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package peast;
import java.util.Arrays;
//"POST /"(6) + prefix + " HTTP/1.1\r\n"(11)
public class Decryptor {
private String plaintext = "";
private String prefixRight = "";
private static final String prefixLeft = "POST /";
private byte[] ciphertext;
private int blockSize;
private byte[] target;
private byte[] targetIV;
private byte attemptLastByte = 0x20;
private byte[] nextAttempt;
public Decryptor() {
}
public void setCiphertext(byte[] ciphertext) {
// TODO get blocksize from packets
blockSize = 16;
assert (ciphertext.length % blockSize == 0);
this.ciphertext = ciphertext;
target = new byte[blockSize];
targetIV = new byte[blockSize];
setTargetBlock();
}
public byte[] getChosenPrefix() {
// total length mod blockSize = blockSize - 1
int bs = blockSize == 0 ? 16 : blockSize;
int prefixLength = bs
- ((prefixLeft.length() + prefixRight.length() + 1) % bs);
final byte[] prefix = new byte[prefixLength];
Arrays.fill(prefix, (byte) 0x41);
return prefix;
}
private byte[] getNextAttemptBlock() {
byte[] nextAttempt = new byte[blockSize];
String tmp = prefixLeft + new String(getChosenPrefix()) + prefixRight;
System.out.println("tmp: " + tmp);
System.arraycopy(tmp.getBytes(), tmp.length() - (blockSize - 1),
nextAttempt, 0, blockSize - 1);
nextAttempt[blockSize - 1] = attemptLastByte;
System.out
.println("Next attempt plaintext: " + new String(nextAttempt));
return nextAttempt;
}
public byte[] getChosenPlaintext(byte[] lastPacket) {
byte[] iv = Arrays.copyOfRange(lastPacket, lastPacket.length
- blockSize, lastPacket.length);
byte[] cp = new byte[blockSize];
nextAttempt = getNextAttemptBlock();
assert (attemptLastByte < 128);
for (int idx = 0; idx < blockSize; idx++) {
cp[idx] = (byte) (nextAttempt[idx] ^ targetIV[idx] ^ iv[idx]);
}
attemptLastByte += 1;
return cp;
}
public boolean matches(byte[] cipher) {
byte[] m = new byte[blockSize];
System.arraycopy(cipher, 0, m, 0, blockSize);
if (Arrays.equals(m, target)) {
String found = new String(nextAttempt);
System.out.println("match!: " + found);
char ch = found.charAt(found.length() - 1);
plaintext = plaintext + ch;
prefixRight += ch;
attemptLastByte = 0x20;
setTargetBlock();
return true;
}
return false;
}
private void setTargetBlock() {
// which ciphertext block are we trying to guess next?
int index = (prefixLeft.length() + getChosenPrefix().length + prefixRight
.length()) / blockSize;
System.arraycopy(ciphertext, index * blockSize, target, 0, blockSize);
System.arraycopy(ciphertext, (index - 1) * blockSize, targetIV, 0,
blockSize);
}
public void setPrefixRight(String prefix) {
prefixRight = prefix;
}
public String getPrefixRight() {
return prefixRight;
}
public String getPlaintext() {
return plaintext;
}
}