fix(sdk): reject GMAC root signatures instead of trusting the manifest (DSPX-4703) - #1031
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fix(sdk): reject GMAC root signatures instead of trusting the manifest (DSPX-4703)#1031dmihalcik-virtru wants to merge 1 commit into
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📝 WalkthroughWalkthroughThe change separates root and segment integrity algorithms, adds fail-closed validation, preserves stream errors during buffering, and adds coverage for current and legacy TDF integrity behavior. ChangesIntegrity algorithm separation
Stream error preservation
Priority: ⬆️ High Estimated code review effort: 4 (Complex) | ~45 minutes Change: Bug fix · Severity of issue fixed: Medium Suggested reviewers: Sequence Diagram(s)sequenceDiagram
participant Client
participant tdf.ts
participant Manifest
participant WebCryptoService
Client->>tdf.ts: decrypt TDF
tdf.ts->>Manifest: read integrity algorithms and root signature
tdf.ts->>WebCryptoService: compute root HMAC
WebCryptoService-->>tdf.ts: return computed integrity value
tdf.ts->>tdf.ts: validate root and segment integrity
tdf.ts-->>Client: return plaintext or IntegrityError
Merge Risk: 🟠 High · up to Valid legacy 4.2.2 HS256 files can fail decryption, so compatibility should be restored before merge. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches 💡 1📝 Generate docstrings 💡
🧪 Generate unit tests (beta)
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…t (DSPX-4703) A segment's `GMAC` hash is the AES-GCM tag the AEAD just computed over that segment's ciphertext, so reading the trailing 16 bytes back out is legitimate. The root signature covers the aggregate hash — a concatenation of segment hashes that AES-GCM never processed — so applying the same trailing-bytes extraction there returns a keyless copy of the last segment hash rather than anything authenticated. Because `rootSignature.alg` is read from the unauthenticated manifest and unknown values were coerced to HS256, an attacker with no key could rewrite `alg` to `GMAC`, recompute the "signature" from the segment hashes they were handing over anyway, and then truncate, reorder, or duplicate segments undetected. AEAD tags bind no ordering, index, or count, so the ordered segment list is exactly what only the root signature protects. Split the one overloaded `getSignature` into four functions that say what they authenticate — `segmentIntegrity`/`rootIntegrity` and their 4.2.2 hex-then-base64 counterparts — so the aggregate hash no longer has a code path that can read a tag out of it. `segmentIntegrity` accepts HS256 and GMAC; `rootIntegrity` accepts only HS256. On read, `asRootIntegrityAlgorithm` fails closed: anything other than HS256, in any casing, raises an `IntegrityError` rather than being coerced. `asSegmentIntegrityAlgorithm` stays permissive about casing, since existing writers emit both spellings. Manifest types widen `rootSignature.alg` and `segmentHashAlg` to plain strings to reflect that they are attacker-controlled until validated. Also stop `streamToBuffer` from laundering the error it now raises. It drained the plaintext with `new Response(stream).arrayBuffer()`, and Chrome replaces *any* error raised while it pulls a Response body with a bare `TypeError: Failed to fetch`. So in the browser — and only there — a tampered payload arrived at the caller indistinguishable from a dropped connection, which is precisely the distinction this commit exists to make: the first is an attack and the second is a retry. Draining with a reader instead preserves the `IntegrityError`. `toString` went through `Response.text()` and had the same problem; it now buffers first. Verified against the previous commit as a baseline: with this commit's tests run on the unfixed code, 14 pass and 9 fail — every case under `controls` stays green while all 8 `exploit: GMAC root downgrade` cases and the lowercase-`segmentHashAlg` case fail. The `streamToBuffer` fix is pinned the same way: reverting it alone turns 409 karma passes into 6 failures — the two payload-tampering cases plus the four error-propagation cases in `tests/mocha/unit/decorated-readable-stream.spec.ts` — while Node stays green, since the masking only ever reproduced in Chrome. With both fixes the whole suite is green: 409 passing / 6 pending under mocha, 409 under karma, and 253 under web-test-runner. Signed-off-by: Dave Mihalcik <dmihalcik@virtru.com>
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Inline comments:
In `@lib/tdf3/src/tdf.ts`:
- Line 1220: Update the segment integrity calculation around segmentIntegrity to
use segmentIntegrityVersion422 when isTargetSpecLegacyTDF(specVersion) is true,
preserving segmentIntegrity for current TDFs. Compare the legacy function’s
base64-encoded hex result directly against the stored signature so legacy HS256
files validate consistently.
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| /** | ||
| * Normalize a manifest-declared root algorithm, failing *closed*. | ||
| * | ||
| * A ZTDF's `rootSignature.alg` is unauthenticated manifest data. Accepting |
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Suggested change
| * A ZTDF's `rootSignature.alg` is unauthenticated manifest data. Accepting | |
| * A base TDF's `rootSignature.alg` is unauthenticated manifest data. Accepting |
| @@ -0,0 +1,462 @@ | |||
| /** | |||
| * DSPX-4703 — the root signature is the only thing in a ZTDF that authenticates | |||
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Suggested change
| * DSPX-4703 — the root signature is the only thing in a ZTDF that authenticates | |
| * The root signature is the only thing in a base TDF that authenticates |
no jira tickets, some soft guidance that OpenTDF is base TDF
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The bug
A segment's
GMAChash is the AES-GCM tag the AEAD just computed over thatsegment's ciphertext, so reading the trailing 16 bytes back out is legitimate
— the tag is a real MAC over real data under the real key.
