密码输入流跳过

Question

我想加密和解密文件。解密时我希望跳过文件的前几个字节，只解密其余字节。

这里包含三个文件

input_file.txt - 输入文本文件进行加密
output_file.txt - 加密 input_file.txt 文件
decrypted_file.txt - 解密 output_file.txt 文件

示例代码：

import java.security.SecureRandom;
import javax.crypto.Cipher;
import javax.crypto.CipherInputStream;
import javax.crypto.CipherOutputStream;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import java.io.FileInputStream;
import java.io.FileOutputStream;

public class FileReadWrite {

    public static byte[] getAESKey() {
        SecureRandom secureRandom = new SecureRandom();
        byte[] bytes = new byte[32];
        secureRandom.nextBytes(bytes);
        return bytes;
    }
/**
 * Method used to generate a random new iv
 * 
 * @return Randomly generated iv
 */
public static byte[] getAESIV() {
    SecureRandom secureRandom = new SecureRandom();
    byte[] bytes = new byte[16];
    secureRandom.nextBytes(bytes);
    return bytes;
}
public static void main(String[] args) throws Exception {
    // TODO Auto-generated method stub
    FileInputStream fin = new FileInputStream("/Users/emp/Research/Streams/input_file.txt");
    FileOutputStream fout = new FileOutputStream("/Users/emp/Research/Streams/output_file.txt");

    SecretKeySpec keySpec = null;
    IvParameterSpec ivSpec = null;
    Cipher ecipher = null;
    Cipher dcipher = null;
    byte[] keyBytes = getAESKey();
    byte[] ivBytes = getAESIV();
    // Creating keyspec and ivspec for generating cipher
    keySpec = new SecretKeySpec(keyBytes,"AES");
    ivSpec = new IvParameterSpec(ivBytes);
    try {
        ecipher = Cipher.getInstance("AES/CTR/NoPadding");
        ecipher.init(Cipher.ENCRYPT_MODE, keySpec, ivSpec);
    } catch (Exception e) {
        System.out.println("Thus the exception occured during cipher generation is ::: "+e);
    }

    CipherOutputStream cout = new CipherOutputStream(fout, ecipher);
    try {
        int count = 0;
        int BUFFER_SIZE = 1024;
        byte[] bytearray = new byte[BUFFER_SIZE];
        while((count = fin.read(bytearray, 0, BUFFER_SIZE)) != -1) {
            //fout.write(bytearray, 0, count);
            cout.write(bytearray, 0, count);
        }
    } catch(Exception ex) {
        System.out.println("Thus the exception occured is ::: "+ex);
    } finally {
        fin.close();
        fout.close();
        cout.close();
    }

    fin = new FileInputStream("/Users/emp/Research/Streams/output_file.txt");
    fout = new FileOutputStream("/Users/emp/Research/Streams/decrypted_file.txt");

    try {
        dcipher = Cipher.getInstance("AES/CTR/NoPadding");
        dcipher.init(Cipher.DECRYPT_MODE, keySpec, ivSpec);
    } catch (Exception e) {
        System.out.println("Thus the exception occured during cipher generation is ::: "+e);
    }
    //fin.skip(1024);
    CipherInputStream cin = new CipherInputStream(fin, dcipher);
    try {
        int count = 0;
        int BUFFER_SIZE = 1024;
        byte[] bytearray = new byte[BUFFER_SIZE];

        **//cin.read(bytearray, 0, 30);**

        while((count = cin.read(bytearray, 0, BUFFER_SIZE)) != -1) {
            //fout.write(bytearray, 0, count);
            fout.write(bytearray, 0, count);
        }
    } catch(Exception ex) {
        System.out.println("Thus the exception occured is ::: "+ex);
    } finally {
        fin.close();
        cin.close();
        fout.close();
    }

    System.out.println("File read write completed successfully !!! ");
}
}

尝试过：

密码输入流跳过 - cin.skip(no_of_bytes) - 这不起作用 - 这会解密整个文件

跳过文件输入流

 fin.skip(no_of_bytes); CipherInputStream cin = new
 CipherInputStream(fin, cipher);

这不会解密文件。输出文件看起来像加密的。

虚拟读取 - 从密码输入流中读取并忽略 - 这有效
```
//cin.read(bytearray, 0, 30);
```

请澄清以下内容：

跳过和搜索输入流在内部做什么？
为什么我的密码输入流搜索不起作用？
为什么密码输入流需要从流中读取所有字节（我假设是这样）来解密？它是如何工作的？

Answer 1

参考文章，

Random access InputStream using AES CTR mode in android

用偏移值更新 iv 后，解密工作正常。

这是更新后的代码

package Streams;
import java.io.FileOutputStream;
import java.io.FileInputStream;
import java.math.BigInteger;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.SecureRandom;
import java.util.Arrays;

import javax.crypto.Cipher;
import javax.crypto.CipherInputStream;
import javax.crypto.CipherOutputStream;
import javax.crypto.ShortBufferException;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;

public class FileReadWrite {
    private static final int AES_BLOCK_SIZE = 16;

