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.
RepresentativeToPublicKey converts a uniform representative value for a curve25519 public key, as produced by ScalarBaseMult, to a curve25519 public key.
ScalarBaseMult computes a curve25519 public key from a private key and also a uniform representative for that public key. Note that this function will fail and return false for about half of private keys. See http://elligator.cr.yp.to/elligator-20130828.pdf.