Cloud backup with MCrypt and S3cmd

To the Clouds

Article from Issue 171/2015

Encrypted backup is easier than ever with MCrypt and the Amazon Web Service's S3cmd utility.

I will wager that one of the first things a computer teacher or resident computer guru ever said to you was: "Remember to back up your files." Over the years, I've had a couple of noteworthy backup headaches. Thankfully, the episodes were only minor mishaps, not full-fledged disasters, but these close calls made me fully understand the need for a simple and methodical backup process.

The cloud makes backups easier than ever. The configuration is no more difficult than before, and you don't have to fumble around with CDs, SSDs, backup tapes, or other physical media. Several cloud-based services specialize in cloud backup, but if you're integrating your backup system with existing scripts and standard practices, you might want a little more control over how you manage your cloud backup system. In this article, I describe how to encrypt a file or file collection using MCrypt and upload the encrypted file onto the Amazon S3 cloud. To help you better understand the encryption process and examine some additional features let's first look at two simple encryption tools.


MCrypt [1] is a simple file encryption tool that is very suitable for encrypted backup scenarios. On Debian and Ubuntu, you can install MCrypt easily as follows:

# apt-get install mcrypt

This command also dutifully drops the libmcrypt4 package onto your system.

MCrypt, which is a more modern version of crypt, takes a password and a salt. (A salt is random data used as an additional input for the encryption process.) Note that you can increase security by compressing your files before (not after) running MCrypt to increase the encrypted file's security.

Encrypting a file is as simple as typing:

# mcrypt secret_doc.odt

You'll receive the following prompts:

Enter the passphrase (maximum of 512 characters)
Please use a combination of upper and lower case letters and numbers.
Enter passphrase:

Note that MCrypt asks you to repeat a password twice, and you will see the file next to your original document file. The file extension ".nc" probably refers to the letters in the word "eNCrypted."

More Options

If you have gzip installed on your system, you can automatically compress the file using gzip with the -z switch:

# mcrypt -z

The output filename will be called

Throwing a -u flag into the ring means you can "unlink" the original file. Use this option with caution: After the encrypted file is created, it deletes the original file, so there's no going back if that password is incorrect. I find this mode very handy for quickly tidying up from within scripts. To use it, enter

# mcrypt -z -u mymiddlenameisdanger.cfg

and to unwrap your securely wrapped gift, enter:

# mcrypt -d -z

Clearly, the -d option stands for decrypt. Note the -z switch, which is now tasked with uncompressing as opposed to compressing, so you don't have to run it through gzip manually afterward. The resulting file is no longer compressed, and you are presented with the original: mymiddlenameisdanger.cfg.

Always My Favorite

If you open a file encrypted with MCrypt in a text viewer such as less, you will see the following gobbledygook surrounding the default 128-bit encryption algorithm offered by MCrypt:

^@m^C@rijndael-128^@ ^@cbc^@mcrypt-sha1^@^U\<BD>^X<C4

Figure 1 shows the available encryption algorithms.

Figure 1: The mighty MCrypt offers lots of algorithms and different key sizes to suit your needs.

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