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feat(sdk): support ML-KEM rewrap session keys - #388
Part of DSPX-4221 (Session Keys should support ML-KEM). KASClient.unwrap() only branched EC vs RSA for the client's ephemeral rewrap "session key" (as opposed to a KAS-managed wrapping key), so requesting an ML-KEM session key would fail. KeyType already had MLKEM768Key/MLKEM1024Key entries and sdk-pqc-bc already had the BouncyCastle KEM primitives (used for the KAS-managed mechanism-key path), but nothing generated a fresh ML-KEM keypair for the session-key role.
Changes
spi/KemProvider: add generateSessionKey(KeyType), returning a KemSessionKey: the ephemeral rewrap session key. It exposes only the public key (SPKI PEM), unwraps the KAS response itself, and erases its private key on close(). It's a default method that throws UnsupportedOperationException, so existing third-party providers keep compiling and loading. The core sdk module has no compile-time dependency on BouncyCastle (per ADR 0001), so generation lives in the optional sdk-pqc-bc module and is reached via the existing ServiceLoader-based KemProviders registry — same pattern already used for wrapDEK/unwrapDEK.
sdk-pqc-bc: HybridCrypto.generateSessionKey dispatches over all five supported PQC key types (X-Wing, both NIST hybrids, ML-KEM-768/1024), mirroring the wrapDEK/unwrapDEK switch. The new BouncyCastleKemSessionKey holds the raw private key bytes and zeroes them on close(); the private key is never PEM-encoded or held as a String. BouncyCastleKemProvider implements the new SPI method.
KASClient.unwrap(): branch on sessionKeyType.isMLKEM(), generating a session key via KemProviders.get(...).generateSessionKey(...), unwrapping the response with it, and closing it when the rewrap finishes. The long-lived RSA session keypair is now published through a single immutable holder, so a concurrent first caller can't see the private key without its public PEM.
cmdline supports: reports session-key-mlkem only when a provider can actually generate ML-KEM-768 and ML-KEM-1024 session keys (probed at runtime), so the fips build, which excludes sdk-pqc-bc, reports it as unsupported.
New unit tests: HybridCryptoSessionKeyTest round-trips generateSessionKey -> wrapDEK -> KemSessionKey.unwrapDEK for all five PQC key types and checks that a different key, a tampered KEM ciphertext, a tampered sealed DEK, and a closed key all fail. KASClientTest covers concurrent first RSA unwraps and the missing-provider error; KemProviderTest covers the SPI default.
cmdline's --rewrap-key-type already accepts mlkem:768/mlkem:1024 with no code changes needed: picocli's enum candidate list is derived from KeyType#toString(), which already returns "mlkem:768"/"mlkem:1024". Verified by building cmdline.jar and checking help decrypt output.
Test plan
mvn verify
mvn test -P 'fips,!non-fips'
Both pass. cmdline.jar from the default build reports session-key-mlkem via supports; the fips build does not. Cross-SDK coverage comes from opentdf/tests#571's test_session_key_roundtrip, which runs mlkem:768 and mlkem:1024 with Java decrypting when X-Test is dispatched with java-ref: DSPX-4221-pq-sessions.
The PR adds PEM key-pair generation to the KEM provider API and Bouncy Castle implementation. KASClient.unwrap now handles EC, ML-KEM, and RSA session keys. The supports command reports ML-KEM session-key support only when an ML-KEM-768 provider is registered.
Changes: KEM key generation and ML-KEM unwrap
Layer / File(s)
Summary
KEM key-generation contract and implementation sdk/src/main/java/io/opentdf/platform/sdk/spi/KemProvider.java, sdk-pqc-bc/src/main/java/io/opentdf/platform/sdk/pqc/bc/..., sdk-pqc-bc/src/test/java/io/opentdf/platform/sdk/pqc/bc/HybridCryptoGenerateKeyPairTest.java, sdk/src/test/java/io/opentdf/platform/sdk/spi/KemProviderTest.java
KemProvider adds a default key-pair generation method and a PEM key-pair type. HybridCrypto dispatches supported key types, and BouncyCastleKemProvider delegates to it. Tests cover generation, wrap/unwrap, and rejection cases.
