User:David MacQuigg/Sandbox/Email authentication

COPIED to PUBLIC VERSION 10/16/09

This article is a subtopic in a group of articles under Email system. We assume the reader understands the parent article, its terminology, and the roles of different agents in the system.

Secure communications may require any or all of:

1) authentication of the source (individual or organization identity)
2) verification of content (digital signature)
3) confidentiality of content (encryption)
4) originality (no duplicates)
5) timely delivery (no unexpected delays)
6) hidden communication (keeping an enemy unaware)

Solving the problems of bulk email abuse (spamming, phishing and other bulk mail scams) requires that we address requirements 1 and 4. The others may be important in higher security situations, but the major problems with email since 2003 have centered around massive abuse of bulk mail. Email authentication seeks to alleviate these problems by identifying the source and assessing its reputation. Individual recipients may whitelist individual senders, but the identities used in recently developed authentication methods are the domain names of email sending organizations.

To be useful in email authentication, an identity must have three characteristics. It must be unique, verifiable, and suitable for accumulation of reputation. An IP address is unique and verifiable, but difficult to use in a reputation database, because the assignment of IP addresses to specific transmitters is always changing. The problem of identifying "bad" IP addresses has been called "whack-a-mole" - blacklist one IP address, and another pops up. Good IP addresses are usually more stable, but unexpected changes may still occur, and these changes are often not under the control of the email sender. After years of accumulating a good reputation at a particular address, an organization may find it has been assigned a new IP address, perhaps even one that has acquired a bad reputation.

Domain names are unique and suitable for accumulation of reputation. Reputable names can be kept as long as an organization pays a small annual registration fee. The problem with domain names is verifiability in an email. A criminal can too easily forge the name of a reputable domain. That is the problem email authentication methods seek to avoid.

Email authentication methods fall into two categories. Methods like SPF, Sender ID, and CSV rely on the fact that certain IP addresses are firmly under the control of a sender (an individual or organization identified by its domain name). Methods like DKIM rely on a digital signature verifying the entire message and some of its headers. Both depend on the security of DNS. The assumptions are that only the domain owner has access to the DNS records under his name, and that a DNS query by the receiver will return those records unaltered.

|--- Sender's Network ---|           |--------- Recipient's Network --------|
                                /
Author ==> MSA/Transmitter --> / --> Receiver/Forwarder ~~> MDA ==> Recipient
                    /         /        /
                   /       Border     /
                  /                  /
                  ------ DNS -------                     

With IP-based methods, the sender publishes in DNS the IP addresses authorized to transmit using his domain name. With signature-based methods, the sender publishes a public key.

IP methods can be very efficient, rejecting an entire session without transferring any messages, but there must be a "chain of trust" from author to recipient. A "forwarding problem" may occur when the source IP address on the "last hop" is no longer related to the sender's domain name.

Signature methods work "end-to-end" and avoid the forwarding problem. They have a different problem, however. It is not hard for a criminal to get just one signed message through a reputable email service. That message can then be sent via a botnet to millions of recipients, and the signature is still valid. The fundamental advantage of signature methods (path independence) then becomes a fundamental vulnerability.