libsodium-php

A simple, low-level PHP extension for libsodium.

Requires libsodium >= 1.0.9 and PHP >= 7.0.0.

Full documentation here: Using Libsodium in PHP Projects, a guide to using the libsodium PHP extension for modern, secure, and fast cryptography.

Installation

libsodium (and, if you are using binary packages, on some distributions, libsodium-dev as well) have to be installed before this extension.

Then, use the PHP extension manager:

$ sudo pecl install -f libsodium

On some Linux distributions such as Debian, you may have to install PECL (php-pear), the PHP development package (php-dev) and a compiler (build-essential) prior to running this command.

libsodium-php 1.x compatibility API for libsodium-php 2.x

For projects using the 1.x API, or willing to use it, a compatibility layer is available.

Polyfill Libsodium brings the \Sodium\ namespace back.

Examples

Encrypt a single message using a secret key

Encryption:

$secret_key = sodium_crypto_secretbox_keygen();
$message = 'Sensitive information';

$nonce = random_bytes(SODIUM_CRYPTO_SECRETBOX_NONCEBYTES);
$encrypted_message = sodium_crypto_secretbox($message, $nonce, $secret_key);

Decryption:

$decrypted_message = sodium_crypto_secretbox_open($encrypted_message, $nonce, $secret_key);

How it works:

$secret_key is a secret key. Not a password. It's binary data, not something designed to be human readable, but rather to have a key space as large as possible for a given length. The keygen() function creates such a key. That has to remain secret, as it is used both to encrypt and decrypt data.

$nonce is a unique value. Like the secret, its length is fixed. But it doesn't have to be secret, and can be sent along with the encrypted message. The nonce doesn't have to be unpredicable either. It just has to be unique for a given key. With the secretbox() API, using random_bytes() is a totally fine way to generate nonces.

Encrypted messages are slightly larger than unencrypted messages, because they include an authenticator, used by the decryption function to check that the content was not altered.

Encrypt a single message using a secret key, and hide its length

Encryption:

$secret_key = sodium_crypto_secretbox_keygen();
$message = 'Sensitive information';
$block_size = 16;

$nonce = random_bytes(SODIUM_CRYPTO_SECRETBOX_NONCEBYTES);
$padded_message = sodium_pad($padded_message, $block_size);
$encrypted_message = sodium_crypto_secretbox($padded_message, $nonce, $secret_key);

Decryption:

$decrypted_padded_message = sodium_crypto_secretbox_open($encrypted_message, $nonce, $secret_key);
$decrypted_message = sodium_unpad($decrypted_padded_message, $block_size);

How it works:

Sometimes, the length of a message may provide a lot of information about its nature. If a message is one of "yes", "no" and "maybe", encrypting the message doesn't help: knowing the length is enough to know what the message is.

Padding is a technique to mitigate this, by making the length a multiple of a given block size.

Messages must be padded prior to encryption, and unpadded after decryption.

Encrypt a file using a secret key

$secret_key = sodium_crypto_secretstream_xchacha20poly1305_keygen();
$input_file = '/tmp/example.original';
$encrypted_file = '/tmp/example.enc';
$chunk_size = 4096;

$fd_in = fopen($input_file, 'rb');
$fd_out = fopen($encrypted_file, 'wb');

list($stream, $header) = sodium_crypto_secretstream_xchacha20poly1305_init_push($secret_key);

fwrite($fd_out, $header);

$tag = SODIUM_CRYPTO_SECRETSTREAM_XCHACHA20POLY1305_TAG_MESSAGE;
do {
    $chunk = fread($fd_in, $chunk_size);
    if (feof($fd_in)) {
        $tag = SODIUM_CRYPTO_SECRETSTREAM_XCHACHA20POLY1305_TAG_FINAL;
    }
    $encrypted_chunk = sodium_crypto_secretstream_xchacha20poly1305_push($stream, $chunk, '', $tag);
    fwrite($fd_out, $encrypted_chunk);
} while ($tag !== SODIUM_CRYPTO_SECRETSTREAM_XCHACHA20POLY1305_TAG_FINAL);

fclose($fd_out);
fclose($fd_in);

Decrypt the file:

$decrypted_file = '/tmp/example.dec';

$fd_in = fopen($encrypted_file, 'rb');
$fd_out = fopen($decrypted_file, 'wb');

$header = fread($fd_in, SODIUM_CRYPTO_SECRETSTREAM_XCHACHA20POLY1305_HEADERBYTES);

$stream = sodium_crypto_secretstream_xchacha20poly1305_init_pull($header, $secret_key);
do {
    $chunk = fread($fd_in, $chunk_size + SODIUM_CRYPTO_SECRETSTREAM_XCHACHA20POLY1305_ABYTES);
    list($decrypted_chunk, $tag) = sodium_crypto_secretstream_xchacha20poly1305_pull($stream, $chunk);
    fwrite($fd_out, $decrypted_chunk);
} while (!feof($fd_in) && $tag !== SODIUM_CRYPTO_SECRETSTREAM_XCHACHA20POLY1305_TAG_FINAL);
$ok = feof($fd_in);

fclose($fd_out);
fclose($fd_in);

if (!$ok) {
    die('Invalid/corrupted input');
}

How it works:

There's a little bit more code than in the previous examples.

