function decrypt(private_key, public_key, nonce, message, checksum) {
    return crypt(private_key, public_key, nonce, message, checksum).message
}
Spec: http://localhost:3002/steem/@dantheman/how-to-encrypt-a-memo-when-transferring-steem
    @arg {PrivateKey} private_key - required and used for decryption
    @arg {PublicKey} public_key - required and used to calculate the shared secret
    @arg {string} nonce - random or unique uint64, provides entropy when re-using the same private/public keys.
    @arg {Buffer} message - Encrypted or plain text message
    @arg {number} checksum - shared secret checksum
    @throws {Error|TypeError} - "Invalid Key, ..."
    @return {Buffer} - message