cachekit/

config.rs

use std::time::Duration;

use zeroize::Zeroizing;

use crate::error::CachekitError;

/// Maximum number of decrypt-only previous master keys.
///
/// Mirrors `cachekit_core::MAX_DECRYPT_ONLY_KEYS` (spec/encryption.md → "Key
/// Rotation (Keyring)"), which is feature-gated behind `encryption` and so
/// cannot be referenced here unconditionally. A drift-guard test in
/// `config_tests.rs` asserts the two stay equal.
pub const MAX_PREVIOUS_MASTER_KEYS: usize = 3;

// ── CachekitConfig ────────────────────────────────────────────────────────────

/// Configuration values for a CacheKit client.
///
/// A client reads a `CachekitConfig` on one path only: `CacheKit::from_env()`
/// builds one with [`CachekitConfig::from_env`] and applies it. No `CacheKit`
/// constructor takes a `CachekitConfig` value, so a config made with
/// [`CachekitConfigBuilder`] configures no client.
pub struct CachekitConfig {
    /// API key for cachekit.io authentication.
    pub api_key: Option<Zeroizing<String>>,
    /// Base URL of the cachekit.io API.
    pub api_url: String,
    /// Master key used for zero-knowledge encryption (AES-256-GCM).
    pub master_key: Option<Zeroizing<Vec<u8>>>,
    /// Decrypt-only previous master keys for a rotation grace window, in
    /// attempt order. At most [`MAX_PREVIOUS_MASTER_KEYS`] entries.
    ///
    /// These hold key material only. See
    /// [`CachekitConfigBuilder::previous_master_keys`] for the paths that
    /// apply previous keys to a client.
    pub previous_master_keys: Vec<Zeroizing<Vec<u8>>>,
    /// Default TTL for cache entries when none is specified at call site.
    pub default_ttl: Duration,
    /// Optional namespace prefix applied to all cache keys.
    pub namespace: Option<String>,
    /// Maximum number of entries in the L1 in-process cache.
    pub l1_capacity: usize,
    /// Maximum allowed payload size in bytes. Larger payloads are rejected.
    pub max_payload_bytes: usize,
}

impl std::fmt::Debug for CachekitConfig {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        let api_key_repr = if self.api_key.is_some() {
            "[REDACTED]"
        } else {
            "None"
        };
        let master_key_repr = if self.master_key.is_some() {
            "[REDACTED]"
        } else {
            "None"
        };

        f.debug_struct("CachekitConfig")
            .field("api_key", &api_key_repr)
            .field("api_url", &self.api_url)
            .field("master_key", &master_key_repr)
            .field(
                "previous_master_keys",
                &format_args!("[REDACTED; {}]", self.previous_master_keys.len()),
            )
            .field("default_ttl", &self.default_ttl)
            .field("namespace", &self.namespace)
            .field("l1_capacity", &self.l1_capacity)
            .field("max_payload_bytes", &self.max_payload_bytes)
            .finish()
    }
}

impl Default for CachekitConfig {
    fn default() -> Self {
        Self {
            api_key: None,
            api_url: "https://api.cachekit.io".to_owned(),
            master_key: None,
            previous_master_keys: Vec::new(),
            default_ttl: Duration::from_secs(300),
            namespace: None,
            l1_capacity: 1000,
            max_payload_bytes: 5 * 1024 * 1024, // 5 MiB
        }
    }
}

impl CachekitConfig {
    /// Build configuration from environment variables.
    ///
    /// | Variable | Description |
    /// |---|---|
    /// | `CACHEKIT_API_KEY` | API key for cachekit.io |
    /// | `CACHEKIT_API_URL` | Override API base URL (must be HTTPS) |
    /// | `CACHEKIT_MASTER_KEY` | Hex-encoded master key (min 32 bytes); set but empty, not hex or not UTF-8 is an error, never unset |
    /// | `CACHEKIT_PREVIOUS_MASTER_KEYS` | Comma-separated hex-encoded decrypt-only previous master keys (max 3; blank value = unset) |
    /// | `CACHEKIT_DEFAULT_TTL` | Default TTL in seconds (min 1) |
    pub fn from_env() -> Result<Self, CachekitError> {
        let mut config = Self::default();

        // API key
        if let Ok(val) = std::env::var("CACHEKIT_API_KEY") {
            config.api_key = Some(Zeroizing::new(val));
        }

