How to encode utf-8 string in Quoted Printable?

I'm writing a mail program supporting UTF-8.
As you know, in message headers (such as Subject) only ASCII characters are
allowed. This is why Quoted Printable and Base64 encodings are used.
In case of Quoted Printable it's well described in the RFCs how to encode
strings written in ISO 8859-x codepages, such as iso-8859-2.
But how to deal with UTF-8 strings which can contain characters represented
by 3 or 4 bytes?

rex wrote:
> I'm writing a mail program supporting UTF-8.
> As you know, in message headers (such as Subject) only ASCII characters are
> allowed. This is why Quoted Printable and Base64 encodings are used.
> In case of Quoted Printable it's well described in the RFCs how to encode
> strings written in ISO 8859-x codepages, such as iso-8859-2.
> But how to deal with UTF-8 strings which can contain characters represented
> by 3 or 4 bytes?

Each of these UTF-8 bytes will have the high bit turned on and will need
to be encoded in the "=xx" QP format.
The recipient (application) ought to decode any QP before attempting to
interpret the header as UTF-8.