[Djigzo users] [OT] Invalid signature because of "Content-Transfer-Encoding: 8bit"

Martijn Brinkers martijn at djigzo.com
Wed Apr 20 15:53:10 CEST 2011

On 01/-10/-28163 08:59 PM, lst_hoe02 at kwsoft.de wrote:
> Zitat von Martijn Brinkers <martijn at djigzo.com>:
>>> today i got a mail fro a well known German Trustcenter with a invalid
>>> signature warning (content altered). A former mail to an other account
>>> from the same Trustcenter was valid. On inspection it looks like someone
>>> altered the encoding because the valid mail has
>>> "Content-Transfer-Encoding: 8bit" and the broken one
>>> "Content-Transfer-Encoding: quoted-printable". As far as i know a SMTP
>>> server should only pass 8bit if the remote site announces 8BITMIME, so i
>>> suspect this is the trouble maker because neither Djigzo nor our Virus
>>> scan announces 8BITMIME :-(
>>> Any comments on this?
>> The application that added the signature is not RFC 3851 compliant.
>> before signing a message the mail agent should convert 8bit mime bodies
>> to 7bit. This is important because if SMTP sees that a server does not
>> support 8bit, it should convert the message to 7bit. Because of this
>> conversion the message has been changed and therefore the signature is
>> no longer valid. So the trouble maker is the application that signed the
>> message :). The problem is that there is not much you can do. In
>> principle you can disable the conversion from 8bit to 7bit in your own
>> gateway (not that I recommend that ;) but you cannot control other
>> intermediate gateways.
> Lead me straight to another question: What does Djigzo do if it is feed
> with 8bit content to sign? Oh, wait... It does not announce 8BITMIME so
> this should not happen at all, no?

Yes you are right. The caller should convert it to 7bit so the
signing/encryption engine only sees 7bit messages :). However lets
suppose that the caller does not convert the message to 7bit. Postfix
will receive the message and the message will then be send to the
internal SMTP (the after queue filter). Because the internal SMTP server
does not announce 8bit, Postfix will convert it to 7bit and therefore
all email will be converted to 7bit before signing.

Kind regards,


Djigzo open source email encryption
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