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/*
* Copyright (C) 2018 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.android.internal.net.utils;
import java.math.BigInteger;
/** BigIntegerUtils provides utility methods for doing Key Exchange. */
public final class BigIntegerUtils {
/**
* Converts the unsigned Hex String to a positive BigInteger.
*
* @param hexString Hex representation of an unsigned value
* @return the argument converted to BigInteger by an unsigned conversion
*/
public static BigInteger unsignedHexStringToBigInteger(String hexString) {
return new BigInteger(hexString, 16);
}
/**
* Converts the unsigned byte array to a positive BigInteger.
*
* @param byteArray byte array that represents an unsigned value
* @return the argument converted to BigInteger by an unsigned conversion
*/
public static BigInteger unsignedByteArrayToBigInteger(byte[] byteArray) {
return new BigInteger(1/** positive */, byteArray);
}
/**
* Returns a byte array containing the unsigned representation of this BigInteger. Zero-pad on
* the left of byte array to desired size.
*
* @param bigInteger input BigInteger
* @param size size of the output byte array
* @return the byte array containing the unsigned representation of this BigInteger.
*/
public static byte[] bigIntegerToUnsignedByteArray(BigInteger bigInteger, int size) {
byte[] byteArrayWithSignBit = bigInteger.toByteArray();
int len = byteArrayWithSignBit.length;
byte[] output = new byte[size];
// {@link BigInteger} provides method {@link toByteArray} that returns a byte array
// containing the two's-complement representation of this BigInteger (minimum number of
// bytes required to represent this BigInteger, including at least one sign bit). This
// method first remove additional byte that caused by this sign bit and zero-pad on the left
// of byte array to desired size.
if (bigInteger.bitLength() % 8 == 0) {
len = len - 1;
System.arraycopy(byteArrayWithSignBit, 1, output, size - len, len);
} else {
System.arraycopy(byteArrayWithSignBit, 0, output, size - len, len);
}
return output;
}
}
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