Red Hat DIRECTORY SERVER 8.1 - 11-01-2010 Bedienungsanleitung PDF herunterladen (Seite 60)

8.5. Notes

Mail headers cannot be encrypted, nor included in the signature

computation. Do not write any sensitive information in the Subject

when sending an encrypted message.

One important point concerns mail header security. Signature and encryption

applies to the mail body only – and also to attachments, if you chose so. That

is, when you sign a message, no mail header (such as the Subject, Date, all

Received headers, etc.) can be included in the signature. In a similar way,

when you encrypt a message, mail headers are not encrypted.

Most mail headers (e.g. the Date) are added by the Mail User Agent (mailclient)

only after that Enigmail processes the payload; other mail headers (e.g. the

Received) are subsequently added by the Mail Transfer Agents at each hop of

the path from sender to receiver; finally, other mail headers are added by the

Mail Delivery Agent at the endpoint. Even mail headers that are user-set at the

time the message is composed (e.g. the Subject) may be legitimately modified

by antispam or antivirus applications on the destination server.

This is to say that mail headers change in transit (therefore they cannot be

signed), and they must be in cleartext (therefore they cannot be encrypted) in

order for the mail message to be processed by intermediary routers and

delivered.

This is not a limitation of Enigmail, neither of GnuPG. The OpenPGP standard

does not describe a way to sign or encrypt the Subject or, for this purpose, any

other mail header.

Thus, please remember that mail headers aren't and cannot be secured, and

they could be snooped and forged in transit like any cleartext mail message.

Note that, for the same reason, Enigmail will not sign and/or encrypt a blank

message: the message body is empty, so there's nothing Enigmail can process.

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Red Hat DIRECTORY SERVER 8.1 - 11-01-2010 Bedienungsanleitung Seite 60