Android OS images use cryptographic signatures in two places:
- Each .apk file inside the image must be signed. Android's Package Manager
uses an .apk signature in two ways:
- When an application is replaced, it must be signed by the same key as the
old application in order to get access to the old application's data. This
holds true both for updating user apps by overwriting the .apk, and for
overriding a system app with a newer version installed under
- If two or more applications want to share a user ID (so they can share data, etc.), they must be signed with the same key.
- When an application is replaced, it must be signed by the same key as the old application in order to get access to the old application's data. This holds true both for updating user apps by overwriting the .apk, and for overriding a system app with a newer version installed under
- OTA update packages must be signed with one of the keys expected by the system or the installation process will reject them.
The Android tree includes test-keys under
build/target/product/security. Building an Android OS image
make will sign all .apk files using the test-keys.
Since the test-keys are publicly known, anybody can sign their own .apk files
with the same keys, which may allow them to replace or hijack system
apps built into your OS image. For this reason it is critical to sign any
publicly released or deployed Android OS image with a special set of
release-keys that only you have access to.
To generate your own unique set of release-keys, run these commands from the root of your Android tree:
for x in releasekey platform shared media; do \ ./development/tools/make_key ~/.android-certs/$x "$subject"; \ done
$subject should be changed to reflect your organization's
information. You can use any directory, but be careful to pick a
location that is backed up and secure. Some vendors choose to encrypt
their private key with a strong passphrase and store the encrypted key
in source control; others store their release keys somewhere else entirely,
such as on an air-gapped computer.
To generate a release image, use:
./build/tools/releasetools/sign_target_files_apks \ -o \ # explained in the next section -d ~/.android-certs out/dist/*-target_files-*.zip \ signed-target_files.zip
sign_target_files_apks script takes a target-files .zip
as input and produces a new target-files .zip in which all the .apks have
been signed with new keys. The newly signed images can be found under
Signing OTA packagesA signed target-files zip can be converted into a signed OTA update zip using the following procedure:
./build/tools/releasetools/ota_from_target_files \ -k ~/.android-certs/releasekey \ signed-target_files.zip \ signed-ota_update.zip
Signatures and sideloading
Sideloading does not bypass recovery's normal package signature verification mechanism—before installing a package, recovery will verify that it is signed with one of the private keys matching the public keys stored in the recovery partition, just as it would for a package delivered over-the-air.
Update packages received from the main system are typically verified twice:
once by the main system, using the
method in the android API, and then again by
recovery. The RecoverySystem API checks the signature against public keys
stored in the main system, in the file
(by default). Recovery checks the signature against public keys stored
in the recovery partition RAM disk, in the file
By default, the target-files .zip produced by the build sets the OTA
certificate to match the test key. On a released image, a
different certificate must be used so that devices can verify the
authenticity of the update package. Passing the
-o flag to
sign_target_files_apks, as shown in the previous section, replaces
the test key certificate with the release key certificate from your certs
Normally the system image and recovery image store the same set of OTA public keys. By adding a key to just the recovery set of keys, it is possible to sign packages that can be installed only via sideloading (assuming the main system's update download mechanism is correctly doing verification against otacerts.zip). You can specify extra keys to be included only in recovery by setting the PRODUCT_EXTRA_RECOVERY_KEYS variable in your product definition:
[...] PRODUCT_EXTRA_RECOVERY_KEYS := vendor/yoyodyne/security/tardis/sideload
This includes the public key
vendor/yoyodyne/security/tardis/sideload.x509.pem in the recovery
keys file so it can install packages signed
with it. The extra key is not included in otacerts.zip though, so
systems that correctly verify downloaded packages do not invoke recovery for
packages signed with this key.
Certificates and private keys
Each key comes in two files: the certificate, which has the extension .x509.pem, and the private key, which has the extension .pk8. The private key should be kept secret and is needed to sign a package. The key may itself be protected by a password. The certificate, in contrast, contains only the public half of the key, so it can be distributed widely. It is used to verify a package has been signed by the corresponding private key.
The standard Android build uses four keys, all of which reside in
- Generic default key for packages that do not otherwise specify a key.
- Test key for packages that are part of the core platform.
- Test key for things that are shared in the home/contacts process.
- Test key for packages that are part of the media/download system.
Individual packages specify one of these keys by setting LOCAL_CERTIFICATE in their Android.mk file. (testkey is used if this variable is not set.) You can also specify an entirely different key by pathname, e.g.:
[...] LOCAL_CERTIFICATE := device/yoyodyne/security/special
Now the build uses the
key to sign SpecialApp.apk. The build can use only private keys that
are not password protected.
