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

// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// https://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! A wrapper around another RNG that reseeds it after it
//! generates a certain number of random bytes.

use {Rng, Error};
#[cfg(feature="std")]
use NewRng;

/// A wrapper around any RNG which reseeds the underlying RNG after it
/// has generated a certain number of random bytes.
#[derive(Debug)]
pub struct ReseedingRng<R, Rsdr> {
    rng: R,
    generation_threshold: u64,
    bytes_generated: u64,
    /// Controls the behaviour when reseeding the RNG.
    pub reseeder: Rsdr,
}

impl<R: Rng, Rsdr: Reseeder<R>> ReseedingRng<R, Rsdr> {
    /// Create a new `ReseedingRng` with the given parameters.
    ///
    /// # Arguments
    ///
    /// * `rng`: the random number generator to use.
    /// * `generation_threshold`: the number of bytes of entropy at which to reseed the RNG.
    /// * `reseeder`: the reseeding object to use.
    pub fn new(rng: R, generation_threshold: u64, reseeder: Rsdr) -> ReseedingRng<R,Rsdr> {
        ReseedingRng {
            rng: rng,
            generation_threshold: generation_threshold,
            bytes_generated: 0,
            reseeder: reseeder
        }
    }

    /// Reseed the internal RNG if the number of bytes that have been
    /// generated exceed the threshold.
    pub fn reseed_if_necessary(&mut self) {
        if self.bytes_generated >= self.generation_threshold {
            self.reseeder.reseed(&mut self.rng).unwrap();
            self.bytes_generated = 0;
        }
    }
}


impl<R: Rng, Rsdr: Reseeder<R>> Rng for ReseedingRng<R, Rsdr> {
    fn next_u32(&mut self) -> u32 {
        self.reseed_if_necessary();
        self.bytes_generated += 4;
        self.rng.next_u32()
    }

    fn next_u64(&mut self) -> u64 {
        self.reseed_if_necessary();
        self.bytes_generated += 8;
        self.rng.next_u64()
    }

    fn fill_bytes(&mut self, dest: &mut [u8]) {
        self.reseed_if_necessary();
        self.bytes_generated += dest.len() as u64;
        self.rng.fill_bytes(dest)
    }
    
    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
        self.reseed_if_necessary();
        self.bytes_generated += dest.len() as u64;
        self.rng.try_fill_bytes(dest)
    }
}

/// Something that can be used to reseed an RNG via `ReseedingRng`.
///
/// Note that implementations should support `Clone` only if reseeding is
/// deterministic (no external entropy source). This is so that a `ReseedingRng`
/// only supports `Clone` if fully deterministic.
pub trait Reseeder<R: ?Sized> {
    /// Reseed the given RNG.
    ///
    /// On error, this should just forward the source error; errors are handled
    /// by the caller.
    fn reseed(&mut self, rng: &mut R) -> Result<(), Error>;
}

/// Reseed an RNG using `NewRng` to replace the current instance.
#[cfg(feature="std")]
#[derive(Debug)]
pub struct ReseedWithNew;

#[cfg(feature="std")]
impl<R: Rng + NewRng> Reseeder<R> for ReseedWithNew {
    fn reseed(&mut self, rng: &mut R) -> Result<(), Error> {
        R::new().map(|result| *rng = result)
    }
}

#[cfg(test)]
mod test {
    use {impls, le};
    use super::{ReseedingRng, Reseeder};
    use {SeedableRng, Rng, Error};

    struct Counter {
        i: u32
    }

    impl Rng for Counter {
        fn next_u32(&mut self) -> u32 {
            self.i += 1;
            // very random
            self.i - 1
        }
        fn next_u64(&mut self) -> u64 {
            impls::next_u64_via_u32(self)
        }

        fn fill_bytes(&mut self, dest: &mut [u8]) {
            impls::fill_bytes_via_u64(self, dest)
        }
    }
    impl SeedableRng for Counter {
        type Seed = [u8; 4];
        fn from_seed(seed: Self::Seed) -> Self {
            let mut seed_u32 = [0u32; 1];
            le::read_u32_into(&seed, &mut seed_u32);
            Counter { i: seed_u32[0] }
        }
    }

    #[derive(Debug, Clone)]
    struct ReseedCounter;
    impl Reseeder<Counter> for ReseedCounter {
        fn reseed(&mut self, rng: &mut Counter) -> Result<(), Error> {
            *rng = Counter { i: 0 };
            Ok(())
        }
    }

    #[test]
    fn test_reseeding() {
        let mut rs = ReseedingRng::new(Counter {i:0}, 400, ReseedCounter);

        let mut i = 0;
        for _ in 0..1000 {
            assert_eq!(rs.next_u32(), i % 100);
            i += 1;
        }
    }
}