docs(km): add docs on cryptographic details

Author: enzzc

pages/key-manager/reference-content/cryptographic-details-key-manager.mdx

@@ -0,0 +1,66 @@
+---
+meta:
+  title: Cryptographic details of Scaleway Key Manager
+  description: This document details the cryptographic mechanisms of Scaleway Key Manager with adherence to ANSSI-PA-079 recommendations
+content:
+  h1: Cryptographic details of Scaleway Key Manager
+  paragraph: This document details the cryptographic mechanisms of Scaleway Key Manager with adherence to ANSSI-PA-079 recommendations
+tags: key-manager security
+dates:
+  validation: 2025-04-30
+  posted: 2025-04-30
+---
+
+## Cryptographic primitives
+
+### Random number generation
+
+#### Generating KEKs
+
+Scaleway Key Manager uses a **Cryptographically Secure Pseudorandom Number Generator (CSPRNG)**
+to generate both keying material for managed keys and cryptographic unique IVs.
+
+The CSPRNG relies on recent Linux provided ChaCha-based PRNG, which is seeded with high-entropy and unpredictable sources:
+ - Timing variations from hardware events
+ - True Random Number Generators (TRNGs), such as the `RDSEED`/`RDRAND` instructions on AMD64 family processors
+
+<Message type="note">
+Conforms to ANSSI-PA-079 R14.
+</Message>
+
+<Message type="warning">
+That section does not apply to key imported by users via the Bring Your Own Key (BYOK) mechanism.
+</Message>
+
+
+#### Customer-provided KEKs (BYOK)
+
+Scaleway's Key Manager supports Bring Your Own Key (BYOK), so customer can import their
+own key material without relying on Scaleway to generate keys for them. In this case,
+the user has the responsibility to provide a strong key material.
+
+The provided material is not used as is. It is derived using the **HKDF algorithm with SHA2-256**
+as the hash algorithm, with secure random bytes injected as salt.
+
+### Symmetric encryption
+
+Scaleway Key Manager uses **AES-256-GCM** (AES with 256-bit key in Galois Counter Mode) with
+authenticated associated data (AEAD) to encrypt and decrypt user-provided payload.
+
+To reduce the risk of key overuse, plaintext payloads cannot exceed 64 KiB.
+
+For each encryption operation, a 96-bit Initialization Vector (IV)
+is generated using the CSPRNG described in the previous section.
+
+<Message type="note">
+Conforms to ANSSI-PA-079 R1, R4, R12.
+</Message>
+
+<Message type="warning">
+The Key Manager uses AES-256-GCM algorithm internally, but this does not constrain users to the same algorithm for their DEKs.
+We recommend using cryptographic libraries like Tink, which handle DEK management with robust and reviewed algorithms.
+Scaleway provides a Tink integrations for Go and Python, which is the preferred integration method.
+ - [Tink Python integration](https://github.com/scaleway/tink-py-scwkms)
+ - [Ting Go integration](https://github.com/scaleway/tink-go-scwkms)
+
+</Message>