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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

#41

Post by daneel_w »

woodruffw wrote: post #37
daneel_w wrote: post #17

Earlier quoted context omitted.

Probably because RSA 2048 is not yet broken, and once there we still have RSA 4096 to lean back on which is since quite some time the most common key size for most things using RSA (DKIM being one of the exceptions). In the context of DKIM we're waiting for Ed25519 to reach major adoption, which will solve a lot of annoyances for everyone.

Ed25519 has seen broad adoption in TLS and other stacks that are used pervasively where DKIM is also used. What’s blocking it for DKIM uniquely? (This isn’t intended as a leading question.)

Everyone has to be onboard before the switch can be made, and not everyone is happy about the somewhat messy solution of running dual-stack RSA+Ed25519 setups in the interim - it's a bit different than e.g. supporting multiple methods for key exchange or multiple ciphers for symmetric crypto. It's just one of those things that take time to happen.

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

#43

Post by asim »

I think the cynic in me says "so what" mostly because dkim as an ancient technology is hardly secure. I don't think we're any less prone to email fakery and spam. I'd be interested to see a possible new solution or a revamping of email as a protocol but that's unlikely. We're more likely to keep it like snail mail as we prioritise different forms of communication. Unfortunately nothing has beat the universal email address on the internet. Here's hoping we come up with a chat standard that sticks and people run a user@server.com chat server where you can communicate from anywhere. Sorry xmpp, you don't count.

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

#44

Post by formerly_proven »

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…

Compare the time it takes to generate or decrypt/encrypt 4096 bit RSA versus 16384 bit RSA (it's not four times slower).

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

#45

Post by Havoc »

formerly_proven wrote: post #44
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…

Compare the time it takes to generate or decrypt/encrypt 4096 bit RSA versus 16384 bit RSA (it's not four times slower).

Indeed. There has got to be some middle ground there though that is both an incremental improvement and still within reason on cost

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

#47

Post by GTP »

matthewdgreen wrote: post #39
jgrahamc wrote: post #21

Me writing over 14 years ago: https://blog.jgc.org/2010/06/facebooks-dkim-rsa-key-should-b... This was doable 14 years ago for 512-bit keys.

For a number of years it was (non-officially) thought to be a feature to use weak DKIM keys. Some folks argued that short keys allowed you to preserve deniability, since DKIM signatures would only be short-lived and nobody would be able to use DKIM signatures to prove that any email was authentic. (I’m not saying that this is why most companies used short keys, just that there was a general view that short keys were…

Isn't deniability at odds with DKIM's goal? What would be the point of setting DKIM then? Sure, it helps with spam scores. But most companies rely on a major email provider to send emails, so maybe they wouldn't have deliverability issues anyway?

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

#48

Post by awulf »

unwind wrote: post #14
sva_ wrote: post #12

Earlier quoted context omitted.

First sentence: > In our study on the SPF, DKIM, and DMARC records of the top 1M websites, we were surprised to uncover more than 1,700 public DKIM keys that were shorter than 1,024 bits in length

So 1,700 out of 1,000,000, i.e. around 0.2%. "Not common" is one way of putting it, I guess.

I agree, but to be precise, it was 1,726 out of 476,617 DKIM keys found across those 1M domains, or about 0.36%. Since it's impossible to determine all DKIM records a domain might have from the outside, I used a list of commonly used selectors (e.g., "default' or 'key1") for the search. It's likely there are additional short keys under selectors I didn't check.

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

#49

Post by [deleted] »

croes wrote: post #16
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…

> Even simple things like forcing TLS 1.3 instead of 1.2 from client side breaks things...including hn site. That’s the reason, it breaks things, and some of them are important and can’t simply be updated.

[deleted]

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

#50

Post by paulnpace »

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…

In the case of DKIM, Ed25519.
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