AES GCM解密绕过JAVA中的身份验证

Question

我有一些AES/GCM加密数据,想要解密它.我希望绕过身份验证解密它,因为数据不包含身份验证信息(数据由第三方应用程序加密).我尝试使用javax.crypto包进行解密,并且总是抛出标签不匹配错误.有没有办法绕过这个标签检查和解密数据.数据使用AES128加密,并使用12字节初始化向量.

编辑:我有一个临时解决方案来解决这个问题.不确定这是否是正确的方法.

            Key key = new SecretKeySpec(hlsKey, "AES");
            GCMParameterSpec gCMParameterSpec = new     GCMParameterSpec(96, initialisationVector);
            final Cipher c = Cipher.getInstance("AES/GCM/NoPadding", "BC");
            c.init(Cipher.DECRYPT_MODE, key, gCMParameterSpec);

            byte[] nodata = new byte[len * 2];
            System.arraycopy(cipherText, 0, nodata, 0, len);
            byte[] plaindata = new byte[len * 2];
            try { 

                int decrypted_index = 0;
                while (decrypted_index < len) {
                    int cp = c.update(nodata, decrypted_index, nodata.length - decrypted_index, plaindata, decrypted_index);//doFinal(nodata);
                    decrypted_index += cp;
                }
               if(decrypted_index>=len){
                   System.arraycopy(plaindata, 0, plainText, 0, len);
                   retvalue=1;
               }
            } catch (Exception e) {
                e.printStackTrace();
            }

Answer 1

是的,可以在没有认证标签的情况下解密消息:如果您阅读GCM规范,您可以看到CTR的IV仅仅是IV,附加了四个字节00000002(即计数器从零开始,增加1用于计算身份验证标记,再次为加密计数器的起始值).

所以这是代码,inc我用它来验证我的计数器代码两次; 当然可以简单地将最后一个字节设置为值0x02.

package nl.owlstead.so;

import java.nio.charset.StandardCharsets;

import javax.crypto.Cipher;
import javax.crypto.SecretKey;
import javax.crypto.spec.GCMParameterSpec;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;

import org.bouncycastle.util.Arrays;

public class DecryptGCMWithoutVerification {

    private static final int TAG_SIZE = 128;

    public DecryptGCMWithoutVerification() {
        // TODO Auto-generated constructor stub
    }

    public static void main(String[] args) throws Exception {

        // --- encryption using GCM

        Cipher gcm = Cipher.getInstance("AES/GCM/NoPadding");
        SecretKey key = new SecretKeySpec(new byte[16], "AES");
        byte[] ivBytes = new byte[12];
        GCMParameterSpec iv = new GCMParameterSpec(TAG_SIZE, ivBytes);
        gcm.init(Cipher.ENCRYPT_MODE, key, iv);
        byte[] ct = gcm.doFinal("owlstead".getBytes(StandardCharsets.US_ASCII));

        // --- decryption using underlying CTR mode

        Cipher ctr = Cipher.getInstance("AES/CTR/NoPadding");

        // WARNING: this is only correct for a 12 byte IV in GCM mode
        byte[] counter = Arrays.concatenate(ivBytes, new byte[4]);
        inc(counter);
        inc(counter);
        IvParameterSpec ctrIV = new IvParameterSpec(counter);
        ctr.init(Cipher.DECRYPT_MODE, key, ctrIV);

        byte[] pt = ctr.doFinal(ct, 0, ct.length - TAG_SIZE / Byte.SIZE);

        System.out.println(new String(pt, StandardCharsets.US_ASCII));
    }

    private static final byte inc(byte[] counter) {
        for (int i = counter.length - 1; i >= 0; i--) {
            if (++counter[i] != 0) {
                return 0;
            }
        }
        return 1;
    }

}

编辑:此代码用于无效标记或无法重新计算的标记(例如,可能缺少AAD).删除- TAG_SIZE / Byte.SIZE从doFinal在标签被完全丢失.