Building and implementing a Single Sign-On solution

http://merbist.com/2012/04/04/building-and-implementing-a-single-sign-on-solution/

by raganwald • 14 years ago

67 42 14 years ago

tptacek15• 14 years ago

The cookie doesn’t need to contain a lot of data, its value can contain the account id, a timestamp (to know when authentication happened and a trusted signature) and a signature. The signature is critical here since this cookie will allow users to be automatically logged in other sites. I’d recommend the HMAC or DSA encryptions to generate the signature. The DSA encryption, very much like the RSA encryption is an asymmetrical encryption relying on a public/private key. This approach offers more security than a shared secret like HMAC does.

"Yes, definitely build custom crypto for your single signon system, especially if you think HMAC is an "encryption" but just "not as secure" as DSA because it's "shared secret"."

SSO schemes are one of the primary reasons people build crypto for their web apps (they don't want the database dependency between the shared apps). These schemes usually fail calamitously, sometimes a year or two into production, when your app coughs up the ability for one person to log in as any other person without a password.

If you have a real app with a real customer base or doing real revenue and feel the need to put something like this in the field, please ping me. I'm not going to try to charge you. I may try to talk you out of it, though. I will absolutely talk you out of using RSA or DSA.

ovi256• 14 years ago

Thanks for the expert feedback. From my very limited sec experience, I saw account_id in cookie, and identified that as a red flag. OTOH, I think a random token in the cookie, identifying a server-side session that stores the account id, would be a good approach, as it's the classical HTTP auth implementation. For example, what the Rails tutorial describes [1].

For SSO, the other apps can ping the cookie-originating app: "Hey, who is this browser with this cookie token" ?

[1] http://ruby.railstutorial.org/

tptacek15• 14 years ago

That's a fine approach, but developers push back on it because it requires all the apps in the SSO federation to be connected at all times. Most shops should just accept that requirement and get on with their lives, though.

route66• 14 years ago

This is more or less the way SAML [1] works, at least in one of it's protocols. The authentication server redirects (after successful login) to the initiating service and supplies an assertion-token, often with limited validity. With that assertion token more data can be asked from the servers to the auth-server directly. No account_id's go over the wire, the cookie needs only to be valid on the authentication server. And you should really need it, it's not much fun to implement.

1 http://en.wikipedia.org/wiki/SAML_2.0#Artifact_Resolution_Pr...

julianb• 14 years ago

Any thoughts on Atlassian Crowd?

http://www.atlassian.com/software/crowd/overview

tptacek15• 14 years ago

I've never gotten a chance to look at it. Presumably someone has reviewed it; Atlassian's a decently big company. You could probably ask them.

(Note also that if we had reviewed them, we probably wouldn't be able to talk about it here. But: we haven't.)

mattetti• 14 years ago

Thanks for correcting my abuse of language. I should have reviewed my post in detail. I did fix that now.

That said, while I appreciate your perspective, I'm quite disappointed that you aren't offering any solutions besides asking people to contact you.

tptacek15• 14 years ago

"Abuse of language"? You suggested that HMAC was an alternative to DSA. That's not a semantic nit. Signatures, by the way, are not the same thing as message authentication codes. And like I said: nobody should be using RSA or DSA for SSO tokens.

Offering solutions on how to implement a secure single signon scheme with RSA or DSA on Hacker News is like explaining how to perform an appendectomy in Youtube comments.

Sorry, not interested in hurting people.

I know I sound super snippy here, and I'm sorry about that, but this problem comes up a lot. We keep finding applications that get almost everything right; locked down database layer, bulletproof authorization, reliable output filtering to stop Javascript injection... and then they go build single signon, and 3 days after we see that we can turn a guest account into the sitewide admin.

People would be a lot better off if they did not try to build this particular feature themselves. (We don't build SSO systems for people, in case you think this is a sales pitch.)

euroclydon2• 14 years ago

Encryption is some funny stuff. I worked on a team that implemented SSO between an ASP.NET app and a PHP app. We used AES with a shared key and IV. That was it. We just shoved the encrypted user token and timestamp into a domain cookie.

But I guess we should have also signed the encrypted value to know that we generated it?

What's the process to break this scheme?

1) Know what type of value is being encrypted: email, username, tokenid 2) Gather several pairs of usernames/encrypted usernames 3) Use some brute force method to uncover the key and IV 4) generated fake cookies since you know the key/IV

tptacek15• 14 years ago

It's trivial to figure out what encryption scheme† is being used for a cookie or HTTP parameter as long as any portion of the plaintext being encrypted is influenced by something an attacker can control directly or indirectly.

If you're not authenticating your ciphertext, you might as well not encrypt it; attackers can rewrite the plaintext, if not outright decrypt it (the adaptive chosen plaintext attacks on AES block modes all rely on messages not being authenticated).