        // API URL — must be HTTPS
        if let Ok(val) = std::env::var("CACHEKIT_API_URL") {
            validate_https(&val)?;
            config.api_url = val;
        }

        // Master key — hex-decode and validate length >= 32 bytes.
        // Deliberately NO blank-value tolerance here (unlike the previous-keys
        // var below): a blank CACHEKIT_MASTER_KEY treated as unset would
        // silently turn encryption off. Set but not UTF-8 (a raw key put in
        // the environment without hex-encoding it) is an error for the same
        // reason, never unset.
        match std::env::var("CACHEKIT_MASTER_KEY") {
            Ok(val) => {
                // `env::var` hands back an owned copy of the hex secret. Wrap it
                // so that copy is wiped on drop too — the decoded bytes below are
                // already `Zeroizing`, but the hex form is the same key material.
                // Defence in depth over the heap copy only: the process `environ`
                // block still holds the identical hex for the process lifetime
                // and is not wiped here, so this narrows post-lifetime recovery
                // (core dumps, swap, heap reuse), it does not eliminate the
                // exposure.
                let val = Zeroizing::new(val);
                config.master_key = Some(decode_master_key_hex(&val, "CACHEKIT_MASTER_KEY")?);
            }
            Err(std::env::VarError::NotPresent) => {}
            Err(std::env::VarError::NotUnicode(_)) => {
                return Err(CachekitError::Config(
                    "CACHEKIT_MASTER_KEY is not valid UTF-8: set it to the key's 64 hex chars"
                        .to_owned(),
                ))
            }
        }

        config.previous_master_keys =
            previous_master_keys_from_env(config.master_key.as_deref().map(Vec::as_slice))?;

        // Default TTL — minimum 1 second
        if let Ok(val) = std::env::var("CACHEKIT_DEFAULT_TTL") {
            let secs: u64 = val.parse().map_err(|e| {
                CachekitError::Config(format!("CACHEKIT_DEFAULT_TTL must be an integer: {e}"))
            })?;
            if secs < 1 {
                return Err(CachekitError::Config(
                    "CACHEKIT_DEFAULT_TTL must be at least 1 second".to_owned(),
                ));
            }
            config.default_ttl = Duration::from_secs(secs);
        }

        Ok(config)
    }
}

// ── CachekitConfigBuilder ─────────────────────────────────────────────────────

/// Fluent builder for a [`CachekitConfig`] value.
///
/// The builder validates and holds values. No `CacheKit` constructor takes
/// the [`CachekitConfig`] it builds, so nothing set here (keys, namespace,
/// TTL, capacity) reaches a client. To configure a client, use
/// `CacheKit::builder()` or `CacheKit::from_env()`.
#[derive(Default)]
#[must_use]
pub struct CachekitConfigBuilder {
    inner: CachekitConfig,
}

impl CachekitConfigBuilder {
    /// Create a new builder with defaults.
    pub fn new() -> Self {
        Self {
            inner: CachekitConfig::default(),
        }
    }

    /// Set the API key.
    pub fn api_key(mut self, key: impl Into<String>) -> Self {
        self.inner.api_key = Some(Zeroizing::new(key.into()));
        self
    }

    /// Set the API base URL. Must use HTTPS.
    pub fn api_url(mut self, url: impl Into<String>) -> Result<Self, CachekitError> {
        let url = url.into();
        validate_https(&url)?;
        self.inner.api_url = url;
        Ok(self)
    }

    /// Set the master key from a hex string. Must decode to at least 32 bytes.
    pub fn master_key(mut self, hex_key: &str) -> Result<Self, CachekitError> {
        let bytes = decode_master_key_hex(hex_key, "master_key")?;
        validate_previous_master_keys(
            Some(bytes.as_slice()),
            &self.inner.previous_master_keys,
            "previous_master_keys",
        )?;
        self.inner.master_key = Some(bytes);
        Ok(self)
    }

