A binary to text encoding is an encoding of data in plain text. More precisely, it is an encoding of binary data in a sequence of ASCII printable characters. These encodings are necessary for transmission of data when the channel or the protocol only allows ASCII printable characters, such as e-mail or usenet.
Description
The ASCII text-encoding standard uses 128 unique values (0–127) to represent the alphabetic, numeric, and punctuation characters commonly used in the English language, plus a selection of 'control codes' which do not represent printable characters. For example, the capital letter A is ASCII character 65, the numeral 2 is ASCII 50, the character } is ASCII 125, and the metacharacter carriage return is ASCII 13. Systems based on ASCII use seven bits to represent these values digitally.
By contrast, most computers store data in memory organised in eight-bit bytes, and, in the case of machine-executable code and non-textual data formats where maximum storage density is desirable, use the full range of 256 possible values in each eight-bit byte. Many computer programs came to rely on this distinction between seven-bit text and eight-bit binary data, and would not function properly if non-ASCII characters appeared in data that was expected to include only ASCII text. For example, the value of the eighth bit might not be preserved, or the program might interpret a byte value above 127 as a flag telling it to perform some function.
It is often desirable, however, to be able to send non-textual data through text-based systems, such as when one might attach an image file to an e-mail message. To accomplish this, the data are encoded in some way, such that eight-bit data are encoded into seven-bit ASCII characters (generally using only alphanumeric and punctuation characters). Upon safe arrival at its destination, it is then decoded back to its eight-bit form. This process is referred to as ASCII Armoring.
One older method of accomplishing this is the uuencode standard. Today, the Base64 encoding is among the most commonly used methods. Other special-purpose encoding standards include Ascii85, xxcode, BinHex, Quoted-printable and yEnc. Some older and today uncommon formats include BOO, BTOA, and USR encoding. Many programs perform ASCII armoring to allow for data-transport, such as PGP and GNU Privacy Guard (GPG). PGP uses Radix-64 conversion in which 3 characters or bytes(24 bits) are converted in to 4 parts having 6 bits each.
Encoding standards
The most used forms of binary to text encodings are:
Most of these encodings generate text not containing all ASCII printable characters: for example, the base64 encoding generates text that only contains upper case and lower case letters, (A–Z, a–z), numerals (0–9), and the "+", "/", and "=" symbols.
Some of these encoding (quoted-printable and percent encoding) are based on a set of allowed characters and a single escape character. The allowed characters are left unchanged, while all other characters are converted into a string starting with the escape character. This kind of conversion allows the resulting text to be almost readable, in that letters and digits are part of the allowed characters, and are therefore left as they are in the encoded text.
Some other encodings (base64, uuencoding) are based on mapping all possible sequences of six bits into different printable characters. Since there are more than printable characters, this is possible. A given sequence of bytes is translated by viewing it as stream of bits, breaking this stream in six chunks and generating the sequence of corresponding characters. The different encodings differ in the mapping between sequences of bits and characters and in how the resulting text is formatted. Some encodings (the original version of BinHex) use four bits instead of six. This leads to a 33% longer output but simplifies the procedure of encoding, as the byte boundries in the source data and the character boundries in the output only line up every second output character instead of every third.
