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, cpu, poly1305, Aad, Nonce, Tag,
35};
36use crate::{
37 constant_time,
38 error::{self, InputTooLongError},
39 polyfill::slice,
40};
41
42/// A key for sealing packets.
43pub struct SealingKey {
44 key: Key,
45}
46
47impl SealingKey {
48 /// Constructs a new `SealingKey`.
49 pub fn new(key_material: &[u8; KEY_LEN]) -> Self {
50 Self {
51 key: Key::new(key_material),
52 }
53 }
54
55 /// Seals (encrypts and signs) a packet.
56 ///
57 /// On input, `plaintext_in_ciphertext_out` must contain the unencrypted
58 /// `packet_length||plaintext` where `plaintext` is the
59 /// `padding_length||payload||random padding`. It will be overwritten by
60 /// `encrypted_packet_length||ciphertext`, where `encrypted_packet_length`
61 /// is encrypted with `K_1` and `ciphertext` is encrypted by `K_2`.
62 ///
63 /// # Panics
64 ///
65 /// Panics if `plaintext_in_ciphertext_out.len() < PACKET_LENGTH_LEN`.
66 ///
67 /// Panics if `plaintext_in_ciphertext_out` is longer than the maximum
68 /// input size for ChaCha20-Poly1305. Note that this limit is much,
69 /// much larger than SSH's 256KB maximum record size.
70 pub fn seal_in_place(
71 &self,
72 sequence_number: u32,
73 plaintext_in_ciphertext_out: &mut [u8],
74 tag_out: &mut [u8; TAG_LEN],
75 ) {
76 // XXX/TODO(SemVer): Refactor API to return an error.
77 let (len_in_out, data_and_padding_in_out): (&mut [u8; PACKET_LENGTH_LEN], _) =
78 slice::split_first_chunk_mut(plaintext_in_ciphertext_out).unwrap();
79
80 let cpu = cpu::features();
81 // XXX/TODO(SemVer): Refactor API to return an error.
82 let (counter, poly_key) = chacha20_poly1305::begin(
83 &self.key.k_2,
84 make_nonce(sequence_number),
85 Aad::from(len_in_out),
86 data_and_padding_in_out,
87 cpu,
88 )
89 .map_err(error::erase::<InputTooLongError>)
90 .unwrap();
91
92 let _: Counter = self.key.k_1.encrypt_single_block_with_ctr_0(
93 make_nonce(sequence_number),
94 len_in_out,
95 cpu,
96 );
97 self.key
98 .k_2
99 .encrypt(counter, data_and_padding_in_out.into(), cpu);
100
101 let Tag(tag) = poly1305::sign(poly_key, plaintext_in_ciphertext_out, cpu);
102 *tag_out = tag;
103 }
104}
105
106/// A key for opening packets.
107pub struct OpeningKey {
108 key: Key,
109}
110
111impl OpeningKey {
112 /// Constructs a new `OpeningKey`.
113 pub fn new(key_material: &[u8; KEY_LEN]) -> Self {
114 Self {
115 key: Key::new(key_material),
116 }
117 }
118
119 /// Returns the decrypted, but unauthenticated, packet length.
120 ///
121 /// Importantly, the result won't be authenticated until `open_in_place` is
122 /// called.
123 pub fn decrypt_packet_length(
124 &self,
125 sequence_number: u32,
126 encrypted_packet_length: [u8; PACKET_LENGTH_LEN],
127 ) -> [u8; PACKET_LENGTH_LEN] {
128 let cpu = cpu::features();
129 let mut packet_length = encrypted_packet_length;
130 let _: Counter = self.key.k_1.encrypt_single_block_with_ctr_0(
131 make_nonce(sequence_number),
132 &mut packet_length,
133 cpu,
134 );
135 packet_length
136 }
137
138 /// Opens (authenticates and decrypts) a packet.
139 ///
140 /// `ciphertext_in_plaintext_out` must be of the form
141 /// `encrypted_packet_length||ciphertext` where `ciphertext` is the
142 /// encrypted `plaintext`. When the function succeeds the ciphertext is
143 /// replaced by the plaintext and the result is `Ok(plaintext)`, where
144 /// `plaintext` is `&ciphertext_in_plaintext_out[PACKET_LENGTH_LEN..]`;
145 /// otherwise the contents of `ciphertext_in_plaintext_out` are unspecified
146 /// and must not be used.
147 pub fn open_in_place<'a>(
148 &self,
149 sequence_number: u32,
150 ciphertext_in_plaintext_out: &'a mut [u8],
151 tag: &[u8; TAG_LEN],
152 ) -> Result<&'a [u8], error::Unspecified> {
153 let (packet_length, after_packet_length): (&mut [u8; PACKET_LENGTH_LEN], _) =
154 slice::split_first_chunk_mut(ciphertext_in_plaintext_out).ok_or(error::Unspecified)?;
155
156 let cpu = cpu::features();
157 let (counter, poly_key) = chacha20_poly1305::begin(
158 &self.key.k_2,
159 make_nonce(sequence_number),
160 Aad::from(packet_length),
161 after_packet_length,
162 cpu,
163 )
164 .map_err(error::erase::<InputTooLongError>)?;
165
166 // We must verify the tag before decrypting so that
167 // `ciphertext_in_plaintext_out` is unmodified if verification fails.
168 // This is beyond what we guarantee.
169 let calculated_tag = poly1305::sign(poly_key, ciphertext_in_plaintext_out, cpu);
170 constant_time::verify_slices_are_equal(calculated_tag.as_ref(), tag)?;
171
172 // Won't panic because the length was checked above.
173 let after_packet_length = &mut ciphertext_in_plaintext_out[PACKET_LENGTH_LEN..];
174
175 self.key
176 .k_2
177 .encrypt(counter, after_packet_length.into(), cpu);
178
179 Ok(after_packet_length)
180 }
181}
182
183struct Key {
184 k_1: chacha::Key,
185 k_2: chacha::Key,
186}
187
188impl Key {
189 fn new(key_material: &[u8; KEY_LEN]) -> Self {
190 // The first half becomes K_2 and the second half becomes K_1.
191 let (k_2: &[u8], k_1: &[u8]) = key_material.split_at(mid:chacha::KEY_LEN);
192 Self {
193 k_1: chacha::Key::new(k_1.try_into().unwrap()),
194 k_2: chacha::Key::new(k_2.try_into().unwrap()),
195 }
196 }
197}
198
199fn make_nonce(sequence_number: u32) -> Nonce {
200 let [s0: u8, s1: u8, s2: u8, s3: u8] = sequence_number.to_be_bytes();
201 let nonce: [u8; 12] = [0, 0, 0, 0, 0, 0, 0, 0, s0, s1, s2, s3];
202 Nonce::assume_unique_for_key(nonce)
203}
204
205/// The length of key.
206pub const KEY_LEN: usize = chacha::KEY_LEN * 2;
207
208/// The length in bytes of the `packet_length` field in a SSH packet.
209pub const PACKET_LENGTH_LEN: usize = 4; // 32 bits
210
211/// The length in bytes of an authentication tag.
212pub const TAG_LEN: usize = super::TAG_LEN;
213