I'd enjoy talking more about the methods to break this stuff but am concerned that HN will read that comment as "well then we just need to stop those attacks and then we're safe". I have roughly 5 years of experience on HN to back that concern up. :)

† (At least, with enough specificity to attack it.)

bdunbar2• 14 years ago

This might be the right place to ask but ... is there a reason people keep reinventing SSO?

It's 2012. The first place I saw SSO was on a Banyan VINES network, in 1990. Years, I hasten to add, before NetWare figured it out.

We've got it, still, in the enterprise. It's now Active Directory, but I can use my same credentials for login and auth on Windows clients, unix machines, two different web apps from as many external providers.

Note, I have no idea how the web apps obtain my login credentials, save that the AD guy and the web guys did some black magic behind the scenes.

Why do people keep re-inventing SSO, sometimes badly? No way to make a black box that Just Works?

bmelton• 14 years ago

What is the 'de facto' answer for environments where AD isn't present?

I was going to say something about AD not supporting SAML, or federation, but it appears that my knowledge is dated. But in an enterprise where there are no Windows machines (to my knowledge,) much less Windows servers, what's the fill-in?

*Edited the snark out.

JoachimSchipper• 14 years ago

I think the standard UNIX-flavoured solution is LDAP+Kerberos, which is pretty much AD without the gratuitous incompatibility and useful extras.

bdunbar2• 14 years ago

Good question.

I'm familiar with two 'unix' networks that stood apart from the windows networks in an enterprisey setting.

- NIS+

- Scripts to copy a 'master' /etc/passwd to all other unix hosts. Keep the master under version control.

But I've been here for over a decade. When I got here we were using vintella which hooks the unix hosts into AD.

kprobst• 14 years ago

A lot of enterprises use SiteMinder (aka ChannelSecure), which used to be Netegrity's before they got bought out by CA. It supports both Apache and IIS, although I'm not 100% sure what the policy servers run on, it might be Windows. But I have seen LAMP apps that integrate with it, for example. It really is just an intercept-and-forward system that functions as a module on the web server and puts a few things on the HTTP headers to allow apps to handle authorization by themselves.

tptacek15• 14 years ago

Because standarized SSO systems would require the maintainers of multiple unrelated app stacks to agree with each other about something.

Spearchucker4• 14 years ago

The only thing that needs agreement is the protocol. There are two choices IMO. SAML, or WS-Federation. SAML2 is pretty good but limits implementers to the passive requestor profile (it relies on the browser for the HTTP 3xx redirects) and means you can't use services[1]. It also doesn't do attribute provisioning (something U-Prove can do). The SAML protocol also mandates use of SAML tokens. WS-Federation on the other hand lets you use any token you like. And it supports services.

[1] TBH I haven't looked at SAML for a long time. It may well have come along by now.

ominous_prime• 14 years ago

Not only what tptacek said, but it's somewhat related to NIH syndrome. People look at the specs for standards like OpenID, OAuth or SAML2; think it's way too complex; and decide they can write something much simpler to suit their needs. After dealing SAML myself for a couple years, I agree it's huge and possibly over-engineered, but it also has years of work put into it, and there are well-tested implementations that you can use, just maybe not in your favorite language.

Note: I'm talking about Web-SSO. Host SSO pretty much consists of LDAP+Krb5 (which is the basis of Active Directory authentication), or just LDAP. Everything else is pretty much dead, though NIS still sticks around the *nix world due to it's ease of configuration.

mattmanser• 14 years ago

With a lot less experience to you I can relate to the NIH syndrome, but I don't think it's just the specs, it's the resulting implementations. Just go look at the .Net one that everyone recommends you use:

https://github.com/AArnott/dotnetopenid/tree/master/src

Over-engineered specs seemingly result in over-engineered code. Not that I really get why they made the client and the provider one big project in the first place.

Having mucked around with OAuth on a couple of .Net projects it's just a nightmare to use. If something goes wrong and you try and dive into that code, woe betide you. It's like a rabbit warren. Just to figure out you aren't properly encoding the &s or something.

Having used the v3 calendar API recently it's interesting to see Google now just release essentially very simple libraries that just deal with the integration for you. It's like even they've given up trying to get people to use OAuth, where the google groups were littered with 'can't get auth working' and so just hand-wave over it.

Personally I think it's at the SOAP stage that data exchange formats went through, sounds simple but somehow is extremely complex. I'm hoping that the JSON equivalent will come out soon.

jondot• 14 years ago

You might also want to examine the OpenID way, in which case you can use for example, this https://github.com/jondot/passage, or this https://github.com/dbloete/masquerade (depending on how complex the project you want to build on)

famoreira• 14 years ago

Would openID be a good option for a SSO solution? If so what would be the best way to implement a openID server using Ruby?

etdebruin• 14 years ago

Check out https://github.com/dbloete/masquerade

I haven't used it.

Building and implementing a Single Sign-On solution

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