• cipher-text only attacks - given a cipher-text, try out all keys to decrypt the cipher-text. this reveals the key, thus enabling decryption of any cipher-text
• known plain-text attacks - given a few <plain-text, cipher-text> pairs may help map the plaintext letter to cipher-text letter and thus enabling the decryption of messages
• chosen plain-text attacks - attacker chooses a plain-text and system reveals the corresponding cipher-text