Base64Decode.c

//Decodes Base64
#include <openssl/bio.h>
#include <openssl/evp.h>
#include <string.h>
#include <stdio.h>

int calcDecodeLength(const char* b64input) { //Calculates the length of a decoded base64 string
  int len = strlen(b64input);
  int padding = 0;

  if (b64input[len-1] == '=' && b64input[len-2] == '=') //last two chars are =
    padding = 2;
  else if (b64input[len-1] == '=') //last char is =
    padding = 1;

  return (int)len*0.75 - padding;
}

int Base64Decode(char* b64message, char** buffer) { //Decodes a base64 encoded string
  BIO *bio, *b64;
  int decodeLen = calcDecodeLength(b64message),
      len = 0;
  *buffer = (char*)malloc(decodeLen+1);
  FILE* stream = fmemopen(b64message, strlen(b64message), "r");

  b64 = BIO_new(BIO_f_base64());
  bio = BIO_new_fp(stream, BIO_NOCLOSE);
  bio = BIO_push(b64, bio);
  BIO_set_flags(bio, BIO_FLAGS_BASE64_NO_NL); //Do not use newlines to flush buffer
  len = BIO_read(bio, *buffer, strlen(b64message));
    //Can test here if len == decodeLen - if not, then return an error
  (*buffer)[len] = '\0';

  BIO_free_all(bio);
  fclose(stream);

  return (0); //success
}

Base64Encode.c

//Encodes Base64
#include <openssl/bio.h>
#include <openssl/evp.h>
#include <string.h>
#include <stdio.h>
#include <math.h>

int Base64Encode(const char* message, char** buffer) { //Encodes a string to base64
  BIO *bio, *b64;
  FILE* stream;
  int encodedSize = 4*ceil((double)strlen(message)/3);
  *buffer = (char *)malloc(encodedSize+1);

  stream = fmemopen(*buffer, encodedSize+1, "w");
  b64 = BIO_new(BIO_f_base64());
  bio = BIO_new_fp(stream, BIO_NOCLOSE);
  bio = BIO_push(b64, bio);
  BIO_set_flags(bio, BIO_FLAGS_BASE64_NO_NL); //Ignore newlines - write everything in one line
  BIO_write(bio, message, strlen(message));
  BIO_flush(bio);
  BIO_free_all(bio);
  fclose(stream);

  return (0); //success
}

Main.c

#include <stdio.h>

int main() {
  //Encode To Base64
  char* base64EncodeOutput;
  Base64Encode("Hello World", &base64EncodeOutput);
  printf("Output (base64): %s\n", base64EncodeOutput);

  //Decode From Base64
  char* base64DecodeOutput;
  Base64Decode("SGVsbG8gV29ybGQ=", &base64DecodeOutput);
  printf("Output: %s\n", base64DecodeOutput);
  
  return(0);
}

Makefile

all:
	gcc -o base64 Main.c Base64Encode.c Base64Decode.c -lcrypto -lm