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<!DOCTYPE html>
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<head>
<title>Weak cipher mode - Roslyn Security Guard</title>
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<h2>Weak cipher mode</h2>
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<section id="rule" class="rule-single">
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<p>
The ciphertext produced is susceptible to alteration by an adversary.
</p>
<h3>Risk</h3>
<p>
The cipher provides no way to detect that the data has been tampered with.
If the ciphertext can be controlled by an attacker, it could be altered without detection. The use of AES in CBC mode
with a HMAC is recommended guaranteeing integrity and confidentiality.
</p>
<h3>Vulnerable Code</h3>
<pre>
using (var aes = new AesManaged {
KeySize = KeyBitSize,
BlockSize = BlockBitSize,
Mode = CipherMode.OFB,
Padding = PaddingMode.PKCS7
})
{
using (var encrypter = aes.CreateEncryptor(cryptKey, new byte[16]))
using (var cipherStream = new MemoryStream())
{
using (var cryptoStream = new CryptoStream(cipherStream, encrypter, CryptoStreamMode.Write))
using (var binaryWriter = new BinaryWriter(cryptoStream))
{
//Encrypt Data
binaryWriter.Write(secretMessage);
}
cipherText = cipherStream.ToArray();
}
}
//Missing HMAC suffix to assure integrity
</pre>
<h3>Solution</h3>
Using bouncy castle:
<pre>
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Engines;
using Org.BouncyCastle.Crypto.Generators;
using Org.BouncyCastle.Crypto.Modes;
using Org.BouncyCastle.Crypto.Parameters;
using Org.BouncyCastle.Security;
public static readonly int BlockBitSize = 128;
public static readonly int KeyBitSize = 256;
public static byte[] SimpleEncrypt(byte[] secretMessage, byte[] key)
{
//User Error Checks
if (key == null || key.Length != KeyBitSize / 8)
throw new ArgumentException(String.Format("Key needs to be {0} bit!", KeyBitSize), "key");
if (secretMessage == null || secretMessage.Length == 0)
throw new ArgumentException("Secret Message Required!", "secretMessage");
//Using random nonce large enough not to repeat
var nonce = new byte[NonceBitSize / 8];
Random.NextBytes(nonce, 0, nonce.Length);
var cipher = new GcmBlockCipher(new AesFastEngine());
var parameters = new AeadParameters(new KeyParameter(key), MacBitSize, nonce, new byte[0]);
cipher.Init(true, parameters);
//Generate Cipher Text With Auth Tag
var cipherText = new byte[cipher.GetOutputSize(secretMessage.Length)];
var len = cipher.ProcessBytes(secretMessage, 0, secretMessage.Length, cipherText, 0);
cipher.DoFinal(cipherText, len);
//Assemble Message
using (var combinedStream = new MemoryStream())
{
using (var binaryWriter = new BinaryWriter(combinedStream))
{
//Prepend Nonce
binaryWriter.Write(nonce);
//Write Cipher Text
binaryWriter.Write(cipherText);
}
return combinedStream.ToArray();
}
}
</pre>
Custom implementation of Encrypt and HMAC
<pre>
using System.IO;
using System.Security.Cryptography;
public static byte[] SimpleEncrypt(byte[] secretMessage, byte[] cryptKey, byte[] authKey, byte[] nonSecretPayload = null)
{
//User Error Checks
if (cryptKey == null || cryptKey.Length != KeyBitSize / 8)
throw new ArgumentException(String.Format("Key needs to be {0} bit!", KeyBitSize), "cryptKey");
if (authKey == null || authKey.Length != KeyBitSize / 8)
throw new ArgumentException(String.Format("Key needs to be {0} bit!", KeyBitSize), "authKey");
if (secretMessage == null || secretMessage.Length < 1)
throw new ArgumentException("Secret Message Required!", "secretMessage");
byte[] cipherText;
byte[] iv;
using (var aes = new AesManaged {
KeySize = KeyBitSize,
BlockSize = BlockBitSize,
Mode = CipherMode.CBC,
Padding = PaddingMode.PKCS7
})
{
//Use random IV
aes.GenerateIV();
iv = aes.IV;
using (var encrypter = aes.CreateEncryptor(cryptKey, iv))
using (var cipherStream = new MemoryStream())
{
using (var cryptoStream = new CryptoStream(cipherStream, encrypter, CryptoStreamMode.Write))
using (var binaryWriter = new BinaryWriter(cryptoStream))
{
//Encrypt Data
binaryWriter.Write(secretMessage);
}
cipherText = cipherStream.ToArray();
}
}
//Assemble encrypted message and add authentication
using (var hmac = new HMACSHA256(authKey))
using (var encryptedStream = new MemoryStream())
{
using (var binaryWriter = new BinaryWriter(encryptedStream))
{
//Prepend IV
binaryWriter.Write(iv);
//Write Ciphertext
binaryWriter.Write(cipherText);
binaryWriter.Flush();
//Authenticate all data
var tag = hmac.ComputeHash(encryptedStream.ToArray());
//Postpend tag
binaryWriter.Write(tag);
}
return encryptedStream.ToArray();
}
}
</pre>
<h3>References</h3>
<p>
<a href="http://www.infobytesec.com/down/paddingoracle_openjam.pdf">Padding Oracles for the masses (by Matias Soler)</a><br />
<a href="http://en.wikipedia.org/wiki/Authenticated_encryption">Wikipedia: Authenticated encryption</a><br />
<a href="http://csrc.nist.gov/groups/ST/toolkit/BCM/modes_development.html#01">NIST: Authenticated Encryption Modes</a><br />
<a href="http://capec.mitre.org/data/definitions/463.html">CAPEC: Padding Oracle Crypto Attack</a><br />
<a href="http://cwe.mitre.org/data/definitions/696.html">CWE-696: Incorrect Behavior Order</a>
</p>
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