Basics of Encryption and decryption

[jacek]

2) I have a plain text file with size say '20kb'. When I encrypt the file the file size is increased. Why?

Because you use a block cipher which can encrypt only whole blocks.

3) I will encrypt a file with say AES::CBC mode and use passpharse 'hello123'.

Still AES will be the strongest part of your encryption scheme. And AES is as strong as the password, which should remain secret.