Review SessionID Security

[helje5]

The way session IDs are generated is probably not status quo. Security experts please ;-)

[mattt]

I'm not a security expert, but reading the current implementation, I think the small number of random bits (2^~10 for 1337 + 2⁶⁴ for random Int) is problematic; the addition of a timestamp doesn't really do much to mitigate an attack, and MD5 isn't cryptographically secure.

Probably the simplest replacement with reasonable security is to use Foundation UUID, whose values are generated on macOS and Linux from 122 random bits read from /dev/urandom (+6 fixed bits for UUIDv4 format) [1], [2], [3].

Here's what this would look like:

import struct Foundation.UUID

extension NIOHostingSession {
  
  // MARK: - Session ID
    
  static func createSessionID() -> String {
    return UUID().uuidString
  }
}

However, something to consider: According to this article, a motivated attacker may compromise 122 random bits in under an hour, so UUID isn't suitable for most production environments. As an alternative, the article suggests Base64-encoding 160 random bits. And I'm pretty sure you could append / prepend a fixed-format timestamp, atomic counter, and / or namespace if you wanted to be able to determine the age or provenance of a given session ID without compromising security.

I'm not familiar with the current best-practices for cross-platform generation of random data in Swift, but I'd be happy to investigate and submit a PR if that's something you'd be interested in.

[helje5]

OK, did the UUID thing, much better, thanks.

I'm still interested in the current best practices, but only mildly :-) So for now I keep that open just in case someone else has extra suggestions.

BTW: We could also couple a session to a WebSocket connection, i.e. use no session-id at all. Though that would forbid reconnects, though we could use rolling reconnect-ids for that. I suppose.