Algorithm Agility?

https://www.tbray.org/ongoing/When/202x/2021/04/24/Algorithm-Agility

by moks • 5 years ago

30 21 5 years ago

tptacek2• 5 years ago

Algorithm agility is bad. You see it happening at the beginning of every TLS session; what you don't see are the long trail of vulnerabilities that have resulted from TLS's overly complicated handshake, and, worse, the essentially nonexistent vulnerabilities algorithm agility has mitigated over TLS's lifetime; in general, when TLS constructions have failed, they've failed in the joinery (block cipher modes, MAC arrangements, etc), and, most importantly, have required updates to the entire protocol anyways.

It's a bad idea and people shouldn't build it into new systems. Version the protocol instead, and plan on making it straightforward to upgrade the protocol and lock out old versions.

MaxBarraclough2• 5 years ago

Relevant Wikipedia article: https://en.wikipedia.org/wiki/Downgrade_attack

QuinnWilton2• 5 years ago

And all of this is to say nothing of the frankly embarrassing problems that have plagued JWT as a result of algorithm agility (alg=none). Removing agility from JWT wouldn't make it a good specification, but it would certainly make it a better specification.

emteycz• 5 years ago

What is a good alternative?

forgotmypw172• 5 years ago

I disagree with locking out old versions. It's a huge problem with older devices trying to access the web via SSL. I think it is important to consider device longevity and also the possibility of the user not being able to upgrade their software. Security is important, but it is not everything. There is also accessibility and compatibility to consider.

It's why I still test my websites with Netscape 3.x and keep http running.

slaymaker19072• 5 years ago

Can Netscape even work on modern Windows?

sneak• 5 years ago

Postel's law has been deprecated.

baby• 5 years ago

It’d be nice to have someone summarize the discussion on crypto agility, which I think started with agl’s post on the subject. And recently filippo started a similar convo on twitter: https://twitter.com/filosottile/status/1386751406758105089?s...

I don’t think it’s just a crypto thing, agility is an issue in protocols that need to remain compatible in general.

slaymaker19072• 5 years ago

Kerberos/AD is another example of this kind of mess. It has both LDAP and LDAPS not to mention the zoo of encryption types available for keytabs and credential caches. You can still use unsalted encryption types and DES!

While on one hand insecure LDAP is convenient for testing, I do think it should really just be removed and require LDAPS.

upofadown3• 5 years ago

If someone suddenly breaks the stuff based on the 25519 curve there are going to be a lot of very unhappy people. I am still waiting for an explanation of why this particular curve is immune to such a breakthrough.

tatersolid2• 5 years ago

A lot of research went in to Curve25519; multiple decades of ECC academic investigation created a set of criteria for safety:

See https://safecurves.cr.yp.to/

upofadown3• 5 years ago

Curve25519 has only been around for something like 10 years. A catastrophic attack on it would presumably involve new ideas specific to that particular or class of curves.

Otherwise any completely new system would have to be considered secure, as it would not suffer from any of the issues of any of the older systems. My understanding is that new elliptic curves can have entirely new characteristics not shared by older curves.

tptacek2• 5 years ago

Nobody thinks it is. Curve25519 being invincible is not the premise behind the shift away from "agility".

RcouF1uZ4gsC2• 5 years ago

should developers just blast the key-bits into base64 and ship that?

I would argue given the key lengths of ed25519, not even to do base64, but just do hex encoding.

Sure it is a little longer, but in return you get much simpler encoding and decoding, no worries about flavors of base64 or padding.

In addition, hex encoding is much easier to verbally communicate to someone versus base64.

In a security context, I think making things as simple as possible has big benefits.

upofadown3• 5 years ago

Split the difference with base 32? More efficient than hex, you can leave out the 4 easiest letters to confuse with digits and no upper/lower case problem.

Verbal communication is a problem with any system that uses letters. In hex for example; 3, B, C, D and E all sound very much the same.

RcouF1uZ4gsC2• 5 years ago

In hex for example; 3, B, C, D and E all sound very much the same.

We have a standard way of dealing with that in the form the NATO phonetic alphabet.

So 3BCDE would be “Tree Bravo Charlie Delta Echo”

skybrian• 5 years ago

I'm not a any kind of crypto expert, but I'm wondering if this looks like a config file? Let's say you have a JSON file that has one field, which is the key, base64-encoded.

Maybe you won't change the algorithm, but still want to leave room to add another config parameter. The file format is obvious in a text editor and it's easy to add more stuff.

Algorithm Agility?

Related Stories