Base 64 encoding and decoding. It's currently set for encoding. If you want to decode then comment encode and uncomment decode. You will also have to swap the input and output text files.

base_64.cpp

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

const char base_64_arr[]="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

const char decode_arr[]="|&&&}rstuvwxyz{&&&&&&&>?@ABCDEFGHIJKLMNOPQRSTUVW&&&&&&XYZ[\\]^_`abcdefghijklmnopq";

void decode(FILE *f_in, FILE *f_out)
{
    int i, len, ch;
    char in[4], out[3];

	*in = (char) 0;
	*out = (char) 0;
    while(!feof(f_in))
    {
        len=0;
        for(i=0; i < 4 && feof(f_in) == 0; i++)
        {
            ch=0;
            while(feof(f_in) == 0 && ch == 0)
            {
                ch = getc(f_in);
                if(ch != EOF)
                {
	                ch = ((ch < 43 || ch>122) ? 0 : (int) decode_arr[ch-43]);
					if(ch != 0)
                    {
                        if(ch == (int)'&')
                            ch = 0;
                        else
                            ch -= 61;
                    }
                }
            }
            if(feof(f_in) == 0)
            {
                len++;
                if(ch != 0) in[i] = (char) (ch-1);
            }
            else in[i] = (char) 0;
        }
        if(len > 0)
        {
            out[0] = (char) (in[0] << 2 | in[1] >> 4);
            out[1] = (char) (in[1] << 4 | in[2] >> 2);
            out[2] = (char) (((in[2] << 6) & 0xc0) | in[3]);
            for(i=0; i < len - 1; i++)
                if(putc((int) out[i], f_out) == 0) break;
        }
    }
}

void encode(FILE *f_in, FILE *f_out)
{
    char in[3], out[4];
    int i, len, six_bits=0;

	*in = (char) 0;
	*out = (char) 0;
    while(feof(f_in) == 0)
    {
        len=0;
        for(i=0; i<3; i++)
        {
            in[i] = (char) getc(f_in);
            
            if(!feof(f_in)) len++;
            else in[i] = (char) 0;
        }
        
        if(len > 0)
        {
            out[0] = (char) base_64_arr[(int)(in[0] >> 2)];
            out[1] = (char) base_64_arr[(int)(((in[0] & 0x03) << 4) | ((in[1] & 0xf0) >> 4))];
            // Add padding if len <= 1
            if(len>1)
                out[2] = (char) base_64_arr[(int)(((in[1] & 0x0f) << 2) | ((in[2] & 0xc0) >> 6))];
            else
                out[2] = '=';
            // Add padding if len <= 2
            if(len>2)
                out[3] = (char) base_64_arr[(int)(in[2] & 0x3f)];    
            else
                out[3] = '=';
            
            for(i=0; i<4; i++)
                if(putc((int)(out[i]), f_out) == 0) break;
            
            six_bits++;
        }
        // printf("number of six bits: %d\n", six_bits);
    }
}

int main()
{
    FILE* f_in = fopen("input_file.txt", "rb");
    FILE* f_out = fopen("output_file.txt", "wb");

    // ENCODE
    encode(f_in, f_out);

    // DECODE
    // decode(f_in, f_out);

    return 0;
}

input_file.txt

OpenCV (Open Source Computer Vision) is a library of programming functions mainly aimed at real-time computer vision, developed by Intel Russia research center in Nizhny Novgorod, and now supported by Willow Garage and Itseez. It is free for use under the open-source BSD license. The library is cross-platform. It focuses mainly on real-time image processing. If the library finds Intel's Integrated Performance Primitives on the system, it will use these proprietary optimized routines to accelerate itself.

-- Taken from Wikipedia

output_file.txt

T3BlbkNWIChPcGVuIFNvdXJjZSBDb21wdXRlciBWaXNpb24pIGlzIGEgbGlicmFyeSBvZiBwcm9ncmFtbWluZyBmdW5jdGlvbnMgbWFpbmx5IGFpbWVkIGF0IHJlYWwtdGltZSBjb21wdXRlciB2aXNpb24sIGRldmVsb3BlZCBieSBJbnRlbCBSdXNzaWEgcmVzZWFyY2ggY2VudGVyIGluIE5pemhueSBOb3Znb3JvZCwgYW5kIG5vdyBzdXBwb3J0ZWQgYnkgV2lsbG93IEdhcmFnZSBhbmQgSXRzZWV6LiBJdCBpcyBmcmVlIGZvciB1c2UgdW5kZXIgdGhlIG9wZW4tc291cmNlIEJTRCBsaWNlbnNlLiBUaGUgbGlicmFyeSBpcyBjcm9zcy1wbGF0Zm9ybS4gSXQgZm9jdXNlcyBtYWlubHkgb24gcmVhbC10aW1lIGltYWdlIHByb2Nlc3NpbmcuIElmIHRoZSBsaWJyYXJ5IGZpbmRzIEludGVsJ3MgSW50ZWdyYXRlZCBQZXJmb3JtYW5jZSBQcmltaXRpdmVzIG9uIHRoZSBzeXN0ZW0sIGl0IHdpbGwgdXNlIHRoZXNlIHByb3ByaWV0YXJ5IG9wdGltaXplZCByb3V0aW5lcyB0byBhY2NlbGVyYXRlIGl0c2VsZi4KCi0tIFRha2VuIGZyb20gV2lraXBlZGlhCg==