1.編譯Openssl原始碼，生成.so庫（筆者使用環境為Ubuntu 18.04 amd64 桌面版，android_ndk_r19c 64位版）：

下載Openssl原始碼，進入原始碼環境然後執行配置指令碼，設定編譯引數，開始編譯：

    export ANDROID_NDK=/home/gj/android-ndk-r19c
    export PATH=/home/gj/android-ndk-r19c/toolchains/llvm/prebuilt/linux-x86_64/bin:$PATH
    ./Configure android-arm shared
    make SHLIB_EXT=.so -j8

　　生成的.so儲存一下。如果想生成其他ABI架構的so，可以clean後再配置、編譯：

    make clean
    ./Configure android-arm64 shared
    make SHLIB_EXT=.so -j8

2.新建Android專案（注意要選中Native C++這個。筆者使用環境為Android Studio 4.0，JDK1.8，gradle 5.6.4，gradle plugin3.4.2），複製openssl原始碼目錄下所有標頭檔案（include目錄），以及上一步生成的所有.so到Android專案中。

專案地址：https://github.com/areful/NdkUseOpenssl

native-lib.cpp示例展示瞭如何使用openssl庫進行RSA加解密、生成簽名並驗證簽名。

程式碼太多，就不一一列出來內容了，可以clone程式碼檢視內容，只列一下build.gradle和CMakeLists.txt主要內容：

build.gradle：

android {
    ......

    defaultConfig {
        ......

        externalNativeBuild {
            cmake {
                cppFlags "-std=c++11 -frtti -fexceptions"
                arguments '-DANDROID_STL=c++_shared'
            }
            ndk {
                stl = "c++_shared"
                abiFilters 'x86', 'x86_64', 'armeabi-v7a', 'arm64-v8a'
            }
        }
    }
    ......


    sourceSets {
        main {
            jniLibs.srcDirs = ['${PROJECT_SOURCE_DIR}/libs']
        }
    }

    packagingOptions {
        pickFirst '**/*.so'
    }

    externalNativeBuild {
        cmake {
            path "src/main/cpp/CMakeLists.txt"
            version "3.10.2"
        }
    }


}

　　CMakeLists.txt:

# For more information about using CMake with Android Studio, read the
# documentation: https://d.android.com/studio/projects/add-native-code.html

cmake_minimum_required(VERSION 3.4.1)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11")

###############################################################
# import openssl library files BEGIN
include_directories(include)

set(SSL_LIB_NAME ssl)
add_library(${SSL_LIB_NAME} SHARED IMPORTED)
set_target_properties(${SSL_LIB_NAME} PROPERTIES IMPORTED_LOCATION ${PROJECT_SOURCE_DIR}/libs/${ANDROID_ABI}/lib${SSL_LIB_NAME}.so)

set(CRYPTO_LIB_NAME crypto)
add_library(${CRYPTO_LIB_NAME} SHARED IMPORTED)
set_target_properties(${CRYPTO_LIB_NAME} PROPERTIES IMPORTED_LOCATION ${PROJECT_SOURCE_DIR}/libs/${ANDROID_ABI}/lib${CRYPTO_LIB_NAME}.so)
# import openssl library files END
###############################################################

find_library(log-lib log)

add_library(native-lib SHARED
        native-lib.cpp
        native-lib.h
        b64/b64.cpp
        b64/b64.h
        b64/test_b64.cpp
        b64/test_b64.h
        rsa/rsa.cpp
        rsa/rsa.h
        rsa/test_rsa.cpp
        rsa/test_rsa.h
        common.h)

target_link_libraries(
        native-lib
        ssl
        crypto
        ${log-lib})