Tuomas AuraT-110.4206 Information security technology

Encrypting stored data

Aalto University, autumn 2013

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Outline1. Scenarios2. File encryption3. Encrypting file system4. Full disk encryption5. 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

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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

FILE ENCRYPTION

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Simple file encryption1. User enters

passphrase2. Passphrase hashed

with a cryptographic hash functionto produce a key

3. File encrypted with the key

– E.g. AES in CBC mode– Decryption with

the same key– Examples:

crypt(1), GPG

1*****

**

2

SHA-1

d70f3619a209b

Our plan is.…3

% gpg --output ciphertext.gpg --symmetric plaintext.docEnter 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

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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

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

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2

PBKDF2

d70f3619a209b15

Our plan is.…

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User name:

Windows

Password:

Log on to:

Username

*********

Domain

OK Cancel Shut Down... Options <<

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4

key

User’s DPAPI*Master Key

User’s PrivateEFS Key

5 FEK

User profile

User profile

$EFS alternate

data stream

EncryptedFile

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Plaintextfile

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 keys12

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FULL DISK ENCRYPTION

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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

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

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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

BitLocker partitions

EncryptedWindows partition

Boot partition

Windows partition contains:Volume metadata with MACEncrypted OSEncrypted page fileEncrypted temp filesEncrypted dataEncrypted hibernation file

Boot partition contains: MBROS loaderBoot utilities

1.5 GB

BitLocker keys

Storage Root Key (SRK) inside TPM1

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2 Volume Master Key (VMK)

3Full 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)

Secure boot with TPM

CRTM

Boot manager

NTFS boot block

NTFS boot sector

MBR

BIOS

measure and load

Static OS Dynamic OSPre-OS

PCRs on TPM

decrypt,verify signatureand load

load volume metadata,

unseal VMK,verify MAC1

on metadata,decrypt FVEK

1MAC keyed with VMK. 2Different loaders for boot, resume etc.

Windows

OS loader2

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 neededBitLocker 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 AD26

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

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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

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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?

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

d70f3619a209b15

Fileattribute

Our plan is.…

User’s PrivateEFS Key

FEK

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EncryptedFile

Our plan is.…

FEK

Recovery Agent’s Private EFS Key

Plaintextfile

Plaintextfile

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

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