/*
* Copyright (C) 2017 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.apksig.internal.asn1.ber;
import java.nio.ByteBuffer;
/**
* {@link BerDataValueReader} which reads from a {@link ByteBuffer} containing BER-encoded data
* values. See {@code X.690} for the encoding.
*/
public class ByteBufferBerDataValueReader implements BerDataValueReader {
private final ByteBuffer mBuf;
public ByteBufferBerDataValueReader(ByteBuffer buf) {
if (buf == null) {
throw new NullPointerException("buf == null");
}
mBuf = buf;
}
@Override
public BerDataValue readDataValue() throws BerDataValueFormatException {
int startPosition = mBuf.position();
if (!mBuf.hasRemaining()) {
return null;
}
byte firstIdentifierByte = mBuf.get();
int tagNumber = readTagNumber(firstIdentifierByte);
boolean constructed = BerEncoding.isConstructed(firstIdentifierByte);
if (!mBuf.hasRemaining()) {
throw new BerDataValueFormatException("Missing length");
}
int firstLengthByte = mBuf.get() & 0xff;
int contentsLength;
int contentsOffsetInTag;
if ((firstLengthByte & 0x80) == 0) {
// short form length
contentsLength = readShortFormLength(firstLengthByte);
contentsOffsetInTag = mBuf.position() - startPosition;
skipDefiniteLengthContents(contentsLength);
} else if (firstLengthByte != 0x80) {
// long form length
contentsLength = readLongFormLength(firstLengthByte);
contentsOffsetInTag = mBuf.position() - startPosition;
skipDefiniteLengthContents(contentsLength);
} else {
// indefinite length -- value ends with 0x00 0x00
contentsOffsetInTag = mBuf.position() - startPosition;
contentsLength =
constructed
? skipConstructedIndefiniteLengthContents()
: skipPrimitiveIndefiniteLengthContents();
}
// Create the encoded data value ByteBuffer
int endPosition = mBuf.position();
mBuf.position(startPosition);
int bufOriginalLimit = mBuf.limit();
mBuf.limit(endPosition);
ByteBuffer encoded = mBuf.slice();
mBuf.position(mBuf.limit());
mBuf.limit(bufOriginalLimit);
// Create the encoded contents ByteBuffer
encoded.position(contentsOffsetInTag);
encoded.limit(contentsOffsetInTag + contentsLength);
ByteBuffer encodedContents = encoded.slice();
encoded.clear();
return new BerDataValue(
encoded,
encodedContents,
BerEncoding.getTagClass(firstIdentifierByte),
constructed,
tagNumber);
}
private int readTagNumber(byte firstIdentifierByte) throws BerDataValueFormatException {
int tagNumber = BerEncoding.getTagNumber(firstIdentifierByte);
if (tagNumber == 0x1f) {
// high-tag-number form, where the tag number follows this byte in base-128
// big-endian form, where each byte has the highest bit set, except for the last
// byte
return readHighTagNumber();
} else {
// low-tag-number form
return tagNumber;
}
}
private int readHighTagNumber() throws BerDataValueFormatException {
// Base-128 big-endian form, where each byte has the highest bit set, except for the last
// byte
int b;
int result = 0;
do {
if (!mBuf.hasRemaining()) {
throw new BerDataValueFormatException("Truncated tag number");
}
b = mBuf.get();
if (result > Integer.MAX_VALUE >>> 7) {
throw new BerDataValueFormatException("Tag number too large");
}
result <<= 7;
result |= b & 0x7f;
} while ((b & 0x80) != 0);
return result;
}
private int readShortFormLength(int firstLengthByte) {
return firstLengthByte & 0x7f;
}
private int readLongFormLength(int firstLengthByte) throws BerDataValueFormatException {
// The low 7 bits of the first byte represent the number of bytes (following the first
// byte) in which the length is in big-endian base-256 form
int byteCount = firstLengthByte & 0x7f;
if (byteCount > 4) {
throw new BerDataValueFormatException("Length too large: " + byteCount + " bytes");
}
int result = 0;
for (int i = 0; i < byteCount; i++) {
if (!mBuf.hasRemaining()) {
throw new BerDataValueFormatException("Truncated length");
}
int b = mBuf.get();
if (result > Integer.MAX_VALUE >>> 8) {
throw new BerDataValueFormatException("Length too large");
}
result <<= 8;
result |= b & 0xff;
}
return result;
}
private void skipDefiniteLengthContents(int contentsLength) throws BerDataValueFormatException {
if (mBuf.remaining() < contentsLength) {
throw new BerDataValueFormatException(
"Truncated contents. Need: " + contentsLength + " bytes, available: "
+ mBuf.remaining());
}
mBuf.position(mBuf.position() + contentsLength);
}
private int skipPrimitiveIndefiniteLengthContents() throws BerDataValueFormatException {
// Contents are terminated by 0x00 0x00
boolean prevZeroByte = false;
int bytesRead = 0;
while (true) {
if (!mBuf.hasRemaining()) {
throw new BerDataValueFormatException(
"Truncated indefinite-length contents: " + bytesRead + " bytes read");
}
int b = mBuf.get();
bytesRead++;
if (bytesRead < 0) {
throw new BerDataValueFormatException("Indefinite-length contents too long");
}
if (b == 0) {
if (prevZeroByte) {
// End of contents reached -- we've read the value and its terminator 0x00 0x00
return bytesRead - 2;
}
prevZeroByte = true;
} else {
prevZeroByte = false;
}
}
}
private int skipConstructedIndefiniteLengthContents() throws BerDataValueFormatException {
// Contents are terminated by 0x00 0x00. However, this data value is constructed, meaning it
// can contain data values which are themselves indefinite length encoded. As a result, we
// must parse the direct children of this data value to correctly skip over the contents of
// this data value.
int startPos = mBuf.position();
while (mBuf.hasRemaining()) {
// Check whether the 0x00 0x00 terminator is at current position
if ((mBuf.remaining() > 1) && (mBuf.getShort(mBuf.position()) == 0)) {
int contentsLength = mBuf.position() - startPos;
mBuf.position(mBuf.position() + 2);
return contentsLength;
}
// No luck. This must be a BER-encoded data value -- skip over it by parsing it
readDataValue();
}
throw new BerDataValueFormatException(
"Truncated indefinite-length contents: "
+ (mBuf.position() - startPos) + " bytes read");
}
}
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