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

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


Are 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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6















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 encription 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 via current user password (sort of like encrypting an arhive) and if password is changed — the whole storage must be re-encrypted. This (apparenty 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?










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    6















    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 encription 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 via current user password (sort of like encrypting an arhive) and if password is changed — the whole storage must be re-encrypted. This (apparenty 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.























      6












      6








      6








      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 encription 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 via current user password (sort of like encrypting an arhive) and if password is changed — the whole storage must be re-encrypted. This (apparenty 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 encription 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 via current user password (sort of like encrypting an arhive) and if password is changed — the whole storage must be re-encrypted. This (apparenty 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









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      share|improve this question









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      edited 1 hour ago









      Moshe Katz

      486311




      486311






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      asked 10 hours ago









      Filipp W.Filipp W.

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      Check out our Code of Conduct.






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      Check out our Code of Conduct.






















          2 Answers
          2






          active

          oldest

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          8














          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



















          • 3





            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
            9 hours ago











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

            – AndrolGenhald
            9 hours ago











          • That, I do not disagree with :)

            – vidarlo
            9 hours ago



















          2














          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.





















            Your Answer








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            2 Answers
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            active

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            2 Answers
            2






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            8














            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



















            • 3





              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
              9 hours ago











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

              – AndrolGenhald
              9 hours ago











            • That, I do not disagree with :)

              – vidarlo
              9 hours ago
















            8














            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



















            • 3





              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
              9 hours ago











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

              – AndrolGenhald
              9 hours ago











            • That, I do not disagree with :)

              – vidarlo
              9 hours ago














            8












            8








            8







            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 9 hours ago









            AndrolGenhaldAndrolGenhald

            12k52837




            12k52837








            • 3





              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
              9 hours ago











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

              – AndrolGenhald
              9 hours ago











            • That, I do not disagree with :)

              – vidarlo
              9 hours ago














            • 3





              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
              9 hours ago











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

              – AndrolGenhald
              9 hours ago











            • That, I do not disagree with :)

              – vidarlo
              9 hours ago








            3




            3





            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
            9 hours ago





            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
            9 hours ago













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

            – AndrolGenhald
            9 hours ago





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

            – AndrolGenhald
            9 hours ago













            That, I do not disagree with :)

            – vidarlo
            9 hours ago





            That, I do not disagree with :)

            – vidarlo
            9 hours ago













            2














            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.

























              2














              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.























                2












                2








                2







                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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                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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