SilcBool silc_cipher_decrypt(SilcCipher cipher,
                                 const unsigned char *src,
                                 unsigned char *dst, SilcUInt32 len,
                                 unsigned char *iv);

    Decrypts data from `src' into `dst' with the specified cipher and
    Initial Vector (IV).  If the `iv' is NULL then the cipher's internal
    IV is used.  The `src' and `dst' maybe same buffer.