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| {{about|the content encoding scheme|the positional numerical format|Binary number}}
| | It's painful to have hemorrhoids. This is a big hindrance for your daily routine. You cannot do all the aspects which we use to do due to the pain that you are going through.<br><br>There are 2 difficulties associated with utilizing lotions. The initially is the fact that some folks experience burning sensations, occasionally so bad that you need to discontinue the use of the cream. The second issue is that lotions never treat the underlying issues which cause hemorrhoids; therefore creams are a temporary [http://hemorrhoidtreatmentfix.com/bleeding-hemorrhoids hemorrhoids bleeding].<br><br>Another suppository is made with strips of the real aloe vera plant which will be cut into strips plus frozen in a container inside a the freezer. This really is thus soothing and ought to be employed evening plus morning. If you have inflamed haemorrhoids, wait until the swelling has gone down.<br><br>The 3rd type of treatments are medications. The purpose is to control blood pressure. This system might have a few side effects, nevertheless generally the medications are used to create the vein cells tight thus which the hemorrhoid will have less issues. It is a top down system which has its advantages. Even though pills produce negative effects, they are clearly quite popular with pharmacies.<br><br>Thankfully, there are folks inside this world with curious minds whom never accept the status quo. Folks that would quite reach the root of a issue rather of placing up with a temporary answer, just to have it re-appear later.<br><br>Believe me I understand. I understand how painful, inconvenient plus embarrassing hemorrhoids is. For me the big issue was the itching. I mean what may you possibly do to relieve the itch when you are sitting down all day in a busy workplace surrounded by colleagues?<br><br>What more are you able to ask for? Here are two of the best treatments to treat a hemorrhoids plus thousands of individuals swear by it. You don't have to be ashamed anymore plus we don't should suffer every single day as a result of the pain. Treat hemorrhoids today, we deserve to have a great, hemorrhoid-free existence. |
| {{tooshort|date=February 2013}}
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| {{condense|date=February 2013}}
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| '''Base64''' is a group of similar [[binary-to-text encoding]] schemes that represent [[binary data]] in an [[ASCII]] string format by translating it into a [[radix]]-64 representation. The term ''Base64'' originates from a specific [[MIME#Content-Transfer-Encoding|MIME content transfer encoding]]. | |
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| Base64 encoding schemes are commonly used when there is a need to encode binary data that needs to be stored and transferred over media that is designed to deal with textual data. This is to ensure that the data remains intact without modification during transport. Base64 is commonly used in a number of applications including [[email]] via [[MIME]], and storing complex data in [[XML]].
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| {{toc limit|2}}
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| ==Design==
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| The particular choice of [[character set]] selected for the 64 characters required for the base varies between implementations. The general rule is to choose a set of 64 characters that is both part of a subset common to most encodings, and also [[Printable character|printable]]. This combination leaves the data unlikely to be modified in transit through information systems, such as email, that were traditionally not [[8-bit clean]].<ref name="autogenerated2006">{{cite IETF |title= The Base16,Base32,and Base64 Data Encodings |rfc= 4648 |year= 2006 |month= October |publisher=[[Internet Engineering Task Force|IETF]] |accessdate= March 18, 2010}}</ref> For example, MIME's Base64 implementation uses <code>A</code>–<code>Z</code>, <code>a</code>–<code>z</code>, and <code>0</code>–<code>9</code> for the first 62 values. Other variations, usually derived from Base64, share this property but differ in the symbols chosen for the last two values; an example is [[UTF-7]].
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| The earliest instances of this type of encoding were created for dialup communication between systems running the same [[operating system|OS]] — e.g. [[Uuencoding|uuencode]] for [[UNIX]], [[BinHex]] for the [[TRS-80]] (later adapted for the [[Macintosh]]) — and could therefore make more assumptions about what characters were safe to use. For instance, uuencode uses uppercase letters, digits, and many punctuation characters, but no lowercase, since UNIX was sometimes used with [[computer terminal|terminals]] that did not support distinct [[letter case]].<ref>{{cite IETF |title= Privacy Enhancement for InternetElectronic Mail: Part I: Message Encryption and Authentication Procedures |rfc= 1421 |year= 1993 |month= February |publisher=[[Internet Engineering Task Force|IETF]] |accessdate= March 18, 2010}}</ref><ref>{{cite IETF |title= Multipurpose Internet Mail Extensions: (MIME) Part One: Format of Internet Message Bodies |rfc= 2045 |year= 1996 |month= November |publisher=[[Internet Engineering Task Force|IETF]] |accessdate= March 18, 2010}}</ref><ref>{{cite IETF |title= The Base16, Base32, and Base64 Data Encodings | rfc= 3548 |year= 2003 |month= July |publisher=[[Internet Engineering Task Force|IETF]] |accessdate= March 18, 2010}}</ref><ref name="autogenerated2006"/>
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| ==Examples==
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| A quote from [[Thomas Hobbes]]' ''[[Leviathan (book)|Leviathan]]'':
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| <code>
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| : Man is distinguished, not only by his reason, but by this singular passion from
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| : other animals, which is a lust of the mind, that by a perseverance of delight
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| : in the continued and indefatigable generation of knowledge, exceeds the short
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| : vehemence of any carnal pleasure.