The root signature covers the aggregate hash: a concatenation of the segment
hashes. AES-GCM never processed those bytes. Applying the same
trailing-bytes extraction to them returns a keyless copy of the last segment
hash — a value the attacker already has, since it is sitting in the manifest
they are handing you.
rootSignature.algis read from the manifest, which is unauthenticated untilthe root signature validates, and unknown values were coerced to
HS256. Sothe attack needs no key at all:
rootSignature.algtoGMAC.segments[]however you like.rootIntegrityreads those bytes back and agrees with itself.AEAD tags bind no ordering, index, or count — each tag says only "this
ciphertext is intact," never "this is segment 3 of 7." The ordered segment
list is exactly and only what the root signature protects, which is why
losing it costs the whole structural guarantee: a 7-segment file becomes a
2-segment file, or the same segment repeated, and every remaining tag still
verifies.
The fix
Split the one overloaded
getSignatureinto four functions that each say whatthey authenticate:
segmentIntegrityrootIntegrityIntegrityErrorplus
segmentIntegrityVersion422/rootIntegrityVersion422for the legacyhex-then-base64 encoding. After the split there is no longer a code path that
can read a tag out of the aggregate hash, so the misuse isn't a policy check
that could be bypassed — it doesn't exist as a function.
On read,
asRootIntegrityAlgorithmfails closed: anything other thanHS256,in any casing, raises
IntegrityErrorinstead of being coerced.asSegmentIntegrityAlgorithmstays permissive about casing, since writers inthe wild emit both
GMACandgmac.rootSignature.algandsegmentHashAlgare widened to plain strings in themanifest types, which is the honest signature: they are attacker-controlled
until validated.
Don't launder the error we just raised
Raising an
IntegrityErroris only half of it; the caller has to receive one.streamToBufferdrained the plaintext withnew Response(stream).arrayBuffer(), and Chrome replaces any error raisedwhile it pulls a Response body with a bare
TypeError: Failed to fetch,discarding the original. So in the browser — and only in the browser — a
tampered payload arrived at the caller indistinguishable from a dropped
connection. That is exactly the distinction this PR exists to draw: the first
is an attack, the second is a retry.
Draining with a reader instead preserves the error the stream was errored
with.
toStringwent throughResponse.text()and had the same problem; itbuffers first now. This is why
DecoratedReadableStream.tsis in the diff ofan otherwise
tdf.ts-shaped PR — without it the two payload-tampering testsbelow pass under Node and fail under karma.
How to test
lib/tests/mocha/root-signature.spec.tsis new — 23 cases in four groups:reorder, and an unforged GMAC downgrade are all caught under HS256.
truncated and with reordered segments, each of the four casings of
GMAC,and an unknown algorithm that must not fall back to HS256.
round-trip and both still catch payload tampering; lowercase
segmentHashAlgstill reads; an unknown segment algorithm is refused.still caught, and the GMAC downgrade is refused there too.
lib/tests/mocha/unit/decorated-readable-stream.spec.tsis also new — 7 casespinning that
streamToBufferconcatenates correctly and that both it andtoBuffer/toStringreject with the identical error instance the streamwas errored with, never a substitute. It lives under
tests/mocha/unit/sowebpack picks it up and it runs under karma as well as Node, which matters:
the masking only ever reproduced in Chrome.
Baseline
Run this PR's tests against #1030's code, i.e. everything here except the
tdf.tsfix:Every case under controls stays green, and the 9 failures are exactly the
8 exploit cases plus the lowercase-
segmentHashAlgcase. The exploit failuresare all "expected an IntegrityError" — the forged files decrypt cleanly on
unfixed code. With the fix, all 23 pass.
The
streamToBufferfix is pinned the same way: reverting it alone turns 409karma passes into 6 failures — the two payload-tampering cases above plus four
of the seven error-propagation cases — while Node stays green throughout.
With both fixes the whole suite is green: 409 passing / 6 pending under mocha,
409 under karma, 253 under web-test-runner, coverage thresholds met, and
tsc --noEmitandnpm run lintclean.Risk
Crypto read path. Files that were previously accepted and are now rejected are
exactly the ones with a non-HS256 root signature, which had no integrity
guarantee to begin with. Legitimate GMAC-segment files are unaffected — that
path is unchanged and covered above. The write path already refused to produce
a GMAC root as of #1030.
Interop note
This SDK rejects an absent or empty
rootSignature.alg; the Go and Java SDKsdefault it to
HS256. Both are safe, since the HMAC still has to verifyeither way, and no golden file in xtest exercises it. Worth reconciling, but
not in this PR.
DSPX-4703
Cross-SDK coverage lives in opentdf/tests#594.
Summary by CodeRabbit
Security
Compatibility
Reliability