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Preparing input files for SP 800-22

Most surprising SP 800-22 results come from the file, not the generator. Here is how to write clean test data from common environments, and how NISTTEST.COM reads each format.

Updated October 20, 2026

How files are read

Aim for 125,000 bytes (1,000,000 bits) so every test can run; the free test accepts up to 500 KB (why that size).

Linux and macOS

head -c 125000 /dev/urandom > urandom.bin
openssl rand -out openssl.bin 125000

Both make good known-random reference files. If your own generator fails a test that these pass, the problem is in your generator or capture.

Python

From a generator function

import struct

with open("rng.bin", "wb") as f:
    for _ in range(31_250):                       # 31,250 × 4 bytes = 125,000 bytes
        f.write(struct.pack(">I", my_rng_u32()))   # one 32-bit output, big-endian

From a CSV or list of byte values (0–255)

text = open("bytes.csv").read().replace("\n", ",")
values = [int(v) for v in text.split(",") if v.strip()]
open("bytes.bin", "wb").write(bytes(values))      # raises if a value is outside 0–255

From a hex dump

text = open("dump.txt").read()
hex_digits = "".join(text.split()).replace("0x", "").replace(",", "")
open("dump.bin", "wb").write(bytes.fromhex(hex_digits))

From a text file of 0/1 characters

bits = "".join(ch for ch in open("bits.txt").read() if ch in "01")
bits = bits[: len(bits) // 8 * 8]                 # whole bytes only
open("bits.bin", "wb").write(int(bits, 2).to_bytes(len(bits) // 8, "big"))

C

#include <stdio.h>
#include <stdint.h>

int main(void) {
    FILE *f = fopen("rng.bin", "wb");          /* "wb": no newline translation */
    for (int i = 0; i < 125000; i++) {
        uint8_t b = my_rng_byte();
        fwrite(&b, 1, 1, f);
    }
    fclose(f);
    return 0;
}

Arduino and ESP32 over serial

Send raw bytes from the board and capture them on the computer. Don't copy them out of a serial monitor: monitors drop and re-encode bytes.

// Board (ESP32 example)
void setup() { Serial.begin(921600); }
void loop() {
  uint32_t r = esp_random();
  Serial.write((uint8_t *)&r, 4);
}

# Computer: pip install pyserial
import serial

with serial.Serial("/dev/ttyUSB0", 921600, timeout=5) as port:
    port.reset_input_buffer()          # drop boot messages already received
    data = port.read(125_000)
open("esp32.bin", "wb").write(data)
print(len(data), "bytes captured")     # check nothing was lost

On ESP32, check Espressif's documentation for when esp_random() draws on a hardware entropy source (it depends on the radio or the bootloader random source being enabled). Testing in the wrong configuration measures a different generator.

Checklist before uploading

• File size matches what you expected to capture (no truncated serial reads).
• No header, timestamps, log lines or 0x prefixes in a .bin file.
• Not the same buffer repeated, and not the same seed each run.
• Raw source and conditioned output tested separately if your design has both.

NISTTEST.COM is an independent implementation of NIST SP 800-22 Rev. 1a and is not affiliated with or endorsed by the National Institute of Standards and Technology.