Unable to Decrypt WhatsApp Flows Server Response

Question

I am working on decrypting WhatsApp flow data using AES-GCM encryption in C#, but I’m encountering an issue when attempting to decrypt the flow data. Below is my Decryptor class, which handles both decryption of the AES key and flow data:

public class Decryptor
{
    public static DecryptedRequest DecryptRequest(EncryptedRequest body, string privatePem, string passphrase)
    {
        Log.Information("Decrypting request with body: {Bodyinitial_vector}", body.initial_vector);

        byte[] encryptedAesKey = Convert.FromBase64String(body?.encrypted_aes_key);
        byte[] encryptedFlowData = Convert.FromBase64String(body?.encrypted_flow_data);
        byte[] initialVector = Convert.FromBase64String(body?.initial_vector);

        byte[] decryptedAesKey;
        try
        {
            // Decrypt AES key using private key
            using (var rsa = CreateRsaPrivateKey(privatePem, passphrase))
            {
                decryptedAesKey = rsa.Decrypt(encryptedAesKey, RSAEncryptionPadding.OaepSHA256);
                Log.Information("Decrypted AES Key: {decryptedAesKey}", decryptedAesKey);
            }
        }
        catch (Exception ex)
        {
            throw new FlowEndpointException(421, "Failed to decrypt the request. Please verify your private key.");
        }

        const int TAG_LENGTH = 16;
        byte[] flowDataBuffer = encryptedFlowData;
        byte[] initialVectorBuffer = initialVector;
        byte[] encryptedFlowDataBody = flowDataBuffer[..^TAG_LENGTH];
        byte[] encryptedFlowDataTag = flowDataBuffer[^TAG_LENGTH..];

        byte[] decryptedData;
        try
        {
            using (var aesGcm = new AesGcm(decryptedAesKey))
            {
                decryptedData = new byte[encryptedFlowDataBody.Length];
                aesGcm.Decrypt(initialVectorBuffer, encryptedFlowDataBody, decryptedData, encryptedFlowDataTag);
            }
        }
        catch (CryptographicException ex)
        {
            Log.Error("Decryption failed: {ErrorMessage}", ex.Message);
            throw new FlowEndpointException(400, "Failed to decrypt the request.");
        }

        string decryptedJSONString = Encoding.UTF8.GetString(decryptedData);
        return new DecryptedRequest
        {
            DecryptedBody = decryptedJSONString,
            AesKeyBuffer = decryptedAesKey,
            InitialVectorBuffer = initialVectorBuffer
        };
    }
}

The decryption fails at the following point:

using (var aesGcm = new AesGcm(decryptedAesKey))
{
    decryptedData = new byte[encryptedFlowDataBody.Length];
    aesGcm.Decrypt(initialVectorBuffer, encryptedFlowDataBody, decryptedData, encryptedFlowDataTag);
}

The error message I get is:

System.ArgumentException: The specified nonce is not a valid size for this algorithm. (Parameter 'nonce')

This error suggests that the initialVector (nonce) being passed to the AES-GCM decryption is of an invalid size. However, I’ve confirmed that the initialVector is being correctly base64-decoded and passed as expected.

Has anyone encountered a similar issue with AES-GCM decryption in C#? What could be the cause of the invalid nonce size, and how can I resolve it?

I tried decrypting this

{
    "encrypted_flow_data": "33oaKOMa2tsRCuh7g6JNsFzVB5icbm4dLq3sgrRXImxChcGOKWaTEZycsv/Tgg/jB/7cxQWH9D8HvJFkCc78X/avxOtEj0KbF+TuFKG28xO/HMR3A4rpzSyD37UAJy3eSmQ5CHZV7NOOMteJQNAOZQ3YEpcG/BcH8mo2iOQa2EA/8g==",
    "encrypted_aes_key": "NQBgEO5GMMXVHsRx0oDb+0MeTYPnHDZM2yJBG0CeaJ2VqN9ennPnLKknbVgpfRm7GfGSxsPHvO9sZeNs/SXlmYIoasDXw3pgKVjhPq3BOk93Fkfjs8bwBhDP763RCgO5IsOSgCqvNeGxeXxyM2BLr8rUAwoE74FYRLa6dxg5RGEUpGJjKLU8VZmBDbXanhPmy6+iVcLEE79rZLsA0+kU7IZ44eeR7PpvqiDCcrnjME/uCF24+3g+iVjsgcL5lA5DdhUQnfPnkjLTiC+x/NPp7ev3e7nP+PzTLQOxnnihjggRfYHUzklp9laVLAQoSQh7vvyKJUPyCv9BRhEmR7BJVQ==",
    "initial_vector": "Vs37z73b5OiBJ5hMV0kyUQ=="
}

and this is what am expecting at least

Decrypted Request:

{
    "data": {
        "farmer_select": "1"
    },
    "flow_token": "TEST",
    "screen": "DEMO_SCREEN",
    "action": "data_exchange",
    "version": "3.0"
}