How is it possible for user's password to be changed after storage was encrypted? (on OS X, Android) The 2019 Stack Overflow Developer Survey Results Are InDEK, KEK and Master key - simple explanationAre there actually any advantages to Android full-disk encryption?Android / CyanogenMod encryption vs GNU/LinuxEncrypted files storage. How to simplify the password management scheme?How is the FileVault master key protected?Is it possible to retrieve flash-based encrypted disks content (SSD, cellphones, USB sticks, …) after password wipe/replacement?How to correctly handle passwords for an Android appBest practice for securing encrypted content on Android appHow does Android 6 full encryption work … when it doesn't ask for the password at start time?How can Android encryption be so fast?Connection between PIN/password and encryption keys in Android

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How is it possible for user's password to be changed after storage was encrypted? (on OS X, Android)



The 2019 Stack Overflow Developer Survey Results Are InDEK, KEK and Master key - simple explanationAre there actually any advantages to Android full-disk encryption?Android / CyanogenMod encryption vs GNU/LinuxEncrypted files storage. How to simplify the password management scheme?How is the FileVault master key protected?Is it possible to retrieve flash-based encrypted disks content (SSD, cellphones, USB sticks, …) after password wipe/replacement?How to correctly handle passwords for an Android appBest practice for securing encrypted content on Android appHow does Android 6 full encryption work … when it doesn't ask for the password at start time?How can Android encryption be so fast?Connection between PIN/password and encryption keys in Android



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28















There are built-in functionalities to encrypt a storage on OS X (FileVault) and Android.



On OS X: to enable encryption current user must have a password protected account. After enabling the encryption, recovery key is generated (something like HHWj-Y8DK-ODO4-BQEN-FQ4V-M4O8). After the encryption is finished (and in all probability before that as well) user is able to change his password, without the need to re-encrypt the storage.



On Android: user is required to set lockscreen protection to either pin or password. After storage encryption is done (again, probably before that as well), user is able to change password, and even switch from password to pin and vice versa.



Now here is what puzzles me: my understanding is that when storage is encrypted, it is done with current user password (sort of like encrypting an archive) and if password is changed — the whole storage must be re-encrypted. This (apparently incorrect) understanding brings me to following questions:



  1. Based on what "key" (since it is not the password itself) encryption is done then?

    • For OS X, I am guessing, it's the recovery key, but how is it connected to the user's password then?


  2. If password is not the basis for encryption, why is it required to set one before encrypting your storage?

  3. How is ability to decrypt storage is maintained (without re-encrypting) after password is changed?









share|improve this question









New contributor




Filipp W. is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.















  • 2





    I'm pretty sure this is a duplicate, but I can't find the question I'm thinking of...

    – forest
    Apr 8 at 2:46











  • @forest It could be a duplicate of this, but honestly I think that this question is better than that one. I also think I remember a question about why a user's encrypted drive was "deleted" so quickly, and it's because just the key was deleted, but I can't find it.

    – mbomb007
    2 days ago












  • Related (Superuser): Does changing the encryption password imply rewriting all the data?

    – Marc.2377
    2 days ago











  • @forest, (before I posted this question) I was pretty sure as well, that such question already have been asked. Though — I was not able to find it :-)

    – Filipp W.
    yesterday

















28















There are built-in functionalities to encrypt a storage on OS X (FileVault) and Android.



On OS X: to enable encryption current user must have a password protected account. After enabling the encryption, recovery key is generated (something like HHWj-Y8DK-ODO4-BQEN-FQ4V-M4O8). After the encryption is finished (and in all probability before that as well) user is able to change his password, without the need to re-encrypt the storage.



On Android: user is required to set lockscreen protection to either pin or password. After storage encryption is done (again, probably before that as well), user is able to change password, and even switch from password to pin and vice versa.



Now here is what puzzles me: my understanding is that when storage is encrypted, it is done with current user password (sort of like encrypting an archive) and if password is changed — the whole storage must be re-encrypted. This (apparently incorrect) understanding brings me to following questions:



  1. Based on what "key" (since it is not the password itself) encryption is done then?

    • For OS X, I am guessing, it's the recovery key, but how is it connected to the user's password then?


