Packet Structure

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.

CTB

Length

Body

An OpenPGP Packet

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:

-----BEGIN PGP ARMORED FILE-----

zQ9mb29AZXhhbXBsZS5vcmc=
=4adU
-----END PGP ARMORED FILE-----

Example 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 see:

New CTB, 15 bytes: User ID Packet
    Value: [email protected]

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 0xcd, or 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.

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