    /// Set decrypt-only previous master keys from hex strings, in attempt
    /// order, on the [`CachekitConfig`] value.
    ///
    /// No client reads keys set here. With the `encryption` feature enabled,
    /// these paths apply previous keys to a client; each one decrypts with
    /// the current key first, then each previous key in order:
    ///
    /// * `CacheKit::from_env()` and `CacheKit::secure_from_env()` read them
    ///   from `CACHEKIT_PREVIOUS_MASTER_KEYS`.
    /// * `CacheKitBuilder::encryption_from_bytes_with_previous()` takes them
    ///   as arguments. It requires every key to be exactly 32 raw bytes,
    ///   while this method accepts hex that decodes to 32 bytes or more.
    ///
    /// Without the `encryption` feature no path accepts a master key, so
    /// none can silently drop one: `CacheKit::secure_from_env()` does not
    /// exist, `CacheKit::from_env()` returns [`CachekitError::Config`] when
    /// `CACHEKIT_MASTER_KEY` is set, and
    /// `CacheKitBuilder::encryption_from_bytes_with_previous()` always
    /// returns [`CachekitError::Config`].
    ///
    /// Validation is identical to [`Self::master_key`] per entry (valid hex,
    /// at least 32 bytes). At most [`MAX_PREVIOUS_MASTER_KEYS`] entries —
    /// more is a [`CachekitError::Config`], never truncated. The current
    /// master key must not reappear here (forward-only rotation: a retired
    /// key is never re-promoted).
    ///
    /// # Examples
    ///
    /// ```
    /// use cachekit::config::CachekitConfigBuilder;
    ///
    /// // k2 is current after rotation; k1 is kept as a decrypt-only previous key.
    /// let k1 = "11".repeat(32);
    /// let k2 = "22".repeat(32);
    ///
    /// let config = CachekitConfigBuilder::new()
    ///     .master_key(&k2)?
    ///     .previous_master_keys(&[k1.as_str()])?
    ///     .build();
    ///
    /// assert_eq!(config.previous_master_keys.len(), 1);
    /// # Ok::<(), cachekit::CachekitError>(())
    /// ```
    pub fn previous_master_keys(mut self, hex_keys: &[&str]) -> Result<Self, CachekitError> {
        let mut previous = Vec::with_capacity(hex_keys.len());
        for hex_key in hex_keys {
            previous.push(decode_master_key_hex(
                hex_key,
                "previous_master_keys entry",
            )?);
        }
        validate_previous_master_keys(
            self.inner.master_key.as_deref().map(Vec::as_slice),
            &previous,
            "previous_master_keys",
        )?;
        self.inner.previous_master_keys = previous;
        Ok(self)
    }

    /// Set the default TTL. Must be non-zero; a positive sub-second TTL is
    /// sent to the backend as 1 second.
    pub fn default_ttl(mut self, ttl: Duration) -> Result<Self, CachekitError> {
        if ttl.is_zero() {
            return Err(CachekitError::Config(
                "default_ttl must be greater than zero".to_owned(),
            ));
        }
        self.inner.default_ttl = ttl;
        Ok(self)
    }

    /// Set the namespace prefix.
    pub fn namespace(mut self, ns: impl Into<String>) -> Self {
        self.inner.namespace = Some(ns.into());
        self
    }

    /// Set the L1 cache capacity (max entries).
    pub fn l1_capacity(mut self, capacity: usize) -> Self {
        self.inner.l1_capacity = capacity;
        self
    }

    /// Finalise and return the [`CachekitConfig`].
    pub fn build(self) -> CachekitConfig {
        self.inner
    }
}

// ── Helpers ───────────────────────────────────────────────────────────────────

/// Hex-decode a master key and require at least 32 bytes. Shared by every
/// master-key hex path — the current and previous keys here, the `secure`
/// preset and the builder's `.encryption()` — so validation cannot drift.
///
/// Returns `Zeroizing` so the decoded key material is wiped on drop for its
/// whole lifetime — including the early-drop paths where a caller's later
/// validation step fails.
pub(crate) fn decode_master_key_hex(
    hex_key: &str,
    what: &str,
) -> Result<Zeroizing<Vec<u8>>, CachekitError> {
    // Decode straight into zeroizing storage sized once: `hex::decode` grows
    // its Vec by reallocation and frees each outgrown buffer unwiped.
    let mut bytes = Zeroizing::new(vec![0u8; hex_key.len() / 2]);
    hex::decode_to_slice(hex_key, &mut bytes).map_err(|e| {
        // `FromHexError`'s Display quotes the offending character — one
        // character of the key in an error string (CWE-532). Name the class.
        // Exhaustive on purpose: a new variant must be reviewed for the same leak.
        let reason = match e {
            hex::FromHexError::InvalidHexCharacter { .. } => "contains a non-hex character",
            hex::FromHexError::OddLength => "odd number of hex digits",
            hex::FromHexError::InvalidStringLength => "invalid length",
        };
        CachekitError::Config(format!("{what} is not valid hex: {reason}"))
    })?;
    if bytes.len() < 32 {
        return Err(CachekitError::Config(format!(
            "{what} must be at least 32 bytes (64 hex chars); got {} bytes",
            bytes.len()
        )));
    }
    Ok(bytes)
}

