PBKDF2 metadata macros are buggy and untested

Mbed-TLS / mbedtls

An open source, portable, easy to use, readable and flexible TLS library, and reference implementation of the PSA Cryptography API. Releases are on a varying cadence, typically around 3 - 6 months between releases.

https://www.trustedfirmware.org/projects/mbed-tls/

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PBKDF2 metadata macros are buggy and untested #8827

Open gilles-peskine-arm opened 6 months ago

gilles-peskine-arm commented 6 months ago

Test gap: PBKDF2 is missing from test_suite_psa_crypto_metadata.data.
This may cause additional test gaps because we use test_suite_psa_crypto_metadata.data as one of the ways to detect available mechanisms for systematically generated test cases.
Bug (originally reported internally against the PSA API specification, which inherited sample definitions of macros from Mbed TLS): PSA_ALG_KEY_AGREEMENT_GET_KDF() and PSA_ALG_KEY_AGREEMENT_GET_BASE() don't give the expected results for PSA_ALG_KEY_AGREEMENT(PSA_ALG_ECDH, PSA_ALG_PBKDF2_xxx).

gilles-peskine-arm commented 6 months ago

For test_suite_psa_crypto_metadata.data:

Key derivation: PBKDF2 using HMAC-SHA-256
depends_on:PSA_WANT_ALG_SHA_256:PSA_WANT_ALG_PBKDF2_HMAC
key_derivation_algorithm:PSA_ALG_PBKDF2_HMAC(PSA_ALG_SHA_256):ALG_IS_PBKDF2_HMAC:ALG_IS_PBKDF2

Key derivation: PBKDF2 using HMAC-SHA-384
depends_on:PSA_WANT_ALG_SHA_384:PSA_WANT_ALG_TLS12_PSK_TO_MS
key_derivation_algorithm:PSA_ALG_PBKDF2_HMAC(PSA_ALG_SHA_384):ALG_IS_PBKDF2_HMAC:ALG_IS_PBKDF2

Key derivation: PBKDF2 using AES-128-CMAC
depends_on:PSA_WANT_ALG_PBKDF2_AES_CMAC_PRF_128
key_derivation_algorithm:PSA_ALG_PBKDF2_AES_CMAC_PRF_128:ALG_IS_PBKDF2

Add the two new ALG_IS_xxx flags to the .function file.

gilles-peskine-arm commented 6 months ago

Note that it doesn't really make sense to chain a key agreement with PBKDF2: PBKDF2 is generally meant for passwords (low entropy), not key agreement shared secrets (high entropy but not fully uniform). However, there may be protocols that use PBKDF2 with a single iteration as a KDF because they're also using PBKDF2 with many iterations to process passwords and they want to minimize the number of primitives. So arguably we should test the chaining of a key agreement with PBKDF2.