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| </code>
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| is represented as a byte sequence of 8-bit-padded [[ASCII]] characters encoded in MIME's Base64 scheme as follows: | |
| | |
| <code>
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| : TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5IGhpcyByZWFzb24sIGJ1dCBieSB0aGlz
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| : IHNpbmd1bGFyIHBhc3Npb24gZnJvbSBvdGhlciBhbmltYWxzLCB3aGljaCBpcyBhIGx1c3Qgb2Yg
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| : dGhlIG1pbmQsIHRoYXQgYnkgYSBwZXJzZXZlcmFuY2Ugb2YgZGVsaWdodCBpbiB0aGUgY29udGlu
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| : dWVkIGFuZCBpbmRlZmF0aWdhYmxlIGdlbmVyYXRpb24gb2Yga25vd2xlZGdlLCBleGNlZWRzIHRo
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| : ZSBzaG9ydCB2ZWhlbWVuY2Ugb2YgYW55IGNhcm5hbCBwbGVhc3VyZS4=
| |
| </code>
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| In the above quote, the encoded value of ''Man'' is ''TWFu''. Encoded in ASCII, the characters ''M'', ''a'', and ''n'' are stored as the bytes <code>77</code>, <code>97</code>, and <code>110</code>, which are the 8-bit binary values <code>01001101</code>, <code>01100001</code>, and <code>01101110</code>. These three values are joined together into a 24-bit string, producing <code>010011010110000101101110</code>. Groups of 6 bits (6 bits have a maximum of 2<sup>6</sup> = 64 different binary values) are [[Binary_numeral_system#Counting_in_binary|converted into individual numbers]] from left to right (in this case, there are four numbers in a 24-bit string), which are then converted into their corresponding Base64 character values.
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| {|class="wikitable"
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| |-
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| !scope="row"| Text content
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| | colspan="8" style="text-align:center;"| '''M'''
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| | colspan="8" style="text-align:center;"| '''a'''
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| | colspan="8" style="text-align:center;"| '''n'''
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| |-
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| !scope="row"| ASCII
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| | colspan="8" style="text-align:center;"| 77 (0x4d)
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| | colspan="8" style="text-align:center;"| 97 (0x61)
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| | colspan="8" style="text-align:center;"| 110 (0x6e)
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| |-
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| !scope="row"| Bit pattern
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| |0||1||0||0||1||1||0||1||0||1||1||0||0||0||0||1||0||1||1||0||1||1||1||0
| |
| |-
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| !scope="row"| Index
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| | colspan="6" style="text-align:center;"| 19
| |
| | colspan="6" style="text-align:center;"| 22
| |
| | colspan="6" style="text-align:center;"| 5
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| | colspan="6" style="text-align:center;"| 46
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| |-
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| !scope="row"| Base64-encoded
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| | colspan="6" style="text-align:center;"| '''T'''
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| | colspan="6" style="text-align:center;"| '''W'''
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| | colspan="6" style="text-align:center;"| '''F'''
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| | colspan="6" style="text-align:center;"| '''u'''
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| |}
| |
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| As this example illustrates, Base64 encoding converts three [[Octet (computing)|octets]] into four encoded characters.
