［面试］如何对资源文件进行混淆？

[supengchao]

借鉴美团的做法：

1、思路：

对系统aapt（Android Asset Packaging Tool）进行干预，自定义文件混淆的规则。然后重新打包，系统会掉用我们自定义的aapt去打包，打包出来的apk反编译后就会发现原来的资源名全部替换为a,b,c,d...

2、具体的做法如下：

（1）修改AAPT在处理资源文件相关的源码
Resource.cpp中makeFileResources()

    static status_t makeFileResources(Bundle* bundle, const sp<AaptAssets>& assets,                                   

    ResourceTable* table, const sp<ResourceTypeSet>& set,const char* resType)  {

    String8 type8(resType);
    String16 type16(resType);

    bool hasErrors = false;

    ResourceDirIterator it(set, String8(resType));
    ssize_t res;
    while ((res=it.next()) == NO_ERROR) {
        if (bundle->getVerbose()) {
            printf("    (new resource id %s from %s)\n",
                   it.getBaseName().string(), it.getFile()->getPrintableSource().string());
        }
        String16 baseName(it.getBaseName());
        const char16_t* str = baseName.string();
        const char16_t* const end = str + baseName.size();
        while (str < end) {
            if (!((*str >= 'a' && *str <= 'z')
                    || (*str >= '0' && *str <= '9')
                    || *str == '_' || *str == '.')) {
                fprintf(stderr, "%s: Invalid file name: must contain only [a-z0-9_.]\n",
                        it.getPath().string());
                hasErrors = true;
            }
            str++;
        }
        String8 resPath = it.getPath();
        resPath.convertToResPath();

        String8 obfuscationName;
        String8 obfuscationPath = getObfuscationName(resPath, obfuscationName);

        table->addEntry(SourcePos(it.getPath(), 0), String16(assets->getPackage()),
                        type16,
                        baseName, // String16(obfuscationName),
                        String16(obfuscationPath), // resPath
                        NULL,
                        &it.getParams());
        assets->addResource(it.getLeafName(), obfuscationPath/*resPath*/, it.getFile(), type8);
    }

    return hasErrors ? UNKNOWN_ERROR : NO_ERROR;
}