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JNI DETECTED ERROR IN APPLICATION: use of invalid jobject when returning Array of String

Description of the intended goal
I'm trying to implement OpenSSL-generated public/private key pairs in Android/Kotlin using JNI, in order to implement user encryption on the information stored to the cloud server. I've compiled all OpenSSL source code and generated all .so files correcly.
The code (C++)
The C++ code to use OpenSSL is shown below. CmakeLists.txt and NDK configuration is working fine.
extern "C"
JNIEXPORT jobjectArray JNICALL
Java_eu_joober_ui_entry_SplashFragment_generateRSAKeyPair(JNIEnv *env, jobject thiz) {
int ret = 0;
RSA *r = nullptr;
BIGNUM *bne = nullptr;
BIO *bp_public = nullptr, *bp_private = nullptr;
int bits = 2048;
unsigned long e = RSA_F4;
jstring public_key_text;
jstring private_key_text;
jobjectArray returnPair = env->NewObjectArray(2, env->FindClass("java/lang/String"),nullptr);
// 1. generate rsa key
bne = BN_new();
ret = BN_set_word(bne,e);
if(ret != 1){
goto free_all;
}
r = RSA_new();
ret = RSA_generate_key_ex(r, bits, bne, nullptr);
if(ret != 1){
goto free_all;
}
// 2. get public key
bp_public = BIO_new(BIO_s_mem());
ret = PEM_write_bio_RSAPublicKey(bp_public, r);
BIO_get_mem_data(bp_public, &public_key_text);
if(ret != 1){
goto free_all;
}
// 3. get private key
bp_private = BIO_new(BIO_s_mem());
ret = PEM_write_bio_RSAPrivateKey(bp_private, r, nullptr, nullptr, 0, nullptr, nullptr);
BIO_get_mem_data(bp_private, &private_key_text);
// Check public and private keys were generated correctly
__android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, "Public key is: \n%s",public_key_text);
__android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, "Private key is: \n%s",private_key_text);
// 4. free
free_all:
BIO_free_all(bp_public);
BIO_free_all(bp_private);
RSA_free(r);
BN_free(bne);
// 5. Return strings using jobjectArray
if (ret == 1) {
env->SetObjectArrayElement(returnPair, 0, public_key_text);
env->SetObjectArrayElement(returnPair, 1, private_key_text);
return returnPair;
}
else {
return nullptr;
}
}
The error
If I check the Android Logcat, both public and private key seem to be generated correctly (as per __android_log_print output) but the app crashes with the following error when env->SetObjectArrayElement(returnPair, 0, public_key_text); is called:
JNI DETECTED ERROR IN APPLICATION: use of invalid jobject
The IDE (Android Studio) does not complain on any error, and the log suggests that key pair is being generated correctly, but I don't know why the keys are not being stored in the jobjectArray correctly. In fact, if I just simply put:
env->SetObjectArrayElement(returnPair, 0, env->NewStringUTF("Hello"));
env->SetObjectArrayElement(returnPair, 1, env->NewStringUTF("World"));
the code works completely fine, my Kotlin code gets the Strings correctly ("Hello" and "World"), and the app does not crash, which makes me think that problem is only on the C++ side.
The question
What I am doing wrong? I have checked other SO questions like JNI converting jstring to char * or jstring return in JNI program with slight modifications and combinations with no luck.
SIDE NOTE: I'm using OpenSSL implementation with C++ because Android/Kotlin code does not provide the private key generated using KeyPairGenerator.getInstance() and generatePair() (only public key can be retrieved from Keystore), which I need to be stored in a different place so that user information can be retrieved even if the app is uninstalled, as every subsequent call to generatePair() will lead to a different key pair. If you know a different approach to this problem I am more than welcome to any suggestions you may provide.

You never created a java string out of public_key_text
Try
char * public_key_text;
...
BIO_get_mem_data(bp_public, &public_key_text);
...
env->SetObjectArrayElement(returnPair, 0, env->NewStringUTF(public_key_text));

cannot initialize a variable of type 'jstring' (aka '_jstring *') with an rvalue of type 'jobject' (aka '_jobject *')

Does anyone know how to solve the error?
JNIEXPORT jstring JNICALL JAVA_com_pfc_camera_ndkmain_MainActivity_compresion(JNIEnv* env, jobject obj, jobjectArray jargv){
//jargv is a Java array of Java strings
int argc = env->GetArrayLength(jargv);
typedef char *pchar;
pchar *argv = new pchar[argc];
int i;
for(i=0; i<argc; i++)
{
jstring js = env->GetObjectArrayElement(jargv, i); //A Java string
const char *pjc = env->GetStringUTFChars(js); //A pointer to a Java-managed char buffer
size_t jslen = strlen(pjc);
argv[i] = new char[jslen+1]; //Extra char for the terminating null
strcpy(argv[i], pjc); //Copy to *our* buffer. We could omit that, but IMHO this is cleaner. Also, const correctness.
env->ReleaseStringUTFChars(js, pjc);
}
//Call main
Principal *pa=Principal::CreateInstance(argc,argv);
pa->Run();
pa->FreeInstance();
//Now free the array
for(i=0;i<argc;i++)
delete [] argv[i];
delete [] argv;
}
I understand that the error can come from not doing a casting but I do not have it very clear
[ https://i.stack.imgur.com/bOWKZ.png][1]
It seems that the problem has been solved, now I get another error but I do not understand if I'm passing two arguments js and pjc
[ https://i.stack.imgur.com/UHCAR.png][1]

