What is Email authentication ?
Email authentication is a collection of techniques aimed at equipping messages of the email transport system with verifiable information. It is a coarse-grained authentication, usually at Administrative Management Domain (ADMD) , and implies no sort of authorization. That is, the purpose of email authentication is to validate the identities of the parties who participated in transferring a message, as they can modify the message. The results of such validation can then be used in delivery decisions, which are beyond the scope of email- authentication proper, and are quite different in nature from content filtering.
Why ?
Ensuring a valid identity on an email has become a vital step in stopping spam (as email can be filtered based on such an identity), forgery, fraud, and even more serious crimes. The Simple Mail Transfer Protocol (SMTP) is continuously evolving, but when it was designed, in the early 1980s, it was the purview of academia and government agencies, and as such, there was no cause to consider security. It provided for no formal verification of sender. Signing emails is a good first step towards identifying the origin of the message, but it does not establish whether that identity has a good reputation or whether it should be trusted. This article explains how email identities are forged and the steps that are being taken now to prevent it.
Authentication methods SPF (Sender Policy Framework)
SPF authenticates the sender IP address.
SPF checks whether the sender's IP address is authorized by one of the identified ADMDs.
The IP address of the sending MTA is guaranteed to be valid by the Transmission Control Protocol, as it establishes the connection by checking that the remote host is reachable. The MX receives the HELO SMTP command right after the connection is set up, and receives a bounce address at the beginning of each message. Both of them can contain a domain name. The SPF verifier queries the Domain Name System (DNS) for an SPF record labelled with that name. An SPF-compliant ADMD should publish that record beforehand, declaring which IP addresses are, or are not, authorized to use the domain name on the label. The verifier then finds the record's directive that matches the IP address of the sending MTA, and returns the associated result. It can be "pass", "fail", or some intermediate result. When the result is "pass", the corresponding domain name is the authenticated identity.
Usually, ADMDs authorize the IP addresses used by their own outbound MTAs, including any proxy or smarthost. That way, messages sent by an ADMD's boxes get authenticated if they flow through the normal path. Otherwise, unless the intermediate relay (sometimes called mediator) takes specific measures, SPF authentication does not succeed. Those specific measures consist of altering the bounce address, which mailing lists routinely do while forwarding services in general do not.
An MX can reject on "fail", but it is demanding to do so while still avoiding false positives, because that implies maintaining a list of legitimate forwarding services.
DKIM (Domain Keys Identified Mail)
DKIM authenticates parts of the message content.
DKIM checks the message content, deploying digital signatures. Rather than using digital certificates, the keys for signature-verification are distributed via the DNS. That way, a message gets associated to a domain name.
A DKIM-compliant ADMD generates one or more pairs of asymmetric keys, then hands private keys to the signing MTA, and publishes public keys on the DNS. The DNS labels are structured as selector._domainkey.example.com, where selector identifies the key pair, and _domainkey is a fixed keyword, followed by the signing domain's name so that publication occurs under the authority of that domain's ADMD. Just before injecting a message into the SMTP transport system, the signing MTA creates a digital signature that covers selected fields of the header and the body (or just its beginning). The signature should cover substantive header fields such as From:, To:, Date:, and Subject:, which can be chosen on a per-message basis, and then is added to the message header itself, as a trace field. Any number of relays can receive and forward the message. At any hop, the signature can be verified by retrieving the public key from the DNS. If the signature verifies successfully, the domain name is the authenticated identity.
The purpose of a DKIM-signature is not to assure message integrity. Often, it does not even guarantee that a message author's data, as per a signed From: field, has a real name or a valid mailbox. The parts to be signed are chosen so as to identify the message unequivocally. A valid signature just states that the message did actually flow through a box operated by that ADMD.
one as intermediate relays don't modify signed parts of a message, its DKIM-signatures remain valid. Any relay who participates in transferring the message can sign it in turn. While intermediate relays usually can add header fields without breaking existing DKIM-signatures, changing character set, adding a tag to the subject, adding a footer, or "fixing" the MIME structure of a message are likely to break them. Many mailing lists do such changes. The protocol cannot guarantee the survivability of signatures after transit, even in the absence of malice, and prescribes no particular action in that case.
ADSP( Author Domain Signing Practices)
ADSP allows to specify a policy for messages signed by the author's domain. A message has to go through DKIM authentication first, then ADSP can demand a punishing treatment if the message is not signed by the author domain(s) —as per the From: header field.
The ADSP record for example.com, if any, is published in the DNS under the label _adsp._domainkey.example.com.
ADSP is designed for domains heavily abused by phishing and similar fraud. They may want to forgo mail facilities such as mailing lists and non delivery reports, which can happen to remain unsigned, in exchange for a cut in abuse.
ADSP was demoted to historic in November 2013.
DMARC (Domain-based Message Authentication, Reporting & Conformance)
DMARC allows to specify a policy for authenticated messages. It considers both DKIM and SPF as a combined authentication method.
The "R" of DMARC, reporting, consists in supplying feedback to the author domain on how its authentication methods do, thereby providing for informed policy crafting.
Authentication-Results
Authentication-Results: is a trace header field where a receiver records the results of email authentication checks that it carried out.Multiple results for multiple methods can be reported in the same field, separated by semicolons and wrapped as appropriate. For example, the following field is purportedly written by example.org and reports SPF and DKIM results:
The first token after the field name, receiver.example.org, is the ID of the authentication server, code-named authserv-id. A receiver supporting RFC 7001 is responsible to remove (or rename) any false header claiming to belong to its domain, so that downstream filters cannot get confused. However, those filters still need to be configured, as they have to know which identities the domain may use.
For a Mail User Agent (MUA), it is slightly harder to learn what identities it can trust. Since users can receive email from multiple domains —if they have multiple email addresses— any of those domains could let Authentication-Results: fields pass through because they looked neutral to them. That way, a malicious sender can forge an autherv-id that the user would trust if the message arrived from a different domain. To keep clear from forged header fields, MUAs should only trust the ones near to the top of the header. A legitimate
Authentication-Results: appears just above a Received: field by the same domain that the message was retrieved from. Additional Received: fields may appear between that and the top of the header, as the message got transferred internally between servers belonging to that same, trusted ADMD.
The Internet Assigned Numbers Authority maintains a registry of Email Authentication Parameters. Not all parameters need to be registered, though. For example, there can be "policy" values designed for a site's internal use only, which need no registration. In addition, this header field is meant to report the results based on data that is already present in the message; therefore, use of this filter to store an additional value —for example, what about the sender is listed in a DNSWL— is not compliant with RFC 7001, and not amenable to standardization.
Criticism
"Authentication cannot stop spam, unless the cop/Reputation Service/Certificate Authority in charge revokes certificates for spamming. If that could happen, then ISPs would also be willing and even enthusiastic about terminating accounts or otherwise controlling (e.g. port block) their spammers. If ISPs would do that, then there would be no spam to need authentication to stop spam and so need for a CA playing cop. As long as ISPs remain unwilling to police their own spamming customers, they would never deal with a CA willing to play cop.
Authentication involving TLS, SMTP-AUTH, or S/MIME cannot stop backscatter for the same reasons SPF, DKIM, and the rest were, are, and always will be powerless against it. Some of those reasons are why Yahoo still does not sign DKIM on all outgoing mail, Hotmail still publishes whishywashy SPF RRs and neither requires their snakeoil forgery solution on incoming mail."
