Description:
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Curve25519 is an elliptic curve variant of Diffie-Hellman key exchanging.

The author of Curve25519 (which is the chosen public key protocol for DNSCurve http://dnscurve.org) has released a public domain C library on his website: http://cr.yp.to/ecdh.html



Test script:
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Desired functionality would be something like this:
<?
$private = openssl_random_pseudo_bytes(32);
$private[0] &= 248;
$private[31] &= 127;
$private[31] |= 64;
$public = curve25519_get_public($private);
// Get Bob's public key here somehow
$shared = curve25519_get_secret($private, $bobPublic);
// Symmetric key ciphers here
?>