In fact, crypto_secretbox() would work to encrypt as file, but only if that file is pretty small. Since we have to provide the entire content as a string, it has to fit in memory.

If the file is large, we can split it into small chunks, and encrypt chunks individually.

By doing do, we can encrypt arbitrary large files. But we need to make sure that chunks cannot be deleted, truncated, duplicated and reordered. In other words, we don't have a single "message", but a stream of messages, and during the decryption process, we need a way to check that the whole stream matches what we encrypted.

So we create a new stream (init_push) and push a sequence of messages into it (push). Each individual message has a tag attached to it, by default TAG_MESSAGE. In order for the decryption process to know where the end of the stream is, we tag the last message with the TAG_FINAL tag.

When we consume the stream (init_pull, then pull for each message), we check that they can be properly decrypted, and retrieve both the decrypted chunks and the attached tags. If we read the last chunk (TAG_FINAL) and we are at the end of the file, we know that we completely recovered the original stream.

Encrypt a file using a key derived from a password:

$password = 'password';
$input_file = '/tmp/example.original';
$encrypted_file = '/tmp/example.enc';
$chunk_size = 4096;

$alg = SODIUM_CRYPTO_PWHASH_ALG_DEFAULT;
$opslimit = SODIUM_CRYPTO_PWHASH_OPSLIMIT_MODERATE;
$memlimit = SODIUM_CRYPTO_PWHASH_MEMLIMIT_MODERATE;
$salt = random_bytes(SODIUM_CRYPTO_PWHASH_SALTBYTES);

$secret_key = sodium_crypto_pwhash(SODIUM_CRYPTO_SECRETSTREAM_XCHACHA20POLY1305_KEYBYTES,
                                   $password, $salt, $opslimit, $memlimit, $alg);

$fd_in = fopen($input_file, 'rb');
$fd_out = fopen($encrypted_file, 'wb');

fwrite($fd_out, pack('C', $alg));
fwrite($fd_out, pack('P', $opslimit));
fwrite($fd_out, pack('P', $memlimit));
fwrite($fd_out, $salt);

list($stream, $header) = sodium_crypto_secretstream_xchacha20poly1305_init_push($secret_key);

fwrite($fd_out, $header);

$tag = SODIUM_CRYPTO_SECRETSTREAM_XCHACHA20POLY1305_TAG_MESSAGE;
do {
    $chunk = fread($fd_in, $chunk_size);
    if (feof($fd_in)) {
        $tag = SODIUM_CRYPTO_SECRETSTREAM_XCHACHA20POLY1305_TAG_FINAL;
    }
    $encrypted_chunk = sodium_crypto_secretstream_xchacha20poly1305_push($stream, $chunk, '', $tag);
    fwrite($fd_out, $encrypted_chunk);
} while ($tag !== SODIUM_CRYPTO_SECRETSTREAM_XCHACHA20POLY1305_TAG_FINAL);

fclose($fd_out);
fclose($fd_in);

Read the stored parameters and decrypt the file:

$decrypted_file = '/tmp/example.dec';

$fd_in = fopen($encrypted_file, 'rb');
$fd_out = fopen($decrypted_file, 'wb');

$alg = unpack('C', fread($fd_in, 1))[1];
$opslimit = unpack('P', fread($fd_in, 8))[1];
$memlimit = unpack('P', fread($fd_in, 8))[1];
$salt = fread($fd_in, SODIUM_CRYPTO_PWHASH_SALTBYTES);

$header = fread($fd_in, SODIUM_CRYPTO_SECRETSTREAM_XCHACHA20POLY1305_HEADERBYTES);

$secret_key = sodium_crypto_pwhash(SODIUM_CRYPTO_SECRETSTREAM_XCHACHA20POLY1305_KEYBYTES,
                                   $password, $salt, $opslimit, $memlimit, $alg);

$stream = sodium_crypto_secretstream_xchacha20poly1305_init_pull($header, $secret_key);
do {
    $chunk = fread($fd_in, $chunk_size + SODIUM_CRYPTO_SECRETSTREAM_XCHACHA20POLY1305_ABYTES);
    $res = sodium_crypto_secretstream_xchacha20poly1305_pull($stream, $chunk);
    if ($res === FALSE) {
       break;
    }
    list($decrypted_chunk, $tag) = $res;
    fwrite($fd_out, $decrypted_chunk);
} while (!feof($fd_in) && $tag !== SODIUM_CRYPTO_SECRETSTREAM_XCHACHA20POLY1305_TAG_FINAL);
$ok = feof($fd_in);

fclose($fd_out);
fclose($fd_in);

if (!$ok) {
    die('Invalid/corrupted input');
}

How it works:

A password cannot be directly used as a secret key. Passwords are short, must be typable on a keyboard, and people who don't use a password manager should be able to remember them.

A 8 characters password is thus way weaker than a 8 bytes key.

The sodium_crypto_pwhash() function perform a computationally intensive operation on a password in order to derive a secret key.

By doing do, brute-forcing all possible passwords in order to find the secret key used to encrypt the data becomes an expensive operation.

Multiple algorithms can be used to derive a key from a password, and for each of them, different parameters can be chosen. It is important to store all of these along with encrypted data. Using the same algorithm and the same parameters, the same secret key can be deterministically recomputed.