OpenPGP data is organized in packets. Every packet starts with a header containing a packet tag, and the length of the packet. In these documents, we will call the packet structure, and the packet headers the frame, or framing.
For historic reasons, CTB stands for Cipher Type Byte. Nowadays, it simply contains the packet’s Tag, and the Tag specifies what kind of packet we are dealing with. If an old-style CTB is used, then it also indicates the kind of length that immediately follows the CTB.
The packet length specifies the extend of the packet, excluding the CTB and the length information. For example, consider the following User ID Packet:
If you click on the button in the top-right corner of the above
figure, or use
sq dump to inspect the above OpenPGP data, you will
New CTB, 15 bytes: User ID Packet Value: email@example.com 00000000 cd 0f frame 00000002 66 6f 6f 40 65 78 61 6d 70 6c 65 2e 6f 72 value 00000010 67
The first byte with the value
0b1100_1101. The most
significant bit tells us that it is a CTB, the next bit tells us
that it is a new-style CTB, and the remaining bits tell us the
packet’s Tag, in this case 13, denoting a User ID Packet.
The next byte,
0x0f specifies the packet’s length, encoded as a
One-Octet New Format Packet Lengths. In this case, the length of
the packet is 15, excluding the CTB and the encoded length itself.
User ID Packets have a very simple structure, the body of the packet is simply the user id encoded using UTF-8.
If you want to learn more about OpenPGP’s packet structure, see section 4 of RFC4880.