kovidgoyal/kitty · error

rsync signature header has zero block size

Error message

rsync signature header has zero block size

What it means

The rsync signature header's block-size field (bytes 8-11, big-endian uint32) read as zero, which is invalid — blocks must have a positive size.

Source

Thrown at tools/rsync/api.go:101

		return consumed, fmt.Errorf("Invalid checksum_type in signature header: %d", csum)
	}
	switch strong_hash := StrongHashType(bin.Uint16(data[4:])); strong_hash {
	case XXH3:
		self.Strong_hash_type = strong_hash
		self.rsync.SetHasher(new_xxh3_64)
	default:
		return consumed, fmt.Errorf("Invalid strong_hash in signature header: %d", strong_hash)
	}
	switch weak_hash := WeakHashType(bin.Uint16(data[6:])); weak_hash {
	case Rsync:
		self.Weak_hash_type = weak_hash
	default:
		return consumed, fmt.Errorf("Invalid weak_hash in signature header: %d", weak_hash)
	}
	block_size := int(bin.Uint32(data[8:]))
	consumed = 12
	if block_size == 0 {
		return consumed, fmt.Errorf("rsync signature header has zero block size")
	}
	if block_size > MaxBlockSize {
		return consumed, fmt.Errorf("rsync signature header has too large block size %d > %d", block_size, MaxBlockSize)
	}
	self.rsync.BlockSize = block_size
	self.signature = make([]BlockHash, 0, 1024)
	return
}

func (self *Api) read_signature_blocks(data []byte) (consumed int) {
	block_hash_size := self.rsync.HashSize() + 12
	for ; len(data) >= block_hash_size; data = data[block_hash_size:] {
		bl := BlockHash{}
		bl.Unserialize(data[:block_hash_size])
		self.signature = append(self.signature, bl)
		consumed += block_hash_size
	}
	return

View on GitHub (pinned to 6d5d0c4406)

Solutions

1. Confirm the data passed to AddSignatureData begins with a real signature header
2. Check for off-by-one offsets or extra framing bytes before the signature
3. Regenerate the signature from the source file

if len(data) < 12 { return fmt.Errorf("header truncated") }
if binary.BigEndian.Uint32(data[8:12]) == 0 { return fmt.Errorf("zero block size") }

if err := differ.AddSignatureData(chunk); err != nil { abort transfer; regenerate signature }

When it happens

Trigger: AddSignatureData receiving a 12+ byte header whose block-size field is 0 — almost always a corrupted, misaligned, or hand-crafted stream rather than a real signature.

Common situations: Stream misalignment (reading from the wrong offset), truncation, or feeding random/test bytes into AddSignatureData without a valid header.

Related errors

• Invalid weak_hash in signature header: %d
• rsync signature header has too large block size %d > %d
• There were %d leftover bytes in the signature data
• There are %d leftover bytes in the delta
• The checksum was not received at the end of the delta data

AI-assisted analysis of kovidgoyal/kitty@6d5d0c4406 (2026-08-27). Data as JSON: /api/errors/3ed1df94c1396a23. Report an issue: GitHub.