aucpace 0.1.1

AuCPace protocol implementation
Documentation 
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253

use crate::{Error, Result};
use curve25519_dalek::constants::RISTRETTO_BASEPOINT_POINT;
use curve25519_dalek::{
    digest::consts::U64,
    digest::{Digest, Output},
    ristretto::RistrettoPoint,
    scalar::Scalar,
};
use password_hash::PasswordHash;
use rand_core::CryptoRngCore;

#[allow(non_snake_case)]
#[inline(always)]
fn H<D: Digest + Default, const N: u32>() -> D {
    let mut hasher: D = Default::default();
    hasher.update(N.to_le_bytes());
    hasher
}

macro_rules! create_h_impl {
    ($name:ident, $n:literal) => {
        #[allow(non_snake_case)]
        pub(crate) fn $name<D: Digest + Default>() -> D {
            H::<D, $n>()
        }
    };
}

// implement H0..H5 hash functions
create_h_impl!(H0, 0);
create_h_impl!(H1, 1);
create_h_impl!(H2, 2);
create_h_impl!(H3, 3);
create_h_impl!(H4, 4);
create_h_impl!(H5, 5);

/// Generate a fixed length nonce using a CSPRNG
#[inline(always)]
pub(crate) fn generate_nonce<CSPRNG, const N: usize>(rng: &mut CSPRNG) -> [u8; N]
where
    CSPRNG: CryptoRngCore,
{
    let mut nonce = [0; N];
    rng.fill_bytes(&mut nonce);
    nonce
}

/// Computes the SSID from two server and client nonces - s and t
#[inline(always)]
pub(crate) fn compute_ssid<D: Digest + Default, const K1: usize>(
    s: [u8; K1],
    t: [u8; K1],
) -> Output<D> {
    let mut hasher: D = H0();
    hasher.update(s);
    hasher.update(t);
    hasher.finalize()
}

/// Generate a Diffie-Hellman keypair for the CPace substep of the protocol
#[inline(always)]
pub(crate) fn generate_keypair<D, CSPRNG, CI>(
    rng: &mut CSPRNG,
    ssid: Output<D>,
    prs: [u8; 32],
    ci: CI,
) -> (Scalar, RistrettoPoint)
where
    D: Digest<OutputSize = U64> + Default,
    CSPRNG: CryptoRngCore,
    CI: AsRef<[u8]>,
{
    let mut hasher: D = H1();
    hasher.update(ssid);
    hasher.update(prs);
    hasher.update(ci);

    let generator = RistrettoPoint::from_hash(hasher);
    let priv_key = Scalar::random(rng);
    let cofactor = Scalar::ONE;
    let pub_key = generator * (priv_key * cofactor);

    (priv_key, pub_key)
}

/// Compute the first session key sk1 from our private key and the other participant's public key
#[inline(always)]
pub(crate) fn compute_first_session_key<D>(
    ssid: Output<D>,
    priv_key: Scalar,
    pub_key: RistrettoPoint,
) -> Output<D>
where
    D: Digest<OutputSize = U64> + Default,
{
    let shared_point = pub_key * priv_key;

    let mut hasher: D = H2();
    hasher.update(ssid);
    hasher.update(shared_point.compress().to_bytes());

    hasher.finalize()
}

/// Compute the two authenticator messages Ta and Tb
#[inline(always)]
pub(crate) fn compute_authenticator_messages<D>(
    ssid: Output<D>,
    sk1: Output<D>,
) -> (Output<D>, Output<D>)
where
    D: Digest<OutputSize = U64> + Default,
{
    let mut ta_hasher: D = H3();
    ta_hasher.update(ssid);
    ta_hasher.update(sk1);

    let mut tb_hasher: D = H4();
    tb_hasher.update(ssid);
    tb_hasher.update(sk1);

    (ta_hasher.finalize(), tb_hasher.finalize())
}

/// Compute the session key - sk
#[inline(always)]
pub(crate) fn compute_session_key<D>(ssid: Output<D>, sk1: Output<D>) -> Output<D>
where
    D: Digest<OutputSize = U64> + Default,
{
    let mut hasher: D = H5();
    hasher.update(ssid);
    hasher.update(sk1);
    hasher.finalize()
}

/// Compute a scalar from a password hash
#[inline(always)]
pub(crate) fn scalar_from_hash(pw_hash: PasswordHash<'_>) -> Result<Scalar> {
    let hash = pw_hash.hash.ok_or(Error::HashEmpty)?;
    let hash_bytes = hash.as_bytes();

    // support both 32 and 64 byte hashes
    match hash_bytes.len() {
        32 => {
            let arr = hash_bytes
                .try_into()
                .expect("slice length invariant broken");
            Ok(Scalar::from_bytes_mod_order(arr))
        }
        64 => {
            let arr = hash_bytes
                .try_into()
                .expect("slice length invariant broken");
            Ok(Scalar::from_bytes_mod_order_wide(arr))
        }
        _ => Err(Error::HashSizeInvalid),
    }
}

/// Generate a keypair (x, X) for the server
#[inline(always)]
pub(crate) fn generate_server_keypair<CSPRNG>(rng: &mut CSPRNG) -> (Scalar, RistrettoPoint)
where
    CSPRNG: CryptoRngCore,
{
    // for ristretto255 the cofactor is 1, for normal curve25519 it is 8
    // this will need to be provided by a group trait in the future
    let cofactor = Scalar::ONE;
    let private = Scalar::random(rng);
    let public = RISTRETTO_BASEPOINT_POINT * (private * cofactor);

    (private, public)
}

// serde_with helper modules for serialising
#[cfg(feature = "serde")]
pub(crate) mod serde_saltstring {
    use core::fmt;
    use password_hash::SaltString;
    use serde::de::{Error, Visitor};
    use serde::{Deserializer, Serializer};

    pub fn serialize<S>(data: &SaltString, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        serializer.serialize_str(data.as_str())
    }

    struct SaltStringVisitor {}

    impl<'de> Visitor<'de> for SaltStringVisitor {
        type Value = SaltString;

        fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(formatter, "a valid ASCII salt string")
        }

        fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
        where
            E: Error,
        {
            SaltString::from_b64(v).map_err(Error::custom)
        }
    }

    pub fn deserialize<'de, D>(deserializer: D) -> Result<SaltString, D::Error>
    where
        D: Deserializer<'de>,
    {
        deserializer.deserialize_str(SaltStringVisitor {})
    }
}

#[cfg(any(feature = "serde"))]
pub(crate) mod serde_paramsstring {
    use core::fmt;
    use password_hash::ParamsString;
    use serde::de::{Error, Visitor};
    use serde::{Deserializer, Serializer};

    pub fn serialize<S>(data: &ParamsString, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        serializer.serialize_str(data.as_str())
    }

    struct ParamsStringVisitor {}

    impl<'de> Visitor<'de> for ParamsStringVisitor {
        type Value = ParamsString;

        fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(formatter, "a valid PHC parameter string")
        }

        fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
        where
            E: Error,
        {
            v.parse().map_err(Error::custom)
        }
    }

    pub fn deserialize<'de, D>(deserializer: D) -> Result<ParamsString, D::Error>
    where
        D: Deserializer<'de>,
    {
        deserializer.deserialize_str(ParamsStringVisitor {})
    }
}