SealShare/app/Services/FileEncryptionService.php

<?php

namespace App\Services;

use Generator;
use RuntimeException;

/**
 * The encrypted file formats and the keys behind them.
 *
 * New files are `SEALCHK2`, written chunk by chunk as the uploader's browser sends them:
 *
 *   [8 bytes: "SEALCHK2" magic]
 *   [4 bytes: chunk size S, uint32 big-endian]
 *   [7 bytes: random nonce prefix]
 *   Per chunk i: [ciphertext (S bytes, fewer on the last chunk)][16 bytes: GCM tag]
 *
 * Chunk i's nonce is the prefix, i as uint32 big-endian and a byte that is 1 on the last chunk
 * and 0 on every other (the STREAM construction), so dropping, reordering or appending chunks
 * fails authentication. The browser encrypts with the same layout (resources/js/share-uploader.js).
 *
 * `SEALCHK1` (a tag before each chunk, the index XORed into a 12-byte nonce, no last-chunk flag)
 * and the single-block legacy format are still read for shares created before.
 */
class FileEncryptionService
{
    public const HEADER_LENGTH = 19;

    public const TAG_LENGTH = 16;

    private const CIPHER = 'aes-256-gcm';

    private const PBKDF2_ITERATIONS = 100000;

    private const KEY_LENGTH = 32;

    private const NONCE_LENGTH = 12;

    private const NONCE_PREFIX_LENGTH = 7;

    private const MAGIC = 'SEALCHK2';

    private const LEGACY_CHUNKED_MAGIC = 'SEALCHK1';

    private const WRAPPED_KEY_ALGORITHM = 'argon2id';

    /**
     * Derive a key from a password and salt using PBKDF2-SHA256, as shares created before
     * envelope encryption were keyed.
     */
    public function deriveKey(string $password, string $salt): string
    {
        return hash_pbkdf2('sha256', $password, hex2bin($salt), self::PBKDF2_ITERATIONS, self::KEY_LENGTH, true);
    }

    /**
     * Generate a random encryption key (32 bytes, returned as hex).
     */
    public function generateRandomKey(): string
    {
        return bin2hex(random_bytes(self::KEY_LENGTH));
    }

    /**
     * Wrap a share's data key with a key derived from its password (Argon2id).
     *
     * The result names its algorithm and parameters, so they can be raised later without breaking
     * shares wrapped before: `argon2id$<opslimit>$<memlimit>$<salt>$<nonce>$<box>`, in hex.
     */
    public function wrapKey(string $dataKeyHex, string $password): string
    {
        $salt = random_bytes(SODIUM_CRYPTO_PWHASH_SALTBYTES);
        $opslimit = SODIUM_CRYPTO_PWHASH_OPSLIMIT_INTERACTIVE;
        $memlimit = SODIUM_CRYPTO_PWHASH_MEMLIMIT_INTERACTIVE;

        $wrappingKey = $this->deriveWrappingKey($password, $salt, $opslimit, $memlimit);
        $nonce = random_bytes(SODIUM_CRYPTO_SECRETBOX_NONCEBYTES);
        $box = sodium_crypto_secretbox(hex2bin($dataKeyHex), $nonce, $wrappingKey);

        sodium_memzero($wrappingKey);

        return implode('$', [self::WRAPPED_KEY_ALGORITHM, $opslimit, $memlimit, bin2hex($salt), bin2hex($nonce), bin2hex($box)]);
    }

    /**
     * Unwrap a share's data key with its password; returns the key as hex.
     */
    public function unwrapKey(string $wrappedKey, string $password): string
    {
        $parts = explode('$', $wrappedKey);

        if (count($parts) !== 6 || $parts[0] !== self::WRAPPED_KEY_ALGORITHM) {
            throw new RuntimeException('Unsupported wrapped key');
        }

        [, $opslimit, $memlimit, $salt, $nonce, $box] = $parts;

        $wrappingKey = $this->deriveWrappingKey($password, hex2bin($salt), (int) $opslimit, (int) $memlimit);
        $dataKey = sodium_crypto_secretbox_open(hex2bin($box), hex2bin($nonce), $wrappingKey);

        sodium_memzero($wrappingKey);

        if ($dataKey === false) {
            throw new RuntimeException('Unwrapping failed - wrong password or corrupted key');
        }

        return bin2hex($dataKey);
    }

    /**
     * The header a new encrypted file starts with, with a fresh random nonce prefix.
     */
    public function createHeader(int $chunkSize): string
    {
        return self::MAGIC.pack('N', $chunkSize).random_bytes(self::NONCE_PREFIX_LENGTH);
    }

