A+ Core 1 3.5: HSM and TPM — Class Notes

2026-09-27 · class-notes, a-plus, comptia, messer, hardware, tpm, hsm, encryption, keys, root-of-trust

Full class notes for A+ Core 1 3.5: protecting cryptographic keys with a TPM (root of trust, BitLocker) and centralising them with an HSM.

Class notes · Professor Messer A+ Core 1 (220-1201) · Section 3, lesson 5

Quick reference: the short version of this lesson lives in the A+ Core 1 Section 3 resource sheets. This is a seventh lesson under objective 3.5, and it follows naturally from the BIOS lessons — the TPM is the security chip you enable in firmware, and the HSM is its data-centre-scale cousin.

Learning objectives

By the end of these notes you should be able to:

1. Explain why protecting the cryptographic key is the central problem in encryption.
2. Describe what a TPM is, what is inside it, and how it is fitted.
3. Explain how a TPM's unique key enables full-disk encryption, a root of trust and remote attestation.
4. Find and manage the TPM settings in the BIOS.
5. Describe an HSM and the roles it plays at scale.
6. Compare a TPM and an HSM and choose the right one for a scenario.

1. Encryption and the problem of the key

So much of technology is about keeping secrets. Data we hold should be readable only by us, and to achieve that we lean on encryption almost everywhere: a phone encrypts what it stores and what it sends over the air, traffic to and from a web server is encrypted, and data on a local drive or SSD can be encrypted too.

The algorithms that do this are usually open, public standards — anyone can read exactly how the encryption and decryption work. That is not the weakness it sounds like. Think of the lock in a door: understanding how the mechanism works does not get you through the door. What gets you through is the one unique thing nobody else has — the key.

Computers use digital keys the same way. Encrypt data with a key and you must have the correct key to decrypt it later. Which raises the real question of the lesson: once your data is protected by a key, how do you protect the key itself?

2. The Trusted Platform Module (TPM)

On a personal computer, one answer is the Trusted Platform Module, or TPM — a standardised piece of hardware built for encryption, with many cryptographic functions on the chip itself. A TPM may be a separate module plugged onto the motherboard, or it may be built into the motherboard directly.

Inside a TPM you find:

• A cryptographic processor that can perform random number generation and generate cryptographic keys on the chip.
• Persistent memory holding keys burned in during manufacture — unique to that chip.
• Versatile memory for storing keys and other information as you use it.
• Security and access control: the TPM is password-protected, with features designed to stop anyone reaching your private keys.

Note (beyond this lesson): the burned-in unique key is called the Endorsement Key (EK), and the TPM also holds Platform Configuration Registers (PCRs) that record measurements of the boot process — the basis of "sealing" a key to a known-good system state. A TPM can also be a firmware TPM (fTPM / Intel PTT) implemented in the CPU rather than a separate chip; Windows 11 requires TPM 2.0, firmware or discrete.

Trusted Platform Module (TPM):

+-------------------------------------------+
|  Crypto processor                         |
|    - random number generation             |
|    - key generation                       |
|                                           |
|  Persistent memory                        |
|    - key(s) burned in at manufacture      |
|                                           |
|  Versatile memory                         |
|    - stores keys and other data in use    |
|                                           |
|  Security / access control (password)     |
+-------------------------------------------+
   Unique secret key per system = root of trust

2.1 What the unique key lets you do

Because a TPM holds a secret key that is unique to your system — no other machine has the same one — your encryption keys become tied to this computer. That enables several things:

• Full-disk encryption. With BitLocker (or another FDE product), the TPM supplies and protects the key that encrypts the drive. A powerful consequence: you cannot pull the drive out and read it in another machine, because the decryption key stays behind in the original computer's TPM.
• A root of trust. Because the TPM's key is unique and bound to the hardware, it anchors trust in that specific machine. You can use it to confirm that a computer you are talking to across the network really is the device you expect.
• Remote attestation. The TPM can be used to remotely determine whether anything has changed on a system, so you know a machine you connect to is the one you think it is, in the state you expect.

