某app rsa 算法逆向踩坑记录

抓包

利用 charles 的 compose 功能 修改一个字符, 发现 403. 但是可以repeat, 但几分钟后 repeat 照样 403, 说明后台校验时间

jadx 全局搜索

进入 this.b.a 看看

是个接口, 查看交叉引用.

第一个很可疑, 我们进入看看

可以看到, 这个方法就是具体实现.

简单分析了一下, 其中 bytes 就是 时间 + url 路径 + app_type + version
关键函数是 e71.a.a, 我们进入这个函数看看.

frida hook 看看

function getJavaClassName(obj) {
    return Java.cast(obj, Java.use("java.lang.Object")).getClass().getName();
}


Java.perform(function () {
    var Base64 = Java.use("android.util.Base64");

    var TargetClass = Java.use("e71");

    TargetClass.a.overload("java.security.PrivateKey", "[B").implementation = function (key22, data) {
        console.log("=== Hooked method a(PrivateKey, byte[]) ===");

        // 打印 data
        var dataHex = hexdump(data);
        var dataBase64 = Base64.encodeToString(data, 0);

        var StringClass = Java.use("java.lang.String");
        var dataStr = StringClass.$new(data, "UTF-8");

        console.log("[*] Data (hex):", dataHex);
        console.log("[*] Data (base64):", dataBase64);
        console.log(111)
        console.log("[*] Data (string):", dataStr.toString());
        console.log(111)

        // 打印 key 信息
        console.log("[*] PrivateKey class:", getJavaClassName(key22));

        try {
            var RSAPrivateKey = Java.use("java.security.interfaces.RSAPrivateKey");
            if (RSAPrivateKey.class.isInstance(key22)) {
                var castKey = Java.cast(key22, RSAPrivateKey);
                var encoded = castKey.getEncoded();
                var keyBase64 = Base64.encodeToString(encoded, 0);
                console.log("[+] PrivateKey (Base64):", keyBase64);
            } else {
                console.log("[-] Not an RSAPrivateKey instance.");
            }
        } catch (e) {
            console.log("[-] Exception extracting key:", e);
        }

        // 调用原函数
        var result = this.a(key22, data);

        // 打印签名结果
        var resultBase64 = Base64.encodeToString(result, 0);
        console.log("[+] Signature (base64):", resultBase64);
        console.log("=========================================");

        return result;
    };

    function hexdump(bytes) {
        return Array.prototype.map.call(Java.array('byte', bytes), function (b) {
            return ('00' + (b & 0xff).toString(16)).slice(-2);
        }).join(' ');
    }
});

注意这里有两个坑， 一个是如何打印 privatekey 的问题， 另外一个是查看类型的问题, 总是 not a function, 我记得之前frida 还可以用

obj.getClass().getName() 来查看一个对象的类型的, 但是这个样本就是 not a function, 然后我降frida-server版本, 从 frida-server 16.6.6,
降为 frida-server 14.2.18, 还是 not a function, 我问了下chatgpt 和 deepseek 是这么解答的

我们再次打印 privatekey 的类型

PrivateKey class: com.android.org.bouncycastle.jcajce.provider.asymmetric.rsa.BCRSAPrivateCrtKey

然后交给chatgpt 问gpt 这个类型要怎么打印私钥.
结合 gpt 给出的答案， 我们整合一下代码， 最终的代码如下

function getJavaClassName(obj) {
    return Java.cast(obj, Java.use("java.lang.Object")).getClass().getName();
}


Java.perform(function () {
    var Base64 = Java.use("android.util.Base64");
    console.log(1222)

    // 替换成你的实际类名，比如 com.example.MyClass
    var TargetClass = Java.use("e71");

    // Hook public final byte[] a(PrivateKey key, byte[] data)
    TargetClass.a.overload("java.security.PrivateKey", "[B").implementation = function (key22, data) {
        console.log("=== Hooked method a(PrivateKey, byte[]) ===");

        // 打印 data
        var dataHex = hexdump(data);
        var dataBase64 = Base64.encodeToString(data, 0);

        var StringClass = Java.use("java.lang.String");
        var dataStr = StringClass.$new(data, "UTF-8");

        console.log("[*] Data (hex):", dataHex);
        console.log("[*] Data (base64):", dataBase64);
        console.log(111)
        console.log("[*] Data (string):", dataStr.toString());
        console.log(111)

        // 打印 key 信息
        console.log("[*] PrivateKey class:", getJavaClassName(key22));

        try {
            const Base64 = Java.use("android.util.Base64");
            const encoded = Java.cast(key22, Java.use("java.security.Key")).getEncoded();
            const keyBase64 = Base64.encodeToString(encoded, 0);
            console.log("[+] PrivateKey PKCS#8 (Base64):");
            console.log(keyBase64);
        } catch (e) {
            console.log("[-] Exception dumping key:", e);
        }

        // 调用原函数
        var result = this.a(key22, data);

        // 打印签名结果
        var resultBase64 = Base64.encodeToString(result, 0);
        console.log("[+] Signature (base64):", resultBase64);
        console.log("=========================================");

        return result;
    };

    function hexdump(bytes) {
        return Array.prototype.map.call(Java.array('byte', bytes), function (b) {
            return ('00' + (b & 0xff).toString(16)).slice(-2);
        }).join(' ');
    }
});