  2. If password is not the basis for encryption, why is it required to set one before encrypting your storage?

  3. How is ability to decrypt storage is maintained (without re-encrypting) after password is changed?









share|improve this question









New contributor




Filipp W. is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.















  • 2





    I'm pretty sure this is a duplicate, but I can't find the question I'm thinking of...

    – forest
    Apr 8 at 2:46











  • @forest It could be a duplicate of this, but honestly I think that this question is better than that one. I also think I remember a question about why a user's encrypted drive was "deleted" so quickly, and it's because just the key was deleted, but I can't find it.

    – mbomb007
    2 days ago












  • Related (Superuser): Does changing the encryption password imply rewriting all the data?

    – Marc.2377
    2 days ago











  • @forest, (before I posted this question) I was pretty sure as well, that such question already have been asked. Though — I was not able to find it :-)

    – Filipp W.
    yesterday













28












28








28


2






There are built-in functionalities to encrypt a storage on OS X (FileVault) and Android.



On OS X: to enable encryption current user must have a password protected account. After enabling the encryption, recovery key is generated (something like HHWj-Y8DK-ODO4-BQEN-FQ4V-M4O8). After the encryption is finished (and in all probability before that as well) user is able to change his password, without the need to re-encrypt the storage.



On Android: user is required to set lockscreen protection to either pin or password. After storage encryption is done (again, probably before that as well), user is able to change password, and even switch from password to pin and vice versa.



Now here is what puzzles me: my understanding is that when storage is encrypted, it is done with current user password (sort of like encrypting an archive) and if password is changed — the whole storage must be re-encrypted. This (apparently incorrect) understanding brings me to following questions:



  1. Based on what "key" (since it is not the password itself) encryption is done then?

    • For OS X, I am guessing, it's the recovery key, but how is it connected to the user's password then?


  2. If password is not the basis for encryption, why is it required to set one before encrypting your storage?

  3. How is ability to decrypt storage is maintained (without re-encrypting) after password is changed?









share|improve this question









New contributor




Filipp W. is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.












There are built-in functionalities to encrypt a storage on OS X (FileVault) and Android.



On OS X: to enable encryption current user must have a password protected account. After enabling the encryption, recovery key is generated (something like HHWj-Y8DK-ODO4-BQEN-FQ4V-M4O8). After the encryption is finished (and in all probability before that as well) user is able to change his password, without the need to re-encrypt the storage.



On Android: user is required to set lockscreen protection to either pin or password. After storage encryption is done (again, probably before that as well), user is able to change password, and even switch from password to pin and vice versa.



Now here is what puzzles me: my understanding is that when storage is encrypted, it is done with current user password (sort of like encrypting an archive) and if password is changed — the whole storage must be re-encrypted. This (apparently incorrect) understanding brings me to following questions:



  1. Based on what "key" (since it is not the password itself) encryption is done then?

    • For OS X, I am guessing, it's the recovery key, but how is it connected to the user's password then?


  2. If password is not the basis for encryption, why is it required to set one before encrypting your storage?

  3. How is ability to decrypt storage is maintained (without re-encrypting) after password is changed?






encryption passwords android disk-encryption macosx






share|improve this question









New contributor




Filipp W. is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











share|improve this question









New contributor




Filipp W. is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









share|improve this question




share|improve this question








edited 2 days ago







Filipp W.













New contributor




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asked Apr 7 at 15:15









Filipp W.Filipp W.

24838




24838




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Filipp W. is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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New contributor





Filipp W. is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






Filipp W. is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.







  • 2





    I'm pretty sure this is a duplicate, but I can't find the question I'm thinking of...

    – forest
    Apr 8 at 2:46











  • @forest It could be a duplicate of this, but honestly I think that this question is better than that one. I also think I remember a question about why a user's encrypted drive was "deleted" so quickly, and it's because just the key was deleted, but I can't find it.

    – mbomb007
    2 days ago












  • Related (Superuser): Does changing the encryption password imply rewriting all the data?

