1. vpn-info.com

2. Introduction to Trusted Platform Module

3. Trusted computing
Is specified by trusted
Computing group (TCG)
From Trusted computing group, NTRU Cryptosystems

4. Motivating examples for using TPM
How do I
Store a key securely, so a user can access it with a password?
Ensure that I am communicating with a particular user with access to a particular machine?
Make sure my software only runs on a specific machine?
Make sure my software runs only on machines in a specific state?
TPM uses tamper-resistant hardware to ensure system integrity
Systems containing TPM chips:
Lenovo (IBM) Thinkpads and desktops
Fujitsu lifebook
HP desktop and notebooks
TPM Chip vendors: (small inexpensive)
Atmel, Infineon, National, STMicro
Intel D875GRH motherboard

5. Boot-time checking App/lib OS OS App/lib Trusted boot or secure boot
invokes
BIOS
(FLASH)
Boot
loader OS
BIOS
(ROM)
invokes
invokes
invokes
A well-defined sequence of software modules get executed at boot time.
BIOS
(FLASH)
Boot
loader OS
BIOS
(ROM)
Checks
& invokes
Checks
& invokes
Checks
& invokes
Checks
& invokes
Each element in the boot sequence checks the integrity of the next before invoking it
App/lib
Need to know the verification process succeeded
Trusted boot or secure boot

6. TPM architecture
From wikipedia
TPM stores secret keys and releases them depending on the state info at PCRs

7. Platform configuration registers (PCRs)
PCRs are used to securely measure software (by computing hash) during boot
Each PCR can contain an SHA-1 hash value (20byte)
At least 16 PCRs
PCRs are reset to 0 at boot time
Write to a PCR # n by extending it – hash extension
TPM_Extend(n,D): PCR[n]  SHA-1 ( PCR[n] || D )
For example OS
OS computes h3 = SHA-1(module3); stores SHA-1(0,h3) -> PCR[3]
Attacker substitutes module3 with module3’, h3’=SHA-1(module3’)
Checks
& invokes
PCR[3] then contains SHA-1(0, h3’)
Attacker cannot find v such that
SHA-1(SHA-1(0, h3’), v) = SHA-1(0, h3)
App/lib

8. At power-up PCR[n] initialized to 0 BIOS boot block executes
Calls PCR_Extend( n, <BIOS code> )
Then loads and runs BIOS post boot code
BIOS executes:
Calls PCR_Extend( n, <MBR code> )
Then runs MBR (master boot record).
MBR executes:
Calls PCR_Extend( n, <OS loader code, config params> )
Then runs OS loader
Which PCRs to use is defined by specifications

9. Using PCR values after boot
Application 1: encrypted (a.k.a sealed) storage.
Step 1: TPM_TakeOwnership( OwnerPassword, … )
Creates 2048-bit RSA Storage Root Key (SRK) on TPM
Cannot run TPM_TakeOwnership again:
Ownership Enabled flag  False
Done once by IT department or computer owner.
(optional) Step 2: TPM_CreateWrapKey
Create more RSA keys on TPM certified by SRK
Each key identified by 32-bit keyhandle
OwnPass (Owner Password) can later be used to change owner.
SRK key handle ID is 0x

10. Main Step: Encrypt data using RSA key on TPM
Protected Storage
Main Step: Encrypt data using RSA key on TPM
TPM_Seal (some) Arguments:
keyhandle: which TPM key to encrypt with
KeyAuth: Password for using key `keyhandle’
PcrValues: PCRs to embed in encrypted blob
data block: at most 256 bytes (2048 bits)
Used to encrypt symmetric key (e.g. AES)
Returns encrypted blob.
Main point: blob can only be decrypted with TPM_Unseal when PCR-reg-vals = PCR-vals in blob.
TPM_Unseal will fail otherwise
TPM_Seal: allows to specify arbitrary PCR values for unseal.

11. Protected Storage
Embedding PCR values in blob ensures that only certain apps can decrypt data.
e.g.: Messing with MBR or OS kernel will change PCR values.
Why can’t attacker disable TPM until after boot, then extend PCRs with whatever he wants?
Root of trust: BIOS boot block.
Note: resetting TPM after boot (by physically sending TPM_Reset on LPC bus), allows arbitrary values to be loaded onto PCR.
Need owner password to write to DIR. Anyone can read DIR. Stored in NV RAM.

12. PCR and attestation Tie a secret to a list of PCR values
TPM will use or reveal a stored secret only if PCRs have specific values
PCRs reflect system configuration and state
Application: Attestation – to prove to a remote party the legitimate version of software is running on a host
E.g., a company allows computers to connect to its network only if they run “authorized” software
E.g., content providers sell digital media only to authorized versions of players -- DRM

13. Endorsement key (EK) and attestation identity key (AIK)
Every TPM has a unique EK
EK is the root of trust for identification
EK is generated by manufacture, never leaves TPM
Cert for EK public-key issued by TPM vendor, generated by a trusted CA
For attestation, user first generates an attestation identity key
Multiple AIKs may be generated
Cert for AIK public key issued only if EK cert is valid
AIK private key is only known to TPM

14. Attestation
Local host answers challenges from a remote party by signing PCR values with AIK private key (after boot)
Local host runs function TPM_Quote with inputs
keyhandle: which AIK key to sign with
KeyAuth: Password for using key `keyhandle’
PCR List: Which PCRs to sign.
Challenge nonce: 20-byte challenge from remote server
Prevents replay of old signatures.
TPM_Quote returns signed data and signature.
Remote party verifies the signature with AIK public key, verifies cert issuer, verifies PCR values in the signature

15. Attestation: how it works
Attestation Request (20-byte challenge)
Generate pub/priv key pair
TPM_Quote(AIK, PcrList, chal, pub-key)
Obtain cert
App
(SSL) Key Exchange using Cert
Validate:
Cert issuer,
PCR vals in cert OS
Communicate with app
using SSL tunnel
TPM
Remote Server PC

16. Some open questions relating to TPM
TPM provides the guarantee of load-time code integrity.
It does not provide detection ability for run-time compromises such as buffer overflow attack
Can the remote party attest to the current state of a running system?
Active research on this topic, e.g., Flicker
Suppose malicious music file exploits bug in Windows Media Player.
Music file is encrypted.
TCG prevents anyone from getting music file in the clear.
Can anti-virus companies block virus without ever seeing its code in the clear?
Example from Dan Boneh

17. Slides credits
Dan Boneh
Danfeng Yao
Sean Smith