diff --git a/crates/proton-sdk/Cargo.toml b/crates/proton-sdk/Cargo.toml
index 2c43f2d..472437d 100644
--- a/crates/proton-sdk/Cargo.toml
+++ b/crates/proton-sdk/Cargo.toml
@@ -21,6 +21,8 @@ base64.workspace = true
 hex.workspace = true
 hmac.workspace = true
 sha2.workspace = true
+aes-gcm.workspace = true
+hkdf.workspace = true
 bcrypt.workspace = true
 pgp.workspace = true
 tracing.workspace = true
@@ -28,3 +30,8 @@ num-bigint.workspace = true
 num-traits.workspace = true
 getrandom.workspace = true
 rand.workspace = true
+
+[dev-dependencies]
+# A one-shot loopback HTTP server backs the telemetry-instrumentation test;
+# the `net` feature isn't needed by the library itself.
+tokio = { workspace = true, features = ["net"] }
diff --git a/crates/proton-sdk/src/cache.rs b/crates/proton-sdk/src/cache.rs
new file mode 100644
index 0000000..9f3b308
--- /dev/null
+++ b/crates/proton-sdk/src/cache.rs
@@ -0,0 +1,436 @@
+//! Generic persistent-cache primitives.
+//!
+//! A string key/value store with tags, mirroring the C# `Proton.Sdk.Caching`
+//! layer: the [`CacheRepository`] trait (C# `ICacheRepository`), an in-memory
+//! implementation ([`InMemoryCacheRepository`], C# `InMemoryCacheRepository`),
+//! and an at-rest-encryption wrapper ([`EncryptedCacheRepository`], C#
+//! `EncryptedCacheRepository`).
+//!
+//! Higher layers (the Drive entity/secret caches) serialize typed values to
+//! JSON strings and store them here; a consumer can supply an on-disk
+//! implementation of [`CacheRepository`] (e.g. SQLite) without changing those
+//! layers.
+
+use std::collections::{HashMap, HashSet};
+use std::sync::{Arc, Mutex};
+
+use aes_gcm::aead::{Aead, KeyInit, Payload};
+use aes_gcm::{Aes256Gcm, Nonce};
+use async_trait::async_trait;
+use base64::Engine;
+use hkdf::Hkdf;
+use sha2::Sha256;
+
+use crate::error::{ProtonError, Result};
+
+/// A string key/value cache with secondary tag indexing.
+///
+/// Mirrors C# `ICacheRepository`. Implementations must be cheap to share across
+/// tasks (hence [`Send`] + [`Sync`]); the SDK holds them as
+/// `Arc<dyn CacheRepository>`.
+#[async_trait]
+pub trait CacheRepository: Send + Sync {
+    /// Store `value` under `key`, replacing any existing entry and its tags.
+    async fn set(&self, key: &str, value: &str, tags: &[String]) -> Result<()>;
+
+    /// Fetch the value stored under `key`, or `None` if absent.
+    async fn get(&self, key: &str) -> Result<Option<String>>;
+
+    /// Remove the entry stored under `key` (no-op if absent).
+    async fn remove(&self, key: &str) -> Result<()>;
+
+    /// Remove every entry carrying `tag`.
+    async fn remove_by_tag(&self, tag: &str) -> Result<()>;
+
+    /// Remove every entry.
+    async fn clear(&self) -> Result<()>;
+
+    /// Return every `(key, value)` whose entry carries **all** of `tags`
+    /// (set intersection, matching C# `GetByTags`). An empty `tags` slice
+    /// returns nothing.
+    async fn get_by_tags(&self, tags: &[String]) -> Result<Vec<(String, String)>>;
+}
+
+/// Convenience: store a value with no tags.
+pub async fn set_untagged(repo: &dyn CacheRepository, key: &str, value: &str) -> Result<()> {
+    repo.set(key, value, &[]).await
+}
+
+/// Thread-safe in-memory [`CacheRepository`]. Mirrors C#
+/// `InMemoryCacheRepository`.