    – Marc.2377
    2 days ago











  • @forest, (before I posted this question) I was pretty sure as well, that such question already have been asked. Though — I was not able to find it :-)

    – Filipp W.
    yesterday












  • 2





    I'm pretty sure this is a duplicate, but I can't find the question I'm thinking of...

    – forest
    Apr 8 at 2:46











  • @forest It could be a duplicate of this, but honestly I think that this question is better than that one. I also think I remember a question about why a user's encrypted drive was "deleted" so quickly, and it's because just the key was deleted, but I can't find it.

    – mbomb007
    2 days ago












  • Related (Superuser): Does changing the encryption password imply rewriting all the data?

    – Marc.2377
    2 days ago











  • @forest, (before I posted this question) I was pretty sure as well, that such question already have been asked. Though — I was not able to find it :-)

    – Filipp W.
    yesterday







2




2





I'm pretty sure this is a duplicate, but I can't find the question I'm thinking of...

– forest
Apr 8 at 2:46





I'm pretty sure this is a duplicate, but I can't find the question I'm thinking of...

– forest
Apr 8 at 2:46













@forest It could be a duplicate of this, but honestly I think that this question is better than that one. I also think I remember a question about why a user's encrypted drive was "deleted" so quickly, and it's because just the key was deleted, but I can't find it.

– mbomb007
2 days ago






@forest It could be a duplicate of this, but honestly I think that this question is better than that one. I also think I remember a question about why a user's encrypted drive was "deleted" so quickly, and it's because just the key was deleted, but I can't find it.

– mbomb007
2 days ago














Related (Superuser): Does changing the encryption password imply rewriting all the data?

– Marc.2377
2 days ago





Related (Superuser): Does changing the encryption password imply rewriting all the data?

– Marc.2377
2 days ago













@forest, (before I posted this question) I was pretty sure as well, that such question already have been asked. Though — I was not able to find it :-)

– Filipp W.
yesterday





@forest, (before I posted this question) I was pretty sure as well, that such question already have been asked. Though — I was not able to find it :-)

– Filipp W.
yesterday










2 Answers
2






active

oldest

votes


















46














At a high level, disk encryption is implemented using a data encryption key (DEK) and a key encryption key (KEK). The DEK is generated randomly and used to encrypt the drive, the KEK is derived from the user's password using a KDF like PBKDF2 or Argon2 and then used to encrypt the DEK.



When changing the password, the DEK is simply encrypted with a new KEK derived from the new password.



Encrypting without a password is likely prohibited to avoid a false sense of security. It'd be a bit like locking your door but leaving the key in the lock.



Of course, if you're changing your password because you believe someone figured it out, and that person also had access to the encrypted device, it's possible they stored a copy of the DEK. In this case it may be necessary to re-encrypt the entire drive, though doing so will likely take some time.






share|improve this answer


















  • 24





    It should be noted that encryption without encrypting the DEK may be useful. It allows for extremely quick secure deletion of content of the drive. Wipe they DEK, and the information stored is effectively wiped as well.

    – vidarlo
    Apr 7 at 16:25






  • 4





    @vidarlo True, but I wouldn't expect the average user to understand this, or to need it.

    – AndrolGenhald
    Apr 7 at 16:32











  • That, I do not disagree with :)

    – vidarlo
    Apr 7 at 16:33






  • 1





    They don't need to understand it, but they experience crypto data wipe when they reset their iOS device, even if they don't have a passcode set. It is also useful to encrypt before a password is set to ensure that when the user enables encryption, there isn't any residual unencrypted data on disk, and the enablement is much faster. This is what Windows Bitlocker does when compatible hardware encryption is available.

    – user71659
    2 days ago


















15














I completely agree with AndrolGenhald's high-level answer. In case you are interested in a complementary low-level walk-through of Android's storage encryption implementation:



Android can do File-Based Encryption (FBE) and Full-Disc Encryption (FDE), with "disc" referring to the /data partition. I will focus on FDE to illustrate the principle. The set-up is done by the Volume Daemon (Vold), specifically in system/vold/cryptfs.cpp.




