The cause: The string’s length is the likely cause of the problem. Base-64 string length needs to be a multiple of 4. The padding character (=) should be added at the end if it isn’t.

Solution: You should use the code block below to check the string length and add padding characters if necessary:

int mod4 = encryptedtext.Length % 4;

if (mod4 > 0 )
{
encryptedtext += new string('=', 4 - mod4);
}

byte[] encryptedBytes = Convert.FromBase64String(encryptedtext);