'auth-source--pad' use 'length' to get byte length of a
string. However, 'length' return the number of characters which is not
the byte length with multibyte characters such as Chinese
character. This may causes an error "GnuTLS cipher
AES-256-CBC/encrypt input block length xxxx is not a multiple of the
required 16".

According to the doc of 'length'. 'length' can be replaced by
'string-bytes'. And a patch is attached.