KAS session-key generation and unwrap sdk/src/main/java/io/opentdf/platform/sdk/KASClient.java, sdk/src/test/java/io/opentdf/platform/sdk/KASClientTest.java
KASClient creates EC or ML-KEM key material per unwrap and reuses a lazily created RSA keypair. Response-key processing now has separate EC, ML-KEM, and RSA paths. Tests cover concurrent RSA unwraps and missing ML-KEM provider handling.
sequenceDiagram
participant KASClient
participant KemProviders
participant BouncyCastleKemProvider
participant KAS
KASClient->>KemProviders: resolve ML-KEM provider
KASClient->>BouncyCastleKemProvider: generate key pair
BouncyCastleKemProvider-->>KASClient: return public and private PEM
KASClient->>KAS: send public PEM in rewrap request
KAS-->>KASClient: return wrapped response key
KASClient->>BouncyCastleKemProvider: unwrap using private PEM
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Possibly related PRs
opentdf/java-sdk#420: Defines the existing command-line support feature names that this PR conditionally extends with session-key-mlkem.
Suggested reviewers:sujankota
Merge Risk: Low · up to 6fab5
With a wrap-only ML-KEM provider, the CLI can claim session-key support even though unwrap fails before contacting KAS. This is a bounded issue to fix or accept before merging.
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A rabbit checks the keys in view, A public PEM and private too. ML-KEM joins the unwrap flow, While RSA keys share one show. The feature list reflects what’s there.
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Inline comments:
In `@sdk/src/main/java/io/opentdf/platform/sdk/KASClient.java`:
- Around line 126-133: Update the unwrap flow around the ML-KEM branch to keep
the generated public key in a local request-scoped variable instead of shared
clientPublicKey. Use that local variable when assigning body.clientPublicKey,
while preserving the existing EC behavior and each call’s private-key pairing.
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In `@sdk/src/main/java/io/opentdf/platform/sdk/KASClient.java`:
- Around line 136-153: Make the RSA lazy initialization in
generateSessionKeyMaterial thread-safe using double-checked locking: retain the
fast null check, synchronize the initialization block on the appropriate shared
lock, and recheck decryptor before generating keys. Mark both decryptor and
clientPublicKey volatile so the initialized key pair is safely published and the
public/private keys remain consistent across concurrent unwrap calls.
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Inline comments:
In `@cmdline/src/main/java/io/opentdf/platform/Command.java`:
- Line 80: Update the FEATURES construction in Command so session-key-mlkem is
included only when the KemProvider registry reports an ML-KEM provider is
registered, using the existing KemProviders.registered() mechanism. Keep dpop
and dpop_nonce_challenge unconditional, and avoid relying on sdk-pqc-bc build
profiles or classpath presence.
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Pull request overview
Adds ML-KEM (FIPS 203) support for the client-generated “session key” path used in KAS rewrap/unwrap, by extending the KEM SPI to allow ephemeral keypair generation and wiring that into KASClient.unwrap(). This keeps the core sdk module provider-agnostic (no BC dependency) while enabling PQC behavior via the existing ServiceLoader registry.
Changes:
Extend KemProvider SPI with generateKeyPair(KeyType) and implement it in sdk-pqc-bc.
Update KASClient.unwrap() to generate ML-KEM session keypairs via KemProviders and unwrap responses via unwrapDEK.
Add PQC unit test coverage for generateKeyPair -> wrapDEK -> unwrapDEK round-trips, and update cmdline supports feature reporting.
Reviewed changes
Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.
