Primitive Component: XoShiRo256++

Overviewtitle

prim_xoshiro256pp is a PRNG with 256 bit state. For more information refer to this page.

Parameters

Name	type	Description
OutputDw	int	Width of the PRNG output. Must be a multiple of 64.
DefaultSeed	logic	Initial 256-bit state of the PRNG, must be nonzero.

Signal Interfaces

Name	In/Out	Description
seed_en_i	input	External seed input enable
seed_i[256]	input	External seed input
xoshiro_en_i	input	PRNG enable
entropy_i[256]	input	Entropy input
data_o[OutputDw]	output	PRNG output
all_zero_o	output	Impossible all-zero state

Theory of Operations

             /----------------\
seed_en_i    |                |
------------>|  xoshiro256++  |
seed_i       |                |  data_o
=====/======>|                |=====/=======>
 [256]       |                |  [OutputDw]
xoshiro_en_i |                |
------------>|    OutputDw    |
entropy_i    |   DefaultSeed  |  all_zero_o
=====/======>|                |------------->
 [256]       |                |
             |                |
             \----------------/

Xoshiro256++ PRNG consists of:

• 256b state
• A single-cycle state-update function.
• A state output function.

The basic xoshiro256++ PRNG has a 64 bit output. This implementation enables the output size to be any multiple of 64 bits. The output size controlled using the OutputDW parameter.

The xoshiro256++ module has an enable input and an additional entropy input that is XOR'ed into the PRNG state (connect to zero if this feature is unused). As the PRNG may jump into the all-zero state (e.g. due to an active attack), the PRNG state-update function contains a lockup protection which re-seeds the state with DefaultSeed and raises the alert signal all_zero_o, once this condition is detected.

When the seed enable signal seed_en_i is raised, the internal state of xoshiro256++ is updated with the value provided at the 256b input ‘seed_i’. The state is internally updated in every clock cycle whenever the enable signal xoshiro_en_i is raised. The timing diagram below visualizes this process.

{
  signal: [
    {name: 'clk', wave: 'p......|....'},
    {name: 'xoshiro_en_i', wave: '0...1..|..0.'},
    {name: 'seed_en_i', wave: '010....|....'},
    {name:'seed_i',     wave: '3..x...|....', data: 'Seed' },
    {name: 'state', wave: 'x.3..45|678.', data: 'Seed'}
  ]
}