It's Dangerous, a Python cryptographic signing module
packages.python.org
packages.python.org
I repeat: do not use SHA-1 in new applications. Do not use this module.
Sometime soon I believe SHA-3 will be chosen and we'll all be able to move onto that.
"NIST also plans to host a final SHA-3 Candidate Conference in the spring of 2012 to discuss the public feedback on these candidates, and select the SHA-3 winner later in 2012." [http://csrc.nist.gov/groups/ST/hash/sha-3/Round3/index.html]
The final SHA-3 Candidate Conference was held March 22-23, so they should be picking the winner any day now.
I bet that as soon as standards bodies (like NIST) actively encourage you to use better hash functions (which they will probably do soon) the developer of this module will decide on what is safe enough for this purpose.
https://github.com/mitsuhiko/itsdangerous/blob/59f3bf7877e21...
And the tests, of course.
Part of security is being up-to-date in regards to things like hashes. If authors don't update their own libraries and you need to tweak them manually, how is it different from "writing your own"?
I strongly agree with Armin: HMAC-SHA1 (note, the combo, it's not just the latter) is still good security for most applications you'd consider this library for.
That is not necessary, I adjusted the API so that you can easily subclass it and change the digestmod.
Probably, but not necessarily. It's not like the other sha versions are fixing the overall problem however.
> Do not use this module.
Even before the 0.13 release I just pushed out where you can override the module easier you could still easily change the hash method with a two line change. Also it's not like maintainers can't change things easily if modules are well defined.
And even if the module would not allow changing the hash method easily SHA1 with HMAC is still incredibly hard to exploit.
You would indeed be better off using SHA256.
But not much. It's worth pointing out though that dogmatism around not using SHA-1 is misplaced. There are still no practical attacks on HMAC-MD5, for instance, even though MD5 itself is effectively broken and its use in (for instance) X.509 certificates is insecure.
As it is, the size of that digest is also 14 bytes more and I don't feel happy truncating the hash. The bigger SHA2 versions are even longer. Considering that thing should go into cookies and URLs I did not feel very happy with that.
> You would indeed be better off using SHA256.
Probably. I was indeed under the impression that SHA2 is based on the same building blocks as SHA1 but I assume that is not the case. That being said: upgrading is not hard if it even comes to the point where it would be necessary.
If people are having trouble they can refer to the Rails ActiveSupport source code for help.
http://codahale.com/a-lesson-in-timing-attacks/
Preemptively:
* Yes, a realistic attack.
* Yes, very difficult over the Internet.
* Still difficult but not implausible if the attacker can colocate near your app; ie, if you deploy anywhere on EC2.
* Measurement bounds are high nanoseconds LAN, tens of usecs WAN.
* HMAC verification, unlike password hash comparisons, is a place where timing actually does matter.
Not to suggest that I occupy the high road when it comes to snarky comments, but consider whether your snarky comment in this case suggests an unearned (and thus dangerous) level of confidence about crypto app security. This problem (HMAC timing) is so well known that it's generated many hundreds of comments over the years on HN.
But, obviously, this code does more than Python's hmac library does, beyond just knowing how to properly verify the MAC itself.
- The signature: SHA1 HMAC of data and, optionally, a timestamp to expire signatures.
- HMAC key derivation: SHA1 of a secret key and a salt.
Note to non-cryptographers: do not use low-entropy (password-ish) secretswith this, they can be bruteforced. (Using bcrypt/scrypt/PBKDF2 instead would fix this.)
When will some people on hackernews ever learn to start using their brain. PBKDF2 is not what you use to verify signatures. I think the name should already give that away.
//EDIT: and when will I ever learn to read properly. I just realized that the comment was referring to the "salt" part, not the signature.
PBKDF2 for this would be possible, but a terrible idea. The reason why PBKDF2 works for password verification purposes on web apps is because you can rate limit the login entry point. Doing that for sessions or activation links like what you would use itsdangerous for is an easy way to have other people DDOS your app.
PBKDF2/bcrypt etc. is not the solution for this problem.
(Alternative: drop the SHA1/bcrypt/whatever and just use a really strong secret key. 128 bits of randomness is impossible to brute-force.)
itsdangerous has nothing to do with passwords. It's about signing small messages and these messages are obviously created at runtime.
> nonsense and just use a really strong secret key
That's how you should use itsdangerous: use a strong secret key.
$ python
>>> import os
>>> os.urandom(64)
dd if=/dev/random bs=64 count=1 2>/dev/null | hexdump
The point of PBKDF2 and friends is to make it expensive to brute force acquired hashes. It effectively 'rate limits' hash-in-hand brute force attempts. It's designed to protect hashes once they are in hostile environments.
