wsts/
util.rs

1use aes_gcm::{aead::Aead, Aes256Gcm, KeyInit, Nonce};
2use elliptic_curve::{
3    hash2curve::{ExpandMsg, ExpandMsgXmd, Expander},
4    Error as EllipticCurveError,
5};
6use rand_core::{CryptoRng, OsRng, RngCore};
7use sha2::{Digest, Sha256};
8
9use crate::{
10    curve::{point::Point, scalar::Scalar},
11    errors::EncryptionError,
12};
13
14/// Size of the AES-GCM nonce
15pub const AES_GCM_NONCE_SIZE: usize = 12;
16
17/// Expands a message using the XMD (eXpandable Message Digest) method with SHA-256, and reduces
18/// the result modulo the curve order to produce a `Scalar`.
19///
20/// # Arguments
21/// - `msg`: The input message to be expanded.
22/// - `dst`: A domain separation tag (DST). Must be 255 bytes or fewer, as required by the
23///   hash-to-curve specification.
24///
25/// # Returns
26/// - `Ok(Scalar)`: A scalar derived by expanding and reducing the message.
27/// - `Err(EllipticCurveError)`: If `dst` is longer than 255 bytes
28pub fn expand_to_scalar(msg: &[u8], dst: &[u8]) -> Result<Scalar, EllipticCurveError> {
29    // The requested output length (32 bytes) means that the underlying hash function will not fail.
30    let mut buf = [0u8; 32];
31    // This will only fail if the dst exceeds 255 bytes.
32    ExpandMsgXmd::<Sha256>::expand_message(&[msg], &[dst], buf.len())?.fill_bytes(&mut buf);
33    Ok(Scalar::from(buf))
34}
35
36#[allow(dead_code)]
37/// Digest the hasher to a Scalar
38pub fn hash_to_scalar(hasher: &mut Sha256) -> Scalar {
39    let h = hasher.clone();
40    let hash = h.finalize();
41    let mut hash_bytes: [u8; 32] = [0; 32];
42    hash_bytes.clone_from_slice(hash.as_slice());
43
44    Scalar::from(hash_bytes)
45}
46
47/// Do a Diffie-Hellman key exchange to create a shared secret from the passed private/public keys
48pub fn make_shared_secret(private_key: &Scalar, public_key: &Point) -> [u8; 32] {
49    let shared_key = private_key * public_key;
50
51    make_shared_secret_from_key(&shared_key)
52}
53
54/// Create a shared secret from the passed Diffie-Hellman shared key
55pub fn make_shared_secret_from_key(shared_key: &Point) -> [u8; 32] {
56    ansi_x963_derive_key(
57        shared_key.compress().as_bytes(),
58        "DH_SHARED_SECRET_KEY/".as_bytes(),
59    )
60}
61
62/// Derive a shared key using the ANSI-x963 standard
63/// <https://www.secg.org/sec1-v2.pdf> (section 3.6.1)
64pub fn ansi_x963_derive_key(shared_key: &[u8], shared_info: &[u8]) -> [u8; 32] {
65    let mut hasher = Sha256::new();
66    let counter = 1u32;
67
68    hasher.update(shared_key);
69    hasher.update(counter.to_be_bytes());
70    hasher.update(shared_info);
71
72    let hash = hasher.finalize();
73    let mut bytes = [0u8; 32];
74
75    bytes.clone_from_slice(hash.as_slice());
76    bytes
77}
78
79/// Encrypt the passed data using the key
80pub fn encrypt<RNG: RngCore + CryptoRng>(
81    key: &[u8; 32],
82    data: &[u8],
83    rng: &mut RNG,
84) -> Result<Vec<u8>, EncryptionError> {
85    let mut nonce_bytes = [0u8; AES_GCM_NONCE_SIZE];
86
87    rng.fill_bytes(&mut nonce_bytes);
88
89    let nonce_vec = nonce_bytes.to_vec();
90    let nonce = Nonce::from_slice(&nonce_vec);
91    let cipher = Aes256Gcm::new(key.into());
