[test] add new fuzz targets for tbor KDF/HMAC/RSA crypto commands

[David Zimmermann (zimmy87)]

Adds four new fuzz targets as well as a fix for a test issue found in fuzz_tbor_attest_key

[Copilot]

Copilot review overview

🟡 Changes recommended

RSA unwrap currently aborts during setup, and several valid RSA/HMAC paths are misclassified or unreachable.

Review effort: Balanced
Findings: 4

Open (4)

• Match encoded operation types, not enum variants · New
• Size RSA operands to the generated modulus · New
• Use algorithm-specific minimum HMAC key lengths · New
• Finalize the partition before retrieving the unwrapping key · New

What changed in this PR

Adds TBOR fuzz coverage for KDF, HMAC, RSA unwrap, and RSA modular exponentiation commands, plus broadens an attestation rejection check.

Changes:

• Adds five crypto fuzz targets and Cargo registrations.
• Updates oversized attestation-key error handling.

File	Description
fuzz/​Cargo.toml	Registers the new fuzz binaries.
fuzz_tbor_attest_key.rs	Accepts device-side oversized-key rejection.
fuzz_tbor_hkdf.rs	Adds HKDF command fuzzing.
fuzz_tbor_kbkdf.rs	Adds KBKDF-equivalent fuzzing through HKDF.
fuzz_tbor_hmac.rs	Adds HMAC fuzzing.
fuzz_tbor_rsa_mod_exp.rs	Adds RSA modular-exponentiation fuzzing.
fuzz_tbor_rsa_unwrap.rs	Adds wrapped-key import fuzzing.

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[Copilot]

Copilot review overview

🔵 Needs a closer look

Valid RSA/ECC unwrap permissions are misclassified, and the KBKDF target only duplicates HKDF coverage.

Review effort: Balanced
Findings: None

Resolved since last review (4)

• Finalize the partition before retrieving the unwrapping key
• Use algorithm-specific minimum HMAC key lengths
• Size RSA operands to the generated modulus
• Match encoded operation types, not enum variants

Previously missed (2)

In code that hasn't changed since last review

Target fuzzes HKDF instead of the advertised KBKDF path

fuzz/​fuzz_targets/​ddi/​tbor/​fuzz_tbor_kbkdf.rs:116

This target does not exercise KBKDF: it sends the same HkdfDerive request as fuzz_tbor_hkdf, whose handler always runs HKDF extract/expand. SP800-108 counter-HMAC KBKDF uses the distinct sp800_108_kdf path (fw/core/lib/src/ddi/mbor/kbkdf_derive.rs:73-80), so mapping label/context to salt/info only duplicates HKDF coverage and leaves the advertised KBKDF behavior unfuzzed. Add a real TBOR KBKDF command/selector, or remove/rename this target so it does not claim that coverage.

Success predicate rejects valid UnwrapKey permission sets

fuzz/​fuzz_targets/​ddi/​tbor/​fuzz_tbor_rsa_unwrap.rs:340

This success predicate rejects two valid permission sets: UnwrapKey also accepts encrypt+decrypt for RSA/RSA-CRT and derive for ECC (fw/core/lib/src/ddi/tbor/unwrap_key.rs:118-125). If Fuzzed produces either set, the firmware correctly succeeds but this target reports a crash. Classify every accepted usage group rather than only the canonical generated one.

[Copilot]

Copilot review overview

🟡 Changes recommended

The implementation adds four fuzz targets despite the PR description promising five.

Review effort: Balanced
Findings: 1

Open (1)

• Add the missing fifth fuzz target or update the PR scope · New

[Copilot]

Copilot review overview

🟡 Changes recommended

The RSA unwrap target’s usage validation conflicts with current firmware behavior and can report a predictable failure.

Review effort: Balanced
Findings: 1

Open (1)

• Handle undefined KeyUsage bits consistently to prevent false fuzz crashes · New

Resolved since last review (1)

• Add the missing fifth fuzz target or update the PR scope

[Copilot]

Copilot review overview

🟢 Approval recommended

The targets align with firmware contracts and the previously identified issues are resolved.

Review effort: Balanced
Findings: None

Resolved since last review (1)

• Handle undefined KeyUsage bits consistently to prevent false fuzz crashes

[Copilot]

Copilot review overview

🟢 Approval recommended

The fuzz targets align with the exercised command contracts, and no unresolved correctness issues were identified.

Review effort: Balanced
Findings: None

[Copilot]

Copilot review overview

🟡 Changes recommended

RSA modular-exponentiation fuzzing misclassifies valid SecurityDomain imports and consequently reports false crashes.

Review effort: Balanced
Findings: 1

Open (1)

• Include provisioned SecurityDomain in import predicate · New

[Copilot]

Copilot review overview

🔵 Needs a closer look

RSA mod-exp setup aborts before exercising several generated-key scope paths.

Review effort: Balanced
Findings: None

Resolved since last review (1)

• Include provisioned SecurityDomain in import predicate

Previously missed (1)

In code that hasn't changed since last review

Provision unwrapping keys for all non-SecurityDomain scopes

fuzz/​fuzz_targets/​ddi/​tbor/​fuzz_tbor_rsa_mod_exp.rs:244

import_rsa_key always calls GetUnwrappingKey, but this setup provisions that key only for Ephemeral, Local, and SecurityDomain. Session, Unspecified, and Internal therefore hit the helper's expect with PendingKeyGeneration before UnwrapKey or RsaModExp is fuzzed. Finalize the partition for every non-SecurityDomain case so both valid Session requests and invalid-scope rejection paths reach the commands under test.

[Copilot]

Copilot review overview

🟢 Approval recommended

The new targets align with firmware contracts and existing fuzz-harness conventions, with no unresolved correctness issues found.

Review effort: Balanced
Findings: None

[Copilot]

🔵 Needs a closer look

The extensive backend-dependent cryptographic fuzz oracles and security-domain setup warrant final human validation.

0 open findings

🧠 Review effort: Balanced