All archive signatures are 8-byte values, with common 7-byte prefix specific for this particular archive type, and last byte determing the particular signature (start of archive, archive descriptor and so on..)

Archive starts with archive start signature, used by file type identification tools. It is optionally followed by arhive header block that holds global information about the archive

Then goes compressed archive data, followed by archive directory describing these data, followed by directory descriptor used to find and decipher the directory. Such sequence of 3 blocks may be repeated several times. Optionally it follows by the archive footer block that finished the archive. Alternative way of archive identification is by signature contained in the archive footer or by signature of archive descriptor that should be looked in the last 4KB of the file

... TBD