Gmail sending out warnings for 512 bit DKIM keys

As an update to yesterday’s post, Gmail is contacting postmasters at domains signing with 512 bit keys to warn them of the upcoming changes. This message also clarifies “DKIM keys failing.” Messages signed with 512 bit keys or less will be treated as unsigned by Gmail in the next week or so.

Hello,

We noticed that your domain is sending email to Gmail users that is DKIM signed with a 512-bit RSA key. RFC 6376 requires DKIM signing mail using RSA keys of at least 1024-bits for long-lived keys (https://tools.ietf.org/html/rfc6376#section-3.3.3). Shorter keys could be factored by an attacker. As you may know, this attack has been publicly reported. US-CERT has also issued an advisory to upgrade all keys lower than 1024-bits (http://www.kb.cert.org/vuls/id/268267).

As such, we strongly encourage you to upgrade your RSA keys to be at least 1024-bits long.

To best protect our users, Gmail will begin treating emails signed with 512-bit keys as unsigned in about a week. If you continue to use your current key, your messages will not DKIM authenticate.

Affected key:
example._domainkey.example.com descriptive text “k=rsa; p=AKDA3adkelLHaK653IuYD aVgIFc/FBvErvNOkCAwEAAQ==;”

Thank you,
Gmail Team

Is Google failing DKIM keys shorter than 512 bits?
Return Path announces new abuse prevention service

Related Posts

DMARC: an authentication framework

A new email industry group was announced this morning. DMARC is a group of industry participants, including large senders, large receivers and relevant intermediaries working on a framework to reduce the harm from phishing.
DMARC is working on a standard to allow senders to publish sending policies and receivers to act on those policies. Currently, senders who want receivers to not deliver unauthenticated email have to negotiate private agreements with the ISPs to make that happen. This is a way to expand the existing programs. Without a published standard, the overhead in managing individual agreements would quickly become prohibitive.
It is an anti-phishing technique built on top of current authentication processes. This is the “next step” in the process and one that most people involved in the authentication process were anticipating and planning for. I’m glad to see so many big players participating.
 

Read More

Gmail shows authentication data to the recipient

Yesterday Gmail rolled out some changes to their interface. One of the changes is that they are now showing end users authentication results in the user screen.
It’s really the next step in email authentication, showing the results to the end user.
So how does Google do this? Google is checking both SPF and DKIM. If mail is authenticated and the authentication matches the from address then they display the email as:
mail from steve to me
If we click on “details” for that message, we find more specific information.
full details of message showing signing domain and spf domainIn this case the mail went through our outgoing mailserver to gmail.
Mailed-by indicates that the message passed SPF and that the IP address is a valid source of mail from wordtothewise.com.
Signed-by shows the domain in the DKIM d=. In this case, we signed with the subdomain dt.wordtothewise.com. That’s what happens when you sign using the domain in the From address (or a subdomain of it).
For a lot of bulk senders, though, their mail is signed using their ESP’s domain instead.  In that case Gmail shows who signed the mail as well as the from address.

And when we click on “details” for that message we see:
3rd party signature detailsThis is an email from a sender using Madmimi as an ESP. Madmimi is handling both the SPF authentication and the DKIM authentication.
As an aside, this particular  sender has a high enough reputation that Gmail is offering me an unsubscribe option in their interface.
Gmail is distinguishing between first party and third party signatures in authentication. If the mail is authenticated, but the authentication appears to be handled by a separate entity, then Gmail is alerting recipients to that fact.
What does this mean for bulk senders?
For senders that are signing with a domain that matches their From: domain, there is no change. Recipients will not see any mention of your ESP in the headers.
However, if you are using an ESP that is signing your mail with a domain they own, then your recipients will see that information displayed in the email interface. If you don’t want this to be displayed by Gmail, then you will need to move to first party signing. Talk to your ESP about this. If they’re unsure of how to manage it, you can point them to DKIM Core for an Email Service Provider.
Gmail blogpost about the changes
Gmail help page about authentication results

Read More

Gmail reports spear phishing attack

No one, it seems, is immune from account compromise attempts. Today Google reported they had identified a systemic campaign to compromise Gmail accounts belonging to “senior U.S. government officials, Chinese political activists, officials in several Asian countries (predominantly South Korea), military personnel and journalists.”
Google offers a number of solutions for users, including the ability to add 2 factor authentication to your Gmail account. I strongly recommend anyone who uses Gmail to do this.
This isn’t a security blog, but email is one of the major vectors used to infect machines. We’ve seen numerous break ins targeting email senders and ESPs, resulting in customer and recipient data being stolen and then used for spam. Everyone who uses email needs to be aware of the risks and maintain their email account integrity. Be careful clicking links in emails. Be careful opening webpages. Keep your antivirus software up to date.
Everyone is a target.
 

Read More