fix(sdk): reject GMAC root signatures on read (DSPX-4703) - #401
fix(sdk): reject GMAC root signatures on read (DSPX-4703)#401dmihalcik-virtru wants to merge 1 commit into
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📝 WalkthroughWalkthroughTDF integrity handling now uses separate segment and root implementations. Segment verification allows HS256 and GMAC. Root signatures require HS256. Tests cover tampering, algorithm validation, legacy compatibility, and configuration checks. ChangesTDF integrity enforcement
Priority: ➖ Normal Estimated code review effort: 4 (Complex) | ~45 minutes Change: Bug fix Merge Risk: 🟠 High · up to A crafted TDF can bypass integrity enforcement and emit attacker-controlled bytes, so this security gap should be fixed before merge. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
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The vulnerability analysis is correct, the fix is at the right layer, and the test suite is unusually strong — mutation-verified, with controls that would catch a vacuous pass. Direction is right. One substantive doc correction, one compatibility gap, and some smaller things.
The reasoning is right, but the aeadTag javadoc overstates the segment case
That javadoc is the load-bearing rationale for the split, so it's worth being exact:
Only then are the last sixteen bytes a MAC of the data they are being used to authenticate.
Not quite. In readPayload, readBuf is sized from segment.encryptedSegmentSize — also unauthenticated manifest data. An attacker can re-chunk the payload arbitrarily and set each hash to the trailing sixteen bytes of their own chunk; the segment.hash comparison passes with no key. The GMAC segment check by itself has exactly the property this PR condemns in the root.
What actually authenticates a GMAC segment is aesGcm.decrypt(new AesGcm.Encrypted(readBuf)) on the next line — a real tag check under the payload key. So the precise statement is: GMAC is acceptable for segments because a separate AEAD verification follows it, and meaningless for the root because nothing follows. That's a stronger argument than the one in the comment, and it makes the asymmetry structural rather than a property of the bytes.
It also reinforces the thesis: since segment-level checking is keyless-forgeable under GMAC, the root HMAC is doing all the work for manifest integrity — which is exactly why the GMAC-root downgrade is as bad as described. No code change, just the comment.
Compatibility gap: java-sdk can already write GMAC-rooted files
The Compatibility section covers the cross-SDK golden corpus, but not files this SDK could have produced. On main there is no withRootIntegrityAlgorithm (that arrives in #400) — but Config.TDFConfig.integrityAlgorithm is a public mutable field, and TDF.java:581-585 faithfully writes alg = "GMAC" whenever it is set:
String alg = kGmacIntegrityAlgorithm;
if (tdfConfig.integrityAlgorithm == Config.IntegrityAlgorithm.HS256) {
alg = kHmacIntegrityAlgorithm;
}Anyone who set that field has files that become permanently unreadable, with no opt-out. That is defensible — they were never integrity-protected — but it should be a stated decision rather than an omission. Two things worth adding to the PR body:
- Do the Go or JS SDKs expose a root-algorithm option that can emit GMAC? If either does, the blast radius is wider than "no honest writer produces this."
- An explicit "no escape hatch; re-encrypt affected files" line. I agree with no escape hatch — a flag that re-enables a keyless root signature is a flag someone will eventually set — but it should be said out loud.
Smaller findings
SegmentSignatureMismatch is the wrong type for an unsupported algorithm. segmentIntegrityAlgorithmFromManifest throws it for segmentHashAlg: "MD5". That is "the manifest declares something I do not implement", not "a signature did not match" — and callers plausibly treat SegmentSignatureMismatch as a tamper signal specifically. The message is clear; the type conflates two conditions.
Exception asymmetry on the read path. rootIntegrityAlgorithmFromManifest throws RootSignatureValidationException; the redundant requireSupportedRootIntegrityAlgorithm throws IllegalArgumentException. On read the second is unreachable by design, so the only way it fires is a future bug — and then loadTDF leaks a raw IAE past whatever callers catch. The mutation test deliberately depends on the types differing, which is a fair argument for keeping it as is; I would just add a clause to the comment saying that if it ever does fire on the read path, the IAE is the intended fail-loud signal.
The unencrypted path has the same shape and is not covered. loadTDF's payload.isEncrypted == false branch computes the root as a keyless SHA-256 over the aggregate hash, with no allowlist at all. Nothing keyed is involved, so truncation there appears to remain forgeable. Out of scope here, but it is the same bug class one branch over — worth a follow-up ticket rather than silence.
createTdfRefusesAGmacRootSetDirectlyOnTheConfig asserts IllegalArgumentException out of createTDF. createTDF is declared throws SDKException; the unchecked IAE escapes fine but adds an undocumented failure mode to a public API. SDKException would be more consistent for the write path.