+#[derive(Default)]
+pub struct InMemoryCacheRepository {
+    state: Mutex<InMemoryState>,
+}
+
+#[derive(Default)]
+struct InMemoryState {
+    entries: HashMap<String, String>,
+    key_to_tags: HashMap<String, HashSet<String>>,
+    tag_to_keys: HashMap<String, HashSet<String>>,
+}
+
+impl InMemoryCacheRepository {
+    /// Create an empty in-memory cache.
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Wrap a new in-memory cache in an [`Arc`] behind the trait object.
+    pub fn shared() -> Arc<dyn CacheRepository> {
+        Arc::new(Self::new())
+    }
+
+    fn clear_tags_for_key(state: &mut InMemoryState, key: &str) {
+        if let Some(tags) = state.key_to_tags.remove(key) {
+            for tag in tags {
+                if let Some(keys) = state.tag_to_keys.get_mut(&tag) {
+                    keys.remove(key);
+                    if keys.is_empty() {
+                        state.tag_to_keys.remove(&tag);
+                    }
+                }
+            }
+        }
+    }
+}
+
+#[async_trait]
+impl CacheRepository for InMemoryCacheRepository {
+    async fn set(&self, key: &str, value: &str, tags: &[String]) -> Result<()> {
+        let mut state = self.state.lock().unwrap();
+        Self::clear_tags_for_key(&mut state, key);
+        state.entries.insert(key.to_owned(), value.to_owned());
+        let tag_set: HashSet<String> = tags.iter().cloned().collect();
+        for tag in &tag_set {
+            state
+                .tag_to_keys
+                .entry(tag.clone())
+                .or_default()
+                .insert(key.to_owned());
+        }
+        state.key_to_tags.insert(key.to_owned(), tag_set);
+        Ok(())
+    }
+
+    async fn get(&self, key: &str) -> Result<Option<String>> {
+        let state = self.state.lock().unwrap();
+        Ok(state.entries.get(key).cloned())
+    }
+
+    async fn remove(&self, key: &str) -> Result<()> {
+        let mut state = self.state.lock().unwrap();
+        state.entries.remove(key);
+        Self::clear_tags_for_key(&mut state, key);
+        Ok(())
+    }
+
+    async fn remove_by_tag(&self, tag: &str) -> Result<()> {
+        let mut state = self.state.lock().unwrap();
+        let keys: Vec<String> = state
+            .tag_to_keys
+            .get(tag)
+            .map(|keys| keys.iter().cloned().collect())
+            .unwrap_or_default();
+        for key in keys {
+            state.entries.remove(&key);
+            Self::clear_tags_for_key(&mut state, &key);
+        }
+        Ok(())
+    }
+
+    async fn clear(&self) -> Result<()> {
+        let mut state = self.state.lock().unwrap();
+        state.entries.clear();
+        state.key_to_tags.clear();
+        state.tag_to_keys.clear();
+        Ok(())
+    }
+
+    async fn get_by_tags(&self, tags: &[String]) -> Result<Vec<(String, String)>> {
+        if tags.is_empty() {
+            return Ok(Vec::new());
+        }
+        let state = self.state.lock().unwrap();
+        let mut candidates: Option<HashSet<String>> = None;
+        for tag in tags {
+            match state.tag_to_keys.get(tag) {
+                Some(keys) => {
+                    candidates = Some(match candidates {
+                        Some(existing) => existing.intersection(keys).cloned().collect(),
+                        None => keys.clone(),
+                    });
+                }
+                None => return Ok(Vec::new()),
+            }
+            if candidates.as_ref().is_some_and(|c| c.is_empty()) {
+                return Ok(Vec::new());
+            }
+        }
+        let candidates = candidates.unwrap_or_default();
+        Ok(candidates
+            .into_iter()
+            .filter_map(|key| state.entries.get(&key).map(|v| (key.clone(), v.clone())))
+            .collect())
+    }
+}
+
+/// At-rest-encryption wrapper around any [`CacheRepository`]. Mirrors C#
+/// `EncryptedCacheRepository`.
