Integrity & discontinuities

FLAC MD5 signature

FLAC can store a signature of its original audio. FlacCompagnon decodes the sound, recomputes the signature and compares both to detect possible alteration.

Reading the result

MD5 OK means the audio matches the signature. It does not guarantee a lossless recording source. Editing tags or cover art does not change this audio signature.

Go deeperMethod, limits, verification and sources

The FLAC MD5 status checks whether decoded audio matches the signature stored in the FLAC STREAMINFO metadata block. It is an integrity check, not an analysis of recording quality, losslessness or provenance.

Calculation#

FLAC stores an MD5 of unencoded audio samples, not an MD5 of the .flac file bytes. During FlacCompagnon's fused FLAC decode pass, the app receives raw decoded integer samples and hashes the exact byte sequence required by the format: interleaved channels, each signed two's-complement sample encoded as ceil(bits per sample / 8) little-endian bytes. This avoids a float round trip and makes the successful comparison equivalent to the audio check performed by flac -t.

The same pass feeds the normal analysis, so MD5 verification does not require a second decode. Audio tags, cover art and container-byte changes do not enter this digest.

Statuses#

Status	Meaning
`MD5 OK`	A stored signature was present and matched the decoded audio
`MD5 MISMATCH`	A stored signature did not match decoded audio; corruption or a non-conforming encoder is possible
`No MD5 signature`	STREAMINFO contains the all-zero signature, so there is nothing to compare
`MD5 present (unverified)`	A signature exists but verification was disabled for this analysis
`MD5 check error`	The file could not be fully decoded for the check

A mismatch concerns audio integrity. A file can have MD5 OK while its file-byte MD5 or CRC changes after retagging, and a file-byte checksum can match while its embedded FLAC signature is absent. FlacCompagnon exposes file fingerprints separately because the questions differ.

Limits and tests#

The check applies only to FLAC. It authenticates neither the recording history nor losslessness: a perfectly intact FLAC can still contain a lossy transcode, an upsampled source or deliberately clipped material.

cargo test -p flaccompagnon-core decode::flac
cargo test -p flaccompagnon-core flac_md5

For a manual independent comparison, run the reference FLAC verifier against a test file and compare its result with the app:

flac -t /path/to/file.flac

Reference: RFC 9639, FLAC frame and sample representation.

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