Two-way encryption in PHP

Question

My application (obviously) uses a unique ID to distinguish records. This UID is passed in URLs (e.g. ./examplepage.php?UID=$example_int), among other things.

While I obviously have server-side validation in place to make sure clients don't access other clients' data, is there a two-way encryption method I can use in PHP to only pass encrypted UIDs (e.g. ./examplepage.php?EUID=$encrypted_int), to further reduce the chance of anyone thinking "hey, what happens if I increment this integer?"

TIA.

Answer 1

PHP 5.3 has introduced a new encryption method that is really easy to use: openssl_encrypt and openssl_decrypt. It's not well-documented here, so here's a simple example:

$textToEncrypt = "My super secret information."; $encryptionMethod = "AES-256-CBC";  // AES is used by the U.S. gov't to encrypt top secret documents. $secretHash = "25c6c7ff35b9979b151f2136cd13b0ff";  //To encrypt $encryptedMessage = openssl_encrypt($textToEncrypt, $encryptionMethod, $secretHash);  //To Decrypt $decryptedMessage = openssl_decrypt($encryptedMessage, $encryptionMethod, $secretHash);  //Result echo "Encrypted: $encryptedMessage <br>Decrypted: $decryptedMessage"; 

I chose 256-AES because it's solid and fast. It's been adopted by the U.S. gov't to encrypt top secret documents. It's fast considering machine and software. Here's a list of available encryption methods:

AES-128-CBC, AES-128-CFB, AES-128-CFB1, AES-128-CFB8, AES-128-ECB, AES-128-OFB, AES-192-CBC, AES-192-CFB, AES-192-CFB1, AES-192-CFB8, AES-192-ECB, AES-192-OFB, AES-256-CBC, AES-256-CFB, AES-256-CFB1, AES-256-CFB8, AES-256-ECB, AES-256-OFB, BF-CBC, BF-CFB, BF-ECB, BF-OFB, CAMELLIA-128-CBC, CAMELLIA-128-CFB, CAMELLIA-128-CFB1, CAMELLIA-128-CFB8, CAMELLIA-128-ECB, CAMELLIA-128-OFB, CAMELLIA-192-CBC, CAMELLIA-192-CFB, CAMELLIA-192-CFB1, CAMELLIA-192-CFB8, CAMELLIA-192-ECB, CAMELLIA-192-OFB, CAMELLIA-256-CBC, CAMELLIA-256-CFB, CAMELLIA-256-CFB1, CAMELLIA-256-CFB8, CAMELLIA-256-ECB, CAMELLIA-256-OFB, CAST5-CBC, CAST5-CFB, CAST5-ECB, CAST5-OFB, DES-CBC, DES-CFB, DES-CFB1, DES-CFB8, DES-ECB, DES-EDE, DES-EDE-CBC, DES-EDE-CFB, DES-EDE-OFB, DES-EDE3, DES-EDE3-CBC, DES-EDE3-CFB, DES-EDE3-CFB1, DES-EDE3-CFB8, DES-EDE3-OFB, DES-OFB, DESX-CBC, RC2-40-CBC, RC2-64-CBC, RC2-CBC, RC2-CFB, RC2-ECB, RC2-OFB, RC4, RC4-40, SEED-CBC, SEED-CFB, SEED-ECB, SEED-OFB, aes-128-cbc, aes-128-cfb, aes-128-cfb1, aes-128-cfb8, aes-128-ecb, aes-128-ofb, aes-192-cbc, aes-192-cfb, aes-192-cfb1, aes-192-cfb8, aes-192-ecb, aes-192-ofb, aes-256-cbc, aes-256-cfb, aes-256-cfb1, aes-256-cfb8, aes-256-ecb, aes-256-ofb, bf-cbc, bf-cfb, bf-ecb, bf-ofb, camellia-128-cbc, camellia-128-cfb, camellia-128-cfb1, camellia-128-cfb8, camellia-128-ecb, camellia-128-ofb, camellia-192-cbc, camellia-192-cfb, camellia-192-cfb1, camellia-192-cfb8, camellia-192-ecb, camellia-192-ofb, camellia-256-cbc, camellia-256-cfb, camellia-256-cfb1, camellia-256-cfb8, camellia-256-ecb, camellia-256-ofb, cast5-cbc, cast5-cfb, cast5-ecb, cast5-ofb, des-cbc, des-cfb, des-cfb1, des-cfb8, des-ecb, des-ede, des-ede-cbc, des-ede-cfb, des-ede-ofb, des-ede3, des-ede3-cbc, des-ede3-cfb, des-ede3-cfb1, des-ede3-cfb8, des-ede3-ofb, des-ofb, desx-cbc, rc2-40-cbc, rc2-64-cbc, rc2-cbc, rc2-cfb, rc2-ecb, rc2-ofb, rc4, rc4-40, seed-cbc, seed-cfb, seed-ecb, seed-ofb

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IMPORTANT UPDATE!!!

Thanks Hobo and Jorwin for pointing out that in PHP 5.3.3 > there is a new parameter that makes this function a little more secure.

Jorwin referenced this link in his comment, and here is an excerpt that is applicable:

In 5.3.3 they added a new parameter, string $iv (initialization vector) Real parameters are: string openssl_encrypt ( string $data , string $method , string $password, bool $raw_output = false, string $iv )

If $iv is missing, a warning is issued: "Using an empty Initialization Vector (iv) is potentially insecure and not recommended".

If $iv is too short, another warning: "IV passed is only 3 bytes long, cipher expects an IV of precisely 8 bytes, padding with \0"

same IV should be used in openssl_decrypt()

Answer 2

You don't need two-way encryption - encryption is for maintaining secrecy, but what you're really looking for here is authenticity.

HMACs (essentially, keyed hashes) are one way of getting cryptographic authenticity. Accompany the UID with a HMAC of the UID (PHP has a HMAC implementation), using a key that only the server knows. At the start of each request, check the HMAC.

Basically, use the right tool for the right job.