+///
+/// Each value is encrypted independently: a random 16-byte salt feeds
+/// HKDF-SHA256 (with the entry key mixed into the `info` parameter) to derive a
+/// fresh AES-256-GCM key and 96-bit nonce; the stored payload is
+/// `base64([salt(16)][ciphertext][tag(16)])`. Keys and tags are stored in the
+/// clear (they drive lookup); only values are protected.
+///
+/// A GCM authentication failure on read is treated as tampering or a changed
+/// encryption key: the inner cache is cleared and the read reported as a miss
+/// (matching the C# behavior).
+pub struct EncryptedCacheRepository {
+    inner: Arc<dyn CacheRepository>,
+    encryption_key: Vec<u8>,
+}
+
+const SALT_LEN: usize = 16;
+const KEY_LEN: usize = 32;
+const NONCE_LEN: usize = 12;
+const TAG_LEN: usize = 16;
+const ENCRYPTION_CONTEXT: &[u8] = b"Drive.EncryptedCacheRepository";
+
+impl EncryptedCacheRepository {
+    /// Wrap `inner`, encrypting values under `encryption_key`.
+    pub fn new(inner: Arc<dyn CacheRepository>, encryption_key: impl Into<Vec<u8>>) -> Self {
+        Self {
+            inner,
+            encryption_key: encryption_key.into(),
+        }
+    }
+
+    /// Wrap `inner` and box the result behind the trait object.
+    pub fn shared(
+        inner: Arc<dyn CacheRepository>,
+        encryption_key: impl Into<Vec<u8>>,
+    ) -> Arc<dyn CacheRepository> {
+        Arc::new(Self::new(inner, encryption_key))
+    }
+
+    /// Derive the per-entry AES key + nonce from the salt and entry key.
+    fn derive(&self, salt: &[u8], entry_key: &str) -> Result<([u8; KEY_LEN], [u8; NONCE_LEN])> {
+        let mut info = ENCRYPTION_CONTEXT.to_vec();
+        info.extend_from_slice(entry_key.as_bytes());
+        let hk = Hkdf::<Sha256>::new(Some(salt), &self.encryption_key);
+        let mut okm = [0u8; KEY_LEN + NONCE_LEN];
+        hk.expand(&info, &mut okm)
+            .map_err(|e| ProtonError::invalid_operation(format!("cache HKDF expand: {e}")))?;
+        let mut key = [0u8; KEY_LEN];
+        let mut nonce = [0u8; NONCE_LEN];
+        key.copy_from_slice(&okm[..KEY_LEN]);
+        nonce.copy_from_slice(&okm[KEY_LEN..]);
+        Ok((key, nonce))
+    }
+
+    fn encrypt(&self, entry_key: &str, plaintext: &str) -> Result<String> {
+        let mut salt = [0u8; SALT_LEN];
+        getrandom::getrandom(&mut salt)
+            .map_err(|e| ProtonError::invalid_operation(format!("cache salt: {e}")))?;
+        let (key, nonce) = self.derive(&salt, entry_key)?;
+        let cipher = Aes256Gcm::new_from_slice(&key)
+            .map_err(|e| ProtonError::invalid_operation(format!("cache cipher: {e}")))?;
+        // aes-gcm appends the 16-byte tag to the ciphertext, so this yields
+        // `ciphertext || tag` — matching the C# `[salt][ciphertext][tag]` layout.
+        let sealed = cipher
+            .encrypt(
+                Nonce::from_slice(&nonce),
+                Payload {
+                    msg: plaintext.as_bytes(),
+                    aad: &[],
+                },
+            )
+            .map_err(|_| ProtonError::invalid_operation("cache encrypt failed"))?;
+        let mut out = Vec::with_capacity(SALT_LEN + sealed.len());
+        out.extend_from_slice(&salt);
+        out.extend_from_slice(&sealed);
+        Ok(base64::engine::general_purpose::STANDARD.encode(out))
+    }
+
+    /// Decrypt a stored value. `Ok(None)` signals a GCM auth failure (tampered
+    /// or stale entry); the caller clears the cache and treats it as a miss.
