Verify-on-read re-hashes the full object on every local read (~8x slower than write). A ReadVerifier seam skips re-verification for blobs verified this process, while scrub still catches bitrot.
CAS keys make verify-on-read free: central verification, x-amz-checksum echo, tombstone GC, and corruption surfacing.
S3 integrity checksums moved from a surface-level adapter into the storage/ingest core via a protocol-agnostic ChecksumProvider seam — every write path verified, replicas re-verify on receive.
x-amz-checksum-CRC32/SHA256 echo on PUT; integrity surfaced to clients so dirty bytes never masquerade as clean. Teaches: client-verifiable integrity, Bolt zero-allocation on read path, Sentinel honesty guarantee.