IsLowOrder checks if the passed group element is of low order.

PrivateKeyToCurve25519 converts an Ed25519 private key into a corresponding curve25519 private key such that the resulting curve25519 public key will equal the result from PublicKeyToCurve25519.

PublicKeyToCurve25519 converts an Ed25519 public key into the curve25519 public key that would be generated from the same private key.