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| <span id="Base64table">The Base64 index table</span>:
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| {|class="wikitable" style="text-align:center"
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| |-
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| !scope="col"| Value !!scope="col"| Char
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| |rowspan="17"|
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| !scope="col"| Value !!scope="col"| Char
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| |rowspan="17"|
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| !scope="col"| Value !!scope="col"| Char
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| |rowspan="17"|
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| !scope="col"| Value !!scope="col"| Char
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| |-
| |
| | 0 || <code>A</code> || 16 || <code>Q</code> || 32 || <code>g</code> || 48 || <code>w</code>
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| |-
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| | 1 || <code>B</code> || 17 || <code>R</code> || 33 || <code>h</code> || 49 || <code>x</code>
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| |-
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| | 2 || <code>C</code> || 18 || <code>S</code> || 34 || <code>i</code> || 50 || <code>y</code>
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| |-
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| | 3 || <code>D</code> || 19 || <code>T</code> || 35 || <code>j</code> || 51 || <code>z</code>
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| |-
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| | 4 || <code>E</code> || 20 || <code>U</code> || 36 || <code>k</code> || 52 || <code>0</code>
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| |-
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| | 5 || <code>F</code> || 21 || <code>V</code> || 37 || <code>l</code> || 53 || <code>1</code>
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| |-
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| | 6 || <code>G</code> || 22 || <code>W</code> || 38 || <code>m</code> || 54 || <code>2</code>
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| |-
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| | 7 || <code>H</code> || 23 || <code>X</code> || 39 || <code>n</code> || 55 || <code>3</code>
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| |-
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| | 8 || <code>I</code> || 24 || <code>Y</code> || 40 || <code>o</code> || 56 || <code>4</code>
| |
| |-
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| | 9 || <code>J</code> || 25 || <code>Z</code> || 41 || <code>p</code> || 57 || <code>5</code>
| |
| |-
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| | 10 || <code>K</code> || 26 || <code>a</code> || 42 || <code>q</code> || 58 || <code>6</code>
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| |-
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| | 11 || <code>L</code> || 27 || <code>b</code> || 43 || <code>r</code> || 59 || <code>7</code>
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| |-
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| | 12 || <code>M</code> || 28 || <code>c</code> || 44 || <code>s</code> || 60 || <code>8</code>
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| |-
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| | 13 || <code>N</code> || 29 || <code>d</code> || 45 || <code>t</code> || 61 || <code>9</code>
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| |-
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| | 14 || <code>O</code> || 30 || <code>e</code> || 46 || <code>u</code> || 62 || <code>+</code>
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| |-
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| | 15 || <code>P</code> || 31 || <code>f</code> || 47 || <code>v</code> || 63 || <code>/</code>
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| |}
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| When the number of bytes to encode is not divisible by three (that is, if there are only one or two bytes of input for the last 24-bit block), then the following action is performed:
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| Add extra bytes with value zero so there are three bytes, and perform the conversion to base64. If there was only one significant input byte, only the first two base64 digits are picked (12 bits), and if there were two significant input bytes, the first three base64 digits are picked (18 bits). '<code>=</code>' characters might be added to make the last block contain four base64 characters.
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| As a result:
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| When the last group contains one octet, the four [[least significant bit]]s of the final 6-bit block are set to zero; and when the last group contains two octets, the two least significant bits of the final 6-bit block are set to zero.
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| ===Padding===
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| The '<code>==</code>' sequence indicates that the last group contained only one byte, and '<code>=</code>' indicates that it contained two bytes. The example below illustrates how truncating the input of the whole of the above quote changes the output padding:
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| <!-- This is the encoding of **THE WHOLE** of the above passage and the ending fits in with both the above encoding and the first line of the following example. Verified using http://www.motobit.com/util/base64-decoder-encoder.asp
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| In the previous version the example started with a space, which was not visible and thus quite misleading. -->
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| Input ends with: ''any carnal plea'''sure.''''' Output ends with: YW55IGNhcm5hbCBwbGVh'''c3VyZS4='''
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| Input ends with: ''any carnal plea'''sure''''' Output ends with: YW55IGNhcm5hbCBwbGVh'''c3VyZQ=='''
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| Input ends with: ''any carnal plea'''sur''''' Output ends with: YW55IGNhcm5hbCBwbGVh'''c3Vy'''
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| Input ends with: ''any carnal plea'''su''''' Output ends with: YW55IGNhcm5hbCBwbGVh'''c3U='''
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| Input ends with: ''any carnal plea'''s''''' Output ends with: YW55IGNhcm5hbCBwbGVh'''cw=='''
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| The same characters will be encoded differently depending on their position within the three-octet group which is encoded to produce the four characters. For example:
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| The input: ''plea'''sure.''''' Encodes to: cGxlY'''XN1cmUu'''
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| The input: ''lea'''sure.''''' Encodes to: bGVh'''c3VyZS4='''
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| The input: ''ea'''sure.''''' Encodes to: ZWF'''zdXJlLg=='''
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| The input: ''a'''sure.''''' Encodes to: Y'''XN1cmUu'''
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| The input: '''''sure.''''' Encodes to: '''c3VyZS4='''
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| The number of output bytes per input byte is <math>\tfrac{4}{3}</math> (33% overhead), up to rounding. Specifically, given an input of ''n'' bytes, the output will be <math>4 \lceil n/3 \rceil</math> bytes long, including padding characters.