  • cryptfs_enable_internal(int crypt_type, const char* passwd, ...) starts the storage encryption, with crypt_type specifying if a pin or password is used (to determine which keyboard to show on the unlock screen) and passwd giving the actual user pin/password. It will set up a footer crypt_ftr to be stored along the encrypted partition, then it calls create_encrypted_random_key to populate the crypt_ftr.




    • create_encrypted_random_key generates a random master key and a random salt and passes them on to encrypt_master_key.


    • encrypt_master_key uses a key-derivation function (e.g. scrypt), that takes the salt and the user pin/password as an input and deterministically derives an intermediate key. The master key is then encrypted with the intermediate key using AES-128-CBC. The encrypted master key and the salt are stored in crypt_ftr, but not the user pin/password.

    • Back in cryptfs_enable_internal, the crypt_ftr is written to the disc. Then the actual storage encryption via Linux' dm-crypt is triggered using the decrypted master key.


  • cryptfs_check_passwd(const char* passwd) starts storage decryption by backtracking the above steps to obtain the decrypted master key. The crypt_ftr has to be read from the disc, containing the encrypted master key and the salt. The user-supplied pin/password plus salt are fed into the key derivation function. This results in an intermediate key that can decrypt the master key (most of this happens in decrypt_master_key_aux).



  • cryptfs_changepw(int crypt_type, const char* newpw) handles changing the user pin/password. It will not generate a new master key, it just encrypts the existing master key via encrypt_master_key using the new user pin/password.

Based on this information, the answers to your questions would be:



  1. The randomly generated master key is used for the actual storage encryption.


  2. We need a user pin/password to encrypt the master key. Thus the user pin/password is needed to later retrieve the master key for decrypting the storage.


  3. Changing the user pin/password will not change the master key, only the encryption of the master key.






share|improve this answer










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f9c69e9781fa194211448473495534 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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  • Thanks for elaborate 'under the hood' explanation for Android!

    – Filipp W.
    yesterday












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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









46














At a high level, disk encryption is implemented using a data encryption key (DEK) and a key encryption key (KEK). The DEK is generated randomly and used to encrypt the drive, the KEK is derived from the user's password using a KDF like PBKDF2 or Argon2 and then used to encrypt the DEK.



When changing the password, the DEK is simply encrypted with a new KEK derived from the new password.



Encrypting without a password is likely prohibited to avoid a false sense of security. It'd be a bit like locking your door but leaving the key in the lock.



Of course, if you're changing your password because you believe someone figured it out, and that person also had access to the encrypted device, it's possible they stored a copy of the DEK. In this case it may be necessary to re-encrypt the entire drive, though doing so will likely take some time.






share|improve this answer


















  • 24





    It should be noted that encryption without encrypting the DEK may be useful. It allows for extremely quick secure deletion of content of the drive. Wipe they DEK, and the information stored is effectively wiped as well.

    – vidarlo
    Apr 7 at 16:25






  • 4





    @vidarlo True, but I wouldn't expect the average user to understand this, or to need it.

    – AndrolGenhald
    Apr 7 at 16:32











  • That, I do not disagree with :)

    – vidarlo
    Apr 7 at 16:33






  • 1





    They don't need to understand it, but they experience crypto data wipe when they reset their iOS device, even if they don't have a passcode set. It is also useful to encrypt before a password is set to ensure that when the user enables encryption, there isn't any residual unencrypted data on disk, and the enablement is much faster. This is what Windows Bitlocker does when compatible hardware encryption is available.

    – user71659
    2 days ago















46














At a high level, disk encryption is implemented using a data encryption key (DEK) and a key encryption key (KEK). The DEK is generated randomly and used to encrypt the drive, the KEK is derived from the user's password using a KDF like PBKDF2 or Argon2 and then used to encrypt the DEK.



When changing the password, the DEK is simply encrypted with a new KEK derived from the new password.



Encrypting without a password is likely prohibited to avoid a false sense of security. It'd be a bit like locking your door but leaving the key in the lock.



