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package xsecretbox
import (
"errors"
"github.com/aead/chacha20/chacha"
"golang.org/x/crypto/curve25519"
)
// SharedKey computes a shared secret compatible with the one used by `crypto_box_xchacha20poly1305``
func SharedKey(secretKey [32]byte, publicKey [32]byte) ([32]byte, error) {
var sharedKey [32]byte
curve25519.ScalarMult(&sharedKey, &secretKey, &publicKey)
c := byte(0)
for i := 0; i < 32; i++ {
c |= sharedKey[i]
}
if c == 0 {
return sharedKey, errors.New("weak public key")
}
var nonce [16]byte
chacha.HChaCha20(&sharedKey, &nonce, &sharedKey)
return sharedKey, nil
}
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