+    fn decrypt(&self, entry_key: &str, encoded: &str) -> Result<Option<String>> {
+        let combined = base64::engine::general_purpose::STANDARD
+            .decode(encoded)
+            .map_err(|e| ProtonError::invalid_operation(format!("cache base64: {e}")))?;
+        if combined.len() < SALT_LEN + TAG_LEN {
+            return Err(ProtonError::invalid_operation("cache value too short"));
+        }
+        let (salt, sealed) = combined.split_at(SALT_LEN);
+        let (key, nonce) = self.derive(salt, entry_key)?;
+        let cipher = Aes256Gcm::new_from_slice(&key)
+            .map_err(|e| ProtonError::invalid_operation(format!("cache cipher: {e}")))?;
+        match cipher.decrypt(
+            Nonce::from_slice(&nonce),
+            Payload {
+                msg: sealed,
+                aad: &[],
+            },
+        ) {
+            Ok(plaintext) => {
+                let text = String::from_utf8(plaintext)
+                    .map_err(|e| ProtonError::invalid_operation(format!("cache utf8: {e}")))?;
+                Ok(Some(text))
+            }
+            // Authentication failure: tampering or a changed key. Signal a miss.
+            Err(_) => Ok(None),
+        }
+    }
+}
+
+#[async_trait]
+impl CacheRepository for EncryptedCacheRepository {
+    async fn set(&self, key: &str, value: &str, tags: &[String]) -> Result<()> {
+        let encrypted = self.encrypt(key, value)?;
+        self.inner.set(key, &encrypted, tags).await
+    }
+
+    async fn get(&self, key: &str) -> Result<Option<String>> {
+        let Some(encrypted) = self.inner.get(key).await? else {
+            return Ok(None);
+        };
+        match self.decrypt(key, &encrypted)? {
+            Some(value) => Ok(Some(value)),
+            None => {
+                self.inner.clear().await?;
+                Ok(None)
+            }
+        }
+    }
+
+    async fn remove(&self, key: &str) -> Result<()> {
+        self.inner.remove(key).await
+    }
+
+    async fn remove_by_tag(&self, tag: &str) -> Result<()> {
+        self.inner.remove_by_tag(tag).await
+    }
+
+    async fn clear(&self) -> Result<()> {
+        self.inner.clear().await
+    }
+
+    async fn get_by_tags(&self, tags: &[String]) -> Result<Vec<(String, String)>> {
+        let entries = self.inner.get_by_tags(tags).await?;
+        let mut out = Vec::with_capacity(entries.len());
+        for (key, encrypted) in entries {
+            match self.decrypt(&key, &encrypted)? {
+                Some(value) => out.push((key, value)),
+                None => {
+                    self.inner.clear().await?;
+                    return Ok(Vec::new());
+                }
+            }
+        }
+        Ok(out)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn tags(values: &[&str]) -> Vec<String> {
+        values.iter().map(|s| s.to_string()).collect()
+    }
+
+    #[tokio::test]
+    async fn in_memory_round_trips_and_overwrites() {
+        let cache = InMemoryCacheRepository::new();
+        cache.set("k", "v1", &[]).await.unwrap();
+        assert_eq!(cache.get("k").await.unwrap().as_deref(), Some("v1"));
+        cache.set("k", "v2", &[]).await.unwrap();
+        assert_eq!(cache.get("k").await.unwrap().as_deref(), Some("v2"));
+        cache.remove("k").await.unwrap();
+        assert_eq!(cache.get("k").await.unwrap(), None);
+    }
+
+    #[tokio::test]
+    async fn in_memory_get_by_tags_intersects() {
+        let cache = InMemoryCacheRepository::new();
+        cache.set("a", "1", &tags(&["x", "y"])).await.unwrap();
+        cache.set("b", "2", &tags(&["x"])).await.unwrap();
+        cache.set("c", "3", &tags(&["y"])).await.unwrap();
+
+        let mut both = cache.get_by_tags(&tags(&["x", "y"])).await.unwrap();
+        both.sort();
+        assert_eq!(both, vec![("a".to_string(), "1".to_string())]);
+
+        let mut just_x = cache.get_by_tags(&tags(&["x"])).await.unwrap();
+        just_x.sort();
+        assert_eq!(
+            just_x,
+            vec![
+                ("a".to_string(), "1".to_string()),
+                ("b".to_string(), "2".to_string())
+            ]
+        );
+
+        assert!(cache.get_by_tags(&[]).await.unwrap().is_empty());
+    }
+
+    #[tokio::test]
+    async fn in_memory_remove_by_tag_drops_only_tagged() {
+        let cache = InMemoryCacheRepository::new();
+        cache.set("a", "1", &tags(&["x"])).await.unwrap();
+        cache.set("b", "2", &tags(&["y"])).await.unwrap();
+        cache.remove_by_tag("x").await.unwrap();
+        assert_eq!(cache.get("a").await.unwrap(), None);
+        assert_eq!(cache.get("b").await.unwrap().as_deref(), Some("2"));
+        // The tag index is cleaned up too: re-querying yields nothing.
