PHP Two Way Encryption With Checksum

Question

I am trying to pass a JSON string from one web application to another using URL parameters (for an internal SSO server).

What I need to do is be able to encrypt the JSON string (which is a user payload object) with a pre-shared key, forward the user to the service provider application with the payload attached as a URL parameter and then on the service provider application decrypt the payload back into a JSON string to get the required information.

Now this part isn't as much of an issue thanks to all of PHP's built in encryption functions but the next part is the difficulty. I am needing to embed a checksum within the encrypted string which can be checked when decrypting it so that if it has been modified in transit then I can raise an exception.

The purpose of this is to make sure that the user payload has not been modified in transit either accidentally or deliberately.

Answer 1

You want to provide more than a "checksum" (usually defined as "calculable by any party"); you want to provide an authentication tag or message authentication code (MAC). You have a couple options:

1. Use an "authenticated encryption" (AE) or "authenticated encryption with associated data" (AEAD) cipher to do this. AE(AD) ciphers provide an "authentication tag" over the cipher text, either in a single pass or with a repeated process over the encrypted cipher text. Examples (probably available in whichever PHP cipher library you're using) are GCM, EAX, and CCM. This is recommended, as the decryption operation will fail if the authentication tag is not verified, and only one shared secret (key) is necessary.
2. You can construct the system yourself using cryptographic primitives. This is less ideal, as you are responsible for more independent pieces, you need to manage more keys (if you have access to an OMAC implementation, you can use the same key), and your individual construction is not vetted by third parties (aka the collective work of the internet). If you follow this path, you need to keep some key details in mind:
   • Use a strong hash-based message authentication code (HMAC) such as HMAC/SHA-256, -384, or -512. Do not use SHA-1 or MD5, as these are easily brute forced.
   • Verify the HMAC before decrypting the cipher text. Any HMAC that fails means the entire cipher text should be discarded. You can remember this (on the generating side) as Encrypt Then MAC, and if you search for it, you'll see that not following this advice is the source of many cryptographic vulnerabilities and implementation exploits.
   • Verify the HMAC with a constant-time algorithm (i.e. do not use a short-circuit string equality comparison, the default in Java). PHP provides hash_equals to do this. Here's a quick explanation of timing attacks and a code review of a PHP example.

For either choice you'll want to encode the resulting cipher text and authentication tag with URL-safe Base64 in order to avoid data loss or corruption. If your message format is not strictly structured with included lengths, you'll have to pre-share the protocol ahead of time (i.e. for message m of length n bytes -> 16 bytes IV | n-48 bytes cipher text | 32 bytes HMAC).

Last note: always use a unique, non-predictable IV for each message that is encrypted with a key. Many people gloss over this, because it's "easy to just use 0x00 * 16", but any stream cipher mode of operation like CTR used as the foundation of GCM and CCM will lose fundamental security if two messages are encrypted with the same IV and key.