Provider for integration of TPM 2.0 to OpenSSL 3.x

Makes the TPM 2.0 accessible via the standard OpenSSL API and command-line tools, so one can add TPM support to (almost) any OpenSSL 3.x based application.

The tpm2-openssl project

• Implements a provider that integrates the Trusted Platform Module (TPM 2.0) operations to the OpenSSL 3.x, which is the next version of OpenSSL after 1.1.1.

• Follows the new OpenSSL provider API and strictly avoids any legacy API. Therefore this implementation:

  • Is compatible with OpenSSL 3.x and (hopefully) future OpenSSL versions.
  • Does not work with any previous version, including the current OpenSSL 1.1.

• Is based on a major refactoring of the tpm2-tss-engine. The code is still there, but largely reshuffled to match the new OpenSSL API. Therefore this implementation:

  • Retains (almost) all functions of the tpm2-tss-engine, although the command-line interface and the API has changed.
  • Does not modify format of the TSS2 PRIVATE KEY file, so keys created by the previous version still work.
  • Respects the original license and copyright.

• Relies on the Enhanced System API (ESAPI) from the Trusted Computing Groups (TCG) TPM Software Stack (TSS 2.0) and uses the tpm2-tss software stack implementation, version 3.2.0 or later.

Build and Installation Instructions

Several distributions include a tpm2-openssl package. For example, on Debian 12 or Ubuntu 22.04 just run:

apt install tpm2-openssl tpm2-tools tpm2-abrmd libtss2-tcti-tabrmd0

The in-kernel resource manager is not sufficient for complex scenarios such as SSL or X.509 operations. The tpm2-abrmd must be used instead.

Instructions for building and installing the tpm2 provider on other systems are provided in the INSTALL.md file.

Instructions for how releases are conducted, please see the RELEASE.md file.

Features and Documentation

The tpm2 provider functions can be used via the openssl command-line tool, or via the libcrypto API.

No TPM-specific API calls are needed: the applications may be completely unaware that the keys being used are stored within TPM. However, the application has to:

• Load the tpm2 provider with the TPM-based operations,
• When needed, load the base or default provider with operations for file read/write, standard encoders/decoders, symmetric ciphers, and hashes.

For further documentation see latest github docs.

You may also review documentation of the individual TPM2 vendors, such as the OPTIGA™ TPM 2.0 command reference and code examples.

Initialization

Connect to the TPM2 using the openssl -provider option, or using the OSSL_PROVIDER API functions. The TPM2OPENSSL_TCTI environment variable may be used to specify the TPM Command Transmission Interface (TCTI).

The OSSL_PROVIDER_self_test API may be used to invoke the TPM self-test operation.

Symmetric Operations

Provides encryption (TPM2_EncryptDecrypt) using the openssl enc or the EVP_Cipher API. The AES-128, AES-192, AES-256, CAMELLIA-128, CAMELLIA-192 and CAMELLIA-256 algorithm in the ECB, CBC, OFB, CFB or CTR mode is supported.

Provides digest calculation (TPM2_Hash) using the openssl dgst or the EVP_Digest API. The SHA-1, SHA-256, SHA-384 and SHA-512 algorithm is supported.

These operations are disabled by default. The default provider is much faster and should be used instead.

Random Number Generation

Provides a random number generation (TPM2_GetRandom) using the openssl rand or the EVP_RAND API.

Key Operations

Provides key generation (TPM2_Create) using the openssl genpkey or the EVP_PKEY API for the RSA and RSA-PSS keys, as well as the EC keys with a NIST curve P-192, P-224, P-256, P-384 or P-521. The private key gets stored as a PEM (TSS2 PRIVATE KEY) or DER file.

For example, to generate a RSA key using TPM:

openssl genpkey -provider tpm2 -algorithm RSA -out testkey.priv

Provides OSSL_STORE and OSSL_DECODER API to load (TPM2_Load) a private key from a previously generated file, as well as persistent keys generated with the tpm2-tools. Both the hexadecimal key handle as well as the serialized object file may be used. These URI prefixes may be used with any openssl command.

The corresponding public key can be stored using the openssl pkey or the OSSL_ENCODER API. The SubjectPublicKeyInfo (PUBLIC KEY) and PKCS1 (RSA PUBLIC KEY) form, either PEM or DER is supported.

For example, to load a persistent key and export its public portion:

openssl pkey -provider tpm2 -in handle:0x81000000 -pubout -out testkey.pub

Asymmetric Operations

Provides asymmetric signature (TPM2_Sign) using the openssl pkeyutl -sign or the EVP_DigestSign API. The PKCS1 (rsassa) and PSS (rsapss) padding (signing scheme) is supported.

For example, to sign arbitrary data:

openssl pkeyutl -provider tpm2 -inkey handle:0x81000000 \
                -sign -rawin -in testdata -out testdata.sig

Signing using a restricted signing key is possible, e.g. one can sign arbitrary data using the TPM attestation key (AK) created by tpm2_createak. Such keys are compatible with e.g. the strongSwan TPM Plugin. Therefore, OpenSSL could be used to create and deploy VPN keys/certificates.

Provides RSA decryption (TPM2_RSA_Decrypt) using the openssl pkeyutl -decrypt or the EVP_PKEY_decrypt API.

Provides ECDH shared secret derivation (TPM2_ECDH_ZGen) using the openssl pkeyutl -derive or the EVP_PKEY_derive API.

Identity Certificates

Provides all operations required to use a TPM2-based key for:

• Certificate signing with openssl req,
• Certificate Authority (CA) using openssl ca,
• Certificate Management Protocol (CMP) client using openssl cmp,
• Cryptographic Message Standard (S/MIME) processing using openssl cms,
• TLS authentication.

TPM Limitations

Limited Resources

Please mind the limited number of transient key and sequence objects that can be concurrently loaded in the TPM. The number of ongoing digest operations and the number of loaded private keys is limited. The in-kernel resource manager (/dev/tpmrm) is also memory constrained.

Complex scenarios such as SSL or X.509 operations require creation of a large number of transient objects. The in-kernel resource manager is often not sufficient and the user-space resource manager must be used with a sufficiently large --max-transients argument.

Limited Performance

The TPM is a cryptographic processor with a secure key storage. It is not an accelerator. Many operations are slower than a pure software implementation.

For user convenience the tpm2 provider implements also Symmetric Operations that do not use the secure storage, but we recommend using the OpenSSL's default provider instead in performance critical applications.

Limited Set of Algorithms

Not every OpenSSL operation will work with the TPM: some are not specified by the TCG TPM specification, some might not be implemented by your TPM chip.

The list of algorithms supported by the tpm2 provider on your actual TPM can be retrieved using the openssl list commands.

Algorithms that do not require the TPM hardware, such as public key operations, hashes or symmetric ciphers, can be fetched from the OpenSSL's default provider.

Help

When you get stuck, remember: Read-Search-Ask.

1. Read the error message and the documentation
2. Search Google
3. Ask for help

The test scripts provide examples for each implemented functionality. Each test is simple and well-documented.

You can ask a question via an GitHub Issue, or send an email to the TPM2 mailing list.

License

tpm2-openssl is distributed under the BSD 3 Clause License.