There are two input character bytes left in the last triple:įrom those 16 bits, the most significant 2 x 6 bits are encoded to two base64 output characters.To fill up the based64 encoded quadruple, two padding '=' characters are appended. The most significant 6 bits are normally encoded, the other 2 bits are expanded with zeros and also encoded. There is only one input character byte left in the last triple:.So actually there are three cases to distinct: Then each 6bit part which was filled up with zero is encoded with the special padding character '='. In this case, the input buffer is filled up with zeros until it is divisable by three. What to do if the number of input character bytes is not divisible by three? So the string "Sun" has encoded in Base64 the value "U3Vu". Repeat this until no more input character bytes are left.Įxample encoding for the input string "Sun": Take three character bytes from the input stream (24bits), divide them into four 6bit parts and convert each 6bit value according to the table above. In fact three data byte values are encoded to four Base64 values: This makes sense as three 8bit values are in total 24bit, while a value in range 0-63 requires 6bit for representation, thus four Base64 characters fit in 24bits.īefore we look at the encoding process, first here is the Base64 index mapping table: So how to represent a data byte which can hold 256 different values by a set of only 64 characters? Well, of course this is not possible by a 1:1 mapping, so the Base64 encoded data stream needs more characters than the original byte-based. This is where Base64 encoding & decoding could come into play.īase64 uses 64 characters that are available (and printable) in most text encoding schemes to represent its data. However, it is not convenient that a programm shall know whether the data bytes to process are 'text' or 'binary' - and often binary data shall be processed / transmitted via a text-based protocol / algorithm. So if a stream which is expected to contain only ASCII values actually also contain non-ASCII values, this could potentially lead to problems. The usage of the most significant bit of an ascii value is not completely defined (could be used as a parity bit for example). ![]() Still an ASCII value is normally stored in a byte which has eight bits (0-255). the ASCII standard is based on seven-bit values, thus uses a range of 0-127. The origin of such binary-to-text encoding scheme like Base64 is the requirement to send a stream of bytes over a communication channel which does not allow binary data but only text-based data.Į.g. So let's begin …īase64 is an algorithm to convert a stream of bytes into a stream of printable characters (and back). As always, it' probably not the best or most efficient way, but it should be a good starter. I want to share with you my experience to understand and implement Base64 encoding and decoding. Void base64_decode(string & out, string const& encoded_string) įile base64.Welcome to my little article about Base64 encoding. Use this if you know the output should be a valid string Void base64_decode(vector & out, string const& encoded_string) Void base64_encode(string & out, string const& buf) Void base64_encode(string & out, const uint8_t* buf, size_t bufLen) Part of file base64.hpp: void base64_encode(string & out, const vector& buf) It avoids copying the parameters at the expense of a couple of tests.Īdds some handy functions for dealing with strings, although usually the input data is binary and may have zero bytes inside, so typically should not be manipulated as a string (which often implies null-terminated data).Īlso adds some casts to fix compiler warnings (at least on GCC, I haven't run it through MSVC yet). Std::vector decoded = Base64::decode(encoded) Ī bonus here is that the decode function can also decode the URL variant of Base64 encoding. Std::string encoded = Base64::encode(buf, 4) Add the byte to the return value if it isn't part of an '=' character (indicated by 0xff) Size_t encoded_size = encoded_string.size() Make sure string length is a multiple of 4 Expand the return string size to a multiple of 4ī4 = ((b3 & 0x3f) Base64::decode(std::string encoded_string) ![]() Calculate how many bytes that needs to be added to get a multiple of 3 Std::string Base64::encode(const BYTE* buf, unsigned int bufLen) ![]() Return encode(&buf, (unsigned int)buf.size()) Return "" // Avoid dereferencing buf if it's empty Std::string Base64::encode(const std::vector& buf) Std::string base64_encode(BYTE const* buf, unsigned int bufLen) Std::vector base64_decode(std::string const&) And why have I modified it? Well, because it didn't seem appropriate to me that I should work with binary data stored within std::string object ) Here's my modification of the implementation that was originally written by René Nyffenegger.
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