PHP AES-256-CBC解密图片文件后显示损坏的排查与解决求助

我最近在做一个PHP项目，用AES-256-CBC算法对图片文件进行加密和解密，但遇到了一个棘手的问题：把加密后的.aes文件解密还原成图片时，图片总是显示损坏状态，完全无法正常打开。

我的流程是：通过image_encrypt.php处理图片加密生成.aes文件，再用image_decrypt.php读取上传目录里最新的.aes文件，输入密钥后解密还原图片。目前解密过程没有抛出错误，但输出的图片就是损坏的，我已经加了一些调试信息还是没找到根源，麻烦各位帮忙看看代码哪里出问题了！

解密代码（image_decrypt.php） #

<?php
session_start();

if ($_SERVER['REQUEST_METHOD'] == 'POST' && isset($_POST['dec_key'])) {
    $decryptionKey = $_POST['dec_key'];

    // Fetch the latest uploaded .aes file from the uploads directory
    $uploadsDir = __DIR__ . '/uploads/';
    $aesFiles = glob($uploadsDir . '*.aes');

    if (empty($aesFiles)) {
        echo "Error: No .aes files found in the uploads directory.";
        exit;
    }

    // Sort files by modification time, descending
    usort($aesFiles, function ($a, $b) {
        return filemtime($b) - filemtime($a);
    });

    // Use the most recently uploaded .aes file
    $aesFilePath = $aesFiles[0];
    $aesFileName = basename($aesFilePath);

    // Debug: Check the selected file
    echo "Debug: Selected .aes file: $aesFileName<br>";

    if (!file_exists($aesFilePath)) {
        echo "Error: The file does not exist.";
        exit;
    }

    $aesFileContent = file_get_contents($aesFilePath);

    // Debug: Check the .aes file content length
    echo "Debug: .aes file content length: " . strlen($aesFileContent) . "<br>";

    $metadataEndPos = strpos($aesFileContent, "#####");
    if ($metadataEndPos === false) {
        echo "Error: Invalid .aes file format.";
        exit;
    }

    $metadata = substr($aesFileContent, 0, $metadataEndPos);
    $encryptedHex = trim(substr($aesFileContent, $metadataEndPos + 5));

    // Debug: Show metadata
    echo "Debug: Metadata: $metadata<br>";

    preg_match('/Salt:\s([a-f0-9]{32})/', $metadata, $saltMatches);
    preg_match('/IV:\s([a-f0-9]{32})/', $metadata, $ivMatches);

    if (!isset($saltMatches[1]) || !isset($ivMatches[1])) {
        echo "Error: Salt or IV not found in metadata.";
        exit;
    }

    $salt = hex2bin($saltMatches[1]);
    $iv = hex2bin($ivMatches[1]);

    // Debug: Verify salt and IV
    echo "Debug: Salt: " . bin2hex($salt) . "<br>";
    echo "Debug: IV: " . bin2hex($iv) . "<br>";

    $encryptedContent = hex2bin(str_replace(' ', '', $encryptedHex));
    if ($encryptedContent === false) {
        echo "Error: Invalid encrypted content format.";
        exit;
    }

    // Debug: Check encrypted content length
    echo "Debug: Encrypted content length: " . strlen($encryptedContent) . "<br>";

    $cipher = 'aes-256-cbc';
    $key = hash_pbkdf2('sha256', $decryptionKey, $salt, 1000, 32, true);

    // Debug: Key length
    echo "Debug: Key length: " . strlen($key) . "<br>";

    $decryptedContent = openssl_decrypt($encryptedContent, $cipher, $key, OPENSSL_RAW_DATA, $iv);

    if ($decryptedContent === false) {
        echo "Error: Decryption failed. Incorrect key or corrupted data.";
        exit;
    }

    // Debug: Check decrypted content length
    echo "Debug: Decrypted content length: " . strlen($decryptedContent) . "<br>";

    preg_match('/Original File-Extension:\s\.(\w+)/', $metadata, $extMatches);
    $originalExtension = isset($extMatches[1]) ? $extMatches[1] : 'jpg';
    $decryptedFileName = "decrypted_image.$originalExtension";

    if (!is_dir('decrypted_images')) {
        mkdir('decrypted_images', 0777, true);
    }

