PacketTag struct represents a packet tag as per RFC4880.

RFC4880

4.2. Packet Headers

The first octet of the packet header is called the "Packet Tag". It determines the format of the header and denotes the packet contents. Note that the most significant bit is the leftmost bit, called bit 7. A mask for this bit is 0x80 in hexadecimal.

      +---------------+
  PTag |7 6 5 4 3 2 1 0|
      +---------------+
  Bit 7 -- Always one
  Bit 6 -- New packet format if setcopy

Note that old format packets have four bits of packet tags, and new format packets have six; some features cannot be used and still be backward-compatible.

Also note that packets with a tag greater than or equal to 16 MUST use new format packets. The old format packets can only express tags less than or equal to 15.

Old format packets contain:

  Bits 5-2 -- packet tag
  Bits 1-0 -- length-typecopy

New format packets contain:

  Bits 5-0 -- packet tagcopy

4.2.1. Old Format Packet Lengths

The meaning of the length-type in old format packets is:

0 - The packet has a one-octet length. The header is 2 octets long.

1 - The packet has a two-octet length. The header is 3 octets long.

2 - The packet has a four-octet length. The header is 5 octets long.

3 - The packet is of indeterminate length. The header is 1 octet

long, and the implementation must determine how long the packet
is.  If the packet is in a file, this means that the packet
extends until the end of the file.  In general, an implementation
SHOULD NOT use indeterminate-length packets except where the end
of the data will be clear from the context, and even then it is
better to use a definite length, or a new format header.  The new
format headers described below have a mechanism for precisely
encoding data of indeterminate length.copy

4.3. Packet Tags

The packet tag denotes what type of packet the body holds. Note that old format headers can only have tags less than 16, whereas new format headers can have tags as great as 63. The defined tags (in decimal) are as follows:

0        -- Reserved - a packet tag MUST NOT have this value
1        -- Public-Key Encrypted Session Key Packet
2        -- Signature Packet
3        -- Symmetric-Key Encrypted Session Key Packet
4        -- One-Pass Signature Packet
5        -- Secret-Key Packet
6        -- Public-Key Packet
7        -- Secret-Subkey Packet
8        -- Compressed Data Packet
9        -- Symmetrically Encrypted Data Packet
10       -- Marker Packet
11       -- Literal Data Packet
12       -- Trust Packet
13       -- User ID Packet
14       -- Public-Subkey Packet
17       -- User Attribute Packet
18       -- Sym. Encrypted and Integrity Protected Data Packet
19       -- Modification Detection Code Packet
60 to 63 -- Private or Experimental Valuescopy