1. Encrypting stored data
Aalto University, autumn 2013

2. Outline Scenarios File encryption Encrypting file system
Full disk encryption
Data recovery
[Acknowledgement: These slides are partly based on Microsoft material.]
Simple application of cryptography — but a good example of how difficult it is to build secure system

3. Scenarios for data encryption
Lost and stolen laptops
Contain confidential data and access credentials
Physically compromised servers
Contain business secrets, customer data and PII
Unauthorized insiders have physical access
Decommissioned hard disks
Secure decommissioning is expensive
Hardware recycling is typically done in the cheapest and fastest way: no time for secure disk wipe
Old PCs from the US are shipped to China for recycling

4. Data encryption Scenarios:  Risk of disclosure of confidential data
lost and stolen laptop computers
stolen servers
decommissioning hard disks
 Risk of disclosure of confidential data
The obvious solution: encrypt data on disk
But computer security is never quite so simple:
Security often conflicts with usability
Security often conflicts with reliability; plan for data recovery is needed
System design mistakes or programming errors could compromise data

5. file encryption

6. Simple file encryption
User enters passphrase
Passphrase hashed with a cryptographic hash function to produce a key
File encrypted with the key
E.g. AES in CBC mode
Decryption with the same key
Examples: crypt(1), GPG 1
*******
SHA-1 2
d70f3619a209b
Our plan is.… 3
% gpg --output ciphertext.gpg --symmetric plaintext.doc Enter passphrase:

7. Limitations of file encryption
Encrypting a file normally creates an encrypted copy; what happens to the old plaintext file?
No guarantee that the plaintext is not left on the disk
Word processors and other software create temporary files and backup copies
Unencrypted versions and fragments of the file may be left in locations that the user does not even know about
There are tools for deleting temporary files and for wiping free disk space, but none is completely reliable
Cloud storage keep all old data

8. Wiping files
Deleting a file simply marks the space free but does not erase the contents: raw data is still on the disk
Overwriting a file does not always erase the old contents:
File system may organize data in unexpected ways: backups, revision control, copy on write, journal, etc.
Solid state disks (SSD) write in complex patterns
Wiping all empty disk space by overwriting
Deletes most data but no guarantee
Disk drive behavior is not always controllable by the file system driver: bad block replacement, optimizations
Magnetic data remanence: magnetic medium may retain traces of previous contents even after overwritten
Physical destruction: grinding disks, heating magnetic medium above Curie temperature
Flash memory (SSD) fragments may retain data

9. Encrypting file system

10. Windows encrypting file system (EFS)
Encryption is a file attribute
Possible to enable encryption for all files in a folder  new files encrypted
Files are readable only when the user is logged in
Encryption and decryption are transparent to applications
Similar products exist for Unix 

11. EFS key management PBKDF2 User logs in, enters password
*) DPAPI = Data Protection application programming interface 1
PBKDF2
User logs in, enters password
Hashed to produce key
Used to decrypt User’s Master Key
Used to decrypt User’s Private EFS Key
Used to decrypt File Encryption Key (FEK)
Used to encrypt on write and decrypt on read 2
key
User’s DPAPI* Master Key 3
User profile
User’s Private EFS Key 4
User profile
RSA
$EFS alternate data stream
FEK 5
Plaintext file 6
d70f3619a209b15
Encrypted File
Our plan is.…
AES or 3DES

12. EFS limitations Encrypts contents of specific files only
User login credentials (password) needed for decryption
System has no access to encrypted files unless user logs in
System cannot index files without the user password
Backups contain encrypted files, not the plaintext
When encrypting plaintext files, the original file is not wiped, just deleted; the data remains on the disk
User should create files in an encrypted folder
Transparent decryption
e.g. data decrypted transparently when copying to a file share over network or to an un-encrypted FAT partition
Some data is not encrypted:
folder and file names
temp files, earlier unencrypted versions, printer spool
registry, system files and logs
page file can now be encrypted but requires policy configuration
Hibernation file may contain decryption keys

13. EFS and password cracking
EFS security depends on the secrecy of user password
Password hashes are stored in a database on the disk
Password are vulnerable to brute-force attacks
NT hash and historical LM hash use no salt and are therefore especially vulnerable
Rainbow tables (Hellman90, Oechslin03)
Attacker can boot to another OS, extract the password hashes from the hard disk and crack the user password
Notes: resetting user or admin password does not enable access to encrypted files
EFS supports smart cards as alternative login method

14. Trojans, root kits etc.
EFS data is vulnerable to Trojans, viruses and key loggers
Attacker with access to hardware can compromise OS and install a root kit or key logger
Note that these are problems do not apply to lost or stolen laptops

15. EFS summary
Encrypts single files and folders; leaves a lot of information unencrypted
Requires care from user
User must understand what is encrypted and what else happens to the data
User of a non-domain computer must backup keys or risk data loss
Security depends on a strong password
System cannot access encrypted files for admin tasks like backup and indexing
Hibernation breaks the security
Apart from the hibernation issue, EFS would be pretty secure way of encrypting all files on a data disk (D:)

