1// Copyright 2016 Brian Smith.
2//
3// Permission to use, copy, modify, and/or distribute this software for any
4// purpose with or without fee is hereby granted, provided that the above
5// copyright notice and this permission notice appear in all copies.
6//
7// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
8// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
9// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
10// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
11// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
12// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
13// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
14
15//! The [chacha20-poly1305@openssh.com] AEAD-ish construct.
16//!
17//! This should only be used by SSH implementations. It has a similar, but
18//! different API from `ring::aead` because the construct cannot use the same
19//! API as `ring::aead` due to the way the construct handles the encrypted
20//! packet length.
21//!
22//! The concatenation of a and b is denoted `a||b`. `K_1` and `K_2` are defined
23//! in the [chacha20-poly1305@openssh.com] specification. `packet_length`,
24//! `padding_length`, `payload`, and `random padding` are defined in
25//! [RFC 4253]. The term `plaintext` is used as a shorthand for
26//! `padding_length||payload||random padding`.
27//!
28//! [chacha20-poly1305@openssh.com]:
29//! http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.chacha20poly1305?annotate=HEAD
30//! [RFC 4253]: https://tools.ietf.org/html/rfc4253
31
32use super::{
33 chacha::{self, *},
34 chacha20_poly1305::derive_poly1305_key,
35 cpu, poly1305, Nonce, Tag,
36};
37use crate::{constant_time, error};
38
39/// A key for sealing packets.
40pub struct SealingKey {
41 key: Key,
42}
43
44impl SealingKey {
45 /// Constructs a new `SealingKey`.
46 pub fn new(key_material: &[u8; KEY_LEN]) -> Self {
47 Self {
48 key: Key::new(key_material),
49 }
50 }
51
52 /// Seals (encrypts and signs) a packet.
53 ///
54 /// On input, `plaintext_in_ciphertext_out` must contain the unencrypted
55 /// `packet_length||plaintext` where `plaintext` is the
56 /// `padding_length||payload||random padding`. It will be overwritten by
57 /// `encrypted_packet_length||ciphertext`, where `encrypted_packet_length`
58 /// is encrypted with `K_1` and `ciphertext` is encrypted by `K_2`.
59 pub fn seal_in_place(
60 &self,
61 sequence_number: u32,
62 plaintext_in_ciphertext_out: &mut [u8],
63 tag_out: &mut [u8; TAG_LEN],
64 ) {
65 let cpu_features = cpu::features();
66 let mut counter = make_counter(sequence_number);
67 let poly_key = derive_poly1305_key(&self.key.k_2, counter.increment());
68
69 {
70 let (len_in_out, data_and_padding_in_out) =
71 plaintext_in_ciphertext_out.split_at_mut(PACKET_LENGTH_LEN);
72
73 self.key
74 .k_1
75 .encrypt_in_place(make_counter(sequence_number), len_in_out);
76 self.key
77 .k_2
78 .encrypt_in_place(counter, data_and_padding_in_out);
79 }
80
81 let Tag(tag) = poly1305::sign(poly_key, plaintext_in_ciphertext_out, cpu_features);
82 tag_out.copy_from_slice(tag.as_ref());
83 }
84}
85
86/// A key for opening packets.
87pub struct OpeningKey {
88 key: Key,
89}
90
91impl OpeningKey {
92 /// Constructs a new `OpeningKey`.
93 pub fn new(key_material: &[u8; KEY_LEN]) -> Self {
94 Self {
95 key: Key::new(key_material),
96 }
97 }
98
99 /// Returns the decrypted, but unauthenticated, packet length.
100 ///
101 /// Importantly, the result won't be authenticated until `open_in_place` is
102 /// called.
103 pub fn decrypt_packet_length(
104 &self,
105 sequence_number: u32,
106 encrypted_packet_length: [u8; PACKET_LENGTH_LEN],
107 ) -> [u8; PACKET_LENGTH_LEN] {
108 let mut packet_length = encrypted_packet_length;
109 let counter = make_counter(sequence_number);
110 self.key.k_1.encrypt_in_place(counter, &mut packet_length);
111 packet_length
112 }
113
114 /// Opens (authenticates and decrypts) a packet.
115 ///
116 /// `ciphertext_in_plaintext_out` must be of the form
117 /// `encrypted_packet_length||ciphertext` where `ciphertext` is the
118 /// encrypted `plaintext`. When the function succeeds the ciphertext is
119 /// replaced by the plaintext and the result is `Ok(plaintext)`, where
120 /// `plaintext` is `&ciphertext_in_plaintext_out[PACKET_LENGTH_LEN..]`;
121 /// otherwise the contents of `ciphertext_in_plaintext_out` are unspecified
122 /// and must not be used.
123 pub fn open_in_place<'a>(
124 &self,
125 sequence_number: u32,
126 ciphertext_in_plaintext_out: &'a mut [u8],
127 tag: &[u8; TAG_LEN],
128 ) -> Result<&'a [u8], error::Unspecified> {
129 let mut counter = make_counter(sequence_number);
130
131 // We must verify the tag before decrypting so that
132 // `ciphertext_in_plaintext_out` is unmodified if verification fails.
133 // This is beyond what we guarantee.
134 let poly_key = derive_poly1305_key(&self.key.k_2, counter.increment());
135 verify(poly_key, ciphertext_in_plaintext_out, tag)?;
136
137 let plaintext_in_ciphertext_out = &mut ciphertext_in_plaintext_out[PACKET_LENGTH_LEN..];
138 self.key
139 .k_2
140 .encrypt_in_place(counter, plaintext_in_ciphertext_out);
141
142 Ok(plaintext_in_ciphertext_out)
143 }
144}
145
146struct Key {
147 k_1: chacha::Key,
148 k_2: chacha::Key,
149}
150
151impl Key {
152 fn new(key_material: &[u8; KEY_LEN]) -> Self {
153 // The first half becomes K_2 and the second half becomes K_1.
154 let (k_2: &[u8], k_1: &[u8]) = key_material.split_at(mid:chacha::KEY_LEN);
155 Self {
156 k_1: chacha::Key::new(k_1.try_into().unwrap()),
157 k_2: chacha::Key::new(k_2.try_into().unwrap()),
158 }
159 }
160}
161
162fn make_counter(sequence_number: u32) -> Counter {
163 let [s0: u8, s1: u8, s2: u8, s3: u8] = sequence_number.to_be_bytes();
164 let nonce: [u8; 12] = [0, 0, 0, 0, 0, 0, 0, 0, s0, s1, s2, s3];
165 Counter::zero(Nonce::assume_unique_for_key(nonce))
166}
167
168/// The length of key.
169pub const KEY_LEN: usize = chacha::KEY_LEN * 2;
170
171/// The length in bytes of the `packet_length` field in a SSH packet.
172pub const PACKET_LENGTH_LEN: usize = 4; // 32 bits
173
174/// The length in bytes of an authentication tag.
175pub const TAG_LEN: usize = super::TAG_LEN;
176
177fn verify(key: poly1305::Key, msg: &[u8], tag: &[u8; TAG_LEN]) -> Result<(), error::Unspecified> {
178 let Tag(calculated_tag: [u8; 16]) = poly1305::sign(key, input:msg, cpu_features:cpu::features());
179 constant_time::verify_slices_are_equal(a:calculated_tag.as_ref(), b:tag)
180}
181