, uint8_t nonce_counter[16],
                        uint8_t nonce_len, const uint8_t *input, size_t length,
                        uint8_t *output)
 {
@@ -49,7 +49,7 @@ int cipher_encrypt_ctr(cipher_t *cipher, uint8_t nonce_counter[16],
     return offset;
 }
 
-int cipher_decrypt_ctr(cipher_t *cipher, uint8_t nonce_counter[16],
+int cipher_decrypt_ctr(const cipher_t *cipher, uint8_t nonce_counter[16],
                        uint8_t nonce_len, const uint8_t *input, size_t length,
                        uint8_t *output)
 {
diff --git a/sys/include/crypto/modes/ctr.h b/sys/include/crypto/modes/ctr.h
index 85f6cce4af..336b36d9c0 100644
--- a/sys/include/crypto/modes/ctr.h
+++ b/sys/include/crypto/modes/ctr.h
@@ -44,7 +44,7 @@ extern "C" {
  * @return              Length of encrypted data on a successful encryption
  * @return              A negative error code if something went wrong
  */
-int cipher_encrypt_ctr(cipher_t *cipher, uint8_t nonce_counter[16],
+int cipher_encrypt_ctr(const cipher_t *cipher, uint8_t nonce_counter[16],
                        uint8_t nonce_len, const uint8_t *input, size_t length,
                        uint8_t *output);
 
@@ -68,7 +68,7 @@ int cipher_encrypt_ctr(cipher_t *cipher, uint8_t nonce_counter[16],
  * @return              Length of decrypted data on a successful decryption
  * @return              A negative error code if something went wrong
  */
-int cipher_decrypt_ctr(cipher_t *cipher, uint8_t nonce_counter[16],
+int cipher_decrypt_ctr(const cipher_t *cipher, uint8_t nonce_counter[16],
                        uint8_t nonce_len, const uint8_t *input, size_t length,
                        uint8_t *output);