Windows 7 / Getting Started

How BitLocker Encrypts Data

BitLocker encrypts entire volumes. The contents of the volumes can be decrypted only by someone with access to the decryption key, known as the Full Volume Encryption Key (FVEK). Windows 7 actually stores the FVEK in the volume metadata; this is not a problem because the FVEK itself is encrypted using the Volume Master Key (VMK).

Both the FVEK and the VMK are 256 bits. The FVEK always uses AES encryption to protect the volume. By editing the Computer Configuration\Administrative Templates\Windows Components\BitLocker Drive Encryption\Choose Drive Encryption Method And Cipher Strength Group Policy setting, you can set the specific AES encryption strength to one of four values:

  • AES 128 bit with Diffuser (this is the default setting)
  • AES 256 bit with Diffuser (this is the strongest setting, but using it might negatively affect performance)
  • AES 128 bit
  • AES 256 bit

More Info For more information about the encryption algorithms used and the use of diffusers, read "AES-CBC + Elephant Diffuser: A Disk Encryption Algorithm for Windows Vista," at

Windows Vista and Windows 7 encrypt and decrypt disk sectors on the fly as data is read and written (as long as it has access to the FVEK) using the FVE Filter Driver (Fvevol.sys). As shown in Figure below, the FVE Filter Driver, like all filter drivers, resides between the file system (which expects to receive the unencrypted contents of files) and the volume manager (which provides access to the volume). Therefore, applications and users are not aware of encryption when everything is functioning normally.

The FVE Filter Driver transparently encrypts and decrypts disk contents

Encrypting and decrypting data do affect performance. While reading from and writing to a BitLocker-encrypted volume, some processor time will be consumed by the cryptographic operations performed by BitLocker. The actual impact depends on several factors, including caching mechanisms, hard drive speed, and processor performance. However, Microsoft has put great effort into implementing an efficient AES engine so that the performance impact on modern computers is minimal.

[Previous] [Contents] [Next]

In this tutorial:

  1. Managing Disks and File Systems
  2. Overview of Partitioning Disks
  3. How to Choose Between MBR or GPT
  4. Converting from MBR to GPT Disks
  5. GPT Partitions
  6. Choosing Basic or Dynamic Disks
  7. Working with Volumes
  8. How to Create a Simple Volume
  9. How to Create a Spanned Volume
  10. How to Create a Striped Volume
  11. How to Resize a Volume
  12. How to Delete a Volume
  13. How to Create and Use a Virtual Hard Disk
  14. File System Fragmentation
  15. Backup And Restore
  16. How File Backups Work
  17. File and Folder Backup Structure
  18. How System Image Backups Work
  19. How to Start a System Image Backup from the Command Line
  20. How to Restore a System Image Backup
  21. System Image Backup Structure
  22. Best Practices for Computer Backups
  23. How to Manage Backup Using Group Policy Settings
  24. Previous Versions and Shadow Copies
  25. How to Manage Shadow Copies
  26. How to Restore a File with Previous Versions
  27. How to Configure Previous Versions with Group Policy Settings
  28. Windows ReadyBoost
  29. BitLocker Drive Encryption
  30. How BitLocker Encrypts Data
  31. How BitLocker Protects Data
  32. TPM with External Key (Require Startup USB Key At Every Startup)
  33. TPM with PIN (Require PIN At Every Startup)
  34. TPM with PIN and External Key
  35. BitLocker To Go
  36. BitLocker Phases
  37. Requirements for Protecting the System Volume with BitLocker
  38. How to Enable the Use of BitLocker on the System Volume on Computers Without TPM
  39. How to Enable BitLocker Encryption on System Volumes
  40. How to Enable BitLocker Encryption on Data Volumes
  41. How to Manage BitLocker Keys on a Local Computer
  42. How to Manage BitLocker from the Command Line
  43. How to Recover Data Protected by BitLocker
  44. How to Disable or Remove BitLocker Drive Encryption
  45. How to Decommission a BitLocker Drive Permanently
  46. How to Prepare AD DS for BitLocker
  47. How to Configure a Data Recovery Agent
  48. How to Manage BitLocker with Group Policy
  49. The Costs of BitLocker
  50. Windows 7 Encrypting File System
  51. How to Export Personal Certificates
  52. How to Import Personal Certificates
  53. How to Grant Users Access to an Encrypted File
  54. Symbolic Links
  55. How to Create Symbolic Links
  56. How to Create Relative or Absolute Symbolic Links
  57. How to Create Symbolic Links to Shared Folders
  58. How to Use Hard Links
  59. Disk Quotas
  60. How to Configure Disk Quotas on a Single Computer
  61. How to Configure Disk Quotas from a Command Prompt
  62. How to Configure Disk Quotas by Using Group Policy Settings
  63. Disk Tools
  64. EFSDump
  65. SDelete
  66. Streams
  67. Sync
  68. MoveFile and PendMoves