Encrypting and signing e-mail

• Infectious attachments or other technical manipulations
• Fraudulent messages tempt recipients to take actions they would have been better off not taking
• Interception or alteration of messages during transport
• Hacking of an entire mailbox: information about the owner and all correspondents falls into the wrong hands

Who sent you a message is often the deciding factor in whether you open an attachment or comply with a request to do something specific. Attackers take advantage of this normal social behaviour by pretending to be a helpdesk or your bank, for example.

The sender is just as easy to forge in an e-mail as in a paper letter. This is not a technical flaw, but a consequence of global networking. Normally, this is not a problem: messages such as “I have picked up the cinema tickets” or “The exam results for Introduction to Cryptography III are now available” probably do not require excessive identity verification.

In the real world, if it is important, we sign a document to confirm its authenticity. The digital signature does exactly the same thing for e-mail and some more: it ensures that the message has not been altered and that it comes from the person who signed it. (This is why you can’t simply copy a digital signature and put it under another e-mail).

Digital signatures can expose forgeries of the sender and content. In practice, however, this usually does not happen, as two basic requirements are rarely met:

• The sender must sign the message.
• The recipient must check the signature.

There may be several reasons for this, primarily the low demand on the one hand and the necessary learning effort on the other. User-friendliness depends on the procedures and software used, but training is recommended in any case.

If security logos or the like are inserted in the message itself, this is at least worthless, and more likely an alarm sign: Any logos can also be inserted by a fraudster.

If you follow the path of an e-mail, you can recognise three focal points where danger is imminent:

• the transport route
• the mailboxes (mail servers) of the sender and recipient
• their respective PCs or smartphones

On the way from server to server, the e-mail is in fact only partially encrypted during transport. This is not so much a technical defect as a result of the global network: How are the millions of mail server operators supposed to verify each other’s identity, agree on secret keys and keep everything up to date?

However, an e-mail normally only travels for a few seconds and is therefore only vulnerable for a very short time. After that, however, it often remains in the sent and received folders of the respective mailboxes for years and can be attacked.

The state of the art is therefore a combined approach:

• As far as it is easily possible, the infrastructure endeavours to use encrypted transport.
• Really sensitive messages are individually encrypted by the sender (method: S/MIME or GPG) and are therefore safe both during transport and when stored in the mailboxes until they are decrypted by the recipient.

Encrypting individual messages (end-to-end encryption) provides excellent protection against any interception in the infrastructure, including the server. There are also natural limits to this method:

• The sender and recipient must take care of the encryption and decryption.
• It does not protect against malware or eavesdropping devices on the sender’s or recipient’s computer, as the message is unencrypted there.

E-mail encryption is very effective and can be used (almost) without any problems in everyday life after a familiarisation phase. However, introducing it on a large scale (e.g. to organisational units) is not entirely free of pitfalls. If necessary, seek expert advice, for example from IT security.