From Stream -> Base64 -> To Stream

Base64Write.cs

private static byte[] Base64Table = {
    0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4a,
    0x4b, 0x4c, 0x4d, 0x4e, 0x4f, 0x50, 0x51, 0x52, 0x53, 0x54,
    0x55, 0x56, 0x57, 0x58, 0x59, 0x5a,
    0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69, 0x6a,
    0x6b, 0x6c, 0x6d, 0x6e, 0x6f, 0x70, 0x71, 0x72, 0x73, 0x74,
    0x75, 0x76, 0x77, 0x78, 0x79, 0x7a,
    0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39,
    0x2b, 0x2f, 0x3d }; // ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=
//  0x2d, 0x5f, 0x3d }; // ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_=
    
private static void WriteByBase64(Stream from, Stream to)
{
    var buff = new byte[4096];
    int read = 0;
    int i = 0;
    int b;

    long pos = to.Position;

    WriteByBase64_Read(buff, from, ref i, ref read);
    while (true)
    {
        if (i >= read) WriteByBase64_Read(buff, from, ref i, ref read);
        if (i >= read) break;

        b = (buff[i] & 0xFC) >> 2;
        to.WriteByte(Base64Table[b]);                    
        b = (buff[i] & 0x03) << 4;

        if (i + 1 >= read) WriteByBase64_Read(buff, from, ref i, ref read);
        if (i + 1 < read)
        {
            b |= (buff[i + 1] & 0xF0) >> 4;
            to.WriteByte(Base64Table[b]);

            b = (buff[i + 1] & 0x0F) << 2;
            
            if (i + 2 >= read) WriteByBase64_Read(buff, from, ref i, ref read);
            if (i + 2 < read)
            {
                b |= (buff[i + 2] & 0xC0) >> 6;
                to.WriteByte(Base64Table[b]);
                b = buff[i + 2] & 0x3F;
                to.WriteByte(Base64Table[b]);
            }
            else
            {
                to.WriteByte(Base64Table[b]);
                to.WriteByte(Base64Table[64]);
            }
        }
        else
        {
            to.WriteByte(Base64Table[b]);
            to.WriteByte(Base64Table[64]);
            to.WriteByte(Base64Table[64]);
        }

        i += 3;
    }
    
    pos = to.Position - pos;

    while (pos++ % 4 != 0)
        to.WriteByte(Base64Table[64]);
}
private static void WriteByBase64_Read(byte[] buff, Stream from, ref int i, ref int read)
{
    int index = 0;
    int k;
    for (k = i; k < read; ++k) buff[index++] = buff[k];
    
    var nr = from.Read(buff, index, buff.Length - index);
    if (nr > 0)
    {
        read = nr + index;
        i = 0;
    }
}