1 files changed, 72 insertions, 0 deletions
diff --git a/src/signed.rs b/src/signed.rs
new file mode 100644
index 0000000..9f0d184
--- /dev/null
+++ b/src/signed.rs
@@ -0,0 +1,72 @@
+// This code was adapted from the cookie crate which does not make the sign and verify functions public
+// forcing the use of CookieJars, which are akward to work with without a high-level framework.
+// Thanks to Sergio Benitez for writing the original code and releasing it under MIT!
+
+use hmac::{Hmac,NewMac,Mac};
+use sha2::Sha256;
+
+const SIGNED_KEY_LEN: usize = 32;
+const BASE64_DIGEST_LEN: usize = 44;
+
+/// A convenience wrapper around HMAC.
+pub struct Key (pub [u8; SIGNED_KEY_LEN]);
+
+impl Key {
+    const fn zero() -> Self {
+        Key ( [0; SIGNED_KEY_LEN])
+    }
+
+    /// Attempts to generate signing/encryption keys from a secure, random
+    /// source. Keys are generated nondeterministically. If randomness cannot be
+    /// retrieved from the underlying operating system, returns `None`.
+    pub fn try_generate() -> Option<Key> {
+        use rand::RngCore;
+
+        let mut rng = rand::thread_rng();
+        let mut both_keys = [0; SIGNED_KEY_LEN];
+        rng.try_fill_bytes(&mut both_keys).ok()?;
+        Some(Key::from(&both_keys))
+    }
+
+    /// Creates a new Key from a 32 byte cryptographically random string.
+    pub fn from(key: &[u8]) -> Key {
+        if key.len() < SIGNED_KEY_LEN {
+            panic!("bad key length: expected >= 32 bytes, found {}", key.len());
+        }
+
+        let mut output = Key::zero();
+        output.0.copy_from_slice(&key[..SIGNED_KEY_LEN]);
+        output
+    }
+
+    /// Signs the value providing integrity and authenticity.
+    pub fn sign(&self, value: &str) -> String {
+        // Compute HMAC-SHA256 of the cookie's value.
+        let mut mac = Hmac::<Sha256>::new_varkey(&self.0).expect("good key");
+        mac.update(value.as_bytes());
+
+        // Cookie's new value is [MAC | original-value].
+        let mut new_value = base64::encode(&mac.finalize().into_bytes());
+        new_value.push_str(value);
+        new_value
+    }
+
+    /// Extracts the value from a string signed with [`Key::sign`].
+    /// Fails if the string is malformed or the signature doesn't match.
+    pub fn verify(&self, value: &str) -> Result<String, String> {
+        if value.len() < BASE64_DIGEST_LEN {
+            return Err("length of value is <= BASE64_DIGEST_LEN".to_string());
+        }
+
+        // Split [MAC | original-value] into its two parts.
+        let (digest_str, value) = value.split_at(BASE64_DIGEST_LEN);
+        let digest = base64::decode(digest_str).map_err(|_| "bad base64 digest")?;
+
+        // Perform the verification.
+        let mut mac = Hmac::<Sha256>::new_varkey(&self.0).expect("good key");
+        mac.update(value.as_bytes());
+        mac.verify(&digest)
+            .map(|_| value.to_string())
+            .map_err(|_| "value did not verify".to_string())
+    }
+}
+\ No newline at end of file