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Related to this other question about sharing the secret to sign the cookies, my question is more trivial: how really secure is to sign a simple cookie identifier?

In example, expressjs#session library requires a secret, but also says at the begining of the docs:

Session data is not saved in the cookie itself, just the session ID. Session data is stored server-side.

But I don't see any benefit to create an HMAC of an identifier, because this value will be anyway a bearer token for server state.

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I think that an HMAC provides little added security if the session ID is something that is hard to guess. For example, a long secure PRNG. But if the session ID is simple to guess then there is the risk of session hijacking.

As the expressjs framework allows you to provide a session ID creation function that returns IDs with unknown security properties, an HMAC seems like a good strategy.

The default uses the uid2 library which looks like (I'm not a NodeJS user so I'm not certain) a secure PRNG (see source code). An HMAC seems of less value in this case.

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    Just to give an example, suppose the server's token generator is just a counter. If you have token 17, you can be pretty sure 16 and 18 are valid tokens, too. If an attacker needs a MAC of that token, though, then knowing that 16 is a valid token doesn't help (because the token is really 16 and HMAC(16, secret_key)).
    – apsillers
    May 18 '15 at 18:30
  • Then, the only reason, is the "guessing" possibility, and if the session ID is hard enough, signing is useless. I'm right?
    – lucasvc
    May 25 '15 at 6:26
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    Yes @datakey, if the session is crypto secure the HMAC seems unnecessary.
    – Neil Smithline
    May 25 '15 at 21:21

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