(Android™) AES Encryption. AES encryption. The Chilkat encryption component supports 128-bit, 192-bit, and 256-bit AES encryption in ECB (Electronic Cookbook), CBC (Cipher-Block Chaining), and other modes. The encryption algorithm is 128 Advanced Encryption Standard (AES) with cipher-block chaining (CBC) and ESSIV:SHA256. The master key is encrypted with 128-bit AES via calls to the OpenSSL library. You must use 128 bits or more for the key (with 256 being optional). Note: OEMs can use 128-bit or higher to encrypt the master key. Lecture 8: AES: The Advanced Encryption Standard. With regard to using a key length other than 128 bits, the main thing that changes in AES is how you generate the key schedule from the key — an issue I address at the end of Section 8.8.1. The notion of key schedule in AES is explained in Sections. The authenticated encryption operation takes Initialization Vector (IV), Additional Authenticated Data (AAD), Secret Key and 128-bit plaintext and gives a 128-bit ciphertext and authentication tag. Before getting into the implementation of AES GCM encryption algorithm, let’s first understand the basic difference between AES CBC and AES GCM.
Note: Devices running Android 7.0–9support full-disk encryption. New devices running Android10 and higher must use file-based encryption.
Full-disk encryption is the process of encoding all user data on an Android device using anencrypted key. Once a device is encrypted, all user-created data isautomatically encrypted before committing it to disk and all readsautomatically decrypt data before returning it to the calling process.
Full-disk encryption was introduced to Android in 4.4, but Android 5.0 introducedthese new features:
Android Generate Aes Key Length 128 1
Caution: Devices upgraded to Android 5.0 and thenencrypted may be returned to an unencrypted state by factory data reset. New Android 5.0devices encrypted at first boot cannot be returned to an unencrypted state.
How Android full-disk encryption works
Android full-disk encryption is based on
dm-crypt , which is a kernelfeature that works at the block device layer. Because ofthis, encryption works with Embedded MultiMediaCard (eMMC) andsimilar flash devices that present themselves to the kernel as blockdevices. Encryption is not possible with YAFFS, which talks directly to a rawNAND flash chip.
The encryption algorithm is 128 Advanced Encryption Standard (AES) withcipher-block chaining (CBC) and ESSIV:SHA256. The master key is encrypted with128-bit AES via calls to the OpenSSL library. You must use 128 bits or more forthe key (with 256 being optional).
Note: OEMs can use 128-bit or higher to encrypt the master key.
In the Android 5.0 release, there are four kinds of encryption states:
Upon first boot, the device creates a randomly generated 128-bit master keyand then hashes it with a default password and stored salt. The default password is: 'default_password'However, the resultant hash is also signed through a TEE (such as TrustZone),which uses a hash of the signature to encrypt the master key.
You can find the default password defined in the Android Open Source Project cryptfs.cppfile.
When the user sets the PIN/pass or password on the device, only the 128-bit keyis re-encrypted and stored. (ie. user PIN/pass/pattern changes do NOT causere-encryption of userdata.) Note thatmanaged devicemay be subject to PIN, pattern, or password restrictions.
Encryption is managed by
init and vold .init calls vold , and vold sets properties to triggerevents in init. Other parts of the systemalso look at the properties to conduct tasks such as report status, ask for apassword, or prompt to factory reset in the case of a fatal error. To invokeencryption features in vold , the system uses the command line toolvdc ’s cryptfs commands: checkpw ,restart , enablecrypto , changepw ,cryptocomplete , verifypw , setfield ,getfield , mountdefaultencrypted , getpwtype ,getpw , and clearpw .
In order to encrypt, decrypt or wipe
/data , /data must not be mounted. However, in order to show any user interface (UI), theframework must start and the framework requires /data to run. Toresolve this conundrum, a temporary filesystem is mounted on /data .This allows Android to prompt for passwords, show progress, or suggest a datawipe as needed. It does impose the limitation that in order to switch from thetemporary filesystem to the true /data filesystem, the system muststop every process with open files on the temporary filesystem and restart thoseprocesses on the real /data filesystem. To do this, all servicesmust be in one of three groups: core , main , andlate_start .
To trigger these actions, the
vold.decrypt property is set tovarious strings.To kill and restart services, the init commands are:
Flows
There are four flows for an encrypted device. A device is encrypted just onceand then follows a normal boot flow.
In addition to these flows, the device can also fail to encrypt
/data .Each of the flows are explained in detail below.
Encrypt a new device with forceencrypt
This is the normal first boot for an Android 5.0 device.
Encrypt an existing device
This is what happens when you encrypt an unencrypted Android K or earlierdevice that has been migrated to L.
This process is user-initiated and is referred to as “inplace encryption” inthe code. When a user selects to encrypt a device, the UI makes sure thebattery is fully charged and the AC adapter is plugged in so there is enoughpower to finish the encryption process.
Warning: If the device runs out of power and shuts down before it has finishedencrypting, file data is left in a partially encrypted state. The device mustbe factory reset and all data is lost.
To enable inplace encryption,
vold starts a loop to read eachsector of the real block device and then write itto the crypto block device. vold checks to see if a sector is inuse before reading and writing it, which makesencryption much faster on a new device that has little to no data.
State of device: Set
ro.crypto.state = 'unencrypted' and execute the on nonencrypted init trigger to continue booting.
Starting an encrypted device with default encryption
This is what happens when you boot up an encrypted device with no password.Because Android 5.0 devices are encrypted on first boot, there should be no setpassword and therefore this is the default encryption state.
Starting an encrypted device without default encryption
This is what happens when you boot up an encrypted device that has a setpassword. The device’s password can be a pin, pattern, or password.
Failure
A device that fails to decrypt might be awry for a few reasons. The devicestarts with the normal series of steps to boot:
But after the framework opens, the device can encounter some errors:
If these errors are not resolved, prompt user to factory wipe:
If
vold detects an error during the encryption process, and ifno data has been destroyed yet and the framework is up, vold setsthe property vold.encrypt_progress to error_not_encrypted .The UI prompts the user to reboot and alerts them the encryption processnever started. If the error occurs after the framework has been torn down, butbefore the progress bar UI is up, vold will reboot the system. Ifthe reboot fails, it sets vold.encrypt_progress toerror_shutting_down and returns -1; but there will not be anythingto catch the error. This is not expected to happen.
If
vold detects an error during the encryption process, it setsvold.encrypt_progress to error_partially_encrypted and returns -1. The UI should then display a message saying the encryptionfailed and provide a button for the user to factory reset the device.
Aes Key FortniteStoring the encrypted key
The encrypted key is stored in the crypto metadata. Hardware backing isimplemented by using Trusted Execution Environment’s (TEE) signing capability.Previously, we encrypted the master key with a key generated by applying scryptto the user's password and the stored salt. In order to make the key resilientagainst off-box attacks, we extend this algorithm by signing the resultant keywith a stored TEE key. The resultant signature is then turned into an appropriatelength key by one more application of scrypt. This key is then used to encryptand decrypt the master key. To store this key:
Changing the password
When a user elects to change or remove their password in settings, the UI sendsthe command
cryptfs changepw to vold , andvold re-encrypts the disk master key with the new password.
Encryption propertiesvold and init communicate with each other bysetting properties. Here is a list of available properties for encryption.
Aes Key GeneratorVold properties
init properties
Init actionsComments are closed.
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