Nit: aeadTag carries over the missing space — "payload is " + length + "bytes while GMAC is" renders as 0bytes. The line is being rewritten anyway.
Nit: rootIntegrityAlgorithmFromManifest can only ever return HS256, so the return type is decorative. Not worth changing — symmetry with the segment resolver is worth more than the saved line.
Merge-order collision with #402 (DSPX-4584)
Two breakages against the streaming-manifest branch, and the second will not announce itself:
- Conflict — this PR rewrites the exact
calculateSignaturecall sites that DSPX-4584 changed. That branch hoistsaggregateHash.toByteArray()into abyte[] aggregateHash; this one renames the callee torootIntegrity/segmentIntegrityon the same lines. - Compile error, no conflict —
TDFRootSignatureTest.rewrite(...)callswriter.appendManifest(new Gson().toJson(manifest)). DSPX-4584 deletesappendManifestin favour of amanifest()stream, since a manifest with tens of millions of segments cannot be a JavaString. Whichever lands second, that helper stops compiling — and git will not flag it, because the two changes are in different files.
Happy to take the fixup on the DSPX-4584 side either way; just worth knowing before merge order gets decided.
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Inline comments:
In `@sdk/src/main/java/io/opentdf/platform/sdk/TDF.java`:
- Line 577: Validate tdfConfig.segmentIntegrityAlgorithm before any output is
written by adding one shared validation helper, invoking it in the existing
createTDF preflight alongside requireSupportedRootIntegrityAlgorithm and again
at the start of segmentIntegrity. Ensure null and unsupported values are
rejected before the switch or ZipWriter writes ciphertext.
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A ZTDF's root signature is the only thing that authenticates the manifest's
ordered list of segment hashes. AES-GCM tags bind a segment's own bytes and
nothing about its index, its neighbours, or how many segments there are, so
segment-level integrity cannot notice a truncated, reordered, or duplicated
segment list.
One signature routine served both jobs. For a segment, "GMAC" correctly means
reading back the AES-GCM tag the cipher just computed over that segment's
ciphertext. For the root it means nothing: the aggregate hash never passes
through AES-GCM, so there is no tag to recover and the code returned a copy of
the trailing bytes of its own input, i.e. the last segment hash. Manifest data
compared against manifest data, with the payload key never used.
The algorithm was read from `rootSignature.alg` in the manifest, which is not
authenticated. An attacker with no key could therefore take an HS256-rooted
TDF, rewrite the root to "GMAC" with a signature copied from the last segment
hash, and then truncate, reorder, or duplicate segments with the file still
verifying.
Changes:
- Split the routine into segmentIntegrity (HS256 or GMAC) and rootIntegrity
(HS256 only), so tag extraction can no longer be pointed at a non-AEAD
input.
- rootIntegrityAlgorithmFromManifest resolves the root algorithm against an
allowlist and throws SDK.RootSignatureValidationException for anything
else, instead of coercing unknown values to HS256.
- segmentIntegrityAlgorithmFromManifest stays permissive, since both
algorithms are meaningful over ciphertext.
- createTDF validates the configured root algorithm too, so the SDK will not
write a file it would refuse to read.
rootIntegrity validates its own argument in addition to its caller checking
first. The redundancy is deliberate: the check is what makes the function safe,
so it belongs with the function rather than only at today's call sites.
Tests cover truncation, reordering, GMAC in several casings, an unknown
algorithm that must not be coerced, and controls that must keep passing.
Verified by mutation: restoring the old tag-extraction branch inside
rootIntegrity turns exactly the 8 exploit tests red and leaves the other 21
green.
Refs: DSPX-4703, and the write-side controls in DSPX-4736.
Signed-off-by: Dave Mihalcik <dmihalcik@virtru.com>
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Inline comments:
In `@sdk/src/main/java/io/opentdf/platform/sdk/TDF.java`:
- Around line 906-912: Authenticate the payload encryption mode before loadTDF
selects the integrity implementation, rather than trusting payload.isEncrypted
from Manifest.readManifest. Ensure readPayload cannot bypass decryption or use
keyless SHA-256 verification based solely on unauthenticated manifest data,
while preserving the existing unencrypted-TDF behavior if supported.
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Summary
The bug
A ZTDF's root signature is the only thing that authenticates the manifest's ordered list of segment hashes. AES-GCM tags bind a segment's own bytes and say nothing about its index, its neighbours, or how many segments exist — so segment-level integrity structurally cannot notice a truncated, reordered, or duplicated segment list.
One signature routine served both jobs. For a segment, "GMAC" correctly means reading back the AES-GCM tag the cipher just computed over that segment's ciphertext. For the root it means nothing: the aggregate hash never passes through AES-GCM, so there is no tag to recover, and the code returned a copy of the trailing bytes of its own input — the last segment hash. Manifest data compared against manifest data, with the payload key never used.
