Weak Key for a Block Cipher

Define the term Weak Key for a Block Cipher? Illustrate in brief.

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The weak keys are secret keys with a certain value for which the block cipher in question will show certain regularities in the encryption or in other cases, a poor level of encryption. For example, with DES there are four keys for which the encryption is precisely the similar as decryption. This signifies that if one were to encrypt twice with one of such weak keys, then the original plaintext would be recovered. For IDEA there is a class of keys for which the cryptanalysis is deeply facilitated and the key can be recovered. Though, in both cases, the number of weak keys is such a small fraction of all the possible keys that the chance of picking one at arbitrary is exceptionally slight. In these cases, they pose no important threat to the security of the block cipher whenever used for encryption.

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