    public static final void jumpToOffset(final Cipher c, final SecretKeySpec aesKey, final IvParameterSpec iv, final long offset) {
        if (!c.getAlgorithm().toUpperCase().startsWith("AES/CTR")) {
            throw new IllegalArgumentException("Invalid algorithm, only AES/CTR mode supported");
        }
        if (offset < 0) {
            throw new IllegalArgumentException("Invalid offset");
        }
        final int skip = (int) (offset % AES_BLOCK_SIZE);
        final IvParameterSpec calculatedIVForOffset = calculateIVForOffset(iv, offset - skip);
        try {
            c.init(Cipher.ENCRYPT_MODE, aesKey, calculatedIVForOffset);
            final byte[] skipBuffer = new byte[skip];
            c.update(skipBuffer, 0, skip, skipBuffer);
            Arrays.fill(skipBuffer, (byte) 0);
        } catch (ShortBufferException | InvalidKeyException | InvalidAlgorithmParameterException e) {
            throw new IllegalStateException(e);
        }
    }

    private static IvParameterSpec calculateIVForOffset(final IvParameterSpec iv, final long blockOffset) {

        final BigInteger ivBI = new BigInteger(1, iv.getIV());
        final BigInteger ivForOffsetBI = ivBI.add(BigInteger.valueOf(blockOffset / AES_BLOCK_SIZE));
        final byte[] ivForOffsetBA = ivForOffsetBI.toByteArray();
        final IvParameterSpec ivForOffset;
        if (ivForOffsetBA.length >= AES_BLOCK_SIZE) {
            ivForOffset = new IvParameterSpec(ivForOffsetBA, ivForOffsetBA.length - AES_BLOCK_SIZE, AES_BLOCK_SIZE);
        } else {
            final byte[] ivForOffsetBASized = new byte[AES_BLOCK_SIZE];
            System.arraycopy(ivForOffsetBA, 0, ivForOffsetBASized, AES_BLOCK_SIZE - ivForOffsetBA.length, ivForOffsetBA.length);
            ivForOffset = new IvParameterSpec(ivForOffsetBASized);
        }
        return ivForOffset;
    }

    public static byte[] getAESKey() {
        SecureRandom secureRandom = new SecureRandom();
        byte[] bytes = new byte[32];
        secureRandom.nextBytes(bytes);
        return bytes;
    }
    /**
     * Method used to generate a random new iv
     * 
     * @return Randomly generated iv
     */
    public static byte[] getAESIV() {
        SecureRandom secureRandom = new SecureRandom();
        byte[] bytes = new byte[16];
        secureRandom.nextBytes(bytes);
        return bytes;
    }
    public static void main(String[] args) throws Exception {
        // TODO Auto-generated method stub
        FileInputStream fin = new FileInputStream("/Users/emp/Research/Streams/input_file.txt");
        FileOutputStream fout = new FileOutputStream("/Users/emp/Research/Streams/output_file.txt");

        SecretKeySpec keySpec = null;
        IvParameterSpec ivSpec = null;
        Cipher ecipher = null;
        Cipher dcipher = null;
        byte[] keyBytes = getAESKey();
        byte[] ivBytes = getAESIV();
        // Creating keyspec and ivspec for generating cipher
        keySpec = new SecretKeySpec(keyBytes,"AES");
        ivSpec = new IvParameterSpec(ivBytes);
        try {
            ecipher = Cipher.getInstance("AES/CTR/NoPadding");
            ecipher.init(Cipher.ENCRYPT_MODE, keySpec, ivSpec);
        } catch (Exception e) {
            System.out.println("Thus the exception occured during cipher generation is ::: "+e);
        }

        CipherOutputStream cout = new CipherOutputStream(fout, ecipher);
        try {
            int count = 0;
            int BUFFER_SIZE = 1024;
            byte[] bytearray = new byte[BUFFER_SIZE];
            while((count = fin.read(bytearray, 0, BUFFER_SIZE)) != -1) {
                //fout.write(bytearray, 0, count);
                cout.write(bytearray, 0, count);
            }
        } catch(Exception ex) {
            System.out.println("Thus the exception occured is ::: "+ex);
        } finally {
            fin.close();
            fout.close();
            cout.close();
        }

        fin = new FileInputStream("/Users/emp/Research/Streams/output_file.txt");
        fout = new FileOutputStream("/Users/emp/Research/Streams/decrypted_file.txt");

        try {
            dcipher = Cipher.getInstance("AES/CTR/NoPadding");
            dcipher.init(Cipher.DECRYPT_MODE, keySpec, ivSpec);
        } catch (Exception e) {
            System.out.println("Thus the exception occured during cipher generation is ::: "+e);
        }

        fin.skip(1024);
        jumpToOffset(dcipher, keySpec, ivSpec, 1024);
        CipherInputStream cin = new CipherInputStream(fin, dcipher);
        //cin.skip(1024);
        try {
            int count = 0;
            int BUFFER_SIZE = 1024;
            byte[] bytearray = new byte[BUFFER_SIZE];
            //cin.read(bytearray, 0, 30);
            while((count = cin.read(bytearray, 0, BUFFER_SIZE)) != -1) {
                //fout.write(bytearray, 0, count);
                fout.write(bytearray, 0, count);
            }
        } catch(Exception ex) {
            System.out.println("Thus the exception occured is ::: "+ex);
        } finally {
            fin.close();
            cin.close();
            fout.close();
        }

        System.out.println("File read write completed successfully !!! ");
    }
}

密码输入流跳过

Cipher input stream skip

java

encryption

inputstream

aes

seek