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In contrast, most computers store data in memory organized in eight-bit [[byte]]s. Files that contain machine-executable code and non-textual data typically contain all 256 possible eight-bit byte values. 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, if the value of the eighth bit is not preserved, 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—the
==Encoding plain text==
{{See also|Delimiter#ASCII armor|Return-to-libc attack#Protection from return-to-libc attacks}}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
* Still others add [[header (computing)|header]]s or [[trailer (information technology)|trailer]]s 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 " (including trailing space) at the beginning of a line, used to separate mail messages in the [[mbox]] file format.
By using a binary-to-text encoding on messages that are already plain text, then decoding on the other end, one can make such systems appear to be completely [[Transparency (telecommunication)|transparent]]. 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 the number of bits in the input and the number of bits in the encoded output.
{| class="wikitable sortable"
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! Encoding !! Data type !! Efficiency !! Programming language implementations !! Comments
|-
| [[Ascii85]] || Arbitrary || 80% || [http://sites.google.com/site/dannychouinard/Home/unix-linux-trinkets/little-utilities/base64-and-base85-encoding-awk-scripts awk], [http://www.ibiblio.org/pub/packages/ccic/software/unix/utils/btoa.c C], [https://github.com/woolstar/test/blob/master/encode/asc85.c C (2)], [https://web.archive.org/web/20131227071331/http://www.codinghorror.com/blog/2005/10/c-implementation-of-ascii85.html C#], [https://web.archive.org/web/20210927102719/http://blog.wezeku.com/2010/07/01/f-ascii85-module/ F#], [https://pkg.go.dev/encoding/ascii85 Go], [https://web.archive.org/web/20160304035222/http://java.freehep.org/freehep-io/apidocs/org/freehep/util/io/ASCII85.html Java] [https://metacpan.org/pod/Convert::Ascii85 Perl], [https://docs.python.org/3/library/base64.html#base64.a85encode Python], [https://web.archive.org/web/20151208205520/https://code.google.com/p/python-mom/source/browse/mom/codec/base85.py Python (2)]
|-
| [[Base32]] || Arbitrary || 62.5% || [http://sourceforge.net/projects/cyoencode/ ANSI C], [https://github.com/MHumm/DelphiEncryptionCompendium/blob/master/Source/DECFormat.pas Delphi], [https://pkg.go.dev/encoding/base32 Go], [http://commons.apache.org/codec/ Java], [https://docs.python.org/dev/library/base64.html#base64.b32encode Python] || {{space}}
|-
| [[Base36]] || Integer || data-sort-value="64%"|~64% ||
|Uses the [[Arabic numerals]] 0–9 and the [[Latin alphabet|Latin letters]] A–Z (the [[ISO basic Latin alphabet]]). Commonly used by [[URL redirection]] systems like [[TinyURL]] or SnipURL/Snipr as compact alphanumeric identifiers.
|-
| [[Base45]] || Arbitrary || ~67% (97%{{efn|Encoding for QR code generation automatically selects the encoding to match the input character set, encoding 2 alphanumeric characters in 11 bits, and Base45 encodes 16 bits into 3 such characters. The efficiency is thus 32 bits of binary data encoded in 33 bits: 97%.}}) || [https://github.com/Dasio/base45/ Go] || Defined in IETF Specification RFC 9285 for including binary data compactly in a [[QR code]].<ref>{{Cite web|url=https://rfc-editor.org/rfc/rfc9285|title = The Base45 Data Encoding|date = 2022-08-11|last1 = Fältström|first1 = Patrik|last2 = Ljunggren|first2 = Freik|last3 = Gulik|first3 = Dirk-Willem van|quote=Even in Byte mode, a typical QR code reader tries to interpret a byte sequence as text encoded in UTF-8 or ISO/IEC 8859-1. ... Such data has to be converted into an appropriate text before that text could be encoded as a QR code. ... Base45 ... offers a more compact QR code encoding.}}</ref>
|-
| [[Base56]] || Integer || — || [http://rossduggan.ie/blog/codetry/base-56-integer-encoding-in-php/index.html PHP], [https://github.com/jyn514/base56 Python], [https://pkg.go.dev/toolman.org/encoding/base56 Go] || A variant of Base58 encoding which further sheds the lowercase 'i' and 'o' characters in order to minimise the risk of fraud and human-error.<ref>{{cite web |last=Duggan |first=Ross |date=August 18, 2009 |title=Base-56 Integer Encoding in PHP |url=http://rossduggan.ie/blog/codetry/base-56-integer-encoding-in-php/index.html}}</ref>
|-
| {{anchor|Base58}}
[[File:Original source code bitcoin-version-0.1.0 file base58.h.png|400px|thumb|Base58 in the original bitcoin source code]]
|-
