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Which algorithms can be used for key exchange between two wireless nodes and are not secured, meaning they can be eavesdropped?

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    $\begingroup$ Cryptography is a field of study. Saying it cannot break is like saying math or biology cannot break. Besides that, no known ciphers have been fully proven to be unbreakable. I guess you need to study key establishment algorithms and entity authentication for starters. $\endgroup$
    – Maarten Bodewes
    Nov 10, 2019 at 13:27
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    $\begingroup$ (and generally we try to avoid eavesdropping / losing confidentiality, just sending a secret key would be key establishment after which the messages can be eavesdropped). $\endgroup$
    – Maarten Bodewes
    Nov 10, 2019 at 14:15
  • $\begingroup$ I am confused now. By sending a secret key, after key establishment, the messages can(??) be eavesdropped? But after key establishment messages become encrypted, don't they? $\endgroup$
    – someone
    Nov 10, 2019 at 16:28
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    $\begingroup$ I'm really not sure what you mean. Generally we use key exchange to provide security, and the very first thing we protect against is eavesdropping as we want to keep the messages confidential. Now you are asking us to provide a key exchange that does not provide confidentiality? Possibly the question is after DH key exchange, where the key establishment itself can be eavesdropped while the messages cannot? Just encrypting a random secret with RSA public key would also work in that scenario. $\endgroup$
    – Maarten Bodewes
    Nov 10, 2019 at 16:45
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    $\begingroup$ As said, just send a plain secret key. You know, I'll reopen the question, maybe somebody can think of an interesting option such as ECDH with broken parameters. It's at least clear what you mean now. $\endgroup$
    – Maarten Bodewes
    Nov 11, 2019 at 0:19

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Sure, just use Diffie-Hellman with too small parameters. If you can break the discrete logarithm problem for too small keys then the secret should be easy to find.

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  • $\begingroup$ Small parameters you mean the numbers it chooses at the first stages before there exists a key exchange? $\endgroup$
    – someone
    Nov 11, 2019 at 12:16
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    $\begingroup$ Yeah, for instance a logarithmic group of, say, in the hundred-two hundred digit range, easy to crack. A deliberately broken Elliptic Curve would be harder to spot even, but I don't know how to get one of those. $\endgroup$
    – Maarten Bodewes
    Nov 11, 2019 at 12:46
  • $\begingroup$ So, in Bluetooth pairing where there is a few numbers key, that can be eavesdropped? Am I correct? Sorry for the many question, I am totally new to this field. $\endgroup$
    – someone
    Nov 11, 2019 at 14:14
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    $\begingroup$ Generally such few numbers are time and rate limited and used for authentication only (but I don't have the BT standard memorized). $\endgroup$
    – Maarten Bodewes
    Nov 11, 2019 at 15:09

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