The use cases you described (session keys, activation links) are the perfect example of such environments.
It does not matter what hashing function you use for the login. It matters in case your database leaks.
> The use cases you described (session keys, activation links) are the perfect example of such environments.
They are not. Because you don't want to reverse engineer the contents of the message (the message is there in plain text). That's the reverse of what you want to do with passwords. In case of the password you want to brute force what the password is, in case of a signed message you want to forge a signature.
That was my point... I suppose I'm not sure I understood why you brought up login entry point rate limiting?
Also, PBKDF2 has nothing whatsoever to do with rate limiting login attempts. The attack PBKDF2 tries to blunt is offline, not online. PBKDF2 and bcrypt remain effective even when the imposed expense of a single hash compare is less than the network overhead of making a login request.
I am officially an idiot now. I see where the original comment is going with. Namely the "salt" part of the library, not the verification of the signature. You can just use a different secret key or derive one yourself.
I sincerely apologize for missing that and the comments that resulted from that.
That said: the secret key is the important part there, not the salt. The salt is just used to alter the secret key that the use of the serializer in two different places for different purposes does not create the same signature. The intended usage of the whole thing is to not use the salt at all but to provide different secret keys. The "salt" part was taken from the original django implementation with the idea to stay mostly compatible with their format.
Could that be improved by switching to PBKDF2? Probably, but breaking a lengthy secret key is an impossible operation currently, even with SHA1.
> Also, PBKDF2 has nothing whatsoever to do with rate limiting login attempts.
That's not what I meant. I meant that PBKDF2 on a server is feasible because you can rate limit the endpoint. I don't want my servers to be on 100% CPU because someone spams the login form.
I don't know what you mean by "lengthy secret key", but if you're talking about a string that a human would recognize as language, that's not a strong key. Strong keys come from /dev/keygeneration --- er, I mean, /dev/random and from schemes like PBKDF2.
The attack on a human-intelligible crypto key occurs offline, and HMAC verifying keys are usually long-lived, so the attacker has both a lot of time and a lot of incentive to go after that key. Do not use human-intelligible strings as HMAC keys.
I've heard a lot of smart people worry about adaptive password hashes becoming a DOS vector for their application, and while stipulating that they are indeed smart people, they haven't thought this problem through very well. Every application can be DOS'd, usually in much more effective ways than by forcing the server to hash lots of passwords. Availability attack vectors in web apps are so common that they aren't even objectives on penetration tests; what would be the point?
Also, HMAC "authenticates" and "verifies" messages; it doesn't "sign" them. "Signature" has a subtly different meaning in crypto.
`os.urandom(120)` for instance. That's what Flask and Django document. I can see how this is not obvious from the documentation which puts a human readable string in there and I will update it appropriately.
> Also, HMAC "authenticates" and "verifies" messages; it doesn't "sign" them. "Signature" has a subtly different meaning in crypto.
I agree. The use of the word signing for this however is quite widespread and I adopted the same meaning (and implementation) as with the original Django one for compatibility.
And people, STOP USING CRYPTOGRAPHIC PRIMITIVES FROM THINGS THAT ARE NOT OPENSSL.
I'd like your sentence more if it read "STOP USING CRYPTOGRAPHIC PRIMITIVES" and then ended with a period.
Edit: Found quote:
Website security: Use OpenSSL. OpenSSL has a horrible track record for security; but it has the saving grace that because it is so widely used, vendors tend to be very good at making sure that OpenSSL vulnerabilities get fixed promptly. I wish there was a better alternative, but for now at least OpenSSL is the best option available. UPDATE: For added security, terminate SSL connections in restricted environment and pass the raw HTTP over a loopback connection to your web server.
And yes, I'd always recommend using the highest-level possible API. If you have SHA in your code you are probably working at too low a level.
Do you have any recommendations for a better toolkit? Tarsnap uses Colin's own implementations so it's not a very good resource. There are other problems I've found with cryptlib, etc., (e.g., cryptlib is commercial).
1) If there's anything my grad crypto class taught me it's that RSA, specifically padding, is the most god-forsaken idea ever created by man and you will never, ever, ever, ever get it right. If the words RSA are in your code you are in deep shit.
2) S/MIME seems to be a simpler system than any certificate system I have seen. X.509 is an unholy mess. In fact, all PKI is just a complicated disaster waiting to happen.
3) Super simple API -- it can even be done on the command line.
Is there something different you'd recommend?
Edit: Actually, I just thought of another option. GPG has a --sign and --verify option. If GPG can be installed on the system it may be worth trying to integrate that.