Weak keys are keys that result in ciphers that are easy to break. If text is encrypted with a weak key, encrypting the resulting cipher again with the same weak key returns the original text. In 128-bit mode, if the first 64 bits are the same as the last 64 bits, the key is weak. In 192-bit mode, if the first 64 bits are the same as the second 64 bits or if the second 64 bits are the same as the last 64 bits, the key is weak.

Attempting to set a weak key results in a System.Security.Cryptography.CryptographicException. When TripleDESCryptoServiceProvider.GenerateKey is called to create a random key, a weak key will never be returned.

Namespace: System.Security.Cryptography
Assembly: mscorlib (in mscorlib.dll)
Assembly Versions: 1.0.5000.0, 2.0.0.0, 4.0.0.0