I came across this PR while looking at ML-KEM adoption across Java libraries and read the rewrap path against FIPS 203 and the envelope format. The core choices look right: the ML-KEM session key is a fresh ephemeral per unwrap() call (matching the EC path's per-call ECKeyPair, which is the property you want for a rewrap session key); the envelope parsing in HybridCrypto is strict (canonical DER lengths, trailing bytes rejected, bounds-checked reads); KemProviders.get() fails with a clear SDKException when the sdk-pqc-bc module is absent rather than an NPE; and the new round-trip test covers all five key types. Using the FIPS 203 shared secret directly as the wrap key for pure ML-KEM is sound (it is a uniform 32-byte key by construction) and the ADR reference makes the intent clear.
One real issue and two smaller notes.
1. The new double-checked locking leaves a read-side race on clientPublicKey (low, fail-closed). In generateSessionKeyMaterial the initializing thread writes decryptor first and clientPublicKey second (KASClient.java:157-158), but the fast path reads decryptor at :153 and, when it is non-null, returns clientPublicKey at :162 without entering the lock. A thread that arrives between the two writes sees decryptor != null, skips the synchronized block, and returns a SessionKeyMaterial whose publicKeyPem is still null, so the rewrap request is built with body.clientPublicKey = null (:202). This is plain interleaving, not a memory-model corner case, and the window is wider than one instruction: it spans the whole Base64 PEM encoding on the right-hand side of :158. I checked it with a two-thread harness mirroring the exact field and write structure, forcing a reader into that window: it read the null public key in 19,677 of 20,000 forced races, and with the two writes swapped (so the volatile publish of decryptor covers the PEM) it read null in 0 of 20,000. The forced rate is not the production frequency, which depends on thread timing, but it isolates the write-order as the cause and confirms the swap removes it. The window only exists around first use of the RSA path and the result is a rejected request rather than anything unsafe, so this is low severity, but it is the exact class of mixed-state send the comment above the block says the locking is there to prevent. The minimal fix is to swap the two writes (writing clientPublicKey before decryptor makes the decryptor volatile read at :153 guarantee visibility of the PEM); returning the PEM from inside the synchronized block, or storing both fields in a single volatile holder object, also works and is harder to regress.
2. The SPI freezes the private key as an immutable String (design note, worth deciding now).KemProvider.KeyPairPem (KemProvider.java:63-70) carries the ephemeral ML-KEM private key as a PEM String. Strings cannot be zeroized, so the decapsulation key lives on the heap until GC with no way to wipe it after the unwrap completes. The EC path has the same property today via key objects, so this is not a regression, but since this PR is the moment KeyPairPem enters the SPI contract, it is the cheapest time to choose byte[]/char[] (with a wipe after use) over String if you ever want that option; changing it later means breaking the interface.
3. Negative tests (small).HybridCryptoGenerateKeyPairTest round-trips the happy path. Given the parser already has strict failure modes, two cheap additions would pin the security-relevant behavior: unwrap with a different key pair of the same type must fail, and an envelope with a flipped byte in the ciphertext must fail rather than return a DEK. Both are a few lines in the same parameterized style.
The strict envelope parser and the per-call ephemeral are the parts that are easy to get wrong in a rewrap flow, and both look right here.
Part of DSPX-4221 (Session Keys should support ML-KEM). KASClient.unwrap()
only branched EC vs RSA for the client's ephemeral rewrap "session key",
so requesting an ML-KEM session key would fail. KeyType already had
MLKEM768Key/MLKEM1024Key entries and sdk-pqc-bc already had the
BouncyCastle KEM primitives (used for KAS-managed mechanism keys), but
nothing generated a fresh ML-KEM keypair for the session-key role.
- spi/KemProvider: add generateKeyPair(KeyType), returning a new
KeyPairPem (PEM-encoded SPKI/PKCS#8 pair). The core sdk module has no
compile-time dependency on BouncyCastle, so the actual keypair
generation lives in the optional sdk-pqc-bc module and is reached via
the existing ServiceLoader-based KemProviders registry.
