Binary-to-text encoding
A binary-to-text encoding is encoding of data in plain text. More precisely, it is an encoding of binary data in a sequence of printable characters. These encodings are necessary for transmission of data when the channel does not allow binary data or is not 8-bit clean. PGP documentation uses the term "ASCII armor" for binary-to-text encoding when referring to Base64.
Description
The ASCII text-encoding standard uses 128 unique values to represent the alphabetic, numeric, and punctuation characters commonly used in English, 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 ''Encoding plain text
Binary-to-text encoding methods are also used as a mechanism for encoding plain text. For example:- Some systems have a more limited character set they can handle; not only are they not 8-bit clean, some cannot even handle every printable ASCII character.
- Other systems have limits on the number of characters that may appear between line breaks, such as the "1000 characters per line" limit of some SMTP software, as allowed by.
- Still others add headers or trailers to the text.
- A few poorly-regarded but still-used protocols use in-band signaling, causing confusion if specific patterns appear in the message. The best-known is the string "From " at the beginning of a line used to separate mail messages in the mbox file format.
This is sometimes referred to as 'ASCII armoring'. For example, the ViewState component of ASP.NET uses base64 encoding to safely transmit text via HTTP POST,
in order to avoid delimiter collision.
Encoding standards
The table below compares the most used forms of binary-to-text encodings. The efficiency listed is the ratio between number of bits in the input and the number of bits in the encoded output.| Encoding | Data type | Efficiency | Programming language implementations | Comments |
| Ascii85 | Arbitrary | 80% | , , , , , , , , | There exist several variants of this encoding, Base85, btoa, et cetera. |
| Base32 | Arbitrary | 62.5% | , , | |
| Base36 | Arbitrary | ~64% | bash, C, C++, C#, Java, Perl, PHP, Python, Visual Basic, Swift, many others | Uses the Arabic numerals 0–9 and the Latin letters A–Z. Commonly used by URL redirection systems like TinyURL or SnipURL/Snipr as compact alphanumeric identifiers. |
| Base58 | Integer | ~73% | , | Similar to Base64, but modified to avoid both non-alphanumeric characters and letters which might look ambiguous when printed. |
| Base64 | Arbitrary | 75% | , , , , many others | |
| Base85 | Arbitrary | 80% | , | Revised version of Ascii85. |
| BinHex | Arbitrary | 75% | , , | MacOS Classic |
| Decimal | Integer | ~42% | Most languages | Usually the default representation for input/output from/to humans. |
| Hexadecimal | Arbitrary | 50% | Most languages | Exists in uppercase and lowercase variants |
| Intel HEX | Arbitrary | ~<50% | , | Typically used to program EPROM, NOR-Flash memory chips |
| MIME | Arbitrary | See Quoted-printable and Base64 | See Quoted-printable and Base64 | Encoding container for e-mail-like formatting |
| MOS Technology file format | Arbitrary | Typically used to program EPROM, NOR-Flash memory chips. | ||
| Percent encoding | Text, Arbitrary | ~40% | , , probably many others | |
| Quoted-printable | Text | ~33–100% | Probably many | Preserves line breaks; cuts lines at 76 characters |
| S-record | Arbitrary | 49.6% | , | Typically used to program EPROM, NOR-Flash memory chips. 49.6% assumes 255 binary bytes per record. |
| Tektronix hex | Arbitrary | Typically used to program EPROM, NOR-Flash memory chips. | ||
| Uuencoding | Arbitrary | ~60% | Perl, , , probably many others | Largely replaced by MIME and yEnc |
| Xxencoding | Arbitrary | ~75% | Proposed as replacement for Uuencoding to avoid character set translation problems between ASCII and the EBCDIC systems that could corrupt Uuencoded data | |
| yEnc | Arbitrary, mostly non-text | ~98% | Includes a CRC checksum | |
| Arbitrary | 33% | C, ,... | "A Convention for Human-readable 128-bit Keys". A series of small English words is easier for humans to read, remember, and type in than decimal or other binary-to-text encoding systems. Each 64-bit number is mapped to six short words, of one to four characters each, from a public 2048-word dictionary. |
The 95 isprint codes 32 to 126 are known as the ASCII printable characters.
Some older and today uncommon formats include BOO, BTOA, and USR encoding.
Most of these encodings generate text containing only a subset of all ASCII printable characters: for example, the base64 encoding generates text that only contains upper case and lower case letters,, numerals, and the "+", "/", and "=" symbols.
Some of these 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.
These encodings produce the shortest plain ASCII output for input that is mostly printable ASCII.
Some other encodings are based on mapping all possible sequences of six bits into different printable characters. Since there are more than 26 = 64 printable characters, this is possible. A given sequence of bytes is translated by viewing it as stream of bits, breaking this stream in chunks of six bits 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 use four bits instead of six, mapping all possible sequences of 4 bits onto the 16 standard hexadecimal digits.
Using 4 bits per encoded character leads to a 50% longer output than base64, but simplifies encoding and decoding—expanding each byte in the source independently to two encoded bytes is simpler than base64's expanding 3 source bytes to 4 encoded bytes.
Out of PETSCII's first 192 codes, 164 have visible representations when quoted: 5, 17–20 and 28–31, 32–90, 91–127, 129, 133–140, 144–159, and 160–192. This theoretically permits encodings, such as base128, between PETSCII-speaking machines.