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Suppose I have a protocol like the one below between users A and B;

-A sends random nonce G1 to B

-B sends back hash_k(G1) and random none G2

-A verifies hash_k(G1), then sends back hash_k(G1 | G2)

-B verifies and a connection is made

Would this protocol be safe from eavesdroppers and other attacks like man in the midddle?

  • Is hash_k a keyed hash? In that case A and B must already have a secret key between them. What is the purpose of this protocol, identification or key exchange? – S.L. Barth - Reinstate Monica May 12 '15 at 11:08
  • The purpose of this protocol would be to establish a secure connection between the two users, with a secret k they have. – user124627 May 12 '15 at 11:38
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    So they already both have the secret value k? And the purpose is that A can verify that B knows k, and vice versa? – S.L. Barth - Reinstate Monica May 12 '15 at 11:43
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    Why don't you simply stick to a known implementation of TLS ? – Stephane May 12 '15 at 11:46
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    Schneier's law will come quickly into focus here. It's incredibly easy to do encryption/secure transport badly and incredibly hard to do encryption/secure transport well. Choose something well tested or become a statistic. To put it another way, you can through much hard work, come up with something that you can't hack, but is found open to the wide world when others analyze it. – Fiasco Labs May 13 '15 at 2:28
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Your protocol is not safe by any means!

Example for MitM:

  • A sends random nonce G1 to B
  • C intercepts and sends nonce G1c to B
  • B sends back hash_k(G1c) and random nonce G2
  • C intercepts and sends hash_k(G1) and nonce G2c to A
  • A verifies hash_k(G1) (is OK), then sends back hash_k(G1 | G2c)
  • C intercepts and sends back hash_k(G1c | G2)
  • B verifies and a connection is made

So C can now intercept all traffic...

Also eavesdropping is possible with your protocol...

  • I assumed hash_k to be a keyed hash. In that case C could not calculate hash_k(G1). – S.L. Barth - Reinstate Monica May 12 '15 at 11:07
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    @S.L.Barth and the key is exchanged how? – Uwe Plonus May 12 '15 at 11:13
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No. Stick to known protocols such as TLS, Kerberos, SSH & IPSec for key exchanges.

Try researching Diffie-Hellman key exchanges and ECDH (Elliptical Curve Diffie-Hellman, the new method of key exchanges like your example).

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