Symmetric Key to Asymmetric key handoff

Question

I'm not a cryptography expert, I actually only have a little bit of experience using it at all. Anyways, the time has come where one of my applications demands that I have some encryption set up. Please note, the program won't be managing anything super critical that will be able to cause a lot of damage.

Anyways, I was just trying to see if this scheme that I'm using is common and if there are flaws (of which there may be completely stupid & horribly flawed design, that's why I'm asking).

Ok, I have a client -> server communication. The Client I can hard code in the public portion of a 2048-bit RSA key. When the client wants to initiate a secure connection, he sends his username, md5 hash of his password, and a hash of a random UUID, all of which has been encrypted against the server's Public Key. The server receives the information and decrypts using its private key. Checks the database to see if his login + pass work & if they do, create a new entry in the "Sessions" table in the DB. This includes a SessionID, UID (user ID), and the UUID hash. Using the corresponding session ID's UUID as the keyphrase, the server will then send back a message that has the Blowfish encrypted word "Success!" + a random UUID (this message is Digitally Signed so we can determine if it came from the server or not). From that point on, when the client sends info to the server, it will be with a plaintext sess_id & include a Blowfish encrypted message, using the corresponding Session ID's blowfish secret (stored encrypted in the DB) as the key to encrypt / decrypt.

Specifically, I am curious as to whether this system "should work" or if anyone notices that it's glaringly obvious that a vulnerability exists, such as MITM.

Answer 1

From that point on, when the client sends info to the server, it will be with a plaintext sess_id & include a Blowfish encrypted message, using the corresponding Session ID as the key to encrypt / decrypt.

If you're sending the session id in plaintext, and using it as the encryption key, how is that secure?

I see no reason why you can't use standard SSL authentication and let the library implementer worry about the handshaking.

Answer 2

Just use SSL or DTLS, IKEv2, HIP, EAP or some suitable standard protocol. Don't try to invent your own crypto protocols, nobody has enough expertise to do this on their own. Your protocol doesn't have nearly enough entropy in it, so far as I can see, so your resulting keys will be pretty weak.

Answer 3

Issues I can see off the top of my head (although you have left out most of the details, which is where the devil famously resides):

• If you're using a UUID generator rather than a real cryptographic RNG, it likely has insufficient entropy. Don't discount this - in the real world, the favourite way of covertly weakening an encryption system has been to weaken the RNG;

• Your initial RSA encryption sounds like it is susceptible to a small-exponent attack, and potentially other creative attacks. There's too much structure there to be comfortable;

• It sounds like there's numerous opportunities for replay attacks;

• What block cipher mode are you using with Blowfish?

I recommend using TLS/SSL - it's had a lot more friendly eyes looking at it for a lot longer than anything you build yourself ever will.