🛡️ TPM-Backed Scalable Signer: Architecture Overview

This document describes a scalable and reboot-resilient architecture for signing messages using TPM 2.0 without exposing private keys to user space or host.

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🎯 Goals

• ✅ Persist across reboots (keys survive system restart)
• ✅ Store a large number of private keys securely (beyond TPM NVRAM limits)
• ✅ Run multiple signer processes with limited transient handle usage
• ✅ Never expose private keys to RAM or disk in plaintext

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🧠 Key Concepts

Type	Description
Persistent Primary Key	Stored permanently in TPM NVRAM (handle: 0x81000001)
key.priv/.pub pairs	TPM-encrypted private keys and public keys stored on disk
Transient Handles	Temporary TPM objects in RAM used for loaded keys
Signer Context	TPM object handle for active key (created on-the-fly)

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🧱 System Architecture

               [ Disk Storage ]
        +-----------------------------+
        | key1.priv / key1.pub        |
        | key2.priv / key2.pub        |
        | ...                         |
        +-----------------------------+

              |
              v  (load on demand)
    +------------------------------+
    |    Signer Process            |
    |------------------------------|
    | - Loads key.priv/.pub into TPM |
    | - Signs digest                |
    | - Flushes context if needed   |
    +------------------------------+

              |
              v
        [ TPM Transient Handle (RAM) ]
              |
              v
        Signature Output (r, s)

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🔁 Workflow

1. Create Persistent Primary Key (once):

   tpm2_createprimary -C o -G ecc -c primary.ctx
   tpm2_evictcontrol -C o -c primary.ctx 0x81000001

2. Create Private Key for Disk Storage (repeatable):

   tpm2_create -C 0x81000001 -G ecc -u key1.pub -r key1.priv

3. Signer Process Execution (per request):

   tpm2_load -C 0x81000001 -u key1.pub -r key1.priv -c signer.ctx
   tpm2_sign -c signer.ctx -g sha256 -m digest.bin -o signature.bin
   tpm2_flushcontext signer.ctx

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⚙️ Scalability Considerations

Property	Strategy
Max persistent keys	Use only one: the primary key
Unlimited keys	Store key.priv/.pub files on disk
TPM RAM slots	Use one context at a time, flush after use
Parallel signing	Use a worker queue or LRU slot management

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🔐 Security Guarantees

• TPM signs hashes inside the chip
• Private keys never loaded in RAM in plaintext
• Even key.priv is unusable outside TPM
• Supports recovery after reboot

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🧩 Future Extensions

• Signer REST API with key pool loader
• Key rotation and indexing
• PCR-based sealing for specific boot states

Not compatible with secp256k1

sudo tpm2_getcap ecc-curves
TPM2_ECC_NIST_P256: 0x3
TPM2_ECC_NIST_P384: 0x4
TPM2_ECC_BN_P256: 0x10
TPM2_ECC_SM2_P256: 0x20