fwTPM Post-Quantum Support (TPM 2.0 v1.85)

The fwTPM implements the post-quantum additions from TCG TPM 2.0 Library Specification v1.85, using wolfCrypt's FIPS 203 (ML-KEM) and FIPS 204 (ML-DSA) modules. This brings post-quantum keys, signing, and key encapsulation to platforms that have no TPM silicon. These v1.85 commands raise the implemented count from 103 to 111 commands. For the library-wide post-quantum view, see Post-Quantum.

Enable it with --enable-pqc, which the fwTPM build promotes to the full --enable-v185, at configure time. It is also auto-detected when --enable-fwtpm is built against a wolfCrypt that has both ML-DSA and ML-KEM available. Both flags set the internal WOLFTPM_V185 macro that gates the implementation. Pass --disable-pqc to opt out when auto-detect would otherwise enable it.

Algorithms

Alg	Parameter Sets	Use
TPM_ALG_MLKEM (0x00A0)	ML-KEM-512 / 768 / 1024	Key encapsulation (decrypt-only keys)
TPM_ALG_MLDSA (0x00A1)	ML-DSA-44 / 65 / 87	Pure ML-DSA message signing
TPM_ALG_HASH_MLDSA (0x00A2)	HashML-DSA-44 / 65 / 87	Pre-hashed ML-DSA signing

Commands

The eight v1.85 PQC commands live in src/fwtpm/fwtpm_command.c:

Command	CC	Purpose
TPM2_Encapsulate	0x000001A7	ML-KEM encapsulation, returns sharedSecret and ciphertext
TPM2_Decapsulate	0x000001A8	ML-KEM decapsulation from ciphertext (requires USER auth)
TPM2_SignSequenceStart	0x000001AA	Begin ML-DSA sign sequence
TPM2_SignSequenceComplete	0x000001A4	Finalize sign sequence with message buffer
TPM2_VerifySequenceStart	0x000001A9	Begin ML-DSA verify sequence
TPM2_VerifySequenceComplete	0x000001A3	Finalize verify sequence, returns TPMT_TK_VERIFIED
TPM2_SignDigest	0x000001A6	One-shot digest sign (HashML-DSA or ext-mu ML-DSA)
TPM2_VerifyDigestSignature	0x000001A5	Verify digest signature

Primary Key Derivation

PQC primary keys follow the same deterministic derivation model as RSA and ECC: hierarchy seed and template, then a KDFa-derived seed, then FIPS 203 or FIPS 204 key expansion.

• ML-DSA: KDFa(nameAlg, seed, "MLDSA", hashUnique) gives a 32-byte Xi. wc_MlDsaKey_MakeKeyFromSeed turns that into the public key and the expanded private key. The wire format stores only the 32-byte Xi per TCG Part 2 Table 210.
• HashML-DSA: the label is "HASH_MLDSA", with the same seed size and expansion.
• ML-KEM: KDFa(nameAlg, seed, "MLKEM", hashUnique) gives a 64-byte value (d followed by z). wc_MlKemKey_MakeKeyWithRandom turns that into the encapsulation and decapsulation keys. The wire format stores only the 64-byte seed per TCG Part 2 Table 206.

Note

These label strings are an interpretation. TCG Part 4 v185, which would specify them normatively, is unpublished. They are subject to change if a later release candidate or Part 4 v185 prescribes different labels.

Sign and Verify Sequences

Pure ML-DSA sequences are streamable on both sign and verify, so TPM2_SequenceUpdate is accepted. TPM_RC_ONE_SHOT_SIGNATURE applies to multi-pass schemes such as EdDSA, not to pure ML-DSA. A caller can also pass the whole message through the buffer parameter of TPM2_SignSequenceComplete. Verify sequences accumulate the message through TPM2_SequenceUpdate, because TPM2_VerifySequenceComplete has no buffer parameter.

HashML-DSA sequences (both sign and verify) use wolfCrypt's wc_HashAlg context to stream the message into the key's hash algorithm. TPM2_SignSequenceComplete finalizes the hash and calls wc_MlDsaKey_SignCtxHash.

Signature wire formats differ per spec Part 2 Table 217:

• Pure ML-DSA: TPM2B_SIGNATURE_MLDSA, laid out as sigAlg + size + bytes
• HashML-DSA: TPMS_SIGNATURE_HASH_MLDSA, laid out as sigAlg + hashAlg + size + bytes

Buffer Constants

Under WOLFTPM_V185, buffers are lifted to fit ML-DSA-87 signatures (4627 bytes) and public keys (2592 bytes):

Symbol	v1.38	v1.85
FWTPM_MAX_COMMAND_SIZE	4096	8192
FWTPM_MAX_PUB_BUF	512	2720
FWTPM_MAX_DER_SIG_BUF	256	4736
FWTPM_MAX_KEM_CT_BUF	n/a	1600
FWTPM_TIS_FIFO_SIZE	4096	8192
FWTPM_NV_PUBAREA_EST	600	2720

These are the worst-case values. The defaults shrink at compile time to match the parameter sets wolfCrypt was built with. See "v1.85 Embedded RAM Impact" in Building for the per-parameter-set table.

Limitations and Scope

The v1.85 commands are implemented for post-quantum keys only. Non-PQC key types are rejected with TPM_RC_KEY or TPM_RC_SCHEME, even when the v1.85 spec defines the commands generically:

• TPM2_Encapsulate and TPM2_Decapsulate: ML-KEM only. ECC DHKEM (the Table 100 ecdh arm with a non-NULL KDF) is not implemented.
• TPM2_SignSequenceStart, TPM2_VerifySequenceStart, TPM2_SignSequenceComplete, and TPM2_VerifySequenceComplete: ML-DSA and HashML-DSA only. Classical schemes (RSASSA, RSAPSS, ECDSA, SM2, ECSCHNORR, HMAC) that the spec also permits through these commands are not supported.
• TPM2_SignDigest and TPM2_VerifyDigestSignature: ML-DSA and HashML-DSA only. Classical digest signing (RSASSA, RSAPSS, ECDSA) over these new commands is not supported. Use the existing TPM2_Sign and TPM2_VerifySignature commands for those schemes.

Deferred and Out of Scope

Three v1.85 features are deferred, each for a documented reason:

1. ML-KEM-salted sessions. Part 3 Sec.11.1 (TPM2_StartAuthSession) does not describe an ML-KEM bullet alongside the RSA-OAEP and ECDH paths, even though Part 2 Sec.11.4.2 Table 222 defines the mlkem arm of TPMU_ENCRYPTED_SECRET. Part 4 v185, which would specify this normatively, is not yet published. Current behavior: TPM2_StartAuthSession returns TPM_RC_KEY for an ML-KEM tpmKey. Revisit when Part 4 v185 is published.
2. External-mu ML-DSA signing. wolfCrypt has no mu-direct sign API. Part 2 Sec.12.2.3.7 says "512-byte external Mu", but FIPS 204 Algorithm 7 Line 6 produces 64 bytes (SHAKE256 output). This is pending a wolfCrypt API addition and TCG errata confirmation. Current behavior: TPM_RC_SCHEME for ext-mu paths, and TPM_RC_EXT_MU for Pure ML-DSA keys without allowExternalMu.
3. ECC KEM arm of Encapsulate and Decapsulate. Part 2 Sec.10.3.13 Table 100 has both mlkem and ecdh arms, but the table note allows implementations to modify the union based on the algorithms they support. The fwTPM supports the mlkem arm only.

Test Coverage

tests/fwtpm_unit_tests.c includes ten PQC tests that exercise the full path:

• CreatePrimary for ML-KEM-768 and ML-DSA-65
• Full Encapsulate and Decapsulate round-trip (shared secret byte match)
• HashML-DSA SignDigest and VerifyDigestSignature round-trip
• Pure ML-DSA sign sequence and verify sequence round-trip
• Dual-source known-answer tests (NIST ACVP and wolfSSL internal vectors) for ML-DSA-44 verify, ML-DSA-44 keygen determinism, ML-KEM-512 encapsulation with pinned randomness, and ML-KEM-512 keygen determinism
• LoadExternal of a NIST ACVP ML-DSA-44 public key through the fwTPM handler

See Also

• Overview
• Building
• Usage
• SPDM Responder
• Post-Quantum (library-wide)