Why this is not opt-in
The algorithm was read from
rootSignature.algin the manifest, which is unauthenticated, and unknown values were coerced to HS256 rather than rejected. So an attacker with no key could take any HS256-rooted TDF, rewrite the root to"GMAC"with a signature copied from the last segment hash, and then truncate, reorder, or duplicate segments — with the file still verifying.The fix
segmentIntegrity(HS256 or GMAC) androotIntegrity(HS256 only), so tag extraction can no longer be pointed at a non-AEAD input.rootIntegrityAlgorithmFromManifestresolves the root algorithm against an allowlist and throwsSDK.RootSignatureValidationExceptionfor anything else, instead of coercing.segmentIntegrityAlgorithmFromManifestallows both algorithms, because both are meaningful in that position, but still refuses an unrecognised name rather than defaulting it. A GMAC segment hash proves nothing on its own; what makes it trustworthy is that it is bracketed by two keyed checks — the HS256 root signature over the whole hash list, validated inloadTDFbefore any payload is read, and the real AES-GCM tag check under the payload key at decrypt time. The root has neither backstop, which is why the asymmetry between the two positions is structural rather than stylistic.createTDFvalidates both configured algorithms before writing a byte, so the SDK cannot emit a file it would refuse to read, and cannot leave a partial TDF behind on a bad config.On the guard inside
rootIntegrityrootIntegrityvalidates its own argument even thoughcreateTDFand the manifest resolver have both already checked. That is deliberate: the check is what makes the function safe to call, so it belongs with the function rather than only at today's call sites.It is genuinely redundant today, though, and the mutation results below say so plainly — with the resolver in place, a GMAC root cannot reach
rootIntegritythrough any public entry point. It is defence against a future edit, not against a current input. Because nothing else exercises it, this PR adds tests that call the guards directly; without those, a regression that reintroduced tag extraction insiderootIntegritywould leave the entire suite green.The two guards throw
IllegalArgumentExceptionrather thanSDK.TamperException, matching how the config layer already reports bad caller-supplied values (Config.withSegmentSize). On a read they are unreachable, so if one ever does fire it means a bug inTDF, not a hostile file — it should escape loudly rather than arrive at callers wearing an exception type they routinely catch.Compatibility
No well-formed file is affected. Every golden TDF in the cross-SDK corpus is
rootSignature.alg = "HS256"withsegmentHashAlg = "GMAC".Nothing in the ecosystem emits a GMAC root:
RootIntegrityAlgadmits onlyRootHS256; the GMAC constant is deprecated there and annotated as not a legal root algorithm.type RootIntegrityAlgorithm = 'HS256', so it is not expressible.Confighas never exposed a setter for the root algorithm, andnewTDFConfigdefaults it to HS256. The one way to have produced such a file is a caller who reached past the builder and assignedTDFConfig.integrityAlgorithm = GMACon the public field. Those callers now get anIllegalArgumentExceptionfromcreateTDFinstead of silently writing an unverifiable file.There is no escape hatch, by design — no flag, no compatibility mode, no target-mode exemption. A file this rejects is one no honest writer produces, and accepting it would restore the vulnerability for every reader. Anyone holding such a file must re-encrypt it. The legacy hex-encoded root path is still supported, and is covered by a test that confirms it did not become a way around the allowlist.
Test plan
TDFRootSignatureTest: 30 tests, 0 failures, in two groups.25 behavioural tests drive the public API end to end — truncation, reordering, GMAC in several casings, an unknown root algorithm that must not be coerced, an unknown segment algorithm, a legacy hex-encoded GMAC root, a config-level GMAC root, plus controls that must keep passing for reasons unrelated to this change.
5 guard tests call
rootIntegrity/segmentIntegrityand their argument checks directly. These reference methods this PR introduces, so unlike the group above they cannot be run againstmain.Verified by reverting, and by mutation
With this commit's
TDF.javareverted tomainand the test file otherwise untouched, the 25 behavioural tests compile unmodified and 9 fail across 7 methods — the vulnerability is live and reproducible, and the tests are not merely asserting the shape of the new code:The other 16 stay green, so the suite is discriminating rather than strict.
Mutating one layer at a time locates which check is load-bearing:
TDFRootSignatureTestrootIntegrityintactrootIntegrityaccepts GMAC again, resolver intactmain)The resolver is the layer that actually stops a hostile file. The
rootIntegrityguard is caught only by the test written for it — which is the argument for having written it, and an honest statement of what that check is and is not doing.Related
spec/DSPX-4703.mdinopentdf/tests, and feat(xtest): cross-SDK coverage for root and segment integrity algorithms (DSPX-4703, DSPX-4736) tests#594mainSummary by CodeRabbit