    /**
     * Read a `SEALCHK2` header.
     *
     * @return array{chunkSize: int, noncePrefix: string}
     */
    public function parseHeader(string $header): array
    {
        if (strlen($header) !== self::HEADER_LENGTH || ! str_starts_with($header, self::MAGIC)) {
            throw new RuntimeException('Invalid encrypted file header');
        }

        return [
            'chunkSize' => unpack('N', substr($header, 8, 4))[1],
            'noncePrefix' => substr($header, 12, self::NONCE_PREFIX_LENGTH),
        ];
    }

    /**
     * How many chunks a file of this size is sent in; an empty file is one empty chunk.
     */
    public function chunkCount(int $size, int $chunkSize): int
    {
        return max(1, intdiv($size + $chunkSize - 1, $chunkSize));
    }

    /**
     * Where chunk `$index` starts in the encrypted file.
     */
    public function chunkOffset(int $index, int $chunkSize): int
    {
        return self::HEADER_LENGTH + $index * ($chunkSize + self::TAG_LENGTH);
    }

    /**
     * Encrypt one chunk: its ciphertext followed by its tag, as WebCrypto returns it.
     */
    public function encryptChunk(string $plaintext, string $key, string $noncePrefix, int $index, bool $isLast): string
    {
        $tag = '';

        $ciphertext = openssl_encrypt(
            $plaintext,
            self::CIPHER,
            $this->normalizeToBinaryKey($key),
            OPENSSL_RAW_DATA,
            $this->chunkNonce($noncePrefix, $index, $isLast),
            $tag,
            '',
            self::TAG_LENGTH,
        );

        if ($ciphertext === false) {
            throw new RuntimeException('Encryption failed at chunk '.$index);
        }

        return $ciphertext.$tag;
    }

    /**
     * Decrypt one chunk, which fails unless its index and last-chunk flag are the ones it was
     * encrypted with.
     */
    public function decryptChunk(string $chunk, string $key, string $noncePrefix, int $index, bool $isLast): string
    {
        if (strlen($chunk) < self::TAG_LENGTH) {
            throw new RuntimeException('Invalid encrypted file: truncated chunk '.$index);
        }

        $plaintext = openssl_decrypt(
            substr($chunk, 0, -self::TAG_LENGTH),
            self::CIPHER,
            $this->normalizeToBinaryKey($key),
            OPENSSL_RAW_DATA,
            $this->chunkNonce($noncePrefix, $index, $isLast),
            substr($chunk, -self::TAG_LENGTH),
        );

        if ($plaintext === false) {
            throw new RuntimeException('Decryption failed - wrong key or corrupted data');
        }

        return $plaintext;
    }

    /**
     * The decrypted content of a file in any of the three formats, chunk by chunk.
     *
     * @return Generator<int, string>
     */
    public function decryptedChunks(string $encryptedPath, string $key): Generator
    {
        $magic = (string) file_get_contents($encryptedPath, false, null, 0, 8);

        if ($magic === self::MAGIC) {
            yield from $this->decryptChunks($encryptedPath, $key);
        } elseif ($magic === self::LEGACY_CHUNKED_MAGIC) {
            yield from $this->decryptLegacyChunks($encryptedPath, $key);
        } else {
            yield $this->decryptLegacy($encryptedPath, $key);
        }
    }

    /**
     * Normalize a hex key to binary.
     */
    private function normalizeToBinaryKey(string $key): string
    {
        return strlen($key) === 64 ? hex2bin($key) : $key;
    }

    private function deriveWrappingKey(string $password, string $salt, int $opslimit, int $memlimit): string
    {
        return sodium_crypto_pwhash(
            SODIUM_CRYPTO_SECRETBOX_KEYBYTES,
            $password,
            $salt,
            $opslimit,
            $memlimit,
            SODIUM_CRYPTO_PWHASH_ALG_ARGON2ID13,
        );
    }

    /**
     * A `SEALCHK2` chunk's nonce: the file's prefix, the chunk index and the last-chunk flag.
     */
    private function chunkNonce(string $noncePrefix, int $index, bool $isLast): string
    {
        return $noncePrefix.pack('N', $index).($isLast ? "\x01" : "\x00");
    }

