/**
* {@inheritDoc}
*
* <p><strong>Algorithm Description:</strong> hex strings are generated using a 2-step process.
*
* <ol>
* <li>{@code len / 2 + 1} binary bytes are generated using the underlying Random
* <li>Each binary byte is translated into 2 hex digits
* </ol>
*
* @param len the desired string length.
* @return the random string.
* @throws NotStrictlyPositiveException if {@code len <= 0}.
*/
public String nextHexString(int len) throws NotStrictlyPositiveException {
if (len <= 0) {
throw new NotStrictlyPositiveException(LocalizedFormats.LENGTH, len);
}
// Get a random number generator
RandomGenerator ran = getRandomGenerator();
// Initialize output buffer
StringBuilder outBuffer = new StringBuilder();
// Get int(len/2)+1 random bytes
byte[] randomBytes = new byte[(len / 2) + 1];
ran.nextBytes(randomBytes);
// Convert each byte to 2 hex digits
for (int i = 0; i < randomBytes.length; i++) {
Integer c = Integer.valueOf(randomBytes[i]);
/*
* Add 128 to byte value to make interval 0-255 before doing hex
* conversion. This guarantees <= 2 hex digits from toHexString()
* toHexString would otherwise add 2^32 to negative arguments.
*/
String hex = Integer.toHexString(c.intValue() + 128);
// Make sure we add 2 hex digits for each byte
if (hex.length() == 1) {
hex = "0" + hex;
}
outBuffer.append(hex);
}
return outBuffer.toString().substring(0, len);
}
/**
* {@inheritDoc}
*
* <p><strong>Algorithm Description:</strong> hex strings are generated in 40-byte segments using
* a 3-step process.
*
* <ol>
* <li>20 random bytes are generated using the underlying <code>SecureRandom</code>.
* <li>SHA-1 hash is applied to yield a 20-byte binary digest.
* <li>Each byte of the binary digest is converted to 2 hex digits.
* </ol>
*
* @throws NotStrictlyPositiveException if {@code len <= 0}
*/
public String nextSecureHexString(int len) throws NotStrictlyPositiveException {
if (len <= 0) {
throw new NotStrictlyPositiveException(LocalizedFormats.LENGTH, len);
}
// Get SecureRandom and setup Digest provider
final RandomGenerator secRan = getSecRan();
MessageDigest alg = null;
try {
alg = MessageDigest.getInstance("SHA-1");
} catch (NoSuchAlgorithmException ex) {
// this should never happen
throw new MathInternalError(ex);
}
alg.reset();
// Compute number of iterations required (40 bytes each)
int numIter = (len / 40) + 1;
StringBuilder outBuffer = new StringBuilder();
for (int iter = 1; iter < numIter + 1; iter++) {
byte[] randomBytes = new byte[40];
secRan.nextBytes(randomBytes);
alg.update(randomBytes);
// Compute hash -- will create 20-byte binary hash
byte[] hash = alg.digest();
// Loop over the hash, converting each byte to 2 hex digits
for (int i = 0; i < hash.length; i++) {
Integer c = Integer.valueOf(hash[i]);
/*
* Add 128 to byte value to make interval 0-255 This guarantees
* <= 2 hex digits from toHexString() toHexString would
* otherwise add 2^32 to negative arguments
*/
String hex = Integer.toHexString(c.intValue() + 128);
// Keep strings uniform length -- guarantees 40 bytes
if (hex.length() == 1) {
hex = "0" + hex;
}
outBuffer.append(hex);
}
}
return outBuffer.toString().substring(0, len);
}