我们再次 frida hook 一下，结果如下

可以看到 私钥已经出来了， 剩下的就是如何 使用私钥进行 rsa sha256签名了, 下面给出 Java 代码

import java.security.KeyFactory;
import java.security.PrivateKey;
import java.security.Signature;
import java.security.spec.PKCS8EncodedKeySpec;
import java.util.Base64;


class MyClass {
    public static void main(String[] args) throws Exception {
        // 假设你已经有 PrivateKey 对象
        PrivateKey privateKey = loadPrivateKey();  // 你需要从文件或 Base64 解析

        //String data = "hello world";
        String data = "1750927494\n/graphql/v2\nXXX-xxxxxxx\n11.31.0\n";
        byte[] dataBytes = data.getBytes("UTF-8");

        // 创建 SHA256withRSA 签名对象
        Signature signature = Signature.getInstance("SHA256withRSA");
        signature.initSign(privateKey);
        signature.update(dataBytes);
        byte[] signed = signature.sign();

        // 输出 Base64 签名结果
        String signatureBase64 = Base64.getEncoder().encodeToString(signed);
        System.out.println("Signature (Base64): " + signatureBase64);
    }

    // 示例：从 PKCS#8 Base64 字符串中加载私钥
    public static PrivateKey loadPrivateKey() throws Exception {
        String base64Key =
        "MIIEvAxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      + "xxxxxxxxxxxxxxxx==";

    // 用 MIME 解码器自动忽略换行
    byte[] keyBytes = Base64.getMimeDecoder().decode(base64Key);
    PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(keyBytes);
    return KeyFactory.getInstance("RSA").generatePrivate(keySpec);
    }
}

然后运行 结果如下

然后用 java 生成的结果, 替换 charles 中的请求的加密参数. 发现可以成功获取到数据, 到此基本就成功了.

如何用 python 还原一份呢, 下面给出代码. 已下代码经过验证, 可成功通过校验.

import base64
from cryptography.hazmat.primitives import serialization, hashes
from cryptography.hazmat.primitives.asymmetric import padding
from cryptography.hazmat.primitives.asymmetric import rsa
from cryptography.hazmat.backends import default_backend


def load_private_key():
    base64_key = """
        MIIEvXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        XXXXXXXXXXXXXXXXXXXXXXXXXX=="""

    # 移除空白和换行
    base64_key = "".join(base64_key.split())

    # 解码Base64并加载私钥
    key_bytes = base64.b64decode(base64_key)
    private_key = serialization.load_der_private_key(
        key_bytes,
        password=None,
        backend=default_backend()
    )
    return private_key


def main():
    # 加载私钥
    private_key = load_private_key()

    # 准备要签名的数据
    data = "1750929449\n/graphql/v2\nXXXX-XXXXX\n11.31.0\n"
    data_bytes = data.encode('utf-8')

    # 使用SHA256withRSA进行签名
    signature = private_key.sign(
        data_bytes,
        padding.PKCS1v15(),
        hashes.SHA256()
    )

    # 输出Base64编码的签名
    signature_base64 = base64.b64encode(signature).decode('utf-8')
    print("Signature (Base64):", signature_base64)


if __name__ == "__main__":
    main()

python 需要安装 cryptography

$ python3 -m pip install cryptography -i https://pypi.tuna.tsinghua.edu.cn/simple

通过私钥 进行签名, 使用的是 SHA256withRSA

改为 python 后，同样也可以生成加密结果 并且可以通过校验.

总结

• 遇到的核心问题, getClass().getName() 会报错 not a function, 可能是由于android 高版本的优化问题. 并不是 frida-server 和 frida 以及 frida-tools 的问题.
• privateKey 如何打印的问题, 上面已经给出答案. 还有些是通过 keyStore 这种， 这种导出就很麻烦.
• python3.8 安装 frida 14.2.18 会遇到 frida-14.2.18-py3.8-linux-x86_64.egg 问题, 直接下载 放到指定位置就好.
• 关于 RSA private, 或者 public key 也可以考虑 hook base64 的 decode 方法, 一般公钥要base64 解码后然后加载字节数据.

收获

在分析这个样本的过程中, 想借助一些文章, 发现了一些站点, 貌似可以低成本下载文档。

http://toolman.dedyn.io:9192/#/main/csdnPaper
https://github.com/bigintpro/csdn_downloader

充值了一块钱， 结果是个垃圾文章, 骗加公众号的, 无语子.

另外记录一下本人使用的 frida稳定的版本

pip install frida==14.2.18
pip install frida-tools==9.2.5
pip install objection==1.11.0

# 下面是本人使用到的 objection  的脚本, 这里记录一下
objection -g com.xxxxxxx.android explore -P ~/.objection/plugins
plugin wallbreaker objectsearch com.xxxxxxx.android.internal.auth.signing.util.RSASigner
plugin wallbreaker objectdump 0x4142 --fullname


# http://littleshark.space/2024/08/25/Frida%E5%AD%A6%E4%B9%A0%E7%AC%94%E8%AE%B0/Frida%E4%BD%BF%E7%94%A8%E5%A4%A7%E5%85%A8/
# https://bbs.kanxue.com/thread-277929.htm