16. Full disk encryption

17. Full disk encryption Entire disk is encrypted:
Protects all information on disk
Easier to use correctly than EFS
Products are available from various hardware and software vendors including hard disk manufacturers
Password, key or physical token required to boot or to mount disk; thereafter transparent
Usability and reliability issues?
Requires user/admin to be present at boot time
In software-based products:
Password must be strong enough to resist brute-force guessing
Hibernation is a problem
 Hardware solution would be better

18. Trusted platform module
Trusted hardware enables some things that otherwise would be impossible
Trusted platform module (TPM) is a smart-card-like module on the computer motherboard
Holds crypto keys and platform measurements in platform configuration registers (PCR)
Useful TPM operations:
TMP_Seal: encrypt data — in any platform configuration
TPM_Unseal: decrypt the data, but only if the platform configuration is the same as when sealing

19. Windows BitLocker Full-volume encryption in Windows
Uses TPM for key management
Optional PIN input and/or USB dongle at boot time
System volume must be NTFS, data disks can also be FAT
Sealing the entire system partition:
Encrypt data with a symmetric key
Seal the key; store sealed key on disk; unseal when booting
TPM checks the OS integrity before unsealing the key
Can boot to another OS but then cannot unseal the Windows partition  cannot bypass OS access controls
For a stolen laptop, forces the thief to hardware attack against TPM

20. Encrypted Windows partition
BitLocker partitions
Windows partition contains:
Volume metadata with MAC
Encrypted OS
Encrypted page file
Encrypted temp files
Encrypted data
Encrypted hibernation file
1.5 GB
Encrypted Windows partition
Boot partition
Boot partition contains: MBR OS loader Boot utilities

21. BitLocker keys Separate VMK/FVEK adds flexibility — how?
Storage Root Key (SRK) inside TPM 1
Volume Master Key (VMK) 2
Encrypted keys in volume metadata
Full Volume Encryption Key (FVEK) 3
Plaintext data 4
and bring milk …
Separate VMK/FVEK adds flexibility — how?

22. Algorithms and key sizes
Storage root key (SRK) is a 2048-bit RSA key
Volume master key (VMK) is a 256-bit symmetric key
Full volume encrypt key (FVEK) is a 128 or 256-bit symmetric key
The disk in encrypted with AES-CBC
Initialization vector (IV) derived from sector number
No integrity check
Adding a MAC would increase the data size
Disk sectors are pre-processed with a proprietary diffuser algorithm
Makes attacks against integrity more difficult; the whole sector is encrypted as if one cipher block ( bytes)

23. Software authentication with TPM
Measuring platform configuration:
Module n computes hash of module n+1 and extends the hash into a platform configuration register (PCR) in TPM
Module n transfers control to module n+1
At any point, PCRs contain a cumulative fingerprint (hashes) of all software loaded up to that point
Sealing and unsealing data:
TPM binds selected PCR values to the sealed secrets
TPM unseals secrets only if these PCR values have not changed
If attacker tampers with the OS or the boot process, the OS cannot unseal the data
Originally designed as a DRM feature:
Decrypt music only for untampered OS and media player
Slightly different from tranditional secure boot: does not prevent booting to any OS or system configuration

24. Secure boot with TPM Pre-OS Static OS Dynamic OS CRTM measure and load
load volume metadata, unseal VMK,
verify MAC1
on metadata,
decrypt FVEK
BIOS
MBR
NTFS boot sector
NTFS boot block
decrypt, verify signature and load
CRTM = Core Root of Trust Measurement.
BIOS executes code from the first physical sector of the disk, called the Master Boot Record (= MBR = master boot block). The MBR contains with 446 bytes of code and 64 bytes partition table. The code loads the first sector of the boot partition, called boot sector, which contains 512 bytes of code.
BitLocker stores multiple copies of the volume metadata, and the first copy can be located from information in the BIOS Parameter Block (BPB). The BPB is located at the first 0x54 bytes of the first sector of the volume. 
Chapter 30 of the Windows Vista Resource Kit
OS loader is C:\Windows\System32\winload.exe
Boot manager
OS loader2
PCRs on TPM
Windows
1MAC keyed with VMK. 2Different loaders for boot, resume etc.

25. Which PCR values are used?
*PCR 00: CRTM, BIOS and Platform Extensions
(PCR 01: Platform and Motherboard Configuration and Data)
*PCR 02: Option ROM Code
(PCR 03: Option ROM Configuration and Data)
*PCR 04: Master Boot Record (MBR) Code
(PCR 05: Master Boot Record (MBR) Partition Table)
(PCR 06: State Transitions and Wake Events)
(PCR 07: Computer-Manufacturer Specific)
*PCR 08: NTFS Boot Sector
*PCR 09: NTFS Boot Block
*PCR 10: Boot Manager
*PCR 11: BitLocker Critical Components
If any of the *-values has changed, the decryption key will not be unlocked and a recovery password is needed
BitLocker keys will be unlocked before OS upgrade