92    let cipher_vec = cipher.encrypt(nonce, data.to_vec().as_ref())?;
93    let mut bytes = Vec::new();
94
95    bytes.extend_from_slice(&nonce_vec);
96    bytes.extend_from_slice(&cipher_vec);
97
98    Ok(bytes)
99}
100
101/// Decrypt the passed data using the key
102pub fn decrypt(key: &[u8; 32], data: &[u8]) -> Result<Vec<u8>, EncryptionError> {
103    let Some(nonce_data) = data.get(..AES_GCM_NONCE_SIZE) else {
104        return Err(EncryptionError::MissingNonce);
105    };
106    let Some(cipher_data) = data.get(AES_GCM_NONCE_SIZE..) else {
107        return Err(EncryptionError::MissingData);
108    };
109    if cipher_data.is_empty() {
110        return Err(EncryptionError::MissingData);
111    }
112    let nonce = Nonce::from_slice(nonce_data);
113    let cipher = Aes256Gcm::new(key.into());
114
115    Ok(cipher.decrypt(nonce, cipher_data)?)
116}
117
118/// Creates a new random number generator.
119pub fn create_rng() -> impl RngCore + CryptoRng {
120    OsRng
121}
122
123#[cfg(test)]
124mod test {
125    use super::*;
126    use crate::curve::{point::Point, scalar::Scalar};
127
128    #[test]
129    #[allow(non_snake_case)]
130    fn test_shared_secret() {
131        let mut rng = create_rng();
132
133        let x = Scalar::random(&mut rng);
134        let y = Scalar::random(&mut rng);
135
136        let X = Point::from(x);
137        let Y = Point::from(y);
138
139        let xy = make_shared_secret(&x, &Y);
140        let yx = make_shared_secret(&y, &X);
141
142        assert_eq!(xy, yx);
143    }
144
145    #[test]
146    #[allow(non_snake_case)]
147    fn test_encrypt_decrypt() {
148        let mut rng = create_rng();
149        let msg = "It was many and many a year ago, in a kingdom by the sea...";
150
151        let x = Scalar::random(&mut rng);
152        let y = Scalar::random(&mut rng);
153
154        let X = Point::from(x);
155        let Y = Point::from(y);
156
157        let xy = make_shared_secret(&x, &Y);
158        let yx = make_shared_secret(&y, &X);
159
160        let cipher = encrypt(&xy, msg.as_bytes(), &mut rng).unwrap();
161        let plain = decrypt(&yx, &cipher).unwrap();
162
163        assert_eq!(msg.as_bytes(), &plain);
164
165        let missing_nonce = &cipher[..AES_GCM_NONCE_SIZE - 1];
166        match decrypt(&yx, missing_nonce) {
167            Err(EncryptionError::MissingNonce) => {}
168            Err(e) => panic!("expected MissingNonce got Err({e})"),
169            Ok(_) => panic!("expected MissingNonce got Ok()"),
170        }
171
172        let missing_data = &cipher[..AES_GCM_NONCE_SIZE];
173        match decrypt(&yx, missing_data) {
174            Err(EncryptionError::MissingData) => (),
175            Err(e) => panic!("expected MissingData got Err({e})"),
176            Ok(_) => panic!("expected MissingData got Ok()"),
177        }
178
179        let small_data = &cipher[..AES_GCM_NONCE_SIZE + 1];
180        match decrypt(&yx, small_data) {
181            Err(EncryptionError::AesGcm(_)) => (),
182            Err(e) => panic!("expected EncryptionError(AesGcm) got Err({e:?})"),
183            Ok(_) => panic!("expected EncryptionError(AesGcm) got Ok()"),
184        }
185    }
186}
wsts/util.rs

wsts/
util.rs
