You should use a key derivation function, such as PBKDF2. A key derivation function takes a salt and a supplied user password, and produces a key that can be used with a cipher like AES.
To encrypt, you would prompt for a password, generate a random salt, and derive a key using the KDF. You would then use that key with AES in a suitable block cipher mode to encrypt the data, and store only the salt and the encrypted data (and whatever IV the cipher mode requires).
To decrypt, you would prompt for a password, load the salt from the file, and re-derive the key. You would then use that key to decrypt the file.
The purpose of the salt is to prevent precomputation optimisations from being applied to a dictionary attack. It is indeed possible to perform a bruteforce dictionary attack once the salt is known, but the KDF is designed to be slow enough to make this infeasible without precomputation.
Reference:
http://stackoverflow.com/questions/5040130/security-using-aes-with-salted-password-as-key
2017/03/01
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