In C++ you have to use explicit conversion to your desired type.
jstring js = (jstring)env->GetObjectArrayElement(jargv, i);
You can learn about jni programming here
Regarding your other question do the following:
const jbyte *pjc = env->GetStringUTFChars(js, NULL);

JNI - transfer from jstring to byte, from byte to string issue

I have a problem with converting in JNI.
In C++ I'm creating some cipher using AES (Library CryptoPP). I'm converting result to string and returning it. This is how the code getting the string looks like:
JNIEXPORT jbyteArray JNICALL Java_com_example_androidake_MutualAuthenticateChip_prepareEncryptionCPP
(JNIEnv *env, jobject thisObj, jboolean hmm, jboolean jinit) {
string encryption= mac->EncryptCertKey();
jbyteArray returns = env->NewByteArray(encryption.size());
env->SetByteArrayRegion(returns, 0, encryption.length(), (jbyte*) encryption.c_str());
return returns;
};
Above string is converting to jbyteArray which is returned. First I wanted just return string using
env->NewStringUTF(encryption.c_str());
but the application has been crashing. I think it is caused by content of variable 'encryption'. I'm using env->NewStringUTF(encryption.c_str()); in other functions, where returned string is for example just a number or something like that.
Then in Java I'm doing conversion from byte to string:
byte[] cipher = mac_A.prepareEncryptionCPP(true, true);
string cipher_str = new String(cipher);
And I'm putting that string again to the C++ object and compare old cipher with the cipher which is sent from Java:
//Java
boolean result = mac_A.compareEncryption(true, cipher);
//JNI
JNIEXPORT jboolean JNICALL Java_com_example_androidake_MutualAuthenticateChip_compareEncryption
(JNIEnv * env, jobject thisObj, jboolean jinit, jstring cipher){
bool init = jinit;
bool result;
jsize length = env->GetStringUTFLength(cipher);
const char *inCStr_ek = env->GetStringUTFChars(cipher, 0);
string s(inCStr_ek, length);
result = mac->CompareCipher(s);
env->ReleaseStringUTFChars(cipher, inCStr_ek);
return result;
};
Comparing in C++ :
bool MyClass::CompareCipher(std::string cipher_2){
if(cipher == cipher_2){
return true;
}else{
return false;
}
}
And it always returns false. I do not know what I'm doing wrong. I've even sent this cipher from Java to C++ and take it back to Java and the strings are equals, but in C++ side are not.

on your java side code you have
byte[] cipher = mac_A.prepareEncryptionCPP(true, true);
boolean result = mac_A.compareEncryption(true, cipher);
compareEncryption jni function is defined with jstring , not jbytearray.
So from JNI side you are sending a byte array to java, and from java side you send back the same byte array to native side (but uses jstring in call), but then you are using env->GetStringUTFChars(cipher, 0) which converts that byte array into a modified UTF-8 string, so its technically not that same byte array anymore.
if you need strings do the conversions in java side, and just use with same plain byte arrays between jni and java. See this for string encoding issues in Android JNI.

Passing Json char to C using JNI to get Jansson dump of calculated values, Pointer problems

I have implemented Jansson in Android with C and made a function which calculates values from json and that works in C, I tried to use that code in NDK with JNI it builds with no errors, but as i tried to arrange the code to work with JNI it gives me pointer error warning: return from incompatible pointer type. I have read that i need to use jlong for pointers but i cant figure out how that works, it is my first time working in it.
This is my code from C (gives no errors and compiles)
char *doCalc (char *invoice_str) {
json_error_t error;
json_t *invoice = json_loads (invoice_str, JSON_DISABLE_EOF_CHECK, &error);
...
char *result = json_dumps (json_data, JSON_PRESERVE_ORDER);
return result;
}
C code Arranged to work with JNI (gives me error warning: return from incompatible pointer type, which if im correct is because of jchar)
JNIEXPORT jchar JNICALL *Java_com_example_test_doCalc (JNIEnv* env, jobject obj,char const *invoice_str) {
json_error_t error;
json_t *invoice = json_loads (invoice_str, JSON_DISABLE_EOF_CHECK, &error);
...
char *result = json_dumps (json_data, JSON_PRESERVE_ORDER);
return result;
}
Then in my Activity I like to would run doCalc(charJ);, charJ has Json in it. Which would then give me dump of calculated values.
Also I might be looking at this completely wrong, any help is appreciated.

