C# AES Rijndael - detecting invalid passwords

HMAC is what you need. It is exactly made for this purpose. It combines the key and the message (which in this case, will be your password) and hashes them in a way that it will ensure the authenticity and integrity of the content, as long as the hash function used is secure. You can attach the HMAC to the encrypted data, and it can be used later to validate if the decryption was made correctly.

• HMAC, Wikipedia
• System.Security.Cryptography.HMAC

Checksums are exactly for this purpose. Get a hash of your data before encrypting. Encrypt the data and put it along with the hash into storage. After decrypting, get the hash of the decrypted data and compare it with the former. If you use a crypto grade hash (i.e. SHA512) your data will be safe. After all, this is exactly what encrypted compression software does.

For ultimate security, you can encrypt both the hashes and data separately then decrypt and compare. If both data and hash decrypts to corrupted data, there is very minuscule chances that they will match.