---
license: cc0-1.0
pretty_name: Common Crawl document-digest bloom filter
tags:
- bloom-filter
- file-hashes
---

# Common Crawl document-digest bloom filter

Membership filter over payload sha1 digests of **document-type
captures** (PDF ~74%, DOC/XLS/PPT/EPUB and similar) from one Common
Crawl crawl's URL index — the ~1% of CC that isn't HTML. CC digests
are SHA-1 of the (possibly truncated pre-2025) WARC payload; for
static documents this equals the file's raw sha1.

- **30,951,194 sha1 keys** (UPPERCASE hex), p = 1e-4, 71 MiB
- Source: CC-MAIN index `content_digest` column, filtered by MIME
  (crawl data © Common Crawl, generously public; this digest
  compilation CC0)
- One crawl only — a document must have been live and crawled that
  month to appear; more crawls widen coverage

## Rebuild

`cc_extract_job.py` in this repo streams one crawl's CDXJ index
(~1 h locally, $0), `cc_digests_to_hex.py` converts the base32
digests to hex, `./dcso_bloom_build.py cc.sha1.bloom sha1` builds
the filter.

## Format

DCSO bloom filter format (`github.com/DCSO/bloom`), the same format
CIRCL publishes hashlookup in: 48-byte little-endian u64 header
(version, n, p, k, m, N) followed by the bit array. Keys are hex
digest strings — **UPPERCASE for sha1, lowercase for sha256** (the
hashlookup convention). Any DCSO-compatible reader works.

Membership answers are probabilistic: p = 1e-4 false-positive rate,
no false negatives. Confirm anything important against the source.