I am a student of software engineering and working on an encryption method but i also have to look as a hacker on my idea so plz help me that how does a hacker decrypt the encrypted data when the user and sender has key only. Plz note that same key will be used everytime.
The question is too wide. Several techniques are used according the specific encryption algorithm. Since it seems to be you are starting with the topic I would mention the most simple techniques to decrypt a file not having the password. One is brute force; that means to try all possible values until you find one that decrypt the file. The way too be protected against this attack is having very long password which would make it to long to decrypt.
Another simple technique is dictionary attack, where the attacker has a huge list of common passwords and he try all of them hoping to you used one in his list. The way to protect against this attack is to use a very strong password such as having special characters o even having a randomly generated password. By the way, dictionary attack almost never works.
You may check one site here which precisely does that for encrypted rar files.