How can I decrypt an encrypted MCRYPT_RIJNDAEL_256 value in C#, that was encrypted by mcrypt in PHP?

Question

I am trying to read a Base64-Encoded value from a Database table managed on the Linux side. In that table there is a column called first_name. On the Linux side I can decrypt t

Answer 1

As Paŭlo says, ECB mode does not use an IV. If C# insists on one then use all zero bytes.

The key "patient_fn_salt" is 15 characters, 120 bits. Your decryption function is expecting 256 bits of key. You need to be very sure that the extra bits are identical in both systems and are being added in the same place in both systems. Even a single bit wrong will result in garbage decryption. Read the PHP documentation very carefully to determine exactly how "patient_fn_salt" is expanded to a 256 bit key. In particular check if the actual key is SHA256("patient_fn_salt").

As an aside, ECB mode is insecure. Use either CTR mode or CBC mode in preference. CTR mode does not require padding so will probably mean less cyphertext to store.

ETA: on another read through I notice that the C# side is padding with zeros. What padding is the PHP side using? Zero padding is not a good idea, as it cannot recognise a faulty decryption. PKCS7 padding has a much better chance of recognising a faulty output. Best to explicitly specify the padding on both ends rather than rely on defaults.