Alternating step generator and shrinking generator are most commonly used clock controlled based stream ciphers for the generation of key stream. In this study we attempt to provide solution to correlation attack by gradually increasing the lengths of initial input bits of linear feedback shift registers (LFSR’s), which result in the increase of the key length. We implement both the algorithms and found that Shrinking Generator is secure at length of 64 and Alternating Step Generator is secure at the length of 128 against the correlation attack .It is also found that Shrinking Generator is more efficient and secure than Alternating Step Generator.

Correlation Attack, Linear Feedback Shift Registers (LFSR), Linear Complexity, Period, Pseudo Randomness, Stream Cipher.