cryptolib | Cryptomator Crypto Library | Cryptography library

I don't know the libs you mention, but I do know a fair bit about curve25519.

ECDH is of course the act of taking a counterparts public key point (really k[G], where k is their private key (a clamped 256-bit number) and G is the curve's generator point), and multiplying it by your private key, yielding yourK * theirK * G.

This process is commutative which is why it works when the counterpart does the same with your public key and their private key.

Now, as to why the curve parameters seemingly don't matter. curve25519 is a highly highly optimised elliptic curve crypto-system. The scalar multiplication is optimised (variable base scalar multiplication used for ECDH), the point arithmetic is optimised, etc. The multiplication is executed using only the X-coordinate and differential additions. See here for details.

X25519 (curve25519+ECDH) exclusively uses "X-only" scalar multiplication, where points are represented only by their X coordinate. This is one of the fastest and simplest way to do key exchange in constant time, constant time is important for side-channel timing attacks.

The only time the curve is actually needed is when we execute EdDSA point decompression. EdDSA points wire format is made of the Y coordinate, and the sign of the X coordinate.

It's not that the curve is ignored, of course, elliptic curve operations must respect the underlying curve over which they operate and indeed the Galois field over which that curve is defined, it's more that the computation which is being used is guaranteed to stay on the curve by definition.

If you're zeroing all parameters, that's weird, but if you're still leaving the field (Prime in your case), as 2^255 - 19, this must be enough for the ECDH class to know what to do.

Thus, in short, I think it's likely not actually using the curve equation in the ECDH calculations.

It looks like ninja is checking dependencies before the actual build starts and ignores meanwhile file change

Hey! Yes. Because DEPFILEs from C source files are generated at the same time as they are compiled, cmake has no way to know beforehand which file depends on which. It would have to do two passes - first to get dependencies and then to compile (which would be a nice feature, but actually hard to implement). It happens in one pass during compilation (-MD -MT flags), so cmake has no way of knowing that one file depends on that header. I also advise:

• Do not modify files in your source tree. Keep all changes in BINARY_DIR.
• Do not modify files. Generate new files. It's easier and generally, less state = less problems.

Try it such: