Trusted Computing and the Trusted Platform Module Bruce Maggs (with some slides from Bryan Parno)
Bryan Parno’s Travel Story 2
Attestation • How can we know that a system that we would like to use has not been compromised? 3
Bootstrapping Trust is Hard! Challenges: • Hardware App App 1 3 N 5 2 4 assurance • Ephemeral software Module 1 Module 3 • User Interaction Safe? Yes! OS S 1(()) 15 14 13 12 11 10 9 8 7 6 5 4 3 2 Module 4 ^ ) H( 4
Bootstrapping Trust is Hard! Challenges: • Hardware Evil App assurance • Ephemeral software • User Interaction Safe? Evil OS Yes! 5
Trusted Platform Module Components https: //en. wikipedia. org/wiki/Trusted_Platform_Module#/media/File: TPM. svg 6
TPM Chip Often found in business-class laptops https: //en. wikipedia. org/wiki/Trusted_Platform_Module#/media/File: TPM_Asus. jpg 7
Caveat • The TPM is not 100% tamper proof! • Safe use requires physical security • In 2010 Christopher Tarnovsky extracted the private key from an Infineon TPM chip by • soaking the chip in acid to remove plastic • removing RF-shield wire mesh • probing with an extremely small needle 8
Built-In Unique Identifier • “Endorsement Key” permanently embedded in • • TPM RSA public-private key pair Private key never leaves the TPM chip Public key can be certified (e. g. , TPM may include an EKCERT certificate signed by a TPM CA such as the TPM manufacturer) Master “storage root key” (SRK) created when TPM first used 9
On-Chip Algorithms • • • RSA key-pair generation RSA encryption/decryption RSA signing Random number generation SHA-1 hashing Keyed-hash message authentication code (HMAC) 10
Platform Configuration Registers (PCRs) • A TPM contains several 20 -byte PCRs • A PCR is initialized to zero at power on. • The only operation allowed on a PCR is to extend it: • val[PCR] = SHA 1(val[PCR]. newval) • At boot time, a TPM-enabled PC takes a series of measurements and stores them in PCRs 11
HMAC • Hash with two inputs: a key and a block • • of data Typically key is randomly generated and secret Key can be used (for example) to guarantee that the hash was freshly created 12
How HMAC can be used • TPM can hash contents of all storage on computer, or storage in certain places • Disks • Memory • Registers in the CPU • User can choose to execute only from known safe states 13
Applications • Storing and protecting sensitive • • information from modification Trusted boot Attestation 14
![TPM-Based Attestation Example [Gasser et al. ‘ 89], [Arbaugh et al. ‘ 97], [Sailer](https://present5.com/presentation/78f8ee3325102b0513a5f38455974534/image-15.jpg "TPM-Based Attestation Example [Gasser et al. ‘ 89], [Arbaugh et al. ‘ 97], [Sailer")
TPM-Based Attestation Example [Gasser et al. ‘ 89], [Arbaugh et al. ‘ 97], [Sailer et al. ‘ 04], [Marchesini et al. ‘ 04] Module BIOS OS Bootloader App App PCRs TPM KPriv 15
![Establishing Trust via a TPM [Gasser et al. ‘ 89], [Arbaugh et al. ‘](https://present5.com/presentation/78f8ee3325102b0513a5f38455974534/image-16.jpg "Establishing Trust via a TPM [Gasser et al. ‘ 89], [Arbaugh et al. ‘")
Establishing Trust via a TPM [Gasser et al. ‘ 89], [Arbaugh et al. ‘ 97], [Sailer et al. ‘ 04], [Marchesini et al. ‘ 04] random # Accurate!Module K Module Bootloader Sign ( Module BIOS Bootloader Kpriv Guarantees actual TPM logs OS OS Pubp BIOS Bootloader BIOS Module AA p p. A ppp OS random # AA p p. A ppp p ) Guarantees freshness App App Guarantees real TPM PCRs TPM KPriv 16
Microsoft Bit. Locker Drive Encryption • Encryption of volume containing Windows OS, • • • user files, e. g. , C: Separate unencrypted volume contains files needed to load Windows TPM protects disk encryption key by encrypting it TPM releases key only after comparing hash of early (unencrypted) boot files with previous hash Bit. Locker can be used without a TPM – user supplies an encryption password Relies on user having an OS password! 17
Microsoft Secure Boot (Windows 8+) • Enabled by “UEFI” – Unified Extensible • • • Firmware Interface (replacement for traditional BIOS) Manufacturer’s and Microsoft public keys stored in firmware (can add other OS vendors) TPM checks that firmware is signed by the manufacturer TPM checks that hash of boot loader has been signed with Microsoft public key 18
Microsoft Trusted Boot • Takes over after Secure Boot • Verifies all OS components, starting • with Windows kernel verifies boot drivers, start-up files 19
Microsoft Measured Boot • TPM signs measured boot log file • Remote attestation possible by transmitting signed boot log 20
Intel SGX • Intel Software Guard Extensions – • • new instructions added to the x 64 instruction set Incorporated directly into CPU, e. g. , Intel i 7 -6700 K, Dell Inspiron 11 i 3153 (Not a separate chip like TPM. ) Application can created trusted memory “enclave” Only trusted functions (stored in enclave) can see or modify enclave https: //software. intel. com/en-us/sgx/details 21
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