1. Field of the Invention
The present invention relates to computer security and electronic mail. More specifically, the present invention relates to a method and an apparatus for facilitating transmission of an encrypted electronic mail message to anonymous recipients without divulging the identities of the anonymous recipients.
2. Related Art
The advent of computer networks has led to an explosion in the development of applications that facilitate rapid dissemination of information. In particular, electronic mail (email) is becoming the predominant method for communicating textual and other non-voice information. Using email, it is just as easy to send a message to a recipient on another continent as it is to send a message to a recipient within the same building. Furthermore, an email message typically takes only minutes to arrive, instead of the days it takes for conventional mail to snake its way along roads and through airports.
One problem with email is that it is hard to ensure that sensitive information sent through email is kept confidential. This is because an email message can potentially traverse many different computer networks and many different computer systems before it arrives at its ultimate destination. An adversary can potentially intercept an email message at any of these intermediate points along the way.
One way to remedy this problem is to “encrypt” sensitive data using an encryption key so that only someone who possesses a corresponding decryption key can decrypt the message. (Note that for commonly used symmetric encryption mechanisms the encryption key and the decryption key are the same key.) A person sending sensitive data through email can encrypt the sensitive data using the encryption key before it is sent through email. At the other end, the recipient of the email can use the corresponding decryption key to decrypt the sensitive information.
Encryption works well for a message sent to a single recipient. However, encryption becomes more complicated for a message sent to multiple recipients. This is because encryption keys must be managed between the sender and the multiple recipients.
Conventional mail protocols, such as the Pretty Good Privacy (PGP) protocol, send mail to multiple recipients by encrypting a message with a session key (that is randomly selected for the message) to form an encrypted message. The session key is then encrypted with the public key of each of the recipients to form a set of encrypted keys. This set of encrypted keys is sent with the encrypted message to all of the recipients. Each recipient uses one of its private keys to decrypt an encrypted session key and then uses the session key to decrypt the encrypted message.
Note that key identifiers for the public keys that were used to encrypt the encrypted session keys are sent along with the encrypted session keys, so that each recipient can determine whether or not the recipient possesses a corresponding private key that can decrypt the encrypted session key. These identifiers are typically generated by computing as a hash of the public key.
Unfortunately, the key identifiers can also identify a recipient of an email message to other recipients of the email message. This complicates the process of sending an encrypted email message to anonymous recipients, because the recipients of the email message can determine the identities of the anonymous recipients by examining the key identifiers for the anonymous recipients.
What is needed is a method and an apparatus for facilitating transmission of encrypted email to anonymous recipients without divulging the identities of the anonymous recipients.