diff --git a/cbc.c b/cbc.c
index 3312c293e541ee7f25ea1276f39304027029a9ff..cee710937b2a4d683d726dc6cac720d0d6f06bfc 100644
--- a/cbc.c
+++ b/cbc.c
@@ -37,9 +37,7 @@
 #include "nettle-internal.h"
 
 void
-cbc_encrypt(const void *ctx, void (*f)(const void *ctx,
-				       unsigned length, uint8_t *dst,
-				       const uint8_t *src),
+cbc_encrypt(void *ctx, nettle_crypt_func f,
 	    unsigned block_size, uint8_t *iv,
 	    unsigned length, uint8_t *dst,
 	    const uint8_t *src)
@@ -56,9 +54,7 @@ cbc_encrypt(const void *ctx, void (*f)(const void *ctx,
 
 /* Reqires that dst != src */
 static void
-cbc_decrypt_internal(const void *ctx, void (*f)(const void *ctx,
-						unsigned length, uint8_t *dst,
-						const uint8_t *src),
+cbc_decrypt_internal(void *ctx, nettle_crypt_func f,
 		     unsigned block_size, uint8_t *iv,
 		     unsigned length, uint8_t *dst,
 		     const uint8_t *src)
@@ -80,9 +76,7 @@ cbc_decrypt_internal(const void *ctx, void (*f)(const void *ctx,
 #define CBC_BUFFER_LIMIT 4096
 
 void
-cbc_decrypt(const void *ctx, void (*f)(const void *ctx,
-				       unsigned length, uint8_t *dst,
-				       const uint8_t *src),
+cbc_decrypt(void *ctx, nettle_crypt_func f,
 	    unsigned block_size, uint8_t *iv,
 	    unsigned length, uint8_t *dst,
 	    const uint8_t *src)
diff --git a/cbc.h b/cbc.h
index 740e9e194e46d18b9b0b08a5e63ed4126fe62c72..dd19e267247949430c4098ba2ec00571f9725ad6 100644
--- a/cbc.h
+++ b/cbc.h
@@ -32,20 +32,23 @@
 #define cbc_encrypt nettle_cbc_encrypt
 #define cbc_decrypt nettle_cbc_decrypt
 
-/* Uses a void * for cipher contexts. */
+/* Uses a void * for cipher contexts.
+
+   For block ciphers it would make sense with a const void * for the
+   context, but we use the same typedef for stream ciphers where the
+   internal state changes during the encryption. */
+typedef void (*nettle_crypt_func)(void *ctx,
+				  unsigned length, uint8_t *dst,
+				  const uint8_t *src);
 
 void
-cbc_encrypt(const void *ctx, void (*f)(const void *ctx,
-				       unsigned length, uint8_t *dst,
-				       const uint8_t *src),
+cbc_encrypt(void *ctx, nettle_crypt_func f,
 	    unsigned block_size, uint8_t *iv,
 	    unsigned length, uint8_t *dst,
 	    const uint8_t *src);
 
 void
-cbc_decrypt(const void *ctx, void (*f)(const void *ctx,
-				       unsigned length, uint8_t *dst,
-				       const uint8_t *src),
+cbc_decrypt(void *ctx, nettle_crypt_func f,
 	    unsigned block_size, uint8_t *iv,
 	    unsigned length, uint8_t *dst,
 	    const uint8_t *src);