Weak Key for Block Cipher

Question

What is a Weak Key for Block Cipher?

Answer 1

Weak keys are the secret keys with a specific value for which the block cipher in question will display certain regularities in encryption or, in other situations, a poor level of encryption. For example, with DES there are 4 keys for which encryption is precisely similar as decryption. This signifies that when one was to encrypt twice with one of such weak keys, then the original plaintext would be improved. For IDEA there is a group of keys for which cryptanalysis is very much facilitated and the key can be improved. Though, in both such cases, the number of weak keys is such a small fraction of each and every possible key that the option of picking one at arbitrary is exceptionally slight. In such situations, they pose no noteworthy threat to the security of the block cipher whenever employed for encryption.