mbed-tls October 2026

mbed-tls@lists.trustedfirmware.org

5 participants
2 discussions

TLS client authentication with a TPM-held private key (Mbed TLS 4.2): is our approach right, and how do we create the key ID?

by Karimi, Ruien

Hello Mbed TLS team, we would like to ask for your advice on a TLS client use case where the private key must stay in an external TPM. We have read the relevant issues and the roadmap, and would like to confirm our understanding and ask for tips. == 1. Our goal == Our device is a TLS client with mutual authentication (device certificate / LDevID). The system is split over several processors (see the attached diagram): * The "stack processor" runs eCos with Mbed TLS (we are targeting Mbed TLS 4.2 / TF-PSA-Crypto 1.2). * The private LDevID key is stored in a TPM 2.0. The TPM is only reachable from the "application processor" (via the TSS). The two processors communicate through shared memory. Desired handshake flow (as in the diagram): 1) The server sends its hello and a certificate request. 2) Mbed TLS needs to sign the hash of all previous handshake messages (CertificateVerify). 3) Instead of signing locally, the hash is passed via shared memory to the application processor. 4) The application processor has the TPM sign the hash with the private LDevID key and returns the signature. 5) The signature is handed back to Mbed TLS, which sends the CertificateVerify and continues the handshake. Mbed TLS currently expects the private key to be available locally. We want to delegate only the signature operation, without patching the TLS handshake code. Blocking while waiting for the TPM is acceptable for us, so asynchronous operation is not a requirement. == 2. What we found so far == * MBEDTLS_SSL_ASYNC_PRIVATE is server-side only (and not supported with TLS 1.3), so it does not help for a client. * Issues #7889 and #10643 were both closed with the same answer: no client-side async feature; use a PSA opaque (secure element) driver and point the TLS stack at the key in the external element. #10643 also says the driver dispatch layer has to be edited manually. * The roadmap lists "PSA driver - Handle Opaque Persistent Key in Secure Element - Implementation" (2026 CQ2) and "PSA Secure Element, Crypto Accelerator Support Enhancements" (Future). == 3. Our approach == 1) Write a PSA opaque driver with our own key location, with a sign_hash function that sends the hash to the TPM and waits for the signature. 2) Create a PSA key ID that refers to the key in the TPM, so that TLS calls psa_sign_hash() with it and the call reaches our driver. 3) Wrap the key ID with mbedtls_pk_wrap_psa() and pass it with the device certificate to mbedtls_ssl_conf_own_cert(). 4) Add our location to the driver dispatch code so that sign_hash calls our function. == 4. Where we have difficulties == We have difficulties creating a suitable key ID that actually works the way we imagine. The key already exists in the TPM, and we only want PSA to hold a reference to it, so that the handshake signature ends up in our driver function. We do not know how to do this correctly. == 5. Our questions == 1) Is our approach a good one for TLS client authentication with a TPM-held key in Mbed TLS 4.2? Do you have tips or pitfalls to share? 2) How should we create the key ID for a key that already exists in an external device? Are there examples, tests or documentation you would point us to? 3) What exactly is covered by the roadmap item "PSA driver - Handle Opaque Persistent Key in Secure Element - Implementation", and what has been done so far? Is it part of TF-PSA-Crypto 1.2 / Mbed TLS 4.2, and does it address our key-ID problem? 4) Is anything planned to make this easier, so that we do not have to edit the dispatch layer and build so much ourselves? Thank you very much for your help. Best regards, Beste Grüße / Best regards, Ruien Karimi Software Developer Phone: +49 89 45 656 - 307 E-Mail: ruien.karimi(a)softing.com<mailto:ruien.karimi@softing.com> [Softing_Logo_signature] Softing Industrial Automation (SIA) Richard-Reitzner-Allee 6 - 85540 Haar - Deutschland Fax +49 89 45656-492 - www.softing.com<http://www.softing.com/> Sitz: Haar bei München, Amtsgericht München, HRB 127 604 Softing Aktiengesellschaft: Vorsitzender des Aufsichtsrates: Dr. Horst Schiessl; Vorstand: Dr. Wolfgang Trier (Vorsitzender), Ernst Homolk Geschäftsführer: Johann Gschwendtner, Thomas Rummel, Dr. Wolfgang Trier

5 days, 15 hours

[Mbed-tls-announce] Announcement: Future Maintenance of Mbed TLS

by Janos Follath via Mbed-tls-announce

Dear Mbed TLS Users, We are preparing for a transition in the maintenance of TLS and X.509 libraries (both contained in the Mbed TLS project). The existing maintainers will continue to develop and maintain TF-PSA-Crypto, which remains an actively maintained part of the project. The existing maintainers will cease their involvement in their development and maintenance of Mbed TLS from April 2027. Unless new maintainers have joined the project and ramped up by that time, security incident response, security fixes, and releases will cease from April 2027. The future maintenance arrangements for the TLS and X.509 libraries have not yet been determined. We welcome contributions from organisations and individuals interested in helping sustain Mbed TLS, whether through engineering resources, funding, or other forms of support. The existing maintainers are committed to supporting new maintainers as they ramp up and take on responsibility for the libraries. If you or your organisation are interested in contributing to the future maintenance of Mbed TLS, please contact mbed-tls-owner(a)lists.trustedfirmware.org. We recognise the importance of these libraries to their users and the wider ecosystem, and hope that, with community involvement, a sustainable path forward can be established. Kind regards, The Mbed TLS Team IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you. -- Mbed-tls-announce mailing list -- mbed-tls-announce(a)lists.trustedfirmware.org To unsubscribe send an email to mbed-tls-announce-leave(a)lists.trustedfirmware.org

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mbed-tls October 2026