Signatures message binary format

#	Field name	Type	Length in Bytes
1	Packet length (BigEndian)	Int	4
2	Magic Bytes	Bytes	4
3	Content ID (0x15)	Byte	1
4	Payload length	Int	4
5	Payload checksum	Bytes	4
6	Block signatures count (N)	Int	4
7	Block #1 signature	Bytes	64
...	...	...	...
6 + N	Block #N signature	Bytes	64