3GPP 24.501 v20.0.0 — the document's own text
9.3 Security header type
Taught in 7. What a NAS message looks like on the wire (CT1 — the group that writes what your phone says, in depth), 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
Bits 1 to 4 of the second octet of every 5GMM message contain the Security header type IE. This IE includes control information related to the security protection of a 5GMM message. The total size of the Security header type IE is 4 bits.
The Security header type IE can take the values shown in table 9.3.1.
Table 9.3.1: Security header type
| Security header type (octet 1) | ||||
|---|---|---|---|---|
| Bits | ||||
| 4 | 3 | 2 | 1 | |
| 0 | 0 | 0 | 0 | Plain 5GS NAS message, not security protected |
| Security protected 5GS NAS message: | ||||
| 0 | 0 | 0 | 1 | Integrity protected |
| 0 | 0 | 1 | 0 | Integrity protected and ciphered |
| 0 | 0 | 1 | 1 | Integrity protected with new 5G NAS security context (NOTE 1) |
| 0 | 1 | 0 | 0 | Integrity protected and ciphered with new 5G NAS security context (NOTE 2) |
| All other values are reserved. | ||||
| NOTE 1: This codepoint may be used only for a SECURITY MODE COMMAND message.NOTE 2: This codepoint may be used only for a SECURITY MODE COMPLETE message. |
A 5GMM message received with the security header type encoded as 0000 shall be treated as not security protected, plain 5GS NAS message. A protocol entity sending a not security protected 5GMM message shall send the message as plain 5GS NAS message and encode the security header type as 0000.