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I want to encrypt a file using AES-256. How can I do that quickly and easily, and how can I - or someone else -decrypt it again?

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5 Answers 5

up vote 50 down vote accepted

If you want the encryption to be platform independent, you can use openssl:

  • Encryption:

    openssl aes-256-cbc -in attack-plan.txt -out message.enc

  • Decryption:

    openssl aes-256-cbc -d -in message.enc -out plain-text.txt

You can get openssl to base64-encode the message by using the -a switch on both encryption and decryption. This way, you can paste the ciphertext in an email message, for example. It'll look like this:

stefano:~$ openssl aes-256-cbc -in attack-plan.txt -a
enter aes-256-cbc encryption password:
Verifying - enter aes-256-cbc encryption password:
U2FsdGVkX192dXI7yHGs/4Ed+xEC3ejXFINKO6Hufnc=

Note that you have a choice of ciphers and modes of operation. For normal use, I recommend aes 256 in CBC mode. These are the ciphers modes you have available (only counting AES):

aes-128-cbc ← this is okay
aes-128-ecb
aes-192-cbc
aes-192-ecb
aes-256-cbc ← this is recommended
aes-256-ecb

See also:

Please note:

OpenSSL will ask you for a password. This is not an encryption key, it is not limited to 32 bytes! If you're going to transfer files with someone else, your shared secret should be very strong. You can use this site to get a sense of how good your password is:

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7  
openssl aes-256-cbc is shorter than openssl enc -aes-256-cbc and works too. The manual page for this is available by running man enc. Never use ecb for data that should not be tempered with, always use cbc. -salt is redundant since it's default. If you omit -out filename the output will be written to standard output which is useful if you just need to analyze data, but not write it to disk. THe next command shows the line cound for the plaintext: openssl aes-256-cbc -d -in filename | wc -l. (another use, reading a file: openssl aes-256-cbc -d -in filename | less) –  Lekensteyn Sep 9 '11 at 16:37
    
Thanks @Lekensteyn! I incorporated some of that into the answer. –  Stefano Palazzo Sep 10 '11 at 15:19
1  
I use almost exactly this in a script : /usr/bin/openssl enc -aes-256-cbc -a -salt -in $1 -out ${1}.enc with the reverse as you would expect. –  belacq Sep 11 '11 at 0:29
1  
There is no reason to prefer AES 256 to AES 128. @Lekensteyn Never use ECB for confidential data either — basically ECB should never be used (for experts only: ECB should never be used except in some very specific cases). More generally, the openssl command line tool is mostly a proof-of-concept for testing the OpenSSL library. The right answer to this question is either GPG or some archiver such as 7z. –  Gilles Apr 18 at 12:00
1  
I would like to add that openssl enc is actually not that secure if you have a weak password. I previously recommended certain usage of openssl enc, but now suggest to use gpg instead because it uses a KDF in a better way. See this answer. –  Lekensteyn Apr 27 at 22:16

7z (when the password option is used) uses a 256bit AES encryption (with SHA256 key stretching).

Install it (p7zip-full), right click on a file or directory you want to encrypt, and choose Compress, .7z and Other options /Password.

enter image description here

For decryption, right click on the .7z file and choose Extract here.

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I like to use the gpg command:

Encrypt:

gpg --cipher-algo AES256 --symmetric filename.tar.gz

Shorthand:

gpg --cipher-algo AES256 -c filename.tar.gz

This will ask for a passphrase.

Decrypt:

gpg --output filename.tar.gz --decrypt filename.tar.gz.gpg

Shorthand:

gpg -o filename.tar.gz -d filename.tar.gz.gpg

You can also add cipher-algo AES256 to ~/.gnupg/gpg.conf to make AES256 the default. (According to manpage it is CAST5)

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You can also use AES (meaning AES-128). AES-128 is slightly faster and is not less secure. –  Gilles Apr 18 at 12:01

aescrypt

The linked website contains an open-source 256-bit aes encrypt/decrypt tool and is multiplatform - MacOs, Windows, Linux and others through Java.

Encrypt: aescrypt -e -p <password_message> <file>

Decrypt: aescrypt -d -p <password_message> <file>

You could backup and encrypt your home folder using the syntax:

tar -cvf - /home/<home_folder> | aescrypt -e -p <password_message> - > backup.tar.aes

ubuntu installation

Download and extract the source

make
sudo make install

other platforms

Download the binaries or source-code from the website.

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download Truecrypt, make your own container or encrypt your files. works fine in windows and linux.

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