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| In theory, the padding character is not needed for decoding, since the number of missing bytes can be calculated from the number of Base64 digits. In some implementations, the padding character is mandatory, while for others it is not used. One case in which padding characters are required is concatenating multiple Base64 encoded files.
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| ====Decoding Base64 with padding====
| |
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| When decoding Base64 text, four characters are typically converted back to three bytes. The only exceptions are when padding characters exist. A single '<code>=</code>' indicates that the four characters will decode to only two bytes, while '<code>==</code>' indicates that the four characters will decode to only a single byte. For example:
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| Encoded text ends with: YW55IGNhcm5hbCBwbGVh'''cw==''' Block with two '='s decodes to one character: ''any carnal plea'''s'''''
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| Encoded text ends with: YW55IGNhcm5hbCBwbGVh'''c3U=''' Block with one '=' decodes to two characters: ''any carnal plea'''su'''''
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| Encoded text ends with: YW55IGNhcm5hbCBwbGVh'''c3Vy''' Block with no '='s decodes to three characters: ''any carnal plea'''sur'''''
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| ==Implementations and history==
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| ===Variants summary table===
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| Implementations may have some constraints on the alphabet used for representing some bit patterns. This notably concerns the last two characters used in the index table for index 62 and 63, and the character used for padding (which may be mandatory in some protocols, or removed in others). The table below summarizes these known variants, and link to the subsections below.
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| {|class="wikitable" style="text-align:center"
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| |-
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| ! Variant !! Char for index 62 !! Char for index 63 !! ''pad'' char !! Fixed encoded line-length !! Maximum encoded line length !! Line separators !! Characters outside alphabet !! Line checksum
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| |-
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| ! Original Base64 for [[#Privacy-enhanced mail|Privacy-Enhanced Mail (PEM)]] (<nowiki>RFC 1421</nowiki>, deprecated)
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| | <code>+</code> || <code>/</code> || <code>=</code> ''(mandatory)'' || Yes (except last line) || 64 || CR+LF || Forbidden || ''(none)''
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| |-
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| ! Base64 transfer encoding for [[#MIME|MIME]] (<nowiki>RFC 2045</nowiki>)
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| | <code>+</code> || <code>/</code> || <code>=</code> ''(mandatory)'' || No (variable) || 76 || CR+LF || Accepted (discarded) || ''(none)''
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| |-
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| ! Standard 'Base64' encoding for [[#RFC 3548|RFC 3548]] or [[#RFC 4648|RFC 4648]]
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| | <code>+</code> || <code>/</code> || <code>=</code> ''(mandatory)'' || Yes (except last line) || 64 or 76 ''(only if line separators are specified and needed)'' || CR+LF ''(only if specified and needed)'' || Forbidden || ''(none)''
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| |-
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| ! 'Radix-64' encoding for [[#OpenPGP|OpenPGP]] (<nowiki>RFC 4880</nowiki>)
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| | <code>+</code> || <code>/</code> || <code>=</code> ''(mandatory)'' || No (variable) || 76 || CR+LF || Forbidden || 24-bit CRC (Radix-64-encoded, including one ''pad'' character)
| |
| |-
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| ! Modified Base64 encoding for [[#UTF-7|UTF-7]] (<nowiki>RFC 1642</nowiki>, obsoleted)
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| | <code>+</code> || <code>/</code> || ''(none)'' || No (variable) || ''(none)'' || ''(none)'' || Forbidden || ''(none)''
| |
| |-
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| ! Modified Base64 for [[#Filenames|filenames]] (non standard)
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| | <code>+</code> || <code>-</code> || ''(none)'' || No (variable) || ''(filesystem limit, generally 255)'' || ''(none)'' || Forbidden || ''(none)''
| |
| |-
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| ! Base64 with [[#URL applications|URL]] and [[#Filenames|Filename]] Safe Alphabet ([[#RFC 4648|RFC 4648]] 'base64url' encoding)
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| | <code>-</code> || <code>_</code> || ''(optional, not recommended, if present must be [[Percent-encoding|URL encoded]] as ''<code>%3D</code>'')'' || No (variable) || ''(application-dependent)'' || ''(none)'' || Forbidden || ''(none)''
| |
| |-
| |
| ! Non-standard [[#