cryptokit.signature.swift

let privateKey = Curve25519.Signing.PrivateKey()
let publicKey = privateKey.publicKey // publicize freely
let publicKeyData = publicKey.rawRepresentation
let signingPublicKey = try! Curve25519.Signing.PublicKey(rawRepresentation: publicKeyData)

// sign some data with your private key
let data = "All your base are belong to us".data(using: .utf8)!
let signature = try! privateKey.signature(for: data)

if signingPublicKey.isValidSignature(signature, for: data) {
    print("The signature is valid.")
}