Because the TPM is physically part of the hardware, it cannot easily be copied and moved elsewhere — seeing that TPM means you really are looking at that particular physical computer.

Note (beyond this lesson): phones achieve the same ends with equivalent secure hardware — a secure element, TrustZone/TEE, or Apple's Secure Enclave — rather than a literal PC-style TPM. The function (protect keys in tamper-resistant hardware, back screen-lock and encryption) is the same, so treating "the phone uses a TPM" as shorthand is fine, but the component has a different name.

3. Managing the TPM in the BIOS

Firmware is where you turn TPM features on and off. In the example BIOS it is under Security, and scrolling down reveals an option labelled TCG — Trusted Computing Group, the organisation that manages the TPM standards. The system shows a TPM Security Chip 2.0, and from here you can:

• Enable or disable the TPM.
• Clear any data stored in the TPM, with parameters controlling how that deletion happens.

Caution: clearing the TPM destroys the keys it protects. If a drive is BitLocker-encrypted and its key is sealed to the TPM, clearing the TPM (or a firmware change that alters the measured boot state) can force a recovery key prompt. Always have the BitLocker recovery key backed up before touching TPM settings.

4. The Hardware Security Module (HSM)

A TPM is ideal for one device. But a data centre may have hundreds or thousands of devices, each with keys of its own, and that needs a larger-scale solution: the Hardware Security Module, or HSM.

HSMs do several jobs:

• Centralised key storage and backup. Rather than scattering keys across every server, you keep them on one central, protected device — for example, all your web-server keys managed on a single HSM.
• Cryptographic acceleration / offload. An HSM often does the crypto for other systems: instead of a web server spending its own CPU on encryption and decryption, that work happens in hardware on the HSM, offloading it from the software service.
• Personal / lightweight HSMs. Smaller portable HSMs store personal keys and move between computers — a hardware cryptocurrency wallet is a common example, protecting the owner's keys on a small dedicated device.

In the data centre, an HSM is usually a high-end server fitted with cryptographic hardware, deployed to protect the keys that matter most — the keys behind your web servers, or those of a certificate authority.

Note (beyond this lesson): production HSMs are typically FIPS 140-2/140-3 validated and tamper-responsive — they zeroise (wipe) their keys if physically attacked — and keys generated inside them never leave in plaintext. That is exactly why a certificate authority guards its signing key in one.

4.1 Worked example — matching the tool to the job

Two requests, one principle.

1. "Encrypt the finance team's laptops so a stolen laptop is useless." This is single-device protection, so use each laptop's TPM with BitLocker: the key is sealed to the machine, and a thief who removes the SSD cannot read it elsewhere. (Add a startup PIN — see Security perspective.)
2. "Protect the private key our public web servers and CA share, and take the TLS load off the web servers." This spans many systems and needs central, hardened key storage plus offload, so use an HSM: it holds the keys and performs the crypto in hardware.

The rule: TPM for one device's own keys; HSM for many systems' keys and for offloading crypto.

5. TPM vs HSM

6. Security perspective

This lesson is security hardware, so the defender's reading is central:

• The TPM makes a stolen drive worthless — if used well. Sealing the FDE key in the TPM means the disk only decrypts on its own machine, which defeats the "pull the drive and read it elsewhere" attack. The important caveat is that a TPM-only configuration unlocks automatically at boot, so an attacker with the whole laptop still reaches the desktop. Adding a TPM + PIN (or password) ties decryption to something the attacker does not have.
• Discrete TPMs can be bus-sniffed. A separate TPM chip talks to the CPU over a board bus, and an attacker with physical access can interpose on that bus to capture a BitLocker key in transit (the well-documented "TPM sniffing" attack). Mitigate with TPM + PIN, a firmware TPM (no external bus), or bus-encryption where supported. This is the same physical-access theme as the internal TPM header from the Motherboard connections lesson.
• Clearing the TPM is destructive, and that cuts both ways. It is how you safely decommission a machine's keys, but an attacker (or a careless firmware change) that clears or de-syncs the TPM triggers recovery-key prompts and can lock users out — so back up recovery keys and treat TPM clears as a change to control.
• Root of trust and attestation are how you trust a remote machine. Because the key is bound to the hardware, health-attestation and conditional-access systems can require a device to prove its integrity (unchanged boot state) before it joins the network — a strong control against tampered or unknown endpoints.
• An HSM concentrates trust, so protect it accordingly. Centralising CA and server keys in one tamper-responsive, FIPS-validated device massively reduces key sprawl and keeps keys out of software memory — but it also makes that HSM a crown-jewel asset. Access to it, and to its crypto-offload service, must be tightly controlled and logged.

