kopia/kopia · critical

invalid data - corrupted

Error message

invalid data - corrupted

What it means

After recomputing the HMAC-SHA256 over the payload, VerifyAndStrip compares it with the trailing signature using hmac.Equal. When the two differ, the data fails authentication and the error 'invalid data - corrupted' is returned; the payload is never emitted to the output buffer.

Solutions

1. Verify the configured HMAC secret matches the one used when the data was written.
2. Delete the corrupted cache/blob entry and re-fetch it from the source.
3. Check storage hardware/backend for corruption; run repository consistency checks.

Example fix

// before
err := hmac.VerifyAndStrip(entry, secret, out) // fails: corrupted
// after
err := hmac.VerifyAndStrip(entry, secret, out)
if err != nil {
    os.Remove(cachePath) // drop bad entry, refetch
    entry = refetchBlob(blobID)
    err = hmac.VerifyAndStrip(entry, secret, out)
}

err := hmac.VerifyAndStrip(data, secret, out)
if err != nil {
    if strings.Contains(err.Error(), "corrupted") {
        // authenticated failure: drop entry, refetch from canonical source
        os.Remove(cachePath)
        return refetchAndVerify(id)
    }
    return err
}

When it happens

Trigger: Calling VerifyAndStrip on blob data whose content or trailing 32-byte signature was modified — bit rot, truncated-then-overwritten cache files, wrong secret used for verification, or mixing data written with a different key.

Common situations: Failing disk or flaky storage backend corrupting cached blobs; rotating/incorrect HMAC secret in configuration; a cache directory shared between repositories with different secrets.

Understand the failure class

Background: Checksum mismatch errors: "checksum verification failed", "digest mismatch", "expected vs actual checksum" — what they mean and how to fix them — this error's family across 41 libraries.

Related errors

• invalid data - too short
• decrypt
• error hashing
• error reading signature
• unable to decrypt content

AI-assisted analysis of kopia/kopia@82495e54b5 (2026-09-07). Data as JSON: /api/errors/5ae234f6004c46c4. Report an issue: GitHub.

Appendix: source

Thrown at internal/hmac/hmac.go:54

	if _, err := io.CopyN(io.MultiWriter(h, output), r, int64(p)); err != nil {
		return errors.Wrap(err, "error hashing")
	}

	var sigBuf, actualSignature [sha256.Size]byte

	validSignature := h.Sum(sigBuf[:0])

	n, err := r.Read(actualSignature[:])
	if err != nil || n != sha256.Size {
		return errors.Wrap(err, "error reading signature")
	}

	if hmac.Equal(validSignature, actualSignature[:]) {
		return nil
	}

	return errors.New("invalid data - corrupted")
}

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