Of course, if you're changing your password because you believe someone figured it out, and that person also had access to the encrypted device, it's possible they stored a copy of the DEK. In this case it may be necessary to re-encrypt the entire drive, though doing so will likely take some time.






share|improve this answer


















  • 24





    It should be noted that encryption without encrypting the DEK may be useful. It allows for extremely quick secure deletion of content of the drive. Wipe they DEK, and the information stored is effectively wiped as well.

    – vidarlo
    Apr 7 at 16:25






  • 4





    @vidarlo True, but I wouldn't expect the average user to understand this, or to need it.

    – AndrolGenhald
    Apr 7 at 16:32











  • That, I do not disagree with :)

    – vidarlo
    Apr 7 at 16:33






  • 1





    They don't need to understand it, but they experience crypto data wipe when they reset their iOS device, even if they don't have a passcode set. It is also useful to encrypt before a password is set to ensure that when the user enables encryption, there isn't any residual unencrypted data on disk, and the enablement is much faster. This is what Windows Bitlocker does when compatible hardware encryption is available.

    – user71659
    2 days ago













46












46








46







At a high level, disk encryption is implemented using a data encryption key (DEK) and a key encryption key (KEK). The DEK is generated randomly and used to encrypt the drive, the KEK is derived from the user's password using a KDF like PBKDF2 or Argon2 and then used to encrypt the DEK.



When changing the password, the DEK is simply encrypted with a new KEK derived from the new password.



Encrypting without a password is likely prohibited to avoid a false sense of security. It'd be a bit like locking your door but leaving the key in the lock.



Of course, if you're changing your password because you believe someone figured it out, and that person also had access to the encrypted device, it's possible they stored a copy of the DEK. In this case it may be necessary to re-encrypt the entire drive, though doing so will likely take some time.






share|improve this answer













At a high level, disk encryption is implemented using a data encryption key (DEK) and a key encryption key (KEK). The DEK is generated randomly and used to encrypt the drive, the KEK is derived from the user's password using a KDF like PBKDF2 or Argon2 and then used to encrypt the DEK.



When changing the password, the DEK is simply encrypted with a new KEK derived from the new password.



Encrypting without a password is likely prohibited to avoid a false sense of security. It'd be a bit like locking your door but leaving the key in the lock.



Of course, if you're changing your password because you believe someone figured it out, and that person also had access to the encrypted device, it's possible they stored a copy of the DEK. In this case it may be necessary to re-encrypt the entire drive, though doing so will likely take some time.







share|improve this answer












share|improve this answer



share|improve this answer










answered Apr 7 at 16:23









AndrolGenhaldAndrolGenhald

12.2k53138




12.2k53138







  • 24





    It should be noted that encryption without encrypting the DEK may be useful. It allows for extremely quick secure deletion of content of the drive. Wipe they DEK, and the information stored is effectively wiped as well.

    – vidarlo
    Apr 7 at 16:25






  • 4





    @vidarlo True, but I wouldn't expect the average user to understand this, or to need it.

    – AndrolGenhald
    Apr 7 at 16:32











  • That, I do not disagree with :)

    – vidarlo
    Apr 7 at 16:33






  • 1





    They don't need to understand it, but they experience crypto data wipe when they reset their iOS device, even if they don't have a passcode set. It is also useful to encrypt before a password is set to ensure that when the user enables encryption, there isn't any residual unencrypted data on disk, and the enablement is much faster. This is what Windows Bitlocker does when compatible hardware encryption is available.

    – user71659
    2 days ago












  • 24





    It should be noted that encryption without encrypting the DEK may be useful. It allows for extremely quick secure deletion of content of the drive. Wipe they DEK, and the information stored is effectively wiped as well.

    – vidarlo
    Apr 7 at 16:25






  • 4





    @vidarlo True, but I wouldn't expect the average user to understand this, or to need it.

