Re: [psa-crypto] [mbed-tls] Questions about PSA Crypto API and mbed TLS

-- Gilles Peskine > Best, > > François. > > > On 11/2/20 11:01 PM, Gilles Peskine via mbed-tls wrote: >> Hello, >> >> Thank you for your interest in the PSA crypto API. >> >> On 28/10/2020 15:20, François Beerten via mbed-tls wrote: >>> Hi everybody, >>> >>> After reading the PSA Crypto API specs (as on >>> https://armmbed.github.io/mbed-crypto/html/overview/functionality.html) >>> and looking at the mbed TLS library, a few questions came up. >>> >>> Is there some repository with the sources of the PSA Crypto API specs >>> where one can follow the evolution and eventually send proposals and >>> patches ? >>> >> The PSA specification drafts are not public. You can send feedback about >> the PSA Crypto application and driver interfaces on the psa-crypto >> mailing list (psa-crypto@lists.trustedfirmware.org, >> https://lists.trustedfirmware.org/mailman/listinfo/psa-crypto). If you >> prefer to send confidential feedback, you can email mbed-crypto@arm.com >> (feedback at this address will only be discussed inside Arm). An issue >> in the Mbed TLS repository will also reach PSA Crypto architects. >> >>> A note says "Hash suspend and resume is not defined for the SHA3 >>> family of hash algorithms". Why are they not defined for SHA3 ? >>> >> The hash suspend/resume operations marshall the internal state of the >> hash operation. They mimic an existing JavaCard API >> (https://docs.oracle.com/javacard/3.0.5/api/javacard/security/InitializedMess...)). >> >> There is a de facto standard representation of the internal state for >> common Merkle-Damgård constructions, which covers all the currently >> defined hash algorithms except SHA3. If there's interest in this >> functionality, we could standardize a representation for SHA3. >> >>> How can or should one add support in PSA Crypto AP for not yet defined >>> algorithms (for example a KDF) ? >>> >> Answer from a PSA Crypto architect: preferably by requesting an encoding >> for this KDF as a PSA_ALG_xxx value (as well as new >> PSA_KEY_DERIVATION_INPUT_xxx values if applicable). If you can't do >> that, use an encoding in the vendor range (most significant bit set). >> >> The world of key derivation functions is unfortunately messy: there are >> many similar, but not functionally equivalent constructions (such as >> hashing a secret together with a nonce, formatted in all kinds of >> different ways). The set of KDF in PSA Crypto 1.0.0 was the minimum set >> required for the TLS protocol. We expect 1.0.x updates to define more >> KDF algorithms. >> >> Answer from an Mbed TLS maintainer: contributing an implementation would >> be appreciated (but not required). >> >>> In multipart operations, can the user reuse the input buffers >>> immediately after doing an 'update' (for example after >>> psa_hash_update()) ? And can he reuse the input buffers immediately >>> after some "setup" functions like psa_cipher_set_iv() or >>> psa_aead_set_nonce() ? >>> >> Yes. PSA crypto API functions that take a buffer as a parameter never >> take ownership of that buffer. Once the function returns, you can do >> whatever you want with the buffer. >> >> The PSA specification even guarantees that you can use the same buffer, >> or overlapping buffers, as inputs and outputs to the same function call. >> However beware that the Mbed TLS implementation does not always support >> such overlap (https://github.com/ARMmbed/mbedtls/issues/3266). >> >>> Do you plan to support (pure) ED25519 and ED448 only via >>> psa_sign_message() and psa_verify_message() ? What about messages in >>> multiple chunks ? >>> >> We plan to add a multi-part message signature interface, both for the >> sake of pureEdDSA and suitable for Mbed TLS's restartable ECDSA. I >> expect the design to be “what you'd expect” but I haven't yet verified >> that there aren't any gotchas. >> >>> In psa_asymmetric_encrypt(), why is the salt provided explicitely. >>> Shouldn't it be generated randomly internally when needed ? >>> >> Some applications use a fixed or deterministic salt which they check on >> decryption. Note that this parameter is what PKCS#1 calls “label”. >> >>> With PSA Crypto API, you define a flexible API for cryptographic >>> operations. Apparently, other providers could make their own >>> implementation of PSA Crypto API. Will mbed TLS then be able to use >>> those alternate PSA Crypto API implementations ? How would that work >>> practically ? >>> >> The X.509 and TLS layer of Mbed TLS are currently designed to use the >> mbedtls_xxx crypto API. We have already added partial support for the >> psa_xxx crypto API (with MBEDTLS_USE_PSA_CRYPTO), however it is not yet >> possible to fully decouple the X.509/TLS layers from the Mbed TLS crypto >> implementation. (I think this is already possible for a small set of >> cipher suites, but it isn't something that we've tried or currently >> actively support.) Before this can happen, some Mbed TLS APIs need to >> change, which will happen in 2021 with Mbed TLS 3.0. After that, we plan >> to decouple the PSA crypto reference implementation (Mbed TLS's current >> crypto implementation) from the X.509/TLS layer (which will remain “Mbed >> TLS”). Our plans >> (https://developer.trustedfirmware.org/w/mbed-tls/roadmap/) that far >> into the future are still vague and may change. >> >> Note that for the most common case of wanting a different implementation >> of cryptography, which is to leverage hardware such as accelerators and >> secure elements, PSA is defining a driver interface which is currently >> being implemented in Mbed TLS >> (https://github.com/ARMmbed/mbedtls/blob/development/docs/proposed/psa-driver...). >> >> The driver interface lets you combine mechanisms supported by your >> hardware with Mbed TLS's implementation for mechanisms without hardware >> support. >>