- sdk-pqc-bc/HybridCrypto: add a generateKeyPair dispatch mirroring the
existing wrapDEK/unwrapDEK switch, covering all five supported PQC key
types (X-Wing, both NIST hybrids, ML-KEM-768/1024).
- sdk-pqc-bc/BouncyCastleKemProvider: implement the new SPI method.
- KASClient.unwrap(): branch on sessionKeyType.isMLKEM(), generating a
keypair via KemProviders.get(...).generateKeyPair(...) and decrypting
the response with KemProviders.get(...).unwrapDEK(...).
- New unit test (HybridCryptoGenerateKeyPairTest) round-trips
generateKeyPair -> wrapDEK -> unwrapDEK for all five PQC key types.
- cmdline: add "session-key-mlkem" to the `supports` subcommand's
feature list, gated on KemProviders.registered() actually containing an
ML-KEM provider rather than being unconditionally true (so a FIPS-profile
build, which excludes sdk-pqc-bc, correctly reports it as unsupported
instead of claiming a capability that would throw at runtime).
Addresses CodeRabbit review feedback:
- unwrap() previously wrote the generated session public key to the shared
clientPublicKey instance field before serializing the request. A
concurrent unwrap() call on the same KASClient could overwrite that field
first, causing this call to send the wrong public key while decrypting
with its own private key. Refactored so each call's session-key material
(public PEM plus whichever private key it needs for the response) is a
call-scoped SessionKeyMaterial value, threaded explicitly through
generateSessionKeyMaterial()/unwrapResponseKey() rather than shared
mutable fields. This split also resolves a SonarCloud cognitive-complexity
(java:S3776) failure on unwrap() from combining request-build and
response-decrypt branching in one method.
- Follow-up on the same class of bug: the RSA branch's lazy
keypair-initialization was still a check-then-act race on non-volatile
fields (decryptor/clientPublicKey), so two concurrent first callers could
generate distinct keypairs and race to overwrite each other's fields.
Fixed with double-checked locking and volatile fields.
- Command.Supports' FEATURES list unconditionally claimed "session-key-mlkem"
regardless of whether an ML-KEM KemProvider is actually registered (e.g.
under the fips Maven profile, which excludes sdk-pqc-bc). Now computed
dynamically via KemProviders.registered().
Ref: DSPX-4221
Signed-off-by: Dave Mihalcik <dmihalcik@virtru.com>
…s present
The mavenverify job runs the cmdline tests only in its fips step
(`mvn test -P 'fips,!non-fips'`); the non-fips coverage step builds just
the sdk module. The fips profile leaves sdk-pqc-bc off cmdline's
classpath, so `supports` correctly omits session-key-mlkem there, but
CommandTest expected it unconditionally and failed
supports_noArgs_listsFeatures_exits0 and supports_noArgs_json_listsFeatures.
Make the expectation depend on whether sdk-pqc-bc's
BouncyCastleKemProvider class is on the test classpath. The probe is a
plain class lookup, not KemProviders, so a provider jar whose
ServiceLoader registration is broken still fails the test. Also add a
direct `supports session-key-mlkem` exit-code test, the probe the
opentdf/tests xtest suite relies on.
Signed-off-by: Dave Mihalcik <dmihalcik@virtru.com>
Review follow-up (Copilot; Arpan0995's issue 1). The double-checked
locking added for the lazily generated RSA session keypair wrote the
volatile `decryptor` before `clientPublicKey`. A thread arriving between
the two writes saw a non-null decryptor, skipped the lock, and built a
rewrap request with a null client public key. The window spans the PEM
encoding of the public key, and only exists around first RSA use.
Keep the decryptor and its public PEM in one immutable RsaSessionKey,
published through a single AtomicReference, so any thread that sees the
keypair sees both halves. Initialization still double-checks, but now on
a dedicated lock instead of `this`, so a first-use RSA keygen no longer
blocks getStub() and close(). The decryptor is carried in the per-call
SessionKeyMaterial with the PEM it matches, rather than re-read from a
field when the response is processed.
