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use hmac::{Hmac, Mac};
use sha2::Sha256;
const SIGNED_KEY_LEN: usize = 32;
const BASE64_DIGEST_LEN: usize = 44;
/// A convenience wrapper around HMAC.
///
/// This code was adapted from the [`cookie`] crate which does not make the sign and verify functions public
/// forcing the use of [`CookieJar`](cookie::CookieJar)s, 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!
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_from_slice(&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_from_slice(&self.0).expect("good key");
mac.update(value.as_bytes());
mac.verify_slice(&digest)
.map(|_| value.to_string())
.map_err(|_| "value did not verify".to_string())
}
}
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