get file extension from base64 string

Code to Get File Extension from Base64 String in C

To get the file extension from a base64 string in C, you can follow these steps:

1. Decode the base64 string to binary data.
2. Extract the file extension from the binary data.
3. Convert the file extension to a string.

Here's an example implementation:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char getFileExtensionFromBase64(const char base64String) {
    // Decode the base64 string to binary data
    size_t base64Length = strlen(base64String);
    size_t binaryLength = (base64Length * 3) / 4;
    unsigned char binaryData = (unsigned char)malloc(binaryLength);
    // Perform base64 decoding here...

    // Extract the file extension from the binary data
    // Assuming the file extension is located at the end of the binary data
    size_t extensionStartIndex = binaryLength - 4;
    char fileExtension = (char)malloc(5); // Assuming the file extension is at most 4 characters long
    strncpy(fileExtension, (const char*)(binaryData + extensionStartIndex), 4);
    fileExtension[4] = '\0';

    // Convert the file extension to a string
    // Assuming the file extension is in uppercase
    for (int i = 0; i < 4; i++) {
        if (fileExtension[i] >= 'A' && fileExtension[i] <= 'Z') {
            fileExtension[i] += 32; // Convert to lowercase
        }
    }

    free(binaryData);
    return fileExtension;
}

int main() {
    const char* base64String = "SGVsbG8gd29ybGQh"; // Example base64 string
    char* fileExtension = getFileExtensionFromBase64(base64String);
    printf("File Extension: %s\n", fileExtension);
    free(fileExtension);
    return 0;
}

Explanation of the steps:

1. Decoding the base64 string: The base64 string needs to be decoded to binary data before extracting the file extension. The length of the binary data can be calculated based on the length of the base64 string. The binary data can be stored in an unsigned char array.

2. Extracting the file extension: Assuming the file extension is located at the end of the binary data, you can determine the starting index of the file extension. In the example implementation, it is assumed that the file extension is 4 characters long. The file extension is then copied to a separate char array.

3. Converting the file extension to a string: The extracted file extension is converted to a string. In the example implementation, it is assumed that the file extension is in uppercase. The conversion is done by checking each character and adding 32 to convert uppercase letters to lowercase.

Note: The example implementation provided is a simplified version and does not include the actual base64 decoding logic. You will need to implement the base64 decoding part based on your specific requirements or make use of existing libraries or functions for base64 decoding.