remove-ai-watermarks/docs/watermark-forgery-study.md

Watermark forgery study

Audit correction, 2026-09-08: The historical VideoSeal results and cross-scheme conclusions are superseded by the corrected video run below. Historical video observations were taken before encoding rather than from the hashed output artifact (R12); rectangular video decoding also required correction (R01). AudioSeal observations remain separate from those invalidated video comparisons.

Corrected VideoSeal measurements (2026-09-08)

Twelve observations now measure decoded saved artifacts, including controls and double embeds. All 12 recorded SHA-256 digests match the retained MP4s; the decoder also checks that each file remains unchanged during measurement.

Cell	Gradient vs A	Gradient vs B	Real Sora vs A	Real Sora vs B
clean	0.516	0.555	0.559	0.535
marked A	1.000	0.469	1.000	0.469
removed CRF 23	0.559	0.496	1.000	0.469
forged B on clean	0.469	1.000	0.469	1.000
forged B on marked A	0.469	1.000	0.473	0.996
forged B on removed A	0.469	1.000	0.469	1.000

Both marked controls meet the fixed-message A rule. All six B-embedding outputs fall below its 0.9 threshold, while matching B at 0.996 or above. CRF 23 removes the matched A verdict on the gradient but preserves it on this Sora carrier. These two fixed carriers support this scoped observation; they do not establish complete erasure of every trace of A or a universal double-embedding rule. AudioSeal was not rerun in this correction.

The run used the same pinned checkpoint, Torch 2.14.0, ffmpeg 9.0.1, and two CPU threads as the corrected temporal study. No model downloads or provider calls ran. Study source SHA-256: 031d9e0c58af58ab5f9227ea52a500eb86644fc464f9c37be1e894ce0b6b4935. Corrected case rows SHA-256: f8c5a65e5853de0f1ff67c2ef0549fbd81df971e13c79539c85c1b0255eb60a8. Artifacts remain outside the repository.

Historical study (2026-09-07, superseded video results)

Development-only study, run on 2026-09-07 from scripts/watermark_forgery_study.py, opening the forgery/remover-forensics direction from the original research map. It measures the four-state model (clean / marked / removed / forged) and the double-embedding question for both pinned oracles: does embedding message B over an A-marked artifact leave A, B, or mutual interference?

Carriers: the tone-stack audio carrier, the synthetic moving-gradient video clip, and the real committed Sora clip. Message B is the oracle's message generator at seed 8. Everything local; artifacts under .local-eval/.

The double-embedding answer: the last writer wins, completely

Cell	AudioSeal acc vs A	vs B	VideoSeal (gradient) vs A	vs B	VideoSeal (real Sora) vs A	vs B
marked A	1.000	0.44	1.000	0.47	1.000	0.47
removed	0.50 (presence 0.28)	0.44	0.57	0.48	1.000	0.47
forged B on clean	0.44	1.000	0.47	1.000	0.47	1.000
forged B on marked A	0.44	1.000	0.47	1.000	0.48	0.99
forged B on removed	0.56	0.875	0.47	1.000	0.47	1.00

Embedding B over marked A reads as pure B: the original message decays to coin-flip accuracy under both schemes, on synthetic and real content alike. There is no mutual interference and no residue of A. With an unkeyed, openly published embedder this is the expected outcome - and now it is a measurement, not an assumption. The forensic consequence: payload attribution without key control is meaningless against an attacker who can run the same open embedder; only presence detection survives, and even that carries the attacker's payload. Provider schemes that rely on secret keys are a different question this study cannot answer.

Two boundary observations travel with the table:

• The removal transform is carrier-dependent, as the cohorts already showed: 10 dB noise removes the tone stack (presence 0.28) and crf 23 removes the gradient clip (0.57), but the real Sora clip keeps a perfect 1.000 decode through the same crf 23 - the same number the temporal study recorded.
• A forged row reads detected under the AudioSeal presence rule (the label carries the foreign payload) and not_detected under the VideoSeal message-accuracy rule. Both cohort fixtures encode their scheme's correct expected value, and the benchmark docs state the two rules side by side.

Remover-trace probe (scoped first evidence)

High-to-low frequency band energy ratio on clean versus noise-removed audio: the tone stack jumps from 8.6e-05 to 2.3e-02 (a 269x trace), the pinkish carrier 0.016 to 0.039, and the white-noise carrier does not separate at all (0.330 to 0.332) - a noise carrier absorbs a noise remover. This detects the trace of THIS study's gaussian-noise removal only; it is not a general remover detector, and the probe JSON says so in its scope field.

Cohort arms

The audio cohort (v2, 20 cases) and video cohort (v2, 15 cases) now carry the four states as strict manifest rows: removed rows where the transform measurably destroys the mark (audio: noise on tone stack; video: crf 23 on both synthetic carriers), and forged rows embedding message B with the scheme-correct expected verdict. Zero expected-result mismatches in both recorded runs.

Pins

Study script sha256:3985522c862ac44a0c7ef8aa02946a11701439a94d67f5867f00819f638c032b, case rows sha256:03d446c08a5daed623a4be2980e097bb8bbc0cd7a6d3ab3f931d25aa58570a16. Oracles and checkpoints are pinned as in the kernel document.

Follow-ups

• A keyed-scheme perspective needs a scheme with actual key control; WmForger and WMCopier diffusion-attack baselines remain the map's heavyweight item.
• A general remover-trace detector would need features beyond one band ratio and validation against removals this project did not produce.