bitwarden_crypto/keys/
prf.rs

1use crate::{SymmetricCryptoKey, utils::stretch_key};
2
3#[derive(Debug)]
4pub struct InvalidInputError;
5
6/// Takes the output of a PRF and derives a symmetric key.
7///
8/// The PRF output must be at least 32 bytes long. If longer, only the first 32
9/// bytes will be used, and the remainder is discarded.
10pub fn derive_symmetric_key_from_prf(prf: &[u8]) -> Result<SymmetricCryptoKey, InvalidInputError> {
11    let (secret, _) = prf.split_at_checked(32).ok_or(InvalidInputError)?;
12    let secret: [u8; 32] = secret.try_into().expect("length to be 32 bytes");
13    // Don't allow uninitialized PRFs
14    if secret.iter().all(|b| *b == b'\0') {
15        return Err(InvalidInputError);
16    }
17    Ok(SymmetricCryptoKey::Aes256CbcHmacKey(stretch_key(
18        &Box::pin(secret.into()),
19    )))
20}
21
22#[cfg(test)]
23mod tests {
24    use super::*;
25
26    #[test]
27    fn test_prf_succeeds() {
28        let prf = pseudorandom_bytes(32);
29        let key = derive_symmetric_key_from_prf(&prf).unwrap();
30        assert!(matches!(key, SymmetricCryptoKey::Aes256CbcHmacKey(_)));
31    }
32
33    #[test]
34    fn test_zero_key_fails() {
35        let prf: Vec<u8> = (0..32).map(|_| 0).collect();
36        let err = derive_symmetric_key_from_prf(&prf).unwrap_err();
37        assert!(matches!(err, InvalidInputError));
38    }
39
40    #[test]
41    fn test_short_prf_fails() {
42        let prf = pseudorandom_bytes(9);
43        let err = derive_symmetric_key_from_prf(&prf).unwrap_err();
44        assert!(matches!(err, InvalidInputError));
45    }
46
47    #[test]
48    fn test_long_prf_truncated_to_proper_length() {
49        let long_prf = pseudorandom_bytes(33);
50        let prf = pseudorandom_bytes(32);
51        let key1 = derive_symmetric_key_from_prf(&long_prf).unwrap();
52        let key2 = derive_symmetric_key_from_prf(&prf).unwrap();
53        assert_eq!(key1, key2);
54    }
55
56    /// This returns the same bytes deterministically for a given length.
57    fn pseudorandom_bytes(len: usize) -> Vec<u8> {
58        (0..len).map(|x| (x % 255) as u8).collect()
59    }
60}
bitwarden_crypto/keys/prf.rs

bitwarden_crypto/keys/
prf.rs