    – AndrolGenhald
    Apr 7 at 16:32











  • That, I do not disagree with :)

    – vidarlo
    Apr 7 at 16:33






  • 1





    They don't need to understand it, but they experience crypto data wipe when they reset their iOS device, even if they don't have a passcode set. It is also useful to encrypt before a password is set to ensure that when the user enables encryption, there isn't any residual unencrypted data on disk, and the enablement is much faster. This is what Windows Bitlocker does when compatible hardware encryption is available.

    – user71659
    2 days ago







24




24





It should be noted that encryption without encrypting the DEK may be useful. It allows for extremely quick secure deletion of content of the drive. Wipe they DEK, and the information stored is effectively wiped as well.

– vidarlo
Apr 7 at 16:25





It should be noted that encryption without encrypting the DEK may be useful. It allows for extremely quick secure deletion of content of the drive. Wipe they DEK, and the information stored is effectively wiped as well.

– vidarlo
Apr 7 at 16:25




4




4





@vidarlo True, but I wouldn't expect the average user to understand this, or to need it.

– AndrolGenhald
Apr 7 at 16:32





@vidarlo True, but I wouldn't expect the average user to understand this, or to need it.

– AndrolGenhald
Apr 7 at 16:32













That, I do not disagree with :)

– vidarlo
Apr 7 at 16:33





That, I do not disagree with :)

– vidarlo
Apr 7 at 16:33




1




1





They don't need to understand it, but they experience crypto data wipe when they reset their iOS device, even if they don't have a passcode set. It is also useful to encrypt before a password is set to ensure that when the user enables encryption, there isn't any residual unencrypted data on disk, and the enablement is much faster. This is what Windows Bitlocker does when compatible hardware encryption is available.

– user71659
2 days ago





They don't need to understand it, but they experience crypto data wipe when they reset their iOS device, even if they don't have a passcode set. It is also useful to encrypt before a password is set to ensure that when the user enables encryption, there isn't any residual unencrypted data on disk, and the enablement is much faster. This is what Windows Bitlocker does when compatible hardware encryption is available.

– user71659
2 days ago













15














I completely agree with AndrolGenhald's high-level answer. In case you are interested in a complementary low-level walk-through of Android's storage encryption implementation:



Android can do File-Based Encryption (FBE) and Full-Disc Encryption (FDE), with "disc" referring to the /data partition. I will focus on FDE to illustrate the principle. The set-up is done by the Volume Daemon (Vold), specifically in system/vold/cryptfs.cpp.




  • cryptfs_enable_internal(int crypt_type, const char* passwd, ...) starts the storage encryption, with crypt_type specifying if a pin or password is used (to determine which keyboard to show on the unlock screen) and passwd giving the actual user pin/password. It will set up a footer crypt_ftr to be stored along the encrypted partition, then it calls create_encrypted_random_key to populate the crypt_ftr.




    • create_encrypted_random_key generates a random master key and a random salt and passes them on to encrypt_master_key.


    • encrypt_master_key uses a key-derivation function (e.g. scrypt), that takes the salt and the user pin/password as an input and deterministically derives an intermediate key. The master key is then encrypted with the intermediate key using AES-128-CBC. The encrypted master key and the salt are stored in crypt_ftr, but not the user pin/password.

    • Back in cryptfs_enable_internal, the crypt_ftr is written to the disc. Then the actual storage encryption via Linux' dm-crypt is triggered using the decrypted master key.


  • cryptfs_check_passwd(const char* passwd) starts storage decryption by backtracking the above steps to obtain the decrypted master key. The crypt_ftr has to be read from the disc, containing the encrypted master key and the salt. The user-supplied pin/password plus salt are fed into the key derivation function. This results in an intermediate key that can decrypt the master key (most of this happens in decrypt_master_key_aux).



  • cryptfs_changepw(int crypt_type, const char* newpw) handles changing the user pin/password. It will not generate a new master key, it just encrypts the existing master key via encrypt_master_key using the new user pin/password.

Based on this information, the answers to your questions would be:



  1. The randomly generated master key is used for the actual storage encryption.


  2. We need a user pin/password to encrypt the master key. Thus the user pin/password is needed to later retrieve the master key for decrypting the storage.