Try to use jstring instead of char*
JNIEXPORT jchar JNICALL * Java_com_example_test_doCalc(JNIEnv * env, jobject obj, jstring invoice_jstring) {
//convert invoice_jstring to char* link bellow
json_error_t error;
json_t * invoice = json_loads(invoice_str, JSON_DISABLE_EOF_CHECK, & error);
...
char * result = json_dumps(json_data, JSON_PRESERVE_ORDER);
return result;
}
for conversion jstring to char* you can use this answer:
JNI converting jstring to char *

Android JNI - Reliable way to convert jstring to wchar_t

In my Android JNI code, I need to convert jstring to wchar_t. The closest reference I found was How do I convert jstring to wchar_t *.
One can obtain jchar* and the length using the following code:
const jchar *raw = env->GetStringChars(string, 0);
jsize len = env->GetStringLength(string);
wchar_t* wStr = new wchar_t[len+1];
It seems I cannot use wcncpy to copy "raw" into "wStr." Although jchar is 2-bytes long, wchar_t is 4 bytes long on all modern versions of Android OS.
One option is to copy one character at a time in a for loop:
for(int i=0;i<len;i++) {
wStr[i] = raw[i];
}
wStr[len] = 0;
The other option would be to call env->GetStringUTFChars() and use iconv_* routines to convert to wchar_t type.
Can someone please confirm if option 1 is valid? Hope I don't have to resort to option 2. Is there a better option? Regards.

wchar_t specifies an element size but not a character set or encoding. Since you are asking about a 32-bit element, can we assume you want to use Unicode/UTF-32? Regardless, once you decide which encoding you want, standard Java libraries are up to the task.
Use a String.getBytes() overload to get an array of bytes. (It is easier to do this in Java rather than JNI, if you have a choice.) Once you have a jbyteArray, you can copy it to a C buffer and cast to wchar_t *.
On Android, you might want Unicode/UTF-8. But that has an 8-bit code-unit so you probably wouldn't be asking about wchar_t. (BTW-a character in UTF-8 can need 1 or more bytes.)

One way would be to use String.getBytes("UTF-32LE"). Note this is making the ASSUMPTION that wchar_t is 4 bytes and little-endian, but this should be a fairly safe assumption to make.
Here's an example that passes a String from Java to C++, where it is converted to std::wstring, reversed, and passed back to Java:
class MyClass {
private native byte[] reverseString(byte[] arr);
String reverseString(String s) {
try {
return new String(reverseString(s.getBytes("UTF-32")), "UTF-32");
} catch (UnsupportedEncodingException e) {
// TODO Auto-generated catch block
e.printStackTrace();
return "";
}
}
}
On the C++ side you have:
std::wstring toWStr(JNIEnv *env, jbyteArray s)
{
const wchar_t *buf = (wchar_t*) env->GetByteArrayElements(s, NULL);
int n = env->GetArrayLength(s) / sizeof(wchar_t);
// First byte is BOM (0xfeff), so we skip it, hence the "buf + 1".
// There IS NO null-terminator.
std::wstring ret(buf + 1, buf + n);
env->ReleaseByteArrayElements(s, (jbyte*) buf, 0);
return ret;
}
jbyteArray fromWStr(JNIEnv *env, const std::wstring &s)
{
jbyteArray ret = env->NewByteArray((s.size()+1)*sizeof(wchar_t));
// Add the BOM in front.
wchar_t bom = 0xfeff;
env->SetByteArrayRegion(ret, 0, sizeof(wchar_t), (const jbyte*) &bom);
env->SetByteArrayRegion(ret, sizeof(wchar_t), s.size()*sizeof(wchar_t), (const jbyte*) s.c_str());
return ret;
}
extern "C" JNIEXPORT jbyteArray JNICALL Java_MyClass_reverseString(JNIEnv *env, jobject thiz, jbyteArray arr)
{
std::wstring s= toWStr(env, arr);
std::reverse(s.begin(), s.end());
return fromWStr(env, s);
}
I tested it both on my phone, which has Android 4.1.2 and ARM CPU, and on the Android Emulator - Android 4.4.2 and x86 CPU, and this code:
MyClass obj = new MyClass();
Log.d("test", obj.reverseString("hello, здравствуйте, 您好, こんにちは"));
Gave this output:
06-04 17:18:20.605: D/test(8285): はちにんこ ,好您 ,етйувтсвардз ,olleh

As long as all your data is UCS2, you can use option 1. Please see wchar_t for UTF-16 on Linux? for a similar discussion. Note that C++11 provides std::codecvt_utf16 to deal with the situation.

No need to convert. Cast const jchar to (wchar_t *). jni.h define jchar as typedef uint16_t jchar; /* unsigned 16 bits */ which is eventually wchar_t.
You can try this, it worked for me in old project.