ring/aead/
chacha20_poly1305_openssh.rs

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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, k_1) = key_material.split_at(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, s1, s2, s3] = sequence_number.to_be_bytes();
164    let nonce = [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) = poly1305::sign(key, msg, cpu::features());
179    constant_time::verify_slices_are_equal(calculated_tag.as_ref(), tag)
180}
ring/aead/chacha20_poly1305_openssh.rs

ring/aead/
chacha20_poly1305_openssh.rs