  3. Changing the user pin/password will not change the master key, only the encryption of the master key.






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  • Thanks for elaborate 'under the hood' explanation for Android!

    – Filipp W.
    yesterday
















15














I completely agree with AndrolGenhald's high-level answer. In case you are interested in a complementary low-level walk-through of Android's storage encryption implementation:



Android can do File-Based Encryption (FBE) and Full-Disc Encryption (FDE), with "disc" referring to the /data partition. I will focus on FDE to illustrate the principle. The set-up is done by the Volume Daemon (Vold), specifically in system/vold/cryptfs.cpp.




  • cryptfs_enable_internal(int crypt_type, const char* passwd, ...) starts the storage encryption, with crypt_type specifying if a pin or password is used (to determine which keyboard to show on the unlock screen) and passwd giving the actual user pin/password. It will set up a footer crypt_ftr to be stored along the encrypted partition, then it calls create_encrypted_random_key to populate the crypt_ftr.




    • create_encrypted_random_key generates a random master key and a random salt and passes them on to encrypt_master_key.


    • encrypt_master_key uses a key-derivation function (e.g. scrypt), that takes the salt and the user pin/password as an input and deterministically derives an intermediate key. The master key is then encrypted with the intermediate key using AES-128-CBC. The encrypted master key and the salt are stored in crypt_ftr, but not the user pin/password.

    • Back in cryptfs_enable_internal, the crypt_ftr is written to the disc. Then the actual storage encryption via Linux' dm-crypt is triggered using the decrypted master key.


  • cryptfs_check_passwd(const char* passwd) starts storage decryption by backtracking the above steps to obtain the decrypted master key. The crypt_ftr has to be read from the disc, containing the encrypted master key and the salt. The user-supplied pin/password plus salt are fed into the key derivation function. This results in an intermediate key that can decrypt the master key (most of this happens in decrypt_master_key_aux).



  • cryptfs_changepw(int crypt_type, const char* newpw) handles changing the user pin/password. It will not generate a new master key, it just encrypts the existing master key via encrypt_master_key using the new user pin/password.

Based on this information, the answers to your questions would be:



  1. The randomly generated master key is used for the actual storage encryption.


  2. We need a user pin/password to encrypt the master key. Thus the user pin/password is needed to later retrieve the master key for decrypting the storage.


  3. Changing the user pin/password will not change the master key, only the encryption of the master key.






share|improve this answer










New contributor




f9c69e9781fa194211448473495534 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.




















  • Thanks for elaborate 'under the hood' explanation for Android!

    – Filipp W.
    yesterday














15












15








15







I completely agree with AndrolGenhald's high-level answer. In case you are interested in a complementary low-level walk-through of Android's storage encryption implementation:



Android can do File-Based Encryption (FBE) and Full-Disc Encryption (FDE), with "disc" referring to the /data partition. I will focus on FDE to illustrate the principle. The set-up is done by the Volume Daemon (Vold), specifically in system/vold/cryptfs.cpp.




  • cryptfs_enable_internal(int crypt_type, const char* passwd, ...) starts the storage encryption, with crypt_type specifying if a pin or password is used (to determine which keyboard to show on the unlock screen) and passwd giving the actual user pin/password. It will set up a footer crypt_ftr to be stored along the encrypted partition, then it calls create_encrypted_random_key to populate the crypt_ftr.




    • create_encrypted_random_key generates a random master key and a random salt and passes them on to encrypt_master_key.


    • encrypt_master_key uses a key-derivation function (e.g. scrypt), that takes the salt and the user pin/password as an input and deterministically derives an intermediate key. The master key is then encrypted with the intermediate key using AES-128-CBC. The encrypted master key and the salt are stored in crypt_ftr, but not the user pin/password.

    • Back in cryptfs_enable_internal, the crypt_ftr is written to the disc. Then the actual storage encryption via Linux' dm-crypt is triggered using the decrypted master key.


  • cryptfs_check_passwd(const char* passwd) starts storage decryption by backtracking the above steps to obtain the decrypted master key. The crypt_ftr has to be read from the disc, containing the encrypted master key and the salt. The user-supplied pin/password plus salt are fed into the key derivation function. This results in an intermediate key that can decrypt the master key (most of this happens in decrypt_master_key_aux).



