Asymmetric encryption using PHP (the alice and bob story)

code.php

<?php

// define an example, our people, messages and their keys
$people = [
    'alice' => [
        'keys' => gen_keys(),
        'msg' => 'Hi Bob, I\'m sending you a private message'
    ],    
    'bob' => [
        'keys' => gen_keys(),
        'msg' => 'Thanks Alice, message received'
    ]  
];

//
$encrypted = $decrypted = [
    'alice' => '',
    'bob'   => ''
];

// keys get exchanged

// alice encrypts her message to bob
$encrypted['bob'] = encrypt(
    $people['alice']['msg'],         // message to encrypt
    $people['bob']['keys']['public'] // bobs public key, which he sent to alice
);

// message sent to bob

// bob decrypts his message
$decrypted['bob'] = decrypt(
    $encrypted['bob'],                // message to decrypt
    $people['bob']['keys']['private'] // bob's private key, which he uses to decrypt the message
);

// bob now responds

// bob encrypts his message to alice
$encrypted['alice'] = encrypt(
    $people['bob']['msg'],             // message to encrypt
    $people['alice']['keys']['public'] // alice public key, which she sent to bob
);

// alice decrypts her message
$decrypted['alice'] = decrypt(
    $encrypted['alice'],                // message to decrypt
    $people['alice']['keys']['private'] // alice's private key, which she uses to decrypt the message
);

//
print_r($decrypted);

/*
Array
(
    [alice] => Thanks Alice, message received
    [bob] => Hi Bob, I'm sending you a private message
)
*/

/**
 * Functions - wraps for openssl operations
 */
// generate public and private key pair
function gen_keys() {
    $res = openssl_pkey_new(array('private_key_bits' => 2048));

    /* extract the private key */
    openssl_pkey_export($res, $privateKey);

    /* extract the public key */
    $publicKey = openssl_pkey_get_details($res);

    return ['public' => $publicKey["key"], 'private' => $privateKey];
}

// encrypt using public key
function encrypt($msg, $key) {
    $ret = '';
    openssl_public_encrypt(
        $msg, // message to encrypt
        $ret, // &encrypted message
        $key  // public key
    );
    return $ret;
}

// decrypts using private key
function decrypt($msg, $key) {
    $ret = '';
    openssl_private_decrypt(
        $msg, // message to decrypt
        $ret, // &decrypted message
        $key  // private key
    );
    return $ret;
}