# 2.4 Generation

_Peios / Advanced Peios / PCDS / GUID_

> Peios GUIDs are RFC 4122 version 4 — filled from a cryptographic random source, with the version and variant bits fixed afterwards.

GUIDs generated by Peios MUST be version 4 (random) as defined in
RFC 4122 §4.4.

## 2.4.1 Algorithm

To generate a version 4 GUID:

1. Fill all 16 bytes with cryptographically random data.
2. Set the four most significant bits of Data3 to `0100`
   (version 4).
3. Set the two most significant bits of Data4[0] to `10`
   (RFC 4122 variant).

The resulting GUID has 122 random bits, 4 version bits, and
2 variant bits.

> [!NOTE]
> In terms of bit manipulation on the binary layout:
>
> ```
> Data3 = (Data3 & 0x0fff) | 0x4000
> Data4[0] = (Data4[0] & 0x3f) | 0x80
> ```

## 2.4.2 Randomness source

The random data MUST be obtained from a cryptographically secure
source.

In the kernel, this MUST be `get_random_bytes()` or equivalent.

In userspace, this MUST be `getrandom(2)` with no flags (blocking
until the entropy pool is initialised) or equivalent.

GUIDs MUST NOT be generated using a pseudorandom number generator
seeded from a predictable source.