Functions to generate a random private key and calculate the corresponding curve25519 public key.

## Usage

```
keygen(seed = random(32))
pubkey(key)
```

## Details

Asymmetric methods rely on public-private keypairs. The private keys are secret and should never be shared with anyone. The public key on the other hand is not confidential and should be shared with the other parties. Public keys are typically published on the users's website or posted in public directories or keyservers.

The two main applications for public key cryptography are encryption and authentication.

In public key encryption, data that is encrypted using a public key can only be decrypted using the corresponding private key. This allows anyone to send somebody a secure message by encrypting it with the receivers public key. The encrypted message will only be readable by the owner of the corresponding private key. Basic encryption is implemented in simple_encrypt.

Authentication works the other way around. In public key authentication, the owner of the private key creates a 'signature' (an authenticated checksum) for a message in a way that allows anyone who knows the user's public key to verify that this message was indeed signed by the owner of the private key.

If both sender and receiver know each other's public key, the two methods can be combined so that each message going back and forth is signed by the sender and encrypted for the receiver. This protects both against eavesdropping and MITM tampering, creating a fully secure channel.

## Examples

```
# Create keypair
key <- keygen()
pub <- pubkey(key)
# Basic encryption
msg <- serialize(iris, NULL)
ciphertext <- simple_encrypt(msg, pub)
out <- simple_decrypt(ciphertext, key)
stopifnot(identical(msg, out))
```