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Merge pull request #64 from Fethbita/feat-test-fn-rng
Add a new page that explains testing functions that use random generator
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# Testing functions that use RNGs | ||
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Occasionally a function that uses random number generators might need to be tested. For functions that need to be tested with test vectors, the following approach might be adapted: | ||
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```rust | ||
use rand::{RngCore, CryptoRng, rngs::OsRng}; | ||
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pub struct CryptoOperations<R: RngCore + CryptoRng = OsRng> { | ||
rng: R | ||
} | ||
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impl<R: RngCore + CryptoRng> CryptoOperations<R> { | ||
#[must_use] | ||
pub fn new(rng: R) -> Self { | ||
Self { | ||
rng | ||
} | ||
} | ||
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pub fn xor_with_random_bytes(&mut self, secret: &mut [u8; 8]) -> [u8; 8] { | ||
let mut mask = [0u8; 8]; | ||
self.rng.fill_bytes(&mut mask); | ||
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for (byte, mask_byte) in secret.iter_mut().zip(mask.iter()) { | ||
*byte ^= mask_byte; | ||
} | ||
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mask | ||
} | ||
} | ||
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fn main() { | ||
let rng = OsRng; | ||
let mut crypto_ops = <CryptoOperations>::new(rng); | ||
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let mut secret: [u8; 8] = *b"\x00\x01\x02\x03\x04\x05\x06\x07"; | ||
let mask = crypto_ops.xor_with_random_bytes(&mut secret); | ||
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println!("Modified Secret (XORed): {:?}", secret); | ||
println!("Mask: {:?}", mask); | ||
} | ||
``` | ||
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To test this, we can create a `MockCryptoRng` implementing `RngCore` and `CryptoRng` in our testing module. Note that `MockCryptoRng` is private and `#[cfg(test)] mod tests` is cfg-gated to our test environment, thus ensuring that `MockCryptoRng` cannot accidentally be used in production. | ||
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```rust | ||
#[cfg(test)] | ||
mod tests { | ||
use super::*; | ||
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#[derive(Clone, Copy, Debug)] | ||
struct MockCryptoRng { | ||
data: [u8; 8], | ||
index: usize, | ||
} | ||
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impl MockCryptoRng { | ||
fn new(data: [u8; 8]) -> MockCryptoRng { | ||
MockCryptoRng { | ||
data, | ||
index: 0, | ||
} | ||
} | ||
} | ||
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impl CryptoRng for MockCryptoRng {} | ||
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impl RngCore for MockCryptoRng { | ||
fn next_u32(&mut self) -> u32 { | ||
unimplemented!() | ||
} | ||
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fn next_u64(&mut self) -> u64 { | ||
unimplemented!() | ||
} | ||
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fn fill_bytes(&mut self, dest: &mut [u8]) { | ||
for byte in dest.iter_mut() { | ||
*byte = self.data[self.index]; | ||
self.index = (self.index + 1) % self.data.len(); | ||
} | ||
} | ||
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fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), rand::Error> { | ||
unimplemented!() | ||
} | ||
} | ||
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#[test] | ||
fn test_xor_with_mock_rng() { | ||
let mock_crypto_rng = MockCryptoRng::new(*b"\x57\x88\x1e\xed\x1c\x72\x01\xd8"); | ||
let mut crypto_ops = CryptoOperations::new(mock_crypto_rng); | ||
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let mut secret: [u8; 8] = *b"\x00\x01\x02\x03\x04\x05\x06\x07"; | ||
let mask = crypto_ops.xor_with_random_bytes(&mut secret); | ||
let expected_mask = *b"\x57\x88\x1e\xed\x1c\x72\x01\xd8"; | ||
let expected_xored_secret = *b"\x57\x89\x1c\xee\x18\x77\x07\xdf"; | ||
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assert_eq!(secret, expected_xored_secret); | ||
assert_eq!(mask, expected_mask); | ||
} | ||
} | ||
``` |