26. BitLocker modes TPM only: TPM and PIN: TPM (and PIN) and USB stick:
Unsupervised boot (VMK unsealed if the PCR values correct)
Attacker can boot stolen laptop but not log in
 security depends on OS access controls
Very attractive mode of operation enabled by TPM — but see the following slides!
TPM and PIN:
TPM requires a PIN during the secure boot
TMP will be locked after a small number of incorrect PINs
Attacker must break the TPM hardware to decrypt the disk
Attacker may also sniff communication between chips on a live system
TPM (and PIN) and USB stick:
Secure boot and strong keys on a physical token  high security
USB stick without TPM
Traditional software-based full-disk encryption; no secure boot
Network unlock
Server can reboot if on the same network with AD

27. Secure path issues
The PIN input is not secure if the attacker can hack the hardware
Attacker can modify the BIOS or by replace the computer without the user’s knowledge
Key logger on external keyboard can capture the PIN
Similarly, a hacked computer can capture the keys on the USB stick
This requires the attacker to have access to the computer twice: first to install the Trojan, then to use the captured PIN
Inside attacker, e.g. IT support
Not a problem for lost and stolen computers

28. Cold boot attack
Laptop memory is designed for low power consumption  slow refresh rate  data stays in memory for seconds after power loss
Data remanence in DRAM:
Pull out memory from a running computer and plug it into a reader
Some bits will be random but some will retain their values  might be possible to recover most bits of a cryptographic key in the memory
Use cold spray or liquid nitrogen to reduce data loss
Cold boot attack:
Reboot into minimal hacker OS from USB stick or CD
Memory power lost only for a fraction of a second during reboot  memory contents almost unchanged
Lessons:
Breaks full-disk encryption if attacker has access to the running computer
Sleeping laptop = running laptop  most laptops vulnerable
Breaks BitLocker in TPM-only mode even if it is powered down
OS access controls, e.g. screen lock, do not stop a physical attacker

29. Data Revocery

30. Need for data recovery
If the decryption key is lost, encrypted files will be lost
If Admin resets user password, EFS files cannot be read
Password reset and hacking tools have the same effect
User can change the password back to the old one – if remembered
Backup files become unreadable if the user’s old (archived) private key’s is lost
Can happen when rebuilding or cleaning user profile
BitLocker risks: installing Linux boot loader, replacing the motherboard, TPM boot PIN forgotten or mistyped many times, moving disk to another computer
 Good idea to backup decryption keys

31. Data recovery in EFS Windows domain has a data recovery agent (DRA)
FEK is encrypted also with DRA public key
Domain Admin is the default DRA
Other DRAs can be defined in a Group Policy
Standalone machine has no default DRA
Latest password reset disk also recovers EFS private key
User may also export the user’s EFS certificate (including the private key) to a backup disk
Local Admin can configure a DRA on the local machine (see cipher.exe)
Questions:
Win 2000 had Local Admin as default DRA fro non-domain machines; why was this not a good idea?
Local Admin cannot read the users’ encrypted files without the user passwords; can the Admin get around this?

32. Data recovery in EFS
File encryption key (FEK) is encrypted with one or more recovery agents’ public keys
The same mechanism is used for sharing encrypted files between users
Our plan is.…
FEK
Recovery Agent’s Private EFS Key
Plaintext file
User’s Private EFS Key
FEK
File attribute
Plaintext file
d70f3619a209b15
Encrypted File
Our plan is.… 32

33. Data recovery in BitLocker
Recovery password:
User can print a 48-digit recovery password or store it on a USB stick, CD or remote disk; it is actually a 128-bit key
BitLocker encrypts the VMK with the recovery password and stores it with the volume metadata (in the same way as the TMP-sealed VMK)
Multiple backups of volume metadata are stored in the volume in case a part of the volume is corrupted
Organizational recovery policy:
Windows Domain Admin can require the recovery password to be uploaded to the Active Directory
Installing another OS for dual boot will trigger recovery
User can accept the new boot configuration after entering the recovery password

34. Exercises
What secure methods are there for erasing magnetic hard drives and tapes, USB stick or solid-state drives (SSD), and paper documents?
How to delete a specific file from a computer securely without erasing the whole disk?
What security properties does GPG file encryption or EFS provide that full-disk encryption does not?
How vulnerable is EFS to password guessing?
Why do EFS and BitLocker have so many levels of keys? Are some unnecessary?
Compare the security of software-based full-disk encryption and the TPM approach against brute-force password guessing
How to mitigate the risk of cold-boot attacks (both against BitLocker and more generally)?
Explain what effect do powering down the laptop computer, hibernation and sleep mode have on the cold boot attack?
Transparent operation (happens without the user or application even knowing) improves usability of data encryption, but are there risks associated with the transparency?
How would you design the encryption of files in cloud strorage?

35. Related reading Online:
Halderman et al., Lest We Remember: Cold Boot Attacks on Encryption Keys.
Stallings and Brown: Computer security, principles and practice, 2008, chapter 10.5