Getting incorrect decryption value using AesCryptoServiceProvider

Question

I have following code that uses `AesCryptoServiceProvider` for encrypting and decrypting. The `iv` and `key` used are same for both encryption and decryption. Still the decrypted value differ from the source string. - What need to be corrected to get the original value after decrypt? - This code is working when `inputValue = valid128BitString`. But when the `inputString = “Test”` I am getting the following exception `Padding is invalid and cannot be removed.`. How can we correct it? UPDATED QUESTION The following will do the trick based on @jbtule answer. ``` encyptedValue.IV = result.IV; ``` The `IV` value from encryption result changes. Suppose encryption is done in a separate process, how can we know the IV for decryption? Is there a way to make it constant or known? Answer: Your other option is pass a IV in to Encrypt and assign it before you begin your crypto transform, instead of letting aesProvider generate a random one for you. – @Scott Chamberlain ``` aesProvider.IV = Convert…

Accepted Answer

It is easy to make implementation mistakes with cryptographic primitives, people do it all the time, it's best to use a high level library if you can.

I have a snippet that I try to keep reviewed and up to date, that works pretty close to what you're doing. It also does authentication on the cipher text, which I would recommend if there is anyway an adversary could send chosen ciphertext to your decryption implementation, there are a lot of side channel attacks related to modifying the ciphertext.

However, the problem you're having does not have any thing to do with padding, if your ciphertext doesn't matchup to your key and iv, and you didn't authenticate your iv and ciphertext, you'll typically get a padding error (if this is bubbled up a client it's called a padding oracle). You need to change your main statement to:

```
    string valid128BitString = "AAECAwQFBgcICQoLDA0ODw==";
    string inputValue = "Test";
    string keyValue = valid128BitString;

byte[] byteValForString = Encoding.UTF8.GetBytes(inputValue);
    EncryptResult result = Aes128Utility.EncryptData(byteValForString, keyValue);
    EncryptResult encyptedValue = new EncryptResult();
    encyptedValue.IV = result.IV; //<--Very Important
    encyptedValue.EncryptedMsg = result.EncryptedMsg;

string finalResult =Encoding.UTF8.GetString(Aes128Utility.DecryptData(encyptedValue, keyValue));
```

So you use the same IV to decrypt as you did to encrypt.

Getting incorrect decryption value using AesCryptoServiceProvider

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