    /**
     * Decrypt a `SEALCHK2` file; the chunk count comes from the file's length, so a file cut short
     * at a chunk boundary fails on its new last chunk.
     *
     * @return Generator<int, string>
     */
    private function decryptChunks(string $encryptedPath, string $key): Generator
    {
        $handle = fopen($encryptedPath, 'rb');

        if ($handle === false) {
            throw new RuntimeException("Cannot read encrypted file: {$encryptedPath}");
        }

        try {
            ['chunkSize' => $chunkSize, 'noncePrefix' => $noncePrefix] = $this->parseHeader((string) fread($handle, self::HEADER_LENGTH));

            $storedChunkSize = $chunkSize + self::TAG_LENGTH;
            $payloadLength = (int) filesize($encryptedPath) - self::HEADER_LENGTH;
            $chunkCount = intdiv($payloadLength + $storedChunkSize - 1, $storedChunkSize);

            if ($chunkCount === 0) {
                throw new RuntimeException('Invalid encrypted file: no chunks');
            }

            for ($index = 0; $index < $chunkCount; $index++) {
                $chunk = (string) fread($handle, $storedChunkSize);

                yield $this->decryptChunk($chunk, $key, $noncePrefix, $index, $index === $chunkCount - 1);
            }
        } finally {
            fclose($handle);
        }
    }

    /**
     * Decrypt a `SEALCHK1` file.
     *
     * @return Generator<int, string>
     */
    private function decryptLegacyChunks(string $encryptedPath, string $key): Generator
    {
        $handle = fopen($encryptedPath, 'rb');

        if ($handle === false) {
            throw new RuntimeException("Cannot read encrypted file: {$encryptedPath}");
        }

        try {
            fread($handle, 8);

            $chunkSize = unpack('N', (string) fread($handle, 4))[1];
            $baseNonce = (string) fread($handle, self::NONCE_LENGTH);

            if (strlen($baseNonce) !== self::NONCE_LENGTH) {
                throw new RuntimeException('Invalid chunked file: truncated header');
            }

            $binaryKey = $this->normalizeToBinaryKey($key);
            $chunkIndex = 0;

            while (! feof($handle)) {
                $tag = fread($handle, self::TAG_LENGTH);

                if ($tag === false || strlen($tag) === 0) {
                    break;
                }

                if (strlen($tag) !== self::TAG_LENGTH) {
                    throw new RuntimeException('Invalid chunked file: truncated tag at chunk '.$chunkIndex);
                }

                $ciphertext = fread($handle, $chunkSize);

                if ($ciphertext === false || $ciphertext === '') {
                    throw new RuntimeException('Invalid chunked file: missing ciphertext at chunk '.$chunkIndex);
                }

                $plaintext = openssl_decrypt(
                    $ciphertext,
                    self::CIPHER,
                    $binaryKey,
                    OPENSSL_RAW_DATA,
                    $this->legacyChunkNonce($baseNonce, $chunkIndex),
                    $tag,
                );

                if ($plaintext === false) {
                    throw new RuntimeException('Decryption failed - wrong key or corrupted data');
                }

                yield $plaintext;
                $chunkIndex++;
            }
        } finally {
            fclose($handle);
        }
    }

    /**
     * A `SEALCHK1` chunk's nonce: the chunk index XORed into the last 4 bytes of the base nonce.
     */
    private function legacyChunkNonce(string $baseNonce, int $chunkIndex): string
    {
        $nonce = $baseNonce;
        $indexBytes = pack('N', $chunkIndex);

        for ($i = 0; $i < 4; $i++) {
            $nonce[self::NONCE_LENGTH - 4 + $i] = $nonce[self::NONCE_LENGTH - 4 + $i] ^ $indexBytes[$i];
        }

        return $nonce;
    }

    /**
     * Decrypt a legacy single-block encrypted file.
     * Format: [12-byte nonce][16-byte auth tag][ciphertext]
     */
    private function decryptLegacy(string $encryptedPath, string $key): string
    {
        $data = file_get_contents($encryptedPath);

        if ($data === false) {
            throw new RuntimeException("Cannot read encrypted file: {$encryptedPath}");
        }

        $plaintext = openssl_decrypt(
            substr($data, self::NONCE_LENGTH + self::TAG_LENGTH),
            self::CIPHER,
            $this->normalizeToBinaryKey($key),
            OPENSSL_RAW_DATA,
            substr($data, 0, self::NONCE_LENGTH),
            substr($data, self::NONCE_LENGTH, self::TAG_LENGTH),
        );

        if ($plaintext === false) {
            throw new RuntimeException('Decryption failed - wrong key or corrupted data');
        }

        return $plaintext;
    }
}