Symmetrically Encrypted Update Images

SWUpdate allows to symmetrically encrypt update images using the 256 bit AES block cipher in CBC mode.

Building an Encrypted SWU Image

First, create a key via openssl which is part of the OpenSSL project. A complete documentation can be found at the OpenSSL Website.

openssl enc -aes-256-cbc -k <PASSPHRASE> -P -md sha1

The key and initialization vector is generated based on the given <PASSPHRASE>. The output of the above command looks like this:

iv =65D793B87B6724BB27954C7664F15FF3

Then, encrypt an image using this information via

openssl enc -aes-256-cbc -in <INFILE> -out <OUTFILE> -K <KEY> -iv <IV>

where <INFILE> is the unencrypted source image file and <OUTFILE> is the encrypted output image file to be referenced in sw-description. <KEY> is the hex value part of the 2nd line of output from the key generation command above and <IV> is the hex value part of the 3rd line.

Then, create a key file to be supplied to SWUpdate via the -K switch by putting the key and initialization vector hex values on one line separated by whitespace, e.g., for above example values

B78CC67DD3DC13042A1B575184D4E16D6A09412C242CE253ACEE0F06B5AD68FC 65D793B87B6724BB27954C7664F15FF3

For earlier versions of SWUpdate it was falsely noted that passing the SALT as a 3rd parameter would increase security. Key and IV are enough for maximum security, salt doesn’t add any value.

Encryption of UBI volumes

Due to a limit in the Linux kernel api for UBI volumes, the size reserved to be written on disk should be declared before actually write anything. Unfortunately, the size of an encrypted image is not know until the complete decryption, thus preventing to correctly declare the size of the file to be written on disk.

For this reason UBI images can declare the special property “decrypted-size” like this:

images: ( {
                filename = "rootfs.ubifs.enc";
                volume = "rootfs";
                encrypted = true;
                properties = {decrypted-size = "104857600";}

The real size of the decrypted image should be calculated and written to the sw-description before assembling the cpio archive. In this example, 104857600 is the size of the rootfs after the decryption: the encrypted size is by the way larger.

Example sw-description with Encrypted Image

The following example is a (minimal) sw-description for installing a Yocto image onto a Beaglebone. Pay attention to the encrypted = true; setting.

software =
        version = "0.0.1";
        images: ( {
                        filename = "core-image-full-cmdline-beaglebone.ext3.enc";
                        device = "/dev/mmcblk0p3";
                        encrypted = true;

Running SWUpdate with Encrypted Images

Symmetric encryption support is activated by setting the ENCRYPTED_IMAGES option in SWUpdate’s configuration. Use the -K parameter to provide the symmetric key file generated above to SWUpdate.