initial commit: added very basic symmetric key service api

.gitignore

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+/target

Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "symmetric-key-service"
+version = "0.1.0"

Cargo.toml

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+[package]
+name = "symmetric-key-service"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]

src/main.rs

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+// Design a Symmetric Key Distribution Service"                                                                               
+//                                                                                                                             
+// You are building a Key Management Entity (KME)
+// a service responsible for generating, storing, and distributing pre-shared 
+// symmetric keys to authenticated clients (e.g. network gateways, endpoints).
+// Design and implement a core slice of this system in Rust.                                                                                                             
+//
+// Requirements:                                                                                                               
+// Clients authenticate and request a key by providing a session_id and peer_id                                                
+// The KME returns a fresh 256-bit key, ensuring both sides of a session receive the same key                                  
+// Keys are single-use: once consumed, they cannot be re-issued                                                                
+// The service must handle concurrent requests safely                                                                          
+// Keys not consumed within 60 seconds should expire                                                                           
+//                                                                                                                             
+// Deliverables:                                                                                                               
+// * A Rust implementation of the core KME logic
+//   (not necessarily a full HTTP server — a library interface is fine)              
+// * A brief design doc covering: your data structures, concurrency strategy,
+//   and how you'd extend this to multiple KME nodes
+
+use std::collections::HashMap;
+use std::sync::{Arc, Mutex};
+use std::time::{Duration, Instant};
+
+enum KeyEntryState {
+    Active { retrieved_by: Vec<String> },
+    Consumed,
+}
+
+#[derive(Debug, PartialEq)]
+enum KmeError {
+    KeyConsumed,
+    KeyExpired,
+}
+
+struct KeyEntry {
+    key: [u8; 32], // 256 bits
+    creation: Instant,
+    state: KeyEntryState,
+}
+
+#[derive(Debug, PartialEq)]
+struct KeyResponse {
+    pub key_id: String, // stable identifier for this key
+    pub key: [u8; 32],
+}
+
+type SessionId = [u8; 16]; // similar to uuid
+type PeerId = u32;
+type Peer = (SessionId, PeerId);
+
+struct Auth {
+    user_count: u32,
+    session_count: u8,
+}
+
+impl Auth {
+    pub fn login(&mut self) -> Result<(SessionId, PeerId), String>{
+        self.user_count+=1;
+        self.session_count+=1;
+        // give out two session ids in a row to simulate functional auth service
+        // where two peers who want to communicate will be given the same session id
+        let session_id = self.session_count/2;
+        Ok(([session_id;16], self.user_count))
+    }
+}
+
+struct Kme {
+    keys: Arc<Mutex<HashMap<SessionId,KeyEntry>>>,
+}
+
+
+impl Kme {
+    fn new() -> Self {
+        Self { keys: Arc::new(Mutex::new(HashMap::new())) }
+    }
+
+    // todo: service should handle concurrent requests safely
+    pub fn request_key(&self, peer: Peer) -> Result<KeyResponse, KmeError> {
+        let (session_id, _peer_id) = peer;
+        let mut keys = self.keys.lock().unwrap();
+
+        // keys should be single use
+        if let Some(entry) = keys.get_mut(&session_id) {
+            let diff = Instant::now() - entry.creation;
+            // keys will timeout
+            if diff > Duration::from_secs(1) { // 1 second for feasible testing
+                return Err(KmeError::KeyExpired);
+            } 
+            if let KeyEntryState::Consumed = entry.state {
+                // third peer: key already consumed
+                return Err(KmeError::KeyConsumed);
+            }
+            // second peer: return the same key
+            entry.state = KeyEntryState::Consumed; 
+            // todo: update retrieved_by as well? why needed?
+            Ok(KeyResponse { key_id: String::new(), key: entry.key })
+        } else {
+            // first peer: generate and store a new key
+            let key = [0u8; 32]; // TODO: generate real key bytes
+            keys.insert(session_id, KeyEntry {
+                key,
+                creation: Instant::now(),
+                state: KeyEntryState::Active { retrieved_by: vec![] },
+            });
+            Ok(KeyResponse { key_id: String::new(), key })
+        }
+    }
+}
+
+fn main() {
+    let mut auth = Auth{ user_count: 0, session_count: 1};
+    let first_peer = auth.login().unwrap(); // assume fine for now
+    let second_peer = auth.login().unwrap(); // assume fine for now
+
+    let kme = Kme::new();
+    let first_key = kme.request_key((first_peer.0, second_peer.1)).unwrap();
+    let second_key = kme.request_key((second_peer.0, first_peer.1)).unwrap();
+    assert_eq!(first_key, second_key);
+
+    let dupe_attempt = kme.request_key((second_peer.0, first_peer.1)).unwrap_err();
+    assert_eq!(dupe_attempt, KmeError::KeyConsumed);
+
+    let third_peer = auth.login().unwrap();
+    // third_peer somehow hijacks second peer's session_id
+    let third_key = kme.request_key((second_peer.0, third_peer.1)).unwrap_err();
+    assert_eq!(third_key, KmeError::KeyConsumed);
+
+    let forth_peer = auth.login().unwrap();
+    let _third_key = kme.request_key((third_peer.0, forth_peer.1)).unwrap();
+    std::thread::sleep(std::time::Duration::from_secs(2));
+    let forth_key = kme.request_key((forth_peer.0, third_peer.1)).unwrap_err();
+    assert_eq!(forth_key, KmeError::KeyExpired);
+}