List and briefly define types of cryptanalytic attacks based on what is known to the attacker.

Cryptanalysis (from the Greek kryptós, "hidden", and analýein, "to loosen" or "to untie") is the study of methods for obtaining the meaning of encrypted information, without access to the secret information which is normally required to do so. Typically, this involves finding a secret key.

Cryptanalysis can be performed under a number of assumptions about how much can be observed or found out about the system under attack. It it is normally assumed that the general algorithm is known; this is Kerckhoffs' principle of "the enemy knows the system".

There can be many types of attacks and broadly we categorize them as attack models:

Ciphertext-only: the cryptanalyst has access only to a collection of ciphertexts.
Known-plaintext: the attacker has a set of ciphertexts to which he knows the corresponding plaintext.
Chosen-plaintext/chosen-ciphertext: the attacker can get ciphertexts/plaintexts related to an arbitrary set of plaintexts/ciphertexts of his own choosing.

The larger the size of the key space, the more secures a cipher? Justify your answer. key size or key length is the size measured in bits. Keys are used to control the operation of a cipher so that only the correct key can convert encrypted text (cipher text) to plaintext. Many ciphers are based on publicly known algorithms or are open source. A key should therefore be large enough that a brute force attack (possible against any encryption algorithm) is infeasible – i.e., it would take too long to execute to find out the key.
Hence if the key size is larger then the brute force attacker has to try more alternative key for the possibility of finding exact key, this increase the security of encryption.