Add a KASClient test that releases eight concurrent first-time RSA
unwraps at once and asserts they all succeed and send one non-null public
key. Without the lock it fails with eight different keys. It cannot
reliably hit the old write-order window; the single-reference publish
rules that out by construction.
Signed-off-by: Dave Mihalcik <dmihalcik@virtru.com>
Review follow-up (Copilot). KemProvider is a public SPI published in the
sdk artifact, so a new abstract method breaks source compatibility for
any external provider and leaves already-compiled ones open to
AbstractMethodError. Make generateKeyPair a default method that throws
UnsupportedOperationException naming the provider and key type.
BouncyCastleKemProvider already overrides it.
KASClient wraps that exception in an SDKException, so callers that
request an ML-KEM session key see the SDK's usual exception type
whether the provider is missing (KemProviders.get) or can't generate
keypairs.
Signed-off-by: Dave Mihalcik <dmihalcik@virtru.com>
Review follow-up (Arpan0995's note 3). HybridCryptoGenerateKeyPairTest
covered only the happy path. For every hybrid and pure ML-KEM key type,
add three cases that must throw instead of returning a DEK:
- unwrapping with a different generated keypair of the same type
- a flipped byte inside the KEM ciphertext (ML-KEM implicit rejection
gives a wrong wrap key, so AES-GCM fails)
- a flipped byte in the AES-GCM sealed DEK
The tampered envelopes are re-marshalled so they still parse, which
exercises the decapsulation and AEAD paths, not the ASN.1 parser. The
key-type list now comes from KeyType.isHybrid()/isMLKEM(), so a new PQC
type can't skip these tests.
Also add a KASClient test: with no KemProvider on the classpath (the
fips deployment shape), requesting an ML-KEM session key fails with the
actionable SDKException before any rewrap request is sent.
Signed-off-by: Dave Mihalcik <dmihalcik@virtru.com>
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Actionable comments posted: 1
🪄 Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @cmdline/src/main/java/io/opentdf/platform/Command.java:
- Around line 83-99: Update Command.features() so session-key-mlkem is
advertised only when a registered ML-KEM provider can also generate key pairs,
not merely when KemProviders.registered() lists its key type. Add or reuse a
capability check that distinguishes providers whose generateKeyPair() is
unsupported, and use that check alongside the registered-key-type check.
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Replace KemProvider.generateKeyPair / KeyPairPem with generateSessionKey,
which returns a KemSessionKey. It exposes only the public key PEM, unwraps
the rewrap response itself, and erases its private key on close().
KASClient closes it once the rewrap finishes.
The private key used to round-trip through a PKCS#8 PEM String only to be
parsed back into raw bytes for the unwrap. It now stays as raw bytes owned
by the provider and is zeroed after use. generateKeyPair and KeyPairPem
were added by this PR and never released, so nothing else depends on
them. unwrapDEK(String) is unchanged; it has no production callers left.
`supports session-key-mlkem` now probes by generating ML-KEM-768 and
ML-KEM-1024 session keys, so a provider that is registered for those types
but cannot generate session keys no longer makes it report support.
Signed-off-by: Dave Mihalcik <dmihalcik@virtru.com>
@Arpan0995 Thanks for the careful read — all three are addressed:
The RSA session keypair is now published through a single immutable holder behind an AtomicReference, so a thread can never see the private key without its public PEM (11a0d2d).
Took you up on deciding it now. generateKeyPair/KeyPairPem are gone; generateSessionKey returns a KemSessionKey that exposes only the public PEM, unwraps the response itself, and zeroes its raw private key on close(), which KASClient calls when the rewrap finishes. The private key is never PEM-encoded or held as a String. One caveat: BouncyCastle's key parameter objects copy the seed internally and can't be wiped, so those copies are left for GC once the unwrap returns. unwrapDEK(KeyType, String, byte[]) is unchanged and no longer has production callers; if a client-side decap path for KAS-managed keys materializes, we'll add an erasable variant there and deprecate the String one (2454b82).