/// `CACHEKIT_PREVIOUS_MASTER_KEYS`, decoded and validated against the
/// decoded current key. The one reader of the variable, shared by
/// [`CachekitConfig::from_env`] and `CacheKit::secure_from_env` so the two
/// env paths cannot drift. Unset or blank yields no keys.
pub(crate) fn previous_master_keys_from_env(
    master_key: Option<&[u8]>,
) -> Result<Vec<Zeroizing<Vec<u8>>>, CachekitError> {
    // Same reasoning as CACHEKIT_MASTER_KEY in `from_env`: wipe the owned
    // copy of the hex list on drop.
    let val = match std::env::var("CACHEKIT_PREVIOUS_MASTER_KEYS") {
        Ok(val) => Zeroizing::new(val),
        Err(std::env::VarError::NotPresent) => return Ok(Vec::new()),
        // Set but unreadable is not unset: dropping it would silently strip
        // the decrypt-only keys, and every old-key read would fail closed.
        Err(std::env::VarError::NotUnicode(_)) => {
            return Err(CachekitError::Config(
                "CACHEKIT_PREVIOUS_MASTER_KEYS is not valid UTF-8".to_owned(),
            ))
        }
    };
    // A wholly blank value retires the variable (the common way to disable
    // it in shell profiles, Compose files, and k8s manifests) and is treated
    // as unset; a blank entry inside a non-blank list is still an operator
    // mistake.
    if val.trim().is_empty() {
        return Ok(Vec::new());
    }
    let mut previous = Vec::new();
    for entry in val.split(',') {
        let entry = entry.trim();
        if entry.is_empty() {
            return Err(CachekitError::Config(
                "CACHEKIT_PREVIOUS_MASTER_KEYS contains an empty entry".to_owned(),
            ));
        }
        previous.push(decode_master_key_hex(
            entry,
            "CACHEKIT_PREVIOUS_MASTER_KEYS entry",
        )?);
    }
    // Previous keys without a current key is a broken rotation deploy:
    // nothing would ever consume them, and the operator would only find out
    // at the first secure_cache() call. Fail at load.
    let Some(master_key) = master_key else {
        return Err(CachekitError::Config(
            "CACHEKIT_PREVIOUS_MASTER_KEYS requires CACHEKIT_MASTER_KEY to be set".to_owned(),
        ));
    };
    validate_previous_master_keys(Some(master_key), &previous, "CACHEKIT_PREVIOUS_MASTER_KEYS")?;
    Ok(previous)
}

/// Enforce the keyring config invariants: at most [`MAX_PREVIOUS_MASTER_KEYS`]
/// previous keys (rejected, never truncated), and the current master key must
/// not also appear in the previous list (the detectable subset of the
/// forward-only rotation rule — re-promoting a retired key would resume a
/// used, unknowable AES-GCM nonce budget).
///
/// Fail-fast mirror of the checks `cachekit_core::Keyring::new` repeats at
/// client build time; plain equality is fine — both operands are
/// operator-supplied configuration, not secrets under timing attack.
fn validate_previous_master_keys(
    master_key: Option<&[u8]>,
    previous: &[Zeroizing<Vec<u8>>],
    source: &str,
) -> Result<(), CachekitError> {
    if previous.len() > MAX_PREVIOUS_MASTER_KEYS {
        return Err(CachekitError::Config(format!(
            "{source} accepts at most {MAX_PREVIOUS_MASTER_KEYS} entries; got {}",
            previous.len()
        )));
    }
    if let Some(master) = master_key {
        if previous.iter().any(|key| key.as_slice() == master) {
            return Err(CachekitError::Config(format!(
                "the current master key must not appear in {source} \
                 (rotation is forward-only; retired keys are never re-promoted)"
            )));
        }
    }
    Ok(())
}

fn validate_https(url: &str) -> Result<(), CachekitError> {
    if !url.starts_with("https://") {
        return Err(CachekitError::Config(format!(
            "API URL must use HTTPS; got: {url}"
        )));
    }
    Ok(())
}