HashMark: Type-Agnostic Watermarking for Tables with Free-Form Text

Tables are released, copied, and edited far from their owners. A copy found later may have had its rows, values or text changed. We introduce HashMark, a watermark for tables containing both numbers and free-form text. It makes small numerical edits and selects text paraphrases under one secret key. Verification requires only the suspect table, the key, and a public descriptor of how it was marked; neither the original table nor its generator is needed. Column-level statistical tests account for repeated values and dependence between text cells, providing evidence that a suspect table contains watermarked data. On mixed tables, the numbers and the text each keep the verdict when the other is attacked, and no clean control or wrong key is flagged in any mixed-table test. With $80\%$ of the rows or cells of a table replaced, HashMark still detects every table, while the generation-time watermark MUSE detects $26$–$34\%$. A geometric model predicts how many text cells can be marked to within $0.32$ percentage points across four corpora. Numerical marking keeps the data as close to the original as an unwatermarked release, to three decimal places, moves downstream accuracy by at most $0.38$ points, and marks $10,000$ rows in $0.2$ s.