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It is difficult for me to fully understand this whole process.

I am using express.js and express-session.js with Node.

Now if someone logs in, a session is stored in the session store(server side) and as a cookie (client side).

Besides other configuration options, there is:

my session name is : 'session_ID'
and the secret is : 'secret' (*just for this demonstration*)

Example: Server side-> Store: sess:II3l_VBObKhFSN_qo3cu5mL5bjZzCJwL :{\"cookie\":{\"originalMaxAge\":300000,\"expires\":\"2019-04-11T18:42:22.104Z\",\"secure\":false,\"httpOnly\":true,\"path\":\"/\",\"sameSite\":true},\"userId\":\"5cad03dbba90381e7a1c150f\"}

Client side-> Cookie: (see Screenshot)

Now the value of the cookie is the following:

session_ID:"s%3AII3l_VBObKhFSN_qo3cu5mL5bjZzCJwL.1d1i6LfkW%2F%2BcrkvDvJqsPTvybmNenEaMgj87vTRrYVY"

I know that s%3 is some kind of prefix

then there is the sessionId: AII3l_VBObKhFSN_qo3cu5mL5bjZzCJwL

and the last thing after the dot (.): 1d1i6LfkW%2F%2BcrkvDvJqsPTvybmNenEaMgj87vTRrYVY

So correct me if I am wrong but my assumption is:

the sessionID is long enough so it will be safe against bruteforcing a right sessionID and logging in as someone else.

If a client sends a request with the cookie, I can get his data from the session store (here it would be the userId) and make decisions based on that.

So what is the thing after the dot? 1d1i6LfkW%2F%2BcrkvDvJqsPTvybmNenEaMgj87vTRrYVY

Why do I need a secret?

EDIT (as advised by Xander not to do it by an answer):

Okay, what I could have done before asking this question here is just try it out. So I opened two browser B1 and B2.

In B1 I logged in as User1 and in B2 as User2. I have a query where i get the current User.

So I took the session ID of User1 of B1 and replaced it with User2 of B2 and made a query to get the current User -> What I got was no User.

So the assumption of me was wrong.

Is there a helpful site which explains this? I still don't get how this really works "behind the scences".

1

The part before the . is a UUID generated here by uid-safe, using a CSPRNG (by default anyway, the session id generation function appears to be configurable). This is a standard random session identifier created using enough entropy to make guessing one impractical.

The part after the . is a "signature" (actually it's a MAC) created here and validated here using cookie-signature. This means that anyone who actually manages to guess a session id (most likely due to someone using their own subpar generation function) won't be able to make use of it without knowing the secret key used for the MAC.

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