  We show practical attacks against OpenPGP and S/MIME encryption and digital
signatures in the context of email. Instead of targeting the underlying
cryptographic primitives, our attacks abuse legitimate features of the MIME
standard and HTML, as supported by email clients, to deceive the user regarding
the actual message content. We demonstrate how the attacker can unknowingly
abuse the user as a decryption oracle by replying to an unsuspicious looking
email. Using this technique, the plaintext of hundreds of encrypted emails can
be leaked at once. Furthermore, we show how users could be tricked into signing
arbitrary text by replying to emails containing CSS conditional rules. An
evaluation shows that 17 out of 19 OpenPGP-capable email clients, as well as 21
out of 22 clients supporting S/MIME, are vulnerable to at least one attack. We
provide different countermeasures and discuss their advantages and
disadvantages.

Cryptography and Security