Summary

• Encryption is everywhere and its algorithms are public; the secret is the key, so the real problem is protecting the key.
• A TPM is standardised crypto hardware on a PC (a module or built into the board): a crypto processor (RNG, key generation), persistent burned-in keys, versatile key storage, and password-protected access.
• Its unique, hardware-bound key enables full-disk encryption (BitLocker — the drive won't decrypt in another machine), a root of trust, and remote attestation.
• In the BIOS (Security -> TCG, from the Trusted Computing Group) you enable/disable and clear the TPM; a TPM 2.0 chip is current.
• An HSM scales this to many systems: centralised key storage/backup and crypto acceleration/offload, as a data-centre high-end device — plus personal/lightweight HSMs (e.g. crypto wallets).
• TPM = one device's keys; HSM = many systems' keys (web servers, CA).

Glossary

Review questions

1. Why is it usually fine that encryption algorithms are public standards?
2. What is the central problem this lesson addresses?
3. What is a TPM, and what are the two ways it can be fitted to a system?
4. List the main components inside a TPM.
5. How does a TPM make a stolen, encrypted drive useless in another computer?
6. What do "root of trust" and "remote attestation" mean in the context of a TPM?
7. In the BIOS, under what menu do you find TPM settings, and what does TCG stand for?
8. Name two things you can do to the TPM from the BIOS.
9. What is an HSM, and how does its scope differ from a TPM's?
10. Give two roles an HSM plays in a data centre.
11. Give an example of a personal/lightweight HSM.
12. Scenario: you must protect a certificate authority's signing key used by many services. TPM or HSM, and why?
13. Scenario: a laptop uses TPM-only BitLocker. A thief steals the whole laptop. Is the data protected? What would improve it?

Answer key

1. The security rests on the secret key, not on hiding the algorithm — knowing the lock doesn't grant entry without the key. Open review even strengthens the algorithm.
2. How to protect the cryptographic key that protects your data. Everything else follows from that.
3. A standardised on-board crypto hardware module; fitted as a separate module or built into the motherboard. Purpose-built for encryption.
4. A cryptographic processor (RNG, key generation), persistent memory with burned-in keys, versatile memory for stored keys, and password-protected access control. Crypto plus secure storage.
5. The drive's decryption key stays in the original machine's TPM, so the drive won't decrypt elsewhere. The key is bound to that computer.
6. Root of trust: the hardware-bound unique key anchors trust in that specific machine; remote attestation: using it to prove the machine is unchanged and the expected one. Trusting a remote device.
7. Under Security; TCG stands for Trusted Computing Group. They manage the TPM standards.
8. Enable/disable the TPM, and clear its stored data (with deletion parameters). Basic firmware control.
9. A Hardware Security Module — scaled key protection for many systems, versus a TPM's single system. Central vs local.
10. Centralised key storage/backup and cryptographic acceleration/offload — plus protecting infrastructure keys (web servers, CA). Any two.
11. A hardware cryptocurrency wallet. A portable personal HSM.
12. HSM — it protects keys shared across many systems and is a hardened, central device built for exactly that (CA keys). TPM is per-device.
13. Not fully — TPM-only unlocks automatically at boot, so the whole-laptop thief reaches the desktop; add a TPM + PIN. Bind decryption to something the thief lacks.