sso_proxy: document request signing

[mccutchen]

Is your feature request related to a problem? Please describe.

The public key-based request signing functionality added to sso_proxy in #106 is undocumented. In particular, it's not immediately obvious how to a) generate an appropriate keypair or b) validate a signed request in an upstream service.

Describe the solution you'd like

New documentation for this functionality, ideally accompanied by a reference implementation for verifying a signed request.

To start, generating an appropriate keypair for sso_proxy to use for signing requests is as simple as

openssl genrsa -out priv.out 2048

[mccutchen]

Note: The old method for request signing (based on a per-upstream shared secret) is documented here:
https://github.com/buzzfeed/sso/blob/master/docs/sso_config.md#request-signing

Those docs should probably be deprecated or removed in favor of the new approach!

[mccutchen]

To start, generating an appropriate keypair for sso_proxy to use for signing requests is as simple as

openssl genrsa -out priv.out 2048

Turns out the above generates a key in PKCS#1 format (I think?), but we need one in PKCS#8 format, which requires an extra conversion step. Luckily, this is still relatively straightforward to do in one pass:

openssl genrsa 2048 | openssl pkcs8 -topk8 -inform pem -outform pem -nocrypt

Many thanks to the wizards of Stack Overflow for explaining this.

[nebevservian]

I found the documentation pretty sparse and needed a NodeJS application to verify the signatures.

I created a library at: https://www.npmjs.com/package/signature-verifier-buzzfeed-sso

Hopefully this helps someone.

[sporkmonger]

Is it possible to do request signing with not-RSA?