How to use php mcrypt_decrypt() function

In PHP, the mcrypt_decrypt() function is used to decrypt ciphertext using given parameters, the syntax is "mcrypt_decrypt($cipher,$key,$data,$mode,$iv)"; parameter "$key" is the data encryption key and "$data" is the data decrypted using the given "$cipher" and "$mode".

The operating environment of this tutorial: windows7 system, PHP8 version, DELL G3 computer

php mcrypt_decrypt() function

mcrypt_decrypt — Use the given parameters to decrypt ciphertext, syntax format:

mcrypt_decrypt(
    string $cipher,
    string $key,
    string $data,
    string $mode,
    string $iv = ?
): string|false

can decrypt data and return plaintext.

Parameters:

• cipher

  MCRYPT_ciphername One of the constants, or the name of the algorithm as a string value.

• key

  Data encryption key. If the key length is not a valid length supported by the encryption and decryption algorithm, a warning will be generated and false will be returned

• data

  To be decrypted using the given cipher and mode data. If the data size is not n * group size, it is padded by appending '\0'.

• mode

  MCRYPT_MODE_modename One of the constants, or one of the following strings: "ecb", "cbc", "cfb", "ofb", " nofb" and "stream".

• iv

  is used for initialization in CBC, CFB, OFB modes, and some algorithms in STREAM mode. If the link mode does not support the supplied IV size, or if no IV is provided but the link mode requires one, the function will issue a warning and return FALSE.

Return value:

• Return the decrypted data in string format, or return false on failure.

Usage example:

<?php
    # --- 加密 ---

    # 密钥应该是随机的二进制数据，
    # 开始使用 scrypt, bcrypt 或 PBKDF2 将一个字符串转换成一个密钥
    # 密钥是 16 进制字符串格式
    $key = pack(&#39;H*&#39;, "bcb04b7e103a0cd8b54763051cef08bc55abe029fdebae5e1d417e2ffb2a00a3");
    
    # 显示 AES-128, 192, 256 对应的密钥长度：
    #16，24，32 字节。
    $key_size =  strlen($key);
    echo "Key size: " . $key_size . "n";
    
    $plaintext = "This string was AES-256 / CBC / ZeroBytePadding encrypted.";

    # 为 CBC 模式创建随机的初始向量
    $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
    $iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
    

    # 创建和 AES 兼容的密文（Rijndael 分组大小 = 128）
    # 仅适用于编码后的输入不是以 00h 结尾的
    # （因为默认是使用 0 来补齐数据）
    $ciphertext = mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key,
                                 $plaintext, MCRYPT_MODE_CBC, $iv);

    # 将初始向量附加在密文之后，以供解密时使用
    $ciphertext = $iv . $ciphertext;
    
    # 对密文进行 base64 编码
    $ciphertext_base64 = base64_encode($ciphertext);

    echo  $ciphertext_base64 . "n";

    # === 警告 ===

    # 密文并未进行完整性和可信度保护，
    # 所以可能遭受 Padding Oracle 攻击。
    
    # --- 解密 ---
    
    $ciphertext_dec = base64_decode($ciphertext_base64);
    
    # 初始向量大小，可以通过 mcrypt_get_iv_size() 来获得
    $iv_dec = substr($ciphertext_dec, 0, $iv_size);
    
    # 获取除初始向量外的密文
    $ciphertext_dec = substr($ciphertext_dec, $iv_size);

    # 可能需要从明文末尾移除 0
    $plaintext_dec = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key,
                                    $ciphertext_dec, MCRYPT_MODE_CBC, $iv_dec);
    
    echo  $plaintext_dec . "n";
?>

The above routine will output:

Key size: 32
ENJW8mS2KaJoNB5E5CoSAAu0xARgsR1bdzFWpEn+poYw45q+73az5kYi4j+0haevext1dGrcW8Qi59txfCBV8BBj3bzRP3dFCp3CPQSJ8eU=
This string was AES-256 / CBC / ZeroBytePadding encrypted.

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