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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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81

Unfortunately, there is no easy solution to securing your stuff. Think about your use-case, maybe something other than plain AES is better suited.

If you want very simple platform independent encryption, you can use openssl.

Please note: You can use this to hide birthday-gift-ideas.txt from your roommate, but don't expect it to be secure against a determined attacker!

  1. As was pointed out in the comments, this method uses a naive key derivation function, so your password needs to be superlatively good in order for you to have a chance of being secure.
  2. Additionally, this method doesn't authenticate the ciphertext, which means an attacker can modify or corrupt the contents without you noticing.
  3. For many types of security, encryption is simply not enough (e.g. you can't just use encryption to communicate securely)

If you still want to 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:

Warning: I have checked that these sites don't send your password to the server, but that can change at any time. Use these sites with dev tools / inspector and check if they send anything before typing in your strong password.

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  • 9
    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
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    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. – belacqua Sep 11 '11 at 0:29
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    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 '14 at 12:00
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    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 '14 at 22:16
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    -1 For recommending low-level OpenSSL use. It does not provide any HMAC functionality and as @Lekensteyn points out, it lacks proper KDF. – gertvdijk Dec 9 '14 at 13:00
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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 '14 at 12:01
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    +1. Also note that GnuPG also provides message authentication (HMAC) for you when using AES (which OpenSSL lacks being the most upvoted answer at the time of writing). Background info: superuser.com/a/633716/157409 – gertvdijk Dec 9 '14 at 13:01
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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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4

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 <file>

Decrypt: aescrypt -d <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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2

A lot of the suggestions I would have made have already been put forth in this thread. Basically, openssl is really the easiest way to go about encrypting a file or script. However, I would caution against using AES-256 just because it is not available in all versions of openssl on some platforms. Most newer OSes...i.e. Linux have it. But others such as AIX 5.3 do not (i think HP-UX as well). If you intend to use your file or script across different platforms, I strongly recommend using AES-128 because this is available everywhere.

How can you "quickly and easily" encrypt a file using AES-128?

A site like www.ShellScrypt.com uses openssl AES-128 quite intensely to encrypt shell scripts and then makes the encrypted copies of the scripts executable. All you have to do is paste the script to the site, and a zip file will be generated for you. That zip file will contain the encrypted (and executable if it is a script) version of your file. This allows you to "easily" and "conveniently" encrypt a file/script without having to satisfy any package or module requirement on every system you intend to use the script on or run several complex and confusing incantations of openssl commands.

Shown below is a basic encrypt / decrypt openssl command that uses AES-128:

test@test-VirtualBox:~$ 
test@test-VirtualBox:~$ echo precious-content | openssl aes-128-cbc -a -salt -k mypassword
U2FsdGVkX1+K6tvItr9eEI4yC4nZPK8b6o4fc0DR/Vzh7HqpE96se8Fu/BhM314z
test@test-VirtualBox:~$
test@test-VirtualBox:~$ echo U2FsdGVkX1+K6tvItr9eEI4yC4nZPK8b6o4fc0DR/Vzh7HqpE96se8Fu/BhM314z | openssl aes-128-cbc -a -d -salt -k mypassword
precious-content
test@test-VirtualBox:~$ 
test@test-VirtualBox:~$
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  • Just leaving a comment to give a heads-up that “shellscrypt.com” is down and out (aka: the site is gone). – e-sushi Oct 19 '17 at 18:38

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