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| [[Base64]] || Arbitrary || 75% || [http://sites.google.com/site/dannychouinard/Home/unix-linux-trinkets/little-utilities/base64-and-base85-encoding-awk-scripts awk], [http://base64.sourceforge.net/ C], [http://www.fpx.de/fp/Software/UUDeview/ C (2)], [https://github.com/MHumm/DelphiEncryptionCompendium/blob/master/Source/DECFormat.pas Delphi], [https://pkg.go.dev/encoding/base64 Go], [https://docs.python.org/3/library/base64.html#base64.b64encode Python], many others || {{space}}
|-
| [[Base85]] ({{IETF RFC|1924}}) || Arbitrary || 80% ||
| Revised version of [[Ascii85]]. |-
| Base91<ref>{{Cite web |authors=Dake He, Yu Sun, Zhen Jia, Xiuying Yu, Wei Guo, Wei He, Chao Qi, Xianhui Lu |title=A Proposal of Substitute for Base85/64 – Base91 |url=https://www.iiis.org/CDs2010/CD2010SCI/CCCT_2010/PapersPdf/TB100QM.pdf
|-
| basE91<ref>
|-
| Base94<ref>{{cite web |
|-
| Base122<ref>{{cite web |last=Albertson |first=Kevin |date=Nov 26, 2016 |title=Base-122 Encoding |url=http://blog.kevinalbs.com/base122
|-
| BaseXML<ref>{{cite web | url=https://github.com/kriswebdev/BaseXML | title=BaseXML - for XML1.0+ | website=[[GitHub]] | date=16 March 2019 }}</ref> || Arbitrary || 83.5% || [https://github.com/kriswebdev/BaseXML C Python JavaScript] || {{space}}
|-
| Bech32 || Arbitrary || data-sort-value="62.5%"|62.5% + at least 8 chars (label, separator, 6-char [[error correcting code|ECC]]) || C, C++, [[JavaScript]], [[Go (programming language)|Go]], Python, [[Haskell]], [[Ruby (programming language)|Ruby]], [[Rust (programming language)|Rust]]|| Specification.<ref>{{Cite web |date=8 December 2021 |title=bitcoin/bips |url=https://github.com/bitcoin/bips/blob/master/bip-0173.mediawiki#bech32
|-
| [[BinHex]] || Arbitrary || 75%|| [http://metacpan.org/module/Convert::BinHex Perl], [http://www.fpx.de/fp/Software/UUDeview/ C], [http://ibiblio.org/pub/linux/utils/compress/macutils.tar.gz C (2)] || MacOS Classic
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| [[Hexadecimal#Base16 (transfer encoding)|Hexadecimal]] (Base16) || Arbitrary || 50% || Most languages || Exists in [[uppercase]] and [[Letter case#All lowercase|lowercase]] variants
|-
| [[Intel HEX]] || Arbitrary || data-sort-value="50%"|≲50% || [https://github.com/vsergeev/libGIS C library], [http://srecord.sourceforge.net/ C++] || Typically used to program [[EPROM]], [[Flash memory|NOR
|-
| [[MIME]] || Arbitrary || See [[Quoted-printable]] and [[Base64]] || See [[Quoted-printable]] and [[Base64]] || Encoding container for e-mail-like formatting
|-
| [[Percent
|-
| [[Quoted-printable]] || Text || data-sort-value="33%"|~33–100%{{efn|1= One byte stored as =XX. Encoding all but the 94 characters which don't need it (incl. space and tab).}} || Probably many || Preserves line breaks; cuts lines at 76 characters
|-
| [[S-record]] (Motorola hex) || Arbitrary || 49.6% || [https://github.com/vsergeev/libGIS C library], [http://srecord.sourceforge.net/ C++] || Typically used to program [[EPROM]], [[Flash memory|NOR
|-
| [[Tektronix hex]] || Arbitrary || || || Typically used to program [[EPROM]], [[Flash memory|NOR
|-
| [[Uuencoding]] || Arbitrary || data-sort-value="60%"|~60% ([[Uuencoding#Disadvantages|up to 70%]]) || [[Uuencoding#
|-
| [[Xxencoding]] || Arbitrary || data-sort-value="75%"|~75% (similar to Uuencoding) || [http://www.fpx.de/fp/Software/UUDeview/ C], [https://github.com/MHumm/DelphiEncryptionCompendium/blob/master/Source/DECFormat.pas Delphi] || Proposed (and occasionally used) as replacement for Uuencoding to avoid character set translation problems between ASCII and the EBCDIC systems that could corrupt Uuencoded data
|-
| z85 ([https://rfc.zeromq.org/spec/32/ ZeroMQ spec:32/Z85]) || Binary & ASCII || 80% (similar to Ascii85/Base85) || [https://github.com/zeromq/rfc/blob/master/src/spec_32.c C] (original), [https://github.com/coenm/Z85e C#], [https://pub.dev/packages/z85 Dart], [https://github.com/jamesruan/z85/blob/master/src/z85.erl Erlang], [https://github.com/tilinna/z85 Go], [https://github.com/philanc/plc/blob/master/plc/base85.lua Lua], [https://github.com/fxn/z85 Ruby], [https://docs.rs/z85/latest/src/z85/lib.rs.html Rust] and others
|-
| {{IETF RFC|1751}} ([[S/KEY]]) || Arbitrary || 33% || C,<ref name="RFC1760" /> [https://www.dlitz.net/software/pycrypto/doc/#crypto-util-rfc1751 Python]
|
"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.<ref>
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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. These encodings produce the shortest plain ASCII output for input that is mostly printable ASCII.
Some other encodings ([[base64]], [[uuencoding]]) are based on mapping all possible sequences of six [[bit]]s into different printable characters. Since there are more than 2<sup>6</sup> = 64 printable characters, this is possible. A given sequence of bytes is translated by viewing it as a 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 (the original version of BinHex and the recommended encoding for [[CipherSaber]]) 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.
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