Significance of y-coordinates in ECDH public key exchange In the research paper Breaking the Bluetooth Pairing – The Fixed Coordinate Invalid Curve Attack? by Biham and Neumann, 2019,
they talk about attacks in Bluetooth pairing, they state that in the pairing process which involves ECDH key exchange- &quot;sending both the x-coordinate and the y-coordinate during the public key exchange is a design flaw as it is unnecessary and highly inadvisable, and it greatly increases the attack surface&quot;
I'm aware that in other implementations only $x$-coordinate is sent in compressed form. I'm curious to know how sending both coordinates increases the attack surface, how important it is to know the information about the $y$-coordinate and what are the possible attacks related to this.
The paper you reference mentions that although both coordinates are sent, the protocol only requires the x-coordinate to be authenticated and validated. This means that the y-coordinate is free to be manipulated in implementations that do the bare minimum. As mentioned in a comment, &quot;poor programmers and bad quality testers will not consider [this vulnerability]&quot;.
Sending only one coordinate makes it less likely for an implementer to make a mistake. With only one coordinate, it's impossible for the second coordinate to be manipulated, and it's significantly harder to force someone to use a point not on the curve. It's easier to test that given a value, a function always generates a valid point or throws an error than it is to test that a protocol works correctly end-to-end for all possible inputs.