    $filePath = 'decrypted_images/' . $decryptedFileName;
    $fileHandle = fopen($filePath, 'wb');
    if ($fileHandle) {
        fwrite($fileHandle, $decryptedContent);
        fclose($fileHandle);
    } else {
        echo "Error: Failed to open the file for writing.";
        exit;
    }

    // Debug: Check if the file was saved
    echo "Debug: Decrypted file saved at $filePath<br>";

    // Redirect or display success message
    echo "
    <form id='redirectForm' action='image_index.php' method='POST'>
        <input type='hidden' name='decryptedData' value='" . base64_encode($decryptedContent) . "'>
        <input type='hidden' name='decryptedFileName' value='" . htmlspecialchars($decryptedFileName) . "'>
    </form>
    <script>
        document.getElementById('redirectForm').submit();
    </script>";
    exit;
} else {
    echo "Error: Decryption key and/or file is missing.";
}

加密代码（image_encrypt.php） #

<?php
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
    $encryptionKey = $_POST['enc_key'];
    $fileContentBase64 = $_POST['fileContent'];
    $fileName = $_POST['fileName'];

    if (strlen($encryptionKey) < 8 || !preg_match('/^[\w!@#$%^&*()_+\-=\[\]{};:\"\\|,.<>\/?]+$/', $encryptionKey)) {
        echo "Error: Encryption key must be at least 8 characters long and can include letters, numbers, and symbols.";
        exit;
    }

    $fileContent = base64_decode($fileContentBase64);
    $cipher = 'aes-256-cbc';
    $salt = openssl_random_pseudo_bytes(16);
    $key = hash_pbkdf2('sha256', $encryptionKey, $salt, 1000, 32, true);
    $ivLength = openssl_cipher_iv_length($cipher);
    $iv = openssl_random_pseudo_bytes($ivLength);
    $encryptedContent = openssl_encrypt($fileContent, $cipher, $key, OPENSSL_RAW_DATA, $iv);
    $finalEncryptedContent = $iv . $encryptedContent;
    $hexEncryptedContent = strtoupper(bin2hex($finalEncryptedContent));
    $formattedHexOutput = implode(' ', str_split($hexEncryptedContent, 2));
    $currentDateTime = date('d/m/Y h:i:s A');

    $aesFileName = 'encrypted_' . pathinfo($fileName, PATHINFO_FILENAME) . '.aes';
    $aesFilePath = 'encrypted_images/' . $aesFileName;

    if (!is_dir('encrypted_images')) {
        mkdir('encrypted_images', 0777, true);
    }

    // Optional: Include file size and signature in metadata
    $fileSize = strlen($fileContent);
    $fileSignature = bin2hex(substr($fileContent, 0, 4));

    $aesContent = "Original File-Name: $fileName\n";
    $aesContent .= "Original File-Size: $fileSize bytes\n";
    $aesContent .= "Original File-Signature: $fileSignature\n";
    $aesContent .= "Original File-Extension: ." . pathinfo($fileName, PATHINFO_EXTENSION) . "\n";
    $aesContent .= "Encryption Date & Time: $currentDateTime\n";
    $aesContent .= "Salt: " . bin2hex($salt) . "\n";
    $aesContent .= "IV: " . bin2hex($iv) . "\n";
    $aesContent .= "#####\n";
    $aesContent .= "$formattedHexOutput\n";

    // Save the encrypted data to .aes file
    if (file_put_contents($aesFilePath, $aesContent) === false) {
        echo "Error: Failed to save encrypted file.";
        exit;
    }

    $encryptedOutput = $formattedHexOutput;
    include('image_index.php');
}
?>

目前的调试情况 #

我已经在解密代码中添加了一些调试输出，能确认以下几点：

• 程序正确读取到了最新的.aes文件
• 能从.aes文件的metadata中正常提取到Salt和IV
• openssl_decrypt没有返回false，说明解密过程没有触发算法层面的错误
• 解密后的内容也成功写入了目标文件，但打开就是损坏的图片

我怀疑可能是加密或解密时的字节流处理、IV和密钥的使用逻辑哪里出了问题，但自己排查了好久没找到，希望各位能帮忙指出问题所在！

备注：内容来源于stack exchange，提问作者Po Po