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The reason I "rolled my own" key exchange is that I'd want to use the IETF variant of Chacha20-Poly1305 in my protocol and there doesn't seem to be helper functions for key exchange for this algo.

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Unless I'm misunderstanding you, the crypto_secretstream construction [1] that I (and others) have recommended uses ChaCha20Poly1305-IETF as its symmetric cipher. There wouldn't be a key exchange for this algorithm, since it's symmetric. However, the crypto_kx functions [2] generate a 256 bit shared secret, which should work for a ChaCha20Poly1305 key I believe?

1: https://doc.libsodium.org/secret-key_cryptography/secretstre...

2: https://doc.libsodium.org/key_exchange

Probably should have used higher-level functions. Maybe next time :). I don't think I can change the protocol and break compatibility at this time lol.

The crypto_kx functions seem to generate 2 symmetric keys for 2 directions. That creates complications in the code and that's why I didn't use it initially..

Yeahhh, that's a fair point about unnecessary complexity. The docs say if only one symmetric key is necessary, you can just set one of tx/rx to null, and the other will still be the requisite 256 bits, but I appreciate why that's a bit ugly.

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