logo
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

use alloc::string::String;
use alloc::vec;
use alloc::vec::Vec;
use rand::Rng;

use digest::DynDigest;
use subtle::{Choice, ConditionallySelectable, ConstantTimeEq, CtOption};

use crate::algorithms::mgf1_xor;
use crate::errors::{Error, Result};
use crate::key::{self, PrivateKey, PublicKey};

// 2**61 -1 (pow is not const yet)
// TODO: This is the maximum for SHA-1, unclear from the RFC what the values are for other hashing functions.
const MAX_LABEL_LEN: u64 = 2_305_843_009_213_693_951;

/// Encrypts the given message with RSA and the padding
/// scheme from [PKCS#1 OAEP](https://datatracker.ietf.org/doc/html/rfc3447#section-7.1.1).  The message must be no longer than the
/// length of the public modulus minus (2+ 2*hash.size()).
#[inline]
pub fn encrypt<R: Rng, K: PublicKey>(
    rng: &mut R,
    pub_key: &K,
    msg: &[u8],
    digest: &mut dyn DynDigest,
    mgf_digest: &mut dyn DynDigest,
    label: Option<String>,
) -> Result<Vec<u8>> {
    key::check_public(pub_key)?;

    let k = pub_key.size();

    let h_size = digest.output_size();

    if msg.len() + 2 * h_size + 2 > k {
        return Err(Error::MessageTooLong);
    }

    let label = label.unwrap_or_default();
    if label.len() as u64 > MAX_LABEL_LEN {
        return Err(Error::LabelTooLong);
    }

    let mut em = vec![0u8; k];

    let (_, payload) = em.split_at_mut(1);
    let (seed, db) = payload.split_at_mut(h_size);
    rng.fill(seed);

    // Data block DB =  pHash || PS || 01 || M
    let db_len = k - h_size - 1;

    digest.update(label.as_bytes());
    let p_hash = digest.finalize_reset();
    db[0..h_size].copy_from_slice(&*p_hash);
    db[db_len - msg.len() - 1] = 1;
    db[db_len - msg.len()..].copy_from_slice(msg);

    mgf1_xor(db, mgf_digest, seed);
    mgf1_xor(seed, mgf_digest, db);

    pub_key.raw_encryption_primitive(&em, pub_key.size())
}

/// Decrypts a plaintext using RSA and the padding scheme from [pkcs1# OAEP](https://datatracker.ietf.org/doc/html/rfc3447#section-7.1.2)
/// If an `rng` is passed, it uses RSA blinding to avoid timing side-channel attacks.
///
/// Note that whether this function returns an error or not discloses secret
/// information. If an attacker can cause this function to run repeatedly and
/// learn whether each instance returned an error then they can decrypt and
/// forge signatures as if they had the private key. See
/// `decrypt_session_key` for a way of solving this problem.
#[inline]
pub fn decrypt<R: Rng, SK: PrivateKey>(
    rng: Option<&mut R>,
    priv_key: &SK,
    ciphertext: &[u8],
    digest: &mut dyn DynDigest,
    mgf_digest: &mut dyn DynDigest,
    label: Option<String>,
) -> Result<Vec<u8>> {
    key::check_public(priv_key)?;

    let res = decrypt_inner(rng, priv_key, ciphertext, digest, mgf_digest, label)?;
    if res.is_none().into() {
        return Err(Error::Decryption);
    }

    let (out, index) = res.unwrap();

    Ok(out[index as usize..].to_vec())
}

/// Decrypts ciphertext using `priv_key` and blinds the operation if
/// `rng` is given. It returns one or zero in valid that indicates whether the
/// plaintext was correctly structured.
#[inline]
fn decrypt_inner<R: Rng, SK: PrivateKey>(
    rng: Option<&mut R>,
    priv_key: &SK,
    ciphertext: &[u8],
    digest: &mut dyn DynDigest,
    mgf_digest: &mut dyn DynDigest,
    label: Option<String>,
) -> Result<CtOption<(Vec<u8>, u32)>> {
    let k = priv_key.size();
    if k < 11 {
        return Err(Error::Decryption);
    }

    let h_size = digest.output_size();

    if ciphertext.len() != k || k < h_size * 2 + 2 {
        return Err(Error::Decryption);
    }

    let mut em = priv_key.raw_decryption_primitive(rng, ciphertext, priv_key.size())?;

    let label = label.unwrap_or_default();
    if label.len() as u64 > MAX_LABEL_LEN {
        return Err(Error::LabelTooLong);
    }

    digest.update(label.as_bytes());

    let expected_p_hash = &*digest.finalize_reset();

    let first_byte_is_zero = em[0].ct_eq(&0u8);

    let (_, payload) = em.split_at_mut(1);
    let (seed, db) = payload.split_at_mut(h_size);

    mgf1_xor(seed, mgf_digest, db);
    mgf1_xor(db, mgf_digest, seed);

    let hash_are_equal = db[0..h_size].ct_eq(expected_p_hash);

    // The remainder of the plaintext must be zero or more 0x00, followed
    // by 0x01, followed by the message.
    //   looking_for_index: 1 if we are still looking for the 0x01
    //   index: the offset of the first 0x01 byte
    //   zero_before_one: 1 if we saw a non-zero byte before the 1
    let mut looking_for_index = Choice::from(1u8);
    let mut index = 0u32;
    let mut nonzero_before_one = Choice::from(0u8);

    for (i, el) in db.iter().skip(h_size).enumerate() {
        let equals0 = el.ct_eq(&0u8);
        let equals1 = el.ct_eq(&1u8);
        index.conditional_assign(&(i as u32), looking_for_index & equals1);
        looking_for_index &= !equals1;
        nonzero_before_one |= looking_for_index & !equals0;
    }

    let valid = first_byte_is_zero & hash_are_equal & !nonzero_before_one & !looking_for_index;

    Ok(CtOption::new((em, index + 2 + (h_size * 2) as u32), valid))
}