#include <jni.h> 
#include <stdio.h>
#include <windows.h>
#include <wincrypt.h>

JNIEXPORT jobject JNICALL Java_com_example_PrivateKeyRetriever_retrievePrivateKey(JNIEnv *env, jobject thisObj, jstring alias) {
    const char *aliasStr = (*env)->GetStringUTFChars(env, alias, NULL);

    HCRYPTPROV hProv;
    HCRYPTKEY hKey;
    BYTE *pbData;
    DWORD dwDataLen;

    // Acquire a cryptographic context
    if (!CryptAcquireContext(&hProv, NULL, NULL, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT)) {
        printf("CryptAcquireContext failed: %x\n", GetLastError());
        return NULL;
    }

    // Open the private key container
    if (!CryptGetUserKey(hProv, AT_KEYEXCHANGE, &hKey)) {
        printf("CryptGetUserKey failed: %x\n", GetLastError());
        CryptReleaseContext(hProv, 0);
        return NULL;
    }

    // Determine the required buffer size
    if (!CryptExportKey(hKey, 0, PRIVATEKEYBLOB, 0, NULL, &dwDataLen)) {
        printf("CryptExportKey failed: %x\n", GetLastError());
        CryptDestroyKey(hKey);
        CryptReleaseContext(hProv, 0);
        return NULL;
    }

    // Allocate memory for the private key
    pbData = (BYTE *)malloc(dwDataLen);
    if (!pbData) {
        printf("Memory allocation failed.\n");
        CryptDestroyKey(hKey);
        CryptReleaseContext(hProv, 0);
        return NULL;
    }

    // Export the private key
    if (!CryptExportKey(hKey, 0, PRIVATEKEYBLOB, 0, pbData, &dwDataLen)) {
        printf("CryptExportKey failed: %x\n", GetLastError());
        free(pbData);
        CryptDestroyKey(hKey);
        CryptReleaseContext(hProv, 0);
        return NULL;
    }

    // Clean up
    CryptDestroyKey(hKey);
    CryptReleaseContext(hProv, 0);

    // Convert the private key bytes to a Java byte array
    jbyteArray privateKeyBytes = (*env)->NewByteArray(env, dwDataLen);
    (*env)->SetByteArrayRegion(env, privateKeyBytes, 0, dwDataLen, (jbyte *)pbData);
    free(pbData);

    // Create a PrivateKey object using the byte array
    jclass privateKeyClass = (*env)->FindClass(env, "java/security/PrivateKey");
    jmethodID constructor = (*env)->GetMethodID(env, privateKeyClass, "<init>", "([B)V");
    return (*env)->NewObject(env, privateKeyClass, constructor, privateKeyBytes);
}

C Language online compiler

Write, Run & Share C Language code online using OneCompiler's C online compiler for free. It's one of the robust, feature-rich online compilers for C language, running the latest C version which is C18. Getting started with the OneCompiler's C editor is really simple and pretty fast. The editor shows sample boilerplate code when you choose language as 'C' and start coding!

Read inputs from stdin

OneCompiler's C online editor supports stdin and users can give inputs to programs using the STDIN textbox under the I/O tab. Following is a sample C program which takes name as input and print your name with hello.

#include <stdio.h>
int main()
{
    char name[50];
    printf("Enter name:");
    scanf("%s", name);
    printf("Hello %s \n" , name );
    return 0;
    
}

About C

C language is one of the most popular general-purpose programming language developed by Dennis Ritchie at Bell laboratories for UNIX operating system. The initial release of C Language was in the year 1972. Most of the desktop operating systems are written in C Language.

Key features:

  • Structured Programming
  • Popular system programming language
  • UNIX, MySQL and Oracle are completely written in C.
  • Supports variety of platforms
  • Efficient and also handle low-level activities.
  • As fast as assembly language and hence used as system development language.

Syntax help

Loops

1. If-Else:

When ever you want to perform a set of operations based on a condition if-else is used.

if(conditional-expression) {
   // code
} else {
   // code
}

You can also use if-else for nested Ifs and if-else-if ladder when multiple conditions are to be performed on a single variable.

2. Switch:

Switch is an alternative to if-else-if ladder.

switch(conditional-expression) {    
case value1:    
 // code    
 break;  // optional  
case value2:    
 // code    
 break;  // optional  
...    
    
default:     
 // code to be executed when all the above cases are not matched;    
}

3. For:

For loop is used to iterate a set of statements based on a condition.

for(Initialization; Condition; Increment/decrement){  
  // code  
}

4. While:

While is also used to iterate a set of statements based on a condition. Usually while is preferred when number of iterations are not known in advance.

while(condition) {  
 // code 
}

5. Do-While:

Do-while is also used to iterate a set of statements based on a condition. It is mostly used when you need to execute the statements atleast once.

do {
  // code 
} while (condition);

Arrays

Array is a collection of similar data which is stored in continuous memory addresses. Array values can be fetched using index. Index starts from 0 to size-1.

Syntax

One dimentional Array:

data-type array-name[size];

Two dimensional array:

data-type array-name[size][size];

Functions

Function is a sub-routine which contains set of statements. Usually functions are written when multiple calls are required to same set of statements which increases re-usuability and modularity.

Two types of functions are present in C

  1. Library Functions:

Library functions are the in-built functions which are declared in header files like printf(),scanf(),puts(),gets() etc.,

  1. User defined functions:

User defined functions are the ones which are written by the programmer based on the requirement.

How to declare a Function

return_type function_name(parameters);

How to call a Function

function_name (parameters)

How to define a Function

return_type function_name(parameters) {  
  //code
}