ee.sk.smartid.VerificationCodeCalculator Maven / Gradle / Ivy
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package ee.sk.smartid;
import java.nio.ByteBuffer;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
public class VerificationCodeCalculator {
/**
* The Verification Code (VC) is computed as:
*
* integer(SHA256(hash)[−2:−1]) mod 10000
*
* where we take SHA256 result, extract 2 rightmost bytes from it,
* interpret them as a big-endian unsigned integer and take the last 4 digits in decimal for display.
*
* SHA256 is always used here, no matter what was the algorithm used to calculate hash.
*
* @param documentHash hash used to calculate verification code.
* @return verification code.
*/
public static String calculate(byte[] documentHash) {
byte[] digest = DigestCalculator.calculateDigest(documentHash, HashType.SHA256);
ByteBuffer byteBuffer = ByteBuffer.wrap(digest);
int shortBytes = Short.SIZE / Byte.SIZE; // Short.BYTES in java 8
int rightMostBytesIndex = byteBuffer.limit() - shortBytes;
short twoRightmostBytes = byteBuffer.getShort(rightMostBytesIndex);
int positiveInteger = ((int) twoRightmostBytes) & 0xffff;
String code = String.valueOf(positiveInteger);
String paddedCode = "0000" + code;
return paddedCode.substring(code.length());
}
}