# Struct rand_isaac::isaac::IsaacRng

source · `pub struct IsaacRng(_);`

## Expand description

A random number generator that uses the ISAAC algorithm.

ISAAC stands for “Indirection, Shift, Accumulate, Add, and Count” which are
the principal bitwise operations employed. It is the most advanced of a
series of array based random number generator designed by Robert Jenkins
in 1996^{1}^{2}.

ISAAC is notably fast and produces excellent quality random numbers for non-cryptographic applications.

In spite of being designed with cryptographic security in mind, ISAAC hasn’t
been stringently cryptanalyzed and thus cryptographers do not not
consensually trust it to be secure. When looking for a secure RNG, prefer
`Hc128Rng`

instead, which, like ISAAC, is an array-based RNG and one of
the stream-ciphers selected the by eSTREAM contest.

In 2006 an improvement to ISAAC was suggested by Jean-Philippe Aumasson,
named ISAAC+^{3}. But because the specification is not complete, because
there is no good implementation, and because the suggested bias may not
exist, it is not implemented here.

### Overview of the ISAAC algorithm:

(in pseudo-code)

```
Input: a, b, c, s[256] // state
Output: r[256] // results
mix(a,i) = a ^ a << 13 if i = 0 mod 4
a ^ a >> 6 if i = 1 mod 4
a ^ a << 2 if i = 2 mod 4
a ^ a >> 16 if i = 3 mod 4
c = c + 1
b = b + c
for i in 0..256 {
x = s_[i]
a = f(a,i) + s[i+128 mod 256]
y = a + b + s[x>>2 mod 256]
s[i] = y
b = x + s[y>>10 mod 256]
r[i] = b
}
```

Numbers are generated in blocks of 256. This means the function above only runs once every 256 times you ask for a next random number. In all other circumstances the last element of the results array is returned.

ISAAC therefore needs a lot of memory, relative to other non-crypto RNGs. 2 * 256 * 4 = 2 kb to hold the state and results.

This implementation uses `BlockRng`

to implement the `RngCore`

methods.

### References

Bob Jenkins,

*ISAAC: A fast cryptographic random number generator*↩Bob Jenkins,

*ISAAC and RC4*↩Jean-Philippe Aumasson,

*On the pseudo-random generator ISAAC*↩

## Implementations

source### impl IsaacRng

### impl IsaacRng

source#### pub fn new_from_u64(seed: u64) -> Self

👎Deprecated since 0.6.0: use SeedableRng::seed_from_u64 instead

#### pub fn new_from_u64(seed: u64) -> Self

Create an ISAAC random number generator using an `u64`

as seed.
If `seed == 0`

this will produce the same stream of random numbers as
the reference implementation when used unseeded.

## Trait Implementations

source### impl RngCore for IsaacRng

### impl RngCore for IsaacRng

source#### fn fill_bytes(&mut self, dest: &mut [u8])

#### fn fill_bytes(&mut self, dest: &mut [u8])

`dest`

with random data. Read moresource### impl SeedableRng for IsaacRng

### impl SeedableRng for IsaacRng

source#### fn seed_from_u64(seed: u64) -> Self

#### fn seed_from_u64(seed: u64) -> Self

Create an ISAAC random number generator using an `u64`

as seed.
If `seed == 0`

this will produce the same stream of random numbers as
the reference implementation when used unseeded.

#### type Seed = <IsaacCore as SeedableRng>::Seed

#### type Seed = <IsaacCore as SeedableRng>::Seed

`u8`

arrays (we recommend `[u8; N]`

for some `N`

). Read more