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
// Copyright 2016 Brian Smith.
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice appear in all copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

//! The [chacha20-poly1305@openssh.com] AEAD-ish construct.
//!
//! This should only be used by SSH implementations. It has a similar, but
//! different API from `ring::aead` because the construct cannot use the same
//! API as `ring::aead` due to the way the construct handles the encrypted
//! packet length.
//!
//! The concatenation of a and b is denoted `a||b`. `K_1` and `K_2` are defined
//! in the [chacha20-poly1305@openssh.com] specification. `packet_length`,
//! `padding_length`, `payload`, and `random padding` are defined in
//! [RFC 4253]. The term `plaintext` is used as a shorthand for
//! `padding_length||payload||random padding`.
//!
//! [chacha20-poly1305@openssh.com]:
//!    http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.chacha20poly1305?annotate=HEAD
//! [RFC 4253]: https://tools.ietf.org/html/rfc4253

use super::{
    chacha::{self, *},
    chacha20_poly1305::derive_poly1305_key,
    cpu, poly1305, Nonce, Tag,
};
use crate::{constant_time, endian::*, error};
use core::convert::TryInto;

/// A key for sealing packets.
pub struct SealingKey {
    key: Key,
}

impl SealingKey {
    /// Constructs a new `SealingKey`.
    pub fn new(key_material: &[u8; KEY_LEN]) -> SealingKey {
        SealingKey {
            key: Key::new(key_material, cpu::features()),
        }
    }

    /// Seals (encrypts and signs) a packet.
    ///
    /// On input, `plaintext_in_ciphertext_out` must contain the unencrypted
    /// `packet_length||plaintext` where `plaintext` is the
    /// `padding_length||payload||random padding`. It will be overwritten by
    /// `encrypted_packet_length||ciphertext`, where `encrypted_packet_length`
    /// is encrypted with `K_1` and `ciphertext` is encrypted by `K_2`.
    pub fn seal_in_place(
        &self,
        sequence_number: u32,
        plaintext_in_ciphertext_out: &mut [u8],
        tag_out: &mut [u8; TAG_LEN],
    ) {
        let mut counter = make_counter(sequence_number);
        let poly_key =
            derive_poly1305_key(&self.key.k_2, counter.increment(), self.key.cpu_features);

        {
            let (len_in_out, data_and_padding_in_out) =
                plaintext_in_ciphertext_out.split_at_mut(PACKET_LENGTH_LEN);

            self.key
                .k_1
                .encrypt_in_place(make_counter(sequence_number), len_in_out);
            self.key
                .k_2
                .encrypt_in_place(counter, data_and_padding_in_out);
        }

        let Tag(tag) = poly1305::sign(poly_key, plaintext_in_ciphertext_out);
        tag_out.copy_from_slice(tag.as_ref());
    }
}

/// A key for opening packets.
pub struct OpeningKey {
    key: Key,
}

impl OpeningKey {
    /// Constructs a new `OpeningKey`.
    pub fn new(key_material: &[u8; KEY_LEN]) -> OpeningKey {
        OpeningKey {
            key: Key::new(key_material, cpu::features()),
        }
    }

    /// Returns the decrypted, but unauthenticated, packet length.
    ///
    /// Importantly, the result won't be authenticated until `open_in_place` is
    /// called.
    pub fn decrypt_packet_length(
        &self,
        sequence_number: u32,
        encrypted_packet_length: [u8; PACKET_LENGTH_LEN],
    ) -> [u8; PACKET_LENGTH_LEN] {
        let mut packet_length = encrypted_packet_length;
        let counter = make_counter(sequence_number);
        self.key.k_1.encrypt_in_place(counter, &mut packet_length);
        packet_length
    }

    /// Opens (authenticates and decrypts) a packet.
    ///
    /// `ciphertext_in_plaintext_out` must be of the form
    /// `encrypted_packet_length||ciphertext` where `ciphertext` is the
    /// encrypted `plaintext`. When the function succeeds the ciphertext is
    /// replaced by the plaintext and the result is `Ok(plaintext)`, where
    /// `plaintext` is `&ciphertext_in_plaintext_out[PACKET_LENGTH_LEN..]`;
    /// otherwise the contents of `ciphertext_in_plaintext_out` are unspecified
    /// and must not be used.
    pub fn open_in_place<'a>(
        &self,
        sequence_number: u32,
        ciphertext_in_plaintext_out: &'a mut [u8],
        tag: &[u8; TAG_LEN],
    ) -> Result<&'a [u8], error::Unspecified> {
        let mut counter = make_counter(sequence_number);

        // We must verify the tag before decrypting so that
        // `ciphertext_in_plaintext_out` is unmodified if verification fails.
        // This is beyond what we guarantee.
        let poly_key =
            derive_poly1305_key(&self.key.k_2, counter.increment(), self.key.cpu_features);
        verify(poly_key, ciphertext_in_plaintext_out, tag)?;

        let plaintext_in_ciphertext_out = &mut ciphertext_in_plaintext_out[PACKET_LENGTH_LEN..];
        self.key
            .k_2
            .encrypt_in_place(counter, plaintext_in_ciphertext_out);

        Ok(plaintext_in_ciphertext_out)
    }
}

struct Key {
    k_1: chacha::Key,
    k_2: chacha::Key,
    cpu_features: cpu::Features,
}

impl Key {
    fn new(key_material: &[u8; KEY_LEN], cpu_features: cpu::Features) -> Key {
        // The first half becomes K_2 and the second half becomes K_1.
        let (k_2, k_1) = key_material.split_at(chacha::KEY_LEN);
        let k_1: [u8; chacha::KEY_LEN] = k_1.try_into().unwrap();
        let k_2: [u8; chacha::KEY_LEN] = k_2.try_into().unwrap();
        Key {
            k_1: chacha::Key::from(k_1),
            k_2: chacha::Key::from(k_2),
            cpu_features,
        }
    }
}

fn make_counter(sequence_number: u32) -> Counter {
    let nonce = [
        BigEndian::ZERO,
        BigEndian::ZERO,
        BigEndian::from(sequence_number),
    ];
    Counter::zero(Nonce::assume_unique_for_key(*(nonce.as_byte_array())))
}

/// The length of key.
pub const KEY_LEN: usize = chacha::KEY_LEN * 2;

/// The length in bytes of the `packet_length` field in a SSH packet.
pub const PACKET_LENGTH_LEN: usize = 4; // 32 bits

/// The length in bytes of an authentication tag.
pub const TAG_LEN: usize = super::BLOCK_LEN;

fn verify(key: poly1305::Key, msg: &[u8], tag: &[u8; TAG_LEN]) -> Result<(), error::Unspecified> {
    let Tag(calculated_tag) = poly1305::sign(key, msg);
    constant_time::verify_slices_are_equal(calculated_tag.as_ref(), tag)
}