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Cracking a 512-bit DKIM key for less than $8 in the cloud

dmarcchecker.app

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Re: Cracking a 512-bit DKIM key for less than $8 in the cloud

#331

Post by dusted »

dusted wrote: post #269

If you want to try something fun: Provision a 4096-bit DKIM key. Every online DKIM/SPF checker will say all is good when looking at your DNS. They will also fail any test email you send, with more or less excellent descriptions such as: STATUS: Fail DKIM: Pass SPF: Pass There's this fun thing that, apparently: It's permitted and valid to use keys larger than 2048 bits in your DKIM entry. It is not, however, required…

Addendum: You need to set a strict dmarc policy for the checks to fail. Interestingly, the sites will tell you all three are correct and valid, but still fail the mail.. This is probably due to different pieces of software doing the dns record checking and the email validation.

Re: Cracking a 512-bit DKIM key for less than $8 in the cloud

#333

Post by croemer »

ClassyJacket wrote: post #324
layer8 wrote: post #207

Earlier quoted context omitted.

There’s no such guideline. It’s just part of the default title normalization. The submitter can adjust it afterwards.

Altering headlines automatically is really irresponsible.

Is there any documentation on this? @dang?

Re: Cracking a 512-bit DKIM key for less than $8 in the cloud

#334

Post by dang »

croemer wrote: post #186

Slight change of title. The leading "How" was dropped. Original: "How We Cracked a 512-Bit DKIM Key for Less Than $8 in the Cloud"

"How we X'd a Y" is a linkbait trope. We edited that in accordance with the site guidelines: "Please use the original title, unless it is misleading or linkbait". https://news.ycombinator.com/newsguidelines.html

Re: Cracking a 512-bit DKIM key for less than $8 in the cloud

#335

Post by dang »

ClassyJacket wrote: post #324
layer8 wrote: post #207

Earlier quoted context omitted.

There’s no such guideline. It’s just part of the default title normalization. The submitter can adjust it afterwards.

Altering headlines automatically is really irresponsible.

It is if you only count the cases it gets wrong.

Re: Cracking a 512-bit DKIM key for less than $8 in the cloud

#336

Post by sharpshadow »

tsimionescu wrote: post #326
tptacek wrote: post #275

Earlier quoted context omitted.

Those are cases where DKIM is working against the user! I get that we can come up with cases where we're glad some hapless user is undone by DKIM, but when we're discussing messaging security, we generally take the side of the hapless user, not the courts and tort lawyers!

An email exchange has two users: one is the sender, the other the receiver. As the receiver, having proof that I received an email from you is potentially a feature, not a problem. More generally, authenticated communication has a long history of being considered a useful thing for society. Physical mail includes delivery confirmations where the receiver must sign for the receipt, proving to anyone that they did rece…

It would then make sense to encrypt the email content.

Repudiation of clear text messages looks like the easier implementation.

Re: Cracking a 512-bit DKIM key for less than $8 in the cloud

#338

Post by sam_lowry_ »

tredre3 wrote: post #241
sam_lowry_ wrote: post #13

Earlier quoted context omitted.

Email is actually quite secure, just in a different way that web. For instance, once you disregard so called transactional mail and spam, real email is almost all encrypted for all practical purposes. DKIM and DMARC also work quite well for spoofing protection, aside from the corner cases like the above. Average Software Engineers have an outdated idea of email, formed by 1990 era Internet.

I think your view of email is romanticized, or perhaps skewed because of your social circle. Email servers sometimes use TLS to talk with eachother, and emails are signed. But that's the extent of encryption when it comes to "real email". Email content is not encrypted in "almost all" of "real email" because almost nobody uses PGP.

I don't know what social circle I should be part of to consider email unsecure.

Even Russian spies use mail.ru and their emails are compromised not by SMTP MitM but by weak passwords, google for "moscow1 moscow2 password" to see what I am talking about )

Anyway. Back to the technical point. Email servers pretty much always use TLS to talk to each other. The connection may degrade to non-encrypted for backwards compatibility, unlike HTTPS. But it's vanishingly rare.

So, for all practical purposes that affect ordinary citizens: injection, scanning and sensitive information extraction, email in transit is quite secure.

Re: Cracking a 512-bit DKIM key for less than $8 in the cloud

#339

Post by nmadden »

JackSlateur wrote: post #254
Havoc wrote: post #6

Could someone help me understand why we're not dramatically ramping up key sizes across the board on all encryption? Not as a solution, but as a buy-some-time measure. Compute is rapidly increasing, there is continuous chatter about quantum and yet everyone seems to be just staring at their belly buttons. Obviously bigger keys are more expensive in compute, but we've got more too...why only use it on the cracking sid…

Old but still relevant: https://www.schneier.com/blog/archives/2009/09/the_doghouse_... These numbers have nothing to do with the technology of the devices; they are the maximums that thermodynamics will allow. And they strongly imply that brute-force attacks against 256-bit keys will be infeasible until computers are built from something other than matter and occupy something other than space. Long story short, brut…

That’s true for AES-256. But brute force attacks are not the most efficient way to attack RSA, so it’s not true in that case. (Eg quantum computers would break RSA-4096 but not AES-256).

Re: Cracking a 512-bit DKIM key for less than $8 in the cloud

#340

Post by teddyh »

mjl- wrote: post #221
nyrikki wrote: post #199

Earlier quoted context omitted.

RFC1035 imposes a 255 character limit per string on TXT records .

Yes, so you use multiple strings (in a single record) if you need longer values: "first 255 bytes" "second 255 bytes" "etc" DNS clients combine the 255-byte strings back into a single string.

> DNS clients combine the 255-byte strings back into a single string.

No, DKIM clients and SPF clients do that. Generic DNS clients, however, are in theory free to ascribe any semantic meaning they like to the string separations.

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