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Ask HN: Can A give permission to C to access B, without B talking to A?

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Ask HN: Can A give permission to C to access B, without B talking to A?

#1

Post by mangeletti »

### Context

I'm Alpha server, and I can grant permission to access things on Beta server at any point in the future, but I can only talk with Beta server one time, during initial setup (to share a secret, etc.).

Gamma server can talk to me and to Beta server any time. I can also give Gamma server instructions.

### Scenario

Gamma server comes to Beta server and says, "let me in". Beta server says, "go ask Alpha server for permission and return with proof of permission".

### Question

How can the above scenario be achieved? Does this type of permission mechanism already have a name? Would it simply require some shared secret between Alpha server and Beta server and some sort of hash calculation?

I'd also like the permission to expire after a certain period of time (or at a specific datetime).

### Notes

I realize this has similarities to OAuth 2, but since Alpha server can only speak to Beta server one time ever (initial setup), OAuth 2 cannot be used.

Re: Ask HN: Can A give permission to C to access B, without B talking to A?

#2

Post by patio11 »

There exist a variety of ways to do this. How complicated do you want it to be, and how many other concerns does this implicate? Does Gamma have to auth as actually being Gamma to Beta or is that out of scope? etc

The most straightforward way to be is (duh duh) public key crypto. GPG is fine. Gamma talks to Alpha. Alpha tells Gamma: "Give this message to Beta: 'I trust Gamma. Nonce: whatever.' Here's the corresponding signature." Beta has pre-arranged to have Alpha's public key. On presentation of Alpha's message from Gamma and verification of the signature, Beta extends trust to Gamma.

There are a variety of ways to screw this up. Don't roll your own crypto primitives. It is highly likely you have consequential requirements which break the design of this cryptosystem, like e.g. requiring the ability for Alpha to say "Nah, actually, changed my mind about Gamma. They're not authorized anymore."

Re: Ask HN: Can A give permission to C to access B, without B talking to A?

#3

Post by mangeletti »

patio11 wrote: post #2

There exist a variety of ways to do this. How complicated do you want it to be, and how many other concerns does this implicate? Does Gamma have to auth as actually being Gamma to Beta or is that out of scope? etc The most straightforward way to be is (duh duh) public key crypto. GPG is fine. Gamma talks to Alpha. Alpha tells Gamma: "Give this message to Beta: 'I trust Gamma. Nonce: whatever.' Here's the correspondin…

Thanks for your reply. Pertaining your last point, I've actually already added some scope creep to the OP (see the second paragraph of the question section, pertaining expiration, etc.).

Re: Ask HN: Can A give permission to C to access B, without B talking to A?

#5

Post by kmowery »

mangeletti wrote: post #3
patio11 wrote: post #2

There exist a variety of ways to do this. How complicated do you want it to be, and how many other concerns does this implicate? Does Gamma have to auth as actually being Gamma to Beta or is that out of scope? etc The most straightforward way to be is (duh duh) public key crypto. GPG is fine. Gamma talks to Alpha. Alpha tells Gamma: "Give this message to Beta: 'I trust Gamma. Nonce: whatever.' Here's the correspondin…

Thanks for your reply. Pertaining your last point, I've actually already added some scope creep to the OP (see the second paragraph of the question section, pertaining expiration, etc.).

That's still fairly straightforward; change the message A signs to "I trust Gamma. Nonce: whatever. Valid Until: DateTime". B should then 1) check the signature and 2) check the datetime (in that order).

Re: Ask HN: Can A give permission to C to access B, without B talking to A?

#7

Post by kmowery »

mangeletti wrote: post #4

It seems the answer could be to use HMAC. Perhaps, others could expand on this?

I'd agree. An HMAC allows A and B to send messages to each other, through C, without C being able to change or modify the message in flight.

A and B just need to share the HMAC key. Handling (and changing) this key can be tricky; if an attacker acquires it, the entire scheme falls over.

Re: Ask HN: Can A give permission to C to access B, without B talking to A?

#8

Post by anfedorov »

mangeletti wrote: post #4

It seems the answer could be to use HMAC. Perhaps, others could expand on this?

You certainly could. If you want to add something more complicated like an expiration time to prevent replay attacks, it might be a good use case for JSON Web Tokens: http://jwt.io/

Re: Ask HN: Can A give permission to C to access B, without B talking to A?

#9

Post by mangeletti »

kmowery wrote: post #7
mangeletti wrote: post #4

It seems the answer could be to use HMAC. Perhaps, others could expand on this?

I'd agree. An HMAC allows A and B to send messages to each other, through C, without C being able to change or modify the message in flight. A and B just need to share the HMAC key. Handling (and changing) this key can be tricky; if an attacker acquires it, the entire scheme falls over.

Thanks. Is there a specific reason why handling the key is tricky? Or, is this just a matter of keeping it secret?
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