ABSTRACT Stream Ciphers are one of the most important cryptographic techniques for data security due to its efficiency in terms of resources and speed. This study aims to provide a comprehensive survey that summarizes the existing cryptanalysis techniques for stream ciphers. It will also facilitate the security analysis of the existing stream ciphers and provide an opportunity to understand the requirements for developing a secure and efficient stream cipher design. Keywords Stream Cipher, Cryptography, Cryptanalysis, Cryptanalysis Techniques 1. On theINTRODUCTION Cryptography is the primary technique for data and communication security. It becomes indispensable where the communication channels cannot be made perfectly secure. From the ancient times, the two fields of cryptology; cryptography and cryptanalysis are developing side by side. Cryptography studies the design of algorithms for information and communication security at the same time cryptanalysis is concerned with the study of different techniques to search weakness in the cryptographic algorithms and try to break these algorithms. The developed designs or ciphers should be secured against any possible attack in an ideal situation, but this is not possible in the practical world. Any cryptographic primitive can only be tested for all the possible known attacks. Cryptanalysis techniques Cryptographic algorithms can be divided into two categories, Symmetric (Secret) key algorithms and Asymmetric (Public) key algorithms. In some literature cryptographic algorithms have been divided into three categories; Symmetric key, Asymmetric key and Unkeyed algorithms. In Symmetric key algorithm, both the sender and receiver share the secret key and in asymmetric key algorithms; there are two keys; Public key and Private Key. Public key is made public and private key is kept secret with the receiver. The third category i.e. unkeyed algorithms use the hybrid of these two technologies for e.g. hash functions. The symmetric key algorithms are further divided into two categories of Block Cipher and Stream cipher. In this paper we are focusing on Stream ciphers. Cryptanalysis is the technique of deriving the original message from the ciphertext without any prior knowledge of secret key or derivation of key from the ciphertext. A general technique for cryptanalysis, applicable to all cryptographic algorithms is to try all the possible keys until the correct key is matched, it is known as exhaustive key search. With every passing day, the computing ability of hardware is increasing manifold; therefore it becomes necessary to use long keys for avoiding exhaustive key search. All the other attacks applied on stream ciphers are compared to exhaustive key search in terms of data and memory complexity and if its complexity is less than exhaustive key search, then only these are considered as successful. A symmetric key cipher, especially a stream cipher is assumed secure, if the computational capability required for breaking the cipher by best-known attack is greater than or equal to exhaustive key search. There are different Attack scenarios for cryptanalysis based on available resources: 1. Ciphertext only attack 2. Known plain text attack 3. Chosen plaintext attack4. Chosen ciphertext attack basis of intention of the attacker, the attacks can be classified into two categories namely key recovery attack and distinguishing attacks. The motive of key recovery attack is to derive the key but in case of distinguishing attack, the attacker’s motive is only to derive the original from the ciphertext. There are different attacks known for the stream ciphers. Majority of these attacks have been discussed here. This study is conducted due to significance of stream ciphers in data and communication security and will provide an extensive survey of different cryptanalysis techniques and weaknesses of existing stream cipher designs that need to be overcome for developing a secure and efficient stream cipher.