Advanced signing options
When you run the
sign_target_files_apks script, you must
specify on the command line a replacement key for each key used in the build.
dest_key flag specifies key replacements one at a time. The flag
-d dir lets you specify a directory with four keys to
replace all those in
build/target/product/security; it is
equivalent to using
-k four times to specify the mappings:
build/target/product/security/testkey = dir/releasekey build/target/product/security/platform = dir/platform build/target/product/security/shared = dir/shared build/target/product/security/media = dir/media
For the hypothetical tardis product, you need five password-protected keys:
four to replace the four in
one to replace the additional
required by SpecialApp in the example above. If the keys were in the following
vendor/yoyodyne/security/tardis/releasekey.x509.pem vendor/yoyodyne/security/tardis/releasekey.pk8 vendor/yoyodyne/security/tardis/platform.x509.pem vendor/yoyodyne/security/tardis/platform.pk8 vendor/yoyodyne/security/tardis/shared.x509.pem vendor/yoyodyne/security/tardis/shared.pk8 vendor/yoyodyne/security/tardis/media.x509.pem vendor/yoyodyne/security/tardis/media.pk8 vendor/yoyodyne/security/special.x509.pem vendor/yoyodyne/security/special.pk8 # NOT password protected vendor/yoyodyne/security/special-release.x509.pem vendor/yoyodyne/security/special-release.pk8 # password protected
Then you would sign all the apps like this:
./build/tools/releasetools/sign_target_files_apks -d vendor/yoyodyne/security/tardis -k vendor/yoyodyne/special=vendor/yoyodyne/special-release -o tardis-target_files.zip signed-tardis-target_files.zip
This brings up the following:
Enter password for vendor/yoyodyne/security/special-release key> Enter password for vendor/yoyodyne/security/tardis/media key> Enter password for vendor/yoyodyne/security/tardis/platform key> Enter password for vendor/yoyodyne/security/tardis/releasekey key> Enter password for vendor/yoyodyne/security/tardis/shared key> signing: Phone.apk (vendor/yoyodyne/security/tardis/platform) signing: Camera.apk (vendor/yoyodyne/security/tardis/media) signing: Special.apk (vendor/yoyodyne/security/special-release) signing: Email.apk (vendor/yoyodyne/security/tardis/releasekey) [...] signing: ContactsProvider.apk (vendor/yoyodyne/security/tardis/shared) signing: Launcher.apk (vendor/yoyodyne/security/tardis/shared) rewriting SYSTEM/build.prop: replace: ro.build.description=tardis-user Eclair ERC91 15449 test-keys with: ro.build.description=tardis-user Eclair ERC91 15449 release-keys replace: ro.build.fingerprint=generic/tardis/tardis/tardis:Eclair/ERC91/15449:user/test-keys with: ro.build.fingerprint=generic/tardis/tardis/tardis:Eclair/ERC91/15449:user/release-keys signing: framework-res.apk (vendor/yoyodyne/security/tardis/platform) rewriting RECOVERY/RAMDISK/default.prop: replace: ro.build.description=tardis-user Eclair ERC91 15449 test-keys with: ro.build.description=tardis-user Eclair ERC91 15449 release-keys replace: ro.build.fingerprint=generic/tardis/tardis/tardis:Eclair/ERC91/15449:user/test-keys with: ro.build.fingerprint=generic/tardis/tardis/tardis:Eclair/ERC91/15449:user/release-keys using: vendor/yoyodyne/security/tardis/releasekey.x509.pem for OTA package verification done.
After prompting the user for passwords for all password-protected keys, the script re-signs all the .apk files in the input target .zip with the release keys. Before running the command, you can also set the ANDROID_PW_FILE environment variable to a temporary filename; the script then invokes your editor to allow you to enter passwords for all keys (this may be a more convenient way to enter passwords).
sign_target_files_apks also rewrites the build description and
fingerprint in the build properties files to reflect the fact that this is a
signed build. The
-t flag can control what edits are made to the
fingerprint. Run the script with
-h to see documentation on all
Manually generating keys
Android uses 2048-bit RSA keys with public exponent 3. You can generate certificate/private key pairs using the openssl tool from openssl.org:
# generate RSA key
openssl genrsa -3 -out temp.pem 2048Generating RSA private key, 2048 bit long modulus ....+++ .....................+++ e is 3 (0x3) # create a certificate with the public part of the key
openssl req -new -x509 -key temp.pem -out releasekey.x509.pem -days 10000 -subj '/C=US/ST=California/L=San Narciso/O=Yoyodyne, Inc./OU=Yoyodyne Mobility/CN=Yoyodyne/emailAddressfirstname.lastname@example.org'# create a PKCS#8-formatted version of the private key
openssl pkcs8 -in temp.pem -topk8 -outform DER -out releasekey.pk8 -nocrypt# securely delete the temp.pem file
shred --remove temp.pem
The openssl pkcs8 command given above creates a .pk8 file with no
password, suitable for use with the build system. To create a .pk8 secured
with a password (which you should do for all actual release keys), replace the
-nocrypt argument with
-passout stdin; then openssl
will encrypt the private key with a password read from standard input. No
prompt is printed, so if stdin is the terminal the program will appear to hang
when it's really just waiting for you to enter a password. Other values can be
used for the-passout argument to read the password from other locations; for
details, see the
The temp.pem intermediate file contains the private key without any kind of password protection, so dispose of it thoughtfully when generating release keys. In particular, the GNUshred utility may not be effective on network or journaled filesystems. You can use a working directory located in a RAM disk (such as a tmpfs partition) when generating keys to ensure the intermediates are not inadvertently exposed.
Creating image files
Once you have signed-target-files.zip, you need to create the image so you can put it onto a device. To create the signed image from the target files, run the following command from the root of the Android tree:
./build/tools/releasetools/img_from_target_files signed-target-files.zip signed-img.zipThe resulting file,
signed-img.zip, contains all the .img files. To load an image onto a device, use fastboot as follows:
fastboot update signed-img.zip