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POWhy does a bad password cause "Padding is invalid and cannot be removed"?
 

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  1. POWhy does a bad password cause "Padding is invalid and cannot be removed"?
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    <p>I needed some simple string encryption, so I wrote the following code (with a great deal of "inspiration" from <a href="http://www.codeproject.com/KB/security/DotNetCrypto.aspx" rel="noreferrer">here</a>):</p> <pre><code> // create and initialize a crypto algorithm private static SymmetricAlgorithm getAlgorithm(string password) { SymmetricAlgorithm algorithm = Rijndael.Create(); Rfc2898DeriveBytes rdb = new Rfc2898DeriveBytes( password, new byte[] { 0x53,0x6f,0x64,0x69,0x75,0x6d,0x20, // salty goodness 0x43,0x68,0x6c,0x6f,0x72,0x69,0x64,0x65 } ); algorithm.Padding = PaddingMode.ISO10126; algorithm.Key = rdb.GetBytes(32); algorithm.IV = rdb.GetBytes(16); return algorithm; } /* * encryptString * provides simple encryption of a string, with a given password */ public static string encryptString(string clearText, string password) { SymmetricAlgorithm algorithm = getAlgorithm(password); byte[] clearBytes = System.Text.Encoding.Unicode.GetBytes(clearText); MemoryStream ms = new MemoryStream(); CryptoStream cs = new CryptoStream(ms, algorithm.CreateEncryptor(), CryptoStreamMode.Write); cs.Write(clearBytes, 0, clearBytes.Length); cs.Close(); return Convert.ToBase64String(ms.ToArray()); } /* * decryptString * provides simple decryption of a string, with a given password */ public static string decryptString(string cipherText, string password) { SymmetricAlgorithm algorithm = getAlgorithm(password); byte[] cipherBytes = Convert.FromBase64String(cipherText); MemoryStream ms = new MemoryStream(); CryptoStream cs = new CryptoStream(ms, algorithm.CreateDecryptor(), CryptoStreamMode.Write); cs.Write(cipherBytes, 0, cipherBytes.Length); cs.Close(); return System.Text.Encoding.Unicode.GetString(ms.ToArray()); } </code></pre> <p>The code appears to work fine, except that when decrypting data with an incorrect key, I get a CryptographicException - "Padding is invalid and cannot be removed" - on the cs.Close() line in decryptString. </p> <p>example code:</p> <pre><code> string password1 = "password"; string password2 = "letmein"; string startClearText = "The quick brown fox jumps over the lazy dog"; string cipherText = encryptString(startClearText, password1); string endClearText = decryptString(cipherText, password2); // exception thrown </code></pre> <p>My question is, is this to be expected? I would have thought that decrypting with the wrong password would just result in nonsense output, rather than an exception.</p>
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    1. COThis saved me so much time with your comment: `"The code appears to work fine, except that when decrypting data with an incorrect key"` I _swore_ I had copied the keys, but looking 2x I didn't. Hopefully this helps someone else before looking at the padding mechanism or changing code.
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