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I would like to encrypt a file containing 15-16 password, so a really small file, which encryption offer the best security for little file?

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What are you going to do with it once it is encrypted? Do you plan on transmitting it somewhere, sharing it with someone, or is it just going to sit somewhere on a filesystem? – Zoredache Nov 4 '12 at 4:41
You see, encryption functions are defined to take an arbitrary amount of data and scramble the data so it is unreadable until decrypted. This 'arbitrary data' can be either 1 bit or 4GB, it doesn't matter. The efficiency will remain the same. – Fasih Khatib Nov 4 '12 at 8:43
up vote 1 down vote accepted

Are you worried about security or speed of encryption (since the file size is small)?

A good choice would be AES, and if you have a relatively recent processor in your computer, chances are that it supports the AES-NI instruction set where the encryption happens in hardware and is magnitudes of order faster than doing it in software.

Choose a key size of 256 bits if you're looking for the highest possible security.

The fact that AES was even chosen to be implemented in the processors says a lot about its effectiveness and speed.

See A Stick Figure Guide to the Advanced Encryption Standard (AES) for information on the history of AES, comparison with other schemes and the explanation of the algorithm.

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You can use a simple manual cipher like Caesar's cipher where you replace every alphabet/number with another alphabet/number after certain positions.
Like if you take shift = 3 then
A will be replaced with D, D will be replaced with G and so on.
It is safe for storage and transmission as the only way to crack it is if you know the shift. Although trial-and-error method can actually help in breaking the code, it is highly unlikely that someone will decipher what you wrote because you are encrypting passwords and they are not supposed to contain dictionary words.
However the only problem is that you may lose the text case information [uppercase or lowercase] if you do it manually [unless you keep track of it]. I can try to make a simple program to automate it if you want. :)

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