  • cryptfs_changepw(int crypt_type, const char* newpw) handles changing the user pin/password. It will not generate a new master key, it just encrypts the existing master key via encrypt_master_key using the new user pin/password.

Based on this information, the answers to your questions would be:



  1. The randomly generated master key is used for the actual storage encryption.


  2. We need a user pin/password to encrypt the master key. Thus the user pin/password is needed to later retrieve the master key for decrypting the storage.


  3. Changing the user pin/password will not change the master key, only the encryption of the master key.






share|improve this answer










New contributor




f9c69e9781fa194211448473495534 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.










I completely agree with AndrolGenhald's high-level answer. In case you are interested in a complementary low-level walk-through of Android's storage encryption implementation:



Android can do File-Based Encryption (FBE) and Full-Disc Encryption (FDE), with "disc" referring to the /data partition. I will focus on FDE to illustrate the principle. The set-up is done by the Volume Daemon (Vold), specifically in system/vold/cryptfs.cpp.




  • cryptfs_enable_internal(int crypt_type, const char* passwd, ...) starts the storage encryption, with crypt_type specifying if a pin or password is used (to determine which keyboard to show on the unlock screen) and passwd giving the actual user pin/password. It will set up a footer crypt_ftr to be stored along the encrypted partition, then it calls create_encrypted_random_key to populate the crypt_ftr.




    • create_encrypted_random_key generates a random master key and a random salt and passes them on to encrypt_master_key.


    • encrypt_master_key uses a key-derivation function (e.g. scrypt), that takes the salt and the user pin/password as an input and deterministically derives an intermediate key. The master key is then encrypted with the intermediate key using AES-128-CBC. The encrypted master key and the salt are stored in crypt_ftr, but not the user pin/password.

    • Back in cryptfs_enable_internal, the crypt_ftr is written to the disc. Then the actual storage encryption via Linux' dm-crypt is triggered using the decrypted master key.


  • cryptfs_check_passwd(const char* passwd) starts storage decryption by backtracking the above steps to obtain the decrypted master key. The crypt_ftr has to be read from the disc, containing the encrypted master key and the salt. The user-supplied pin/password plus salt are fed into the key derivation function. This results in an intermediate key that can decrypt the master key (most of this happens in decrypt_master_key_aux).



  • cryptfs_changepw(int crypt_type, const char* newpw) handles changing the user pin/password. It will not generate a new master key, it just encrypts the existing master key via encrypt_master_key using the new user pin/password.

Based on this information, the answers to your questions would be:



  1. The randomly generated master key is used for the actual storage encryption.


  2. We need a user pin/password to encrypt the master key. Thus the user pin/password is needed to later retrieve the master key for decrypting the storage.


  3. Changing the user pin/password will not change the master key, only the encryption of the master key.







share|improve this answer










New contributor




f9c69e9781fa194211448473495534 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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share|improve this answer



share|improve this answer








edited Apr 7 at 19:09





















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answered Apr 7 at 18:45









f9c69e9781fa194211448473495534f9c69e9781fa194211448473495534

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1595




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New contributor





f9c69e9781fa194211448473495534 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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f9c69e9781fa194211448473495534 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.












  • Thanks for elaborate 'under the hood' explanation for Android!

    – Filipp W.
    yesterday


















  • Thanks for elaborate 'under the hood' explanation for Android!

    – Filipp W.
    yesterday

















Thanks for elaborate 'under the hood' explanation for Android!

– Filipp W.
yesterday






Thanks for elaborate 'under the hood' explanation for Android!

– Filipp W.
yesterday











Filipp W. is a new contributor. Be nice, and check out our Code of Conduct.









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Filipp W. is a new contributor. Be nice, and check out our Code of Conduct.












Filipp W. is a new contributor. Be nice, and check out our Code of Conduct.











Filipp W. is a new contributor. Be nice, and check out our Code of Conduct.














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