+        assert!(cache.get_by_tags(&tags(&["x"])).await.unwrap().is_empty());
+    }
+
+    #[tokio::test]
+    async fn encrypted_round_trips_and_hides_plaintext() {
+        let inner = InMemoryCacheRepository::shared();
+        let cache = EncryptedCacheRepository::new(inner.clone(), b"hunter2-master-key".to_vec());
+        cache.set("share:1", "secret-value", &tags(&["t"])).await.unwrap();
+
+        // Stored ciphertext is not the plaintext.
+        let stored = inner.get("share:1").await.unwrap().unwrap();
+        assert_ne!(stored, "secret-value");
+
+        // Round-trips through the wrapper.
+        assert_eq!(
+            cache.get("share:1").await.unwrap().as_deref(),
+            Some("secret-value")
+        );
+        // Tags pass through to the inner store.
+        let by_tag = cache.get_by_tags(&tags(&["t"])).await.unwrap();
+        assert_eq!(by_tag, vec![("share:1".to_string(), "secret-value".to_string())]);
+    }
+
+    #[tokio::test]
+    async fn encrypted_wrong_key_is_a_miss_and_clears() {
+        let inner = InMemoryCacheRepository::shared();
+        EncryptedCacheRepository::new(inner.clone(), b"key-one".to_vec())
+            .set("k", "v", &[])
+            .await
+            .unwrap();
+
+        // A different key fails the GCM tag check → treated as a miss, cache cleared.
+        let other = EncryptedCacheRepository::new(inner.clone(), b"key-two".to_vec());
+        assert_eq!(other.get("k").await.unwrap(), None);
+        assert_eq!(inner.get("k").await.unwrap(), None);
+    }
+
+    #[tokio::test]
+    async fn encrypted_salt_is_random_per_write() {
+        let inner = InMemoryCacheRepository::shared();
+        let cache = EncryptedCacheRepository::new(inner.clone(), b"k".to_vec());
+        cache.set("k", "same", &[]).await.unwrap();
+        let first = inner.get("k").await.unwrap().unwrap();
+        cache.set("k", "same", &[]).await.unwrap();
+        let second = inner.get("k").await.unwrap().unwrap();
+        // Random salt per write ⇒ identical plaintext yields different ciphertext.
+        assert_ne!(first, second);
+    }
+}
diff --git a/crates/proton-sdk/src/lib.rs b/crates/proton-sdk/src/lib.rs
index a9dec5c..0fe0d2f 100644
--- a/crates/proton-sdk/src/lib.rs
+++ b/crates/proton-sdk/src/lib.rs
@@ -12,12 +12,14 @@
 
 pub mod account;
 pub mod api;
+pub mod cache;
 pub mod config;
 pub mod crypto;
 pub mod error;
 pub mod http;
 pub mod ids;
 pub mod session;
+pub mod telemetry;
 
 pub use error::{ProtonApiError, ProtonError, Result};
 pub use session::ProtonApiSession;