How to Encrypt and Decrypt a PHP String with Key
Encrypting and decrypting strings in PHP involves utilizing cryptographic techniques to protect sensitive information.
Encryption
To encrypt a string using a secret key:
- Use AES in CTR mode: Encrypt the string using the Advanced Encryption Standard (AES) in Counter (CTR) mode.
- Authenticate the ciphertext: Calculate a Hash-based Message Authentication Code (HMAC) over the IV and ciphertext using HMAC-SHA-256 or Poly1305 (for stream ciphers).
Decryption
To decrypt an encrypted string using the same key:
- Check the MAC: Recalculate the MAC and compare it to the saved one.
- Decrypt the message: Decrypt the ciphertext using AES in CTR mode.
Other Considerations
- Avoid compressing data before encryption.
- Use mb_strlen() and mb_substr() with the '8bit' character set to prevent issues.
- Generate IVs using a CSPRNG and avoid MCRYPT_RAND.
- Always encrypt then MAC to ensure both confidentiality and authenticity.
Libsodium for PHP
If using PHP 7.2 or higher, consider using the libsodium library:
use ParagonIE\Halite\Symmetric\Crypto as SymmetricCrypto; $key = random_bytes(SODIUM_CRYPTO_SECRETBOX_KEYBYTES); $ciphertext = SymmetricCrypto::encrypt($message, $key); $plaintext = SymmetricCrypto::decrypt($ciphertext, $key);
defuse/php-encryption
As an alternative to libsodium:
use Defuse\Crypto\Crypto; $key = Key::createNewRandomKey(); $ciphertext = Crypto::encrypt($message, $key); $plaintext = Crypto::decrypt($ciphertext, $key);
Password Considerations
- For passwords: Hash them using Argon2, scrypt, bcrypt, or PBKDF2-SHA256 with high iterations.
- For authentication: Use HMAC-based algorithms like HKDF2 or hash_hmac() instead of encryption.
- For URL parameters: Use an appropriate framework or library designed for URL parameter validation and tampering protection.
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