Data Encryption and File Security
Encrypting stored data
Aalto University, autumn 2013
Outline
1.
Scenarios
2.
File encryption
3.
Encrypting file system
4.
Full disk encryption
5.
Data recovery
[Acknowledgement:
These slides are partly based on
Microsoft material.]
2
Simple application of
cryptography — but a good
example of how difficult it is
to build
secure system
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 cheapes
t
and fastest way:
no time for secure disk wipe
–
Old PCs from the US are shipped to China for recycling
3
Data encryption
Scenarios:
–
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
4
FILE ENCRYPTION
Simple file encryption
1.
User enters
passphrase
2.
Passphrase hashed
with a cryptographic
hash function
to produce a key
3.
File encrypted
with the key
–
E.g. AES in CBC mode
–
Decryption with
the same key
–
Examples:
crypt(1), GPG
6
1
1
2
2
S
H
A
-
1
d70f3
619a
209b
Our
plan
is.…
3
3
% gpg --output ciphertext.gpg --symmetric plaintext.doc
Enter passphrase:
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
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
8
ENCRYPTING FILE SYSTEM
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
10
EFS key
management
1.
User logs in,
enters password
2.
Hashed to
produce key
3.
Used to decrypt
User’s Master
Key
4.
Used to decrypt
User’s Private
EFS Key
5.
Used to decrypt
File Encryption
Key (FEK)
6.
Used to encrypt
on write and
decrypt on read
1
2
d70f3
619a2
09b15
Our
plan
is.…
6
3
4
key
User’s DPAPI*
Master Key
User’s Private
EFS Key
5
FEK
User profile
User profile
$EFS
alternate
data stream
Encrypted
File
11
Plaintext
file
RSA
AES or 3DES
*) DPAPI = Data Protection
application programming
interface
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
12
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 acces
s to encrypted files
EFS supports
smart cards
as alternative login method
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
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:)
15
FULL DISK ENCRYPTION
16
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
17
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
Windows BitLocker
19
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
BitLocker partitions
Encrypted
Windows
partition
Boot partition
Windows partition contains:
Volume metadata with MAC
Encrypted OS
Encrypted page file
Encrypted temp files
Encrypted data
Encrypted hibernation file
Boot partition contains:
MBR
OS loader
Boot utilities
1.5 GB
BitLocker keys
Storage Root Key (SRK) inside TPM
1
4
2
Volume Master Key (VMK)
3
Full Volume
Encryption Key (FVEK)
Plaintext
data
and
bring
milk …
Separate VMK/FVEK adds flexibility — how?
Encrypted
keys in
volume
metadata
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 (512..8192 bytes)
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 th
e 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
23
Secure boot with TPM
CRTM
Boot manager
NTFS boot block
NTFS boot sector
MBR
BIOS
measure
and load
Static OS
Dynamic OS
Pre-OS
PCRs
on TPM
decrypt,
verify signature
and load
load volume
metadata,
unseal VMK,
verify MAC
1
on metadata,
decrypt FVEK
1
MAC keyed with VMK.
2
Different loaders for boot, resume etc.
Windows
OS loader
2
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
BitLocker modes
TPM only
:
–
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
26
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
27
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
28
DATA REVOCERY
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
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?
31
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
d70f3
619a2
09b15
File
attribute
Our
plan
is.…
User’s Private
EFS Key
FEK
32
Encrypted
File
Plaintext
file
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
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?
34
Related reading
Online:
–
Halderman et al.,
Lest We Remember: Cold Boot
Attacks on Encryption Keys
.
http://citp.princeton.edu/memory/
Stallings and Brown: Computer security,
principles and practice, 2008, chapter 10.5
35
Exploring data encryption and file security, covering topics such as scenarios for encryption, file encryption methods, limitations, and secure file deletion techniques. Highlighting the challenges of balancing security, usability, and reliability in computer systems.
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Presentation Transcript
Encrypting stored data Tuomas Aura T-110.4206 Information security technology Aalto University, autumn 2013
Outline 1. Scenarios 2. File encryption 3. Encrypting file system 4. Full disk encryption 5. Data recovery Simple application of cryptography but a good example of how difficult it is to build secure system [Acknowledgement: These slides are partly based on Microsoft material.] 2
Data encryption Scenarios: 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 4
Simple file encryption 1. 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 2. ***** ** SHA-1 3. 2 d70f3 619a 209b Our plan is. 3 % gpg --output ciphertext.gpg --symmetric plaintext.doc Enter passphrase: 6
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
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 8
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 10
EFS key management *) DPAPI = Data Protection applicationprogramming interface Windows 1 User name: Password: Username ********* Log on to: Domain OK Cancel Shut Down... Options << PBKDF2 1. 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 2. 3. User s DPAPI* Master Key 3 User profile 4. User s Private EFS Key 4 User profile 5. RSA 5 $EFS FEK alternate data stream 6. Plaintext file 6 Encrypted d70f3 619a2 09b15 Our plan is. File AES or 3DES 11
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 12
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 17
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
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 19
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
BitLocker keys Storage Root Key (SRK) inside TPM 1 2Volume Master Key (VMK) Encrypted keys in volume metadata Full Volume Encryption Key (FVEK) 3 Plaintext data 4 and bring milk Separate VMK/FVEK adds flexibility how?
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 (512..8192 bytes)
Secure boot with TPM Pre-OS Static OS Dynamic OS measure and load CRTM load volume metadata, unseal VMK, verify MAC1 on metadata, decrypt FVEK BIOS MBR NTFS boot sector decrypt, verify signature and load NTFS boot block Boot manager OS loader2 PCRs on TPM Windows 1MAC keyed with VMK. 2Different loaders for boot, resume etc.
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
BitLocker modes TPM only: 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 26
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 28
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
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? 31
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 Recovery Agent s Private EFS Key User s Private EFS Key FEK FEK File attribute Plaintext Plaintext file file d70f3 619a2 09b15 Encrypted Our plan is. Our plan is. File 32
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
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? 34
Related reading Online: Halderman et al., Lest We Remember: Cold Boot Attacks on Encryption Keys. http://citp.princeton.edu/memory/ Stallings and Brown: Computer security, principles and practice, 2008, chapter 10.5 35
