ASCII armor
|
|
ASCII Armor is a term used to describe an encoding process, in which data in a binary format is transformed into a textual format, to allow the data to be successfully transmitted through channels designed only for text messages, such as e-mail or usenet.
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 make use of eight-bit bytes to store data, and in the case of machine-executable code and non-textual data formats where maximum storage density is desirable, all eight bits of each byte are used. Many computer systems 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 text. For example, 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 is encoded in some way, such that eight-bit data is 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 coding standards include Ascii85, xxcode, BinHex, and yEnc. Many programs perform ASCII armoring to allow for data-transport, such as PGP and GNU Privacy Guard (GPG).
Another example of ASCII armor (or printable encoding) is the representation of very large numerical values as a sequence of letters, punctuation, and/or digits. For instance, public key encryption systems call for a user to make their public key available for others to use. The key is actually a very large number, and internally, a computer would represent its value as binary data. However, in order to send a public key to others, it is convenient to encode it as a sequence of printable letters and digits.
Today, computers are adopting the Unicode family of standard character encodings, most of which use more than standard ASCII's seven bits for each character. It is still necessary in such situations to protect such data in Internet e-mail and in other areas. Base64 is capable of providing ASCII armor for Unicode data, however, a specific Unicode encoding was created for just this purpose, UTF-7.