---
title: "Generate hardware-based private key (TPM2)"
source_url: https://dev.digicert.com/trustedge/tutorials/generate-hardware-based-private-key-tpm2.html
---

By using the `--tap` option with the `trustedge certificate` command, you can generate hardware-based private keys using a Trusted Application Platform (TAP). These keys are securely stored in a hardware module, such as a TPM, and are never exposed in software. The below steps will guide you through generating an RSA or ECC private key using a TAP. See [TAP/SMP ASN.1 key format](#tapsmp-asn.1-key-format) for more details.


> **Important**
>
> TrustEdge CLI certificate command can be built for one and only one of the following: TPM2 (default build of TrustEdge), PKCS11 (which can handle multiple PKCS11 providers simultaneously), or TAP REMOTE.


## Before you begin

Make sure you understand the following:


- TrustEdge must be [installed](https://dev.digicert.com/md/trustedge/install-and-configure.md) on a [supported device](https://dev.digicert.com/md/trustedge/system-requirements.md).


- The user running TrustEdge CLI commands **must be** a member of the `trustedge` group.

  - Use `groups "$(whoami)"` to see group membership.
  - Use `sudo adduser "$(whoami)" trustedge` to add your user to the `trustedge` group.


- TrustEdge must already own the TPM. See [Take ownership of a TPM](https://dev.digicert.com/md/trustedge/tutorials/take-ownership-of-a-tpm.md).


> **Info**
>
> The [TCG TPM 2.0 Library Specification](https://trustedcomputinggroup.org/resource/tpm-library-specification/) (Rev 1.83, Mar 2024) does not define post-quantum algorithms. As a result, TPM-backed (TAP) keys currently support only RSA and ECC.


## Step 1: Choose the algorithm and key parameters

1. For RSA keys, specify the key size with `--size <size>` and include the `--tap` option.
   ```bash
   trustedge certificate --algorithm RSA --size 2048 --tap --output-file RSA_TAP_2048.pem
   ```
2. For ECC keys, specify the curve type with `--curve <curve>` and include the `--tap` option.
   ```bash
   trustedge certificate --algorithm ECC --curve P256 --tap --output-file ECC_TAP_P256.pem
   ```

## Step 2: Configure optional TAP parameters

You can further customize the TAP key generation by specifying additional TAP-related options, such as key usage or signature schemes.

1. For RSA keys with specific TAP key usage:
   ```bash
   trustedge certificate --algorithm RSA --size 2048 --tap --tap-key-usage TAP_KEY_USAGE_SIGNING --output-file RSA_TAP_SIGN_2048.pem
   ```
2. For ECC keys with a specific TAP signature scheme:
   ```bash
   trustedge certificate --algorithm ECC --curve P256 --tap --tap-sig-scheme TAP_SIG_SCHEME_ECDSA_SHA256 --output-file ECC_TAP_SIGN_P256.pem
   ```


> **Tip**
>
> To enhance the security of your private key, you can protect it with a password by including the `--protect` option with any of the commands above. After running the command, you will be prompted to enter a password to protect the private key.


## Step 3: Verify the key creation

After generating the key, verify that the file was created correctly by listing the contents of the directory:

```bash
ls -l /etc/digicert/keystore/keys/
```

This command will display the files in the `keystore/keys/` directory, including the newly created TAP-protected private key.

<a id="tapsmp-asn.1-key-format"></a>

## TAP/SMP ASN.1 key format

This section describes the ASN.1 module for TAP keys. The `PrivateKeyInfo` structure is defined by [RFC 5208 Public-Key Cryptography Standards (PKCS) #8](https://datatracker.ietf.org/doc/html/rfc5208) and includes an algorithm identifier and the private key data, which is wrapped in an **OCTET STRING**.

These are the Object Identifiers (OIDs) used to build the OIDs in this document:


> **Info**
>
> *mocana* is **1.3.6.1.4.1.14421**


```bash
mocana OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) dod(6) internet(1) private(4) enterprise(1)
mocana (14421)
mocana-tap OBJECT IDENTIFIER ::= { mocana 19 }
```

When the following OIDs are used in an `AlgorithmIdentifier`, the parameters MUST be present and MUST be NULL:

```bash
mocana-tap-rsakey OBJECT IDENTIFIER ::= { mocana-tap 0x01 }
mocana-tap-ecckey OBJECT IDENTIFIER ::= { mocana-tap 0x02 }

mocana-tap-rsaProtectedkey OBJECT IDENTIFIER ::= { mocana-tap 0x21 }
mocana-tap-eccProtectedkey OBJECT IDENTIFIER ::= { mocana-tap 0x22 }
```

**Example definitions:**

For RSA keys, the `PrivateKey` will be an OCTET STRING wrapping data encoded as follows:

```bash
MocanaTapRSAPrivateKey ::= SEQUENCE {
  version Version,
  moduleId ModuleId,
  modulus INTEGER,
  publicExponent INTEGER,
  tapPrivateKey TapPrivateKey
}
```

For ECC keys, the `PrivateKey` will be an OCTET STRING wrapping data encoded as follows (`ECParameters` and `ECPoint` are defined in other standards, such as [RFC 5480](https://datatracker.ietf.org/doc/html/rfc5480)):

```bash
MocanaTapECCPrivateKey ::= SEQUENCE {
  version Version,
  moduleId ModuleId,
  parameters ECParameters,
  publicKey ECPoint,
  tapPrivateKey TapPrivateKey
}
```

## About TPM2 key storage

- TrustEdge creates the private key in the TPM owner hierarchy and persists it to an unused persistent object handle.
- The handle and the `--key-alias` you specify are returned in the command output. Either can be used later by TrustEdge to reference the key.
- Keys never leave the TPM in plaintext. Only public key material is written to /etc/digicert/keystore/keys.