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SHA256Finder.java

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package optjava;

import java.nio.charset.StandardCharsets;

import java.security.MessageDigest;

import java.security.NoSuchAlgorithmException;

import java.util.concurrent.atomic.AtomicBoolean;

public final class SHA256Finder {

private final MessageDigest digest;

public SHA256Finder() {

try {

digest = MessageDigest.getInstance("SHA-256");

} catch (NoSuchAlgorithmException e) {

throw new RuntimeException(e);

}

}

// Threshold mask for remaining bits

public static final int[] thresholdMask = {128, 64, 32, 16, 8, 4, 2, 1};

public static String makeRandomString(int length) {

final StringBuilder sb = new StringBuilder();

for (int j = 0; j < length; j++) {

// Select a random char ch >= 32 && ch < 127

char c = (char)(32 + Math.random() * 96);

sb.append(c);

}

return sb.toString();

}

public static void main(String[] args) throws Exception {

SHA256Finder s = new SHA256Finder();

s.findMatcher(1024, 18);

}

void findMatcher(int length, int target) {

System.out.println("Length: " + length + " ; target: " + target);

long attempts = 0;

// Search indefinitely for a match

String candidate = null;

FOREVER:

while (true) {

candidate = makeRandomString(length);

if (++attempts % 100_000 == 0) {

System.out.print(".");

}

if (belowThreshold(candidate, target)) {

System.out.println();

System.out.println("Length: " + length + " took attempts: " + attempts +" to find a "+ target +" prefix");

System.out.println("Winner: " + candidate);

System.out.println("Winning hash: " + sha256Hash(candidate));

}

}

}

public boolean belowThreshold(final String s, final int numBits) {

final byte[] hash = digest.digest(s.getBytes(StandardCharsets.UTF_8));

int bitsCounted = 0;

final int fullBytes = numBits / 8;

final int remBits = numBits % 8;

// Check full bytes first

for (int i = 0; i < fullBytes; i++) {

// Each hash byte contains 8 bits, so use an int to store them unsigned

int compare = 0xff & hash[i];

if (compare == 0) {

bitsCounted += 8;

if (bitsCounted >= numBits) {

return true;

}

} else {

return false;

}

}

// Now check the bits using one final byte

return thresholdMask[remBits] > (hash[fullBytes] & 0xff);

}

String sha256Hash(String s) {

final byte[] hash = digest.digest(s.getBytes(StandardCharsets.UTF_8));

final StringBuilder sb = new StringBuilder();

for (byte aHash : hash) {

String hex = Integer.toHexString(0xff & aHash);

// We are converting an unsigned byte to a hex string.

// This will give either 1 or 2 hex digits e.g. 'a' or '23' (35 dec).

// If we only get 1 back, we need to include the zero byte so we stay fixed-width as a string.

if (hex.length() == 1) sb.append('0');

sb.append(hex);

}

return sb.toString();

}

}