*
* * @enc@ is called to encrypt a source buffer @s@ and write
* the ciphertext to a destination @d@; @sz@ is the common
- * size of these buffers.
+ * size of these buffers. @d@ might be null, to discard
+ * output; @s@ might be null, to process all-zero input.
*
* * @dec@ is called to decrypt a source buffer @s@ and write
* the recovered plaintext to a destination @d@; @sz@ is the