Added the negative tests: a different key of the same type, a flipped KEM-ciphertext byte, and a flipped sealed-DEK byte all throw for all five PQC types, and a closed session key refuses to unwrap (6fab500, 2454b82).
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Summary
Part of DSPX-4221 (Session Keys should support ML-KEM).
KASClient.unwrap()only branched EC vs RSA for the client's ephemeral rewrap "session key" (as opposed to a KAS-managed wrapping key), so requesting an ML-KEM session key would fail.KeyTypealready hadMLKEM768Key/MLKEM1024Keyentries andsdk-pqc-bcalready had the BouncyCastle KEM primitives (used for the KAS-managed mechanism-key path), but nothing generated a fresh ML-KEM keypair for the session-key role.Changes
spi/KemProvider: addgenerateSessionKey(KeyType), returning aKemSessionKey: the ephemeral rewrap session key. It exposes only the public key (SPKI PEM), unwraps the KAS response itself, and erases its private key onclose(). It's adefaultmethod that throwsUnsupportedOperationException, so existing third-party providers keep compiling and loading. The coresdkmodule has no compile-time dependency on BouncyCastle (per ADR 0001), so generation lives in the optionalsdk-pqc-bcmodule and is reached via the existingServiceLoader-basedKemProvidersregistry — same pattern already used forwrapDEK/unwrapDEK.sdk-pqc-bc:HybridCrypto.generateSessionKeydispatches over all five supported PQC key types (X-Wing, both NIST hybrids, ML-KEM-768/1024), mirroring thewrapDEK/unwrapDEKswitch. The newBouncyCastleKemSessionKeyholds the raw private key bytes and zeroes them onclose(); the private key is never PEM-encoded or held as aString.BouncyCastleKemProviderimplements the new SPI method.KASClient.unwrap(): branch onsessionKeyType.isMLKEM(), generating a session key viaKemProviders.get(...).generateSessionKey(...), unwrapping the response with it, and closing it when the rewrap finishes. The long-lived RSA session keypair is now published through a single immutable holder, so a concurrent first caller can't see the private key without its public PEM.cmdline supports: reportssession-key-mlkemonly when a provider can actually generate ML-KEM-768 and ML-KEM-1024 session keys (probed at runtime), so thefipsbuild, which excludessdk-pqc-bc, reports it as unsupported.HybridCryptoSessionKeyTestround-tripsgenerateSessionKey -> wrapDEK -> KemSessionKey.unwrapDEKfor all five PQC key types and checks that a different key, a tampered KEM ciphertext, a tampered sealed DEK, and a closed key all fail.KASClientTestcovers concurrent first RSA unwraps and the missing-provider error;KemProviderTestcovers the SPI default.cmdline's--rewrap-key-typealready acceptsmlkem:768/mlkem:1024with no code changes needed: picocli's enum candidate list is derived fromKeyType#toString(), which already returns"mlkem:768"/"mlkem:1024". Verified by buildingcmdline.jarand checkinghelp decryptoutput.Test plan
Both pass.
cmdline.jarfrom the default build reportssession-key-mlkemviasupports; thefipsbuild does not. Cross-SDK coverage comes from opentdf/tests#571'stest_session_key_roundtrip, which runsmlkem:768andmlkem:1024with Java decrypting when X-Test is dispatched withjava-ref: DSPX-4221-pq-sessions.Related work
Companion PRs:
test_session_key_roundtrip) gated behind a newsession-key-mlkemfeature flag.rewrap, and adds the equivalent Go SDK branch.Ref: DSPX-4221
Summary by CodeRabbit
New Features
supportscommand to report ML-KEM session-key support when available.Bug Fixes