Billions of emails go back and forth each day without much concern for their security.
On the whole, security is not a major concern for most day-to-day emails, but some do contain personal, proprietary and sensitive information, documents, media, photos, videos and sound files.
Unfortunately, the open nature of email means an attacker can intercept them and if they do not have proper encryption, they can easily read them. Even with PGP — pretty good privacy — encryption scheme first used in 1995, if a sender’s private “key” ends up compromised all their previous emails encrypted with that key can suffer from exposure.
A team, led by computer scientists Duncan Wong and Xiaojian Tian of City University of Hong Kong, defined perfect forward secrecy for email and suggested a technical solution to enable email security to be independent of the server used to send the message:
“An email system provides perfect forward secrecy if any third party, including the email server, cannot recover previous session keys between the sender and the recipient even if the long-term secret keys of the sender and the recipient are compromised,” Wong and Tian said in a paper on the subject.
By building a new email protocol on this principle, the team said it is now possible to exchange emails with almost zero risk of interference from third parties. “Our protocol provides both confidentiality and message authentication in addition to perfect forward secrecy,” Wong and Tian said.
The team’s protocol involves Alice sending Bob an encrypted email with the hope that Charles will not be able to intercept and decrypt the message. Before the email undergoes encryption and then sent, the protocol suggested by Wong and Tian has Alice’s computer send an identification code to the email server. The server creates a random session “hash” used to encrypt the actual encryption key for the email Alice is about to send. Meanwhile, Bob as putative recipient receives the key used to create the hash and bounces back an identification tag. This allows Alice and Bob to verify each other’s identity.
These preliminary steps are all automatically and without Alice or Bob needing to do anything in advance. Now, Alice writes her email, encrypts it using PGP and then “hashes” it using the random key from the server. When Bob receives the encrypted message he uses his version of the hash to unlock the container within which the PGP-encrypted email sits. Bob then uses Alice’s public PGP key to decrypt the message itself.
No snoopers on the Internet between Alice and Bob, not even the email server ever have access to the PGP encrypted email in the open. Moreover, because a different key locks up the PGP encrypted email with a second one-time layer, even if the PGP security ends up compromised, an attacker cannot unlock past emails created with the same key.