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

use core::convert::TryFrom;

use curve25519_dalek::{constants, scalar::Scalar};
use rand_core::{CryptoRng, RngCore};
use sha2::{Digest, Sha512};

use crate::{Error, Signature, VerificationKey, VerificationKeyBytes};

/// An Ed25519 signing key.
///
/// This is also called a secret key by other implementations.
#[derive(Copy, Clone)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
#[cfg_attr(feature = "serde", serde(from = "SerdeHelper"))]
#[cfg_attr(feature = "serde", serde(into = "SerdeHelper"))]
pub struct SigningKey {
    seed: [u8; 32],
    s: Scalar,
    prefix: [u8; 32],
    vk: VerificationKey,
}

impl core::fmt::Debug for SigningKey {
    fn fmt(&self, fmt: &mut core::fmt::Formatter) -> core::fmt::Result {
        fmt.debug_struct("SigningKey")
            .field("seed", &hex::encode(&self.seed))
            .field("s", &self.s)
            .field("prefix", &hex::encode(&self.prefix))
            .field("vk", &self.vk)
            .finish()
    }
}

impl<'a> From<&'a SigningKey> for VerificationKey {
    fn from(sk: &'a SigningKey) -> VerificationKey {
        sk.vk
    }
}

impl<'a> From<&'a SigningKey> for VerificationKeyBytes {
    fn from(sk: &'a SigningKey) -> VerificationKeyBytes {
        sk.vk.into()
    }
}

impl AsRef<[u8]> for SigningKey {
    fn as_ref(&self) -> &[u8] {
        &self.seed[..]
    }
}

impl From<SigningKey> for [u8; 32] {
    fn from(sk: SigningKey) -> [u8; 32] {
        sk.seed
    }
}

impl TryFrom<&[u8]> for SigningKey {
    type Error = Error;
    fn try_from(slice: &[u8]) -> Result<SigningKey, Error> {
        if slice.len() == 32 {
            let mut bytes = [0u8; 32];
            bytes[..].copy_from_slice(slice);
            Ok(bytes.into())
        } else {
            Err(Error::InvalidSliceLength)
        }
    }
}

impl From<[u8; 32]> for SigningKey {
    #[allow(non_snake_case)]
    fn from(seed: [u8; 32]) -> SigningKey {
        // Expand the seed to a 64-byte array with SHA512.
        let h = Sha512::digest(&seed[..]);

        // Convert the low half to a scalar with Ed25519 "clamping"
        let s = {
            let mut scalar_bytes = [0u8; 32];
            scalar_bytes[..].copy_from_slice(&h.as_slice()[0..32]);
            scalar_bytes[0] &= 248;
            scalar_bytes[31] &= 127;
            scalar_bytes[31] |= 64;
            Scalar::from_bits(scalar_bytes)
        };

        // Extract and cache the high half.
        let prefix = {
            let mut prefix = [0u8; 32];
            prefix[..].copy_from_slice(&h.as_slice()[32..64]);
            prefix
        };

        // Compute the public key as A = [s]B.
        let A = &s * &constants::ED25519_BASEPOINT_TABLE;

        SigningKey {
            seed,
            s,
            prefix,
            vk: VerificationKey {
                minus_A: -A,
                A_bytes: VerificationKeyBytes(A.compress().to_bytes()),
            },
        }
    }
}

impl zeroize::Zeroize for SigningKey {
    fn zeroize(&mut self) {
        self.seed.zeroize();
        self.s.zeroize()
    }
}

#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
struct SerdeHelper([u8; 32]);

impl From<SerdeHelper> for SigningKey {
    fn from(helper: SerdeHelper) -> SigningKey {
        helper.0.into()
    }
}

impl From<SigningKey> for SerdeHelper {
    fn from(sk: SigningKey) -> Self {
        Self(sk.into())
    }
}

impl SigningKey {
    /// Generate a new signing key.
    pub fn new<R: RngCore + CryptoRng>(mut rng: R) -> SigningKey {
        let mut bytes = [0u8; 32];
        rng.fill_bytes(&mut bytes[..]);
        bytes.into()
    }

    /// Create a signature on `msg` using this key.
    #[allow(non_snake_case)]
    pub fn sign(&self, msg: &[u8]) -> Signature {
        let r = Scalar::from_hash(Sha512::default().chain(&self.prefix[..]).chain(msg));

        let R_bytes = (&r * &constants::ED25519_BASEPOINT_TABLE)
            .compress()
            .to_bytes();

        let k = Scalar::from_hash(
            Sha512::default()
                .chain(&R_bytes[..])
                .chain(&self.vk.A_bytes.0[..])
                .chain(msg),
        );

        let s_bytes = (r + k * self.s).to_bytes();

        Signature { R_bytes, s_bytes }
    }
}