sqlite.h contains a comment that incorrectly states that sqlite3_rekey() and sqlite3_rekey_v2() are able to decrypt a database:

** Change the key on an open database.  If the current database is not
** encrypted, this routine will encrypt it.  If pNew==0 or nNew==0, the
** database is decrypted.

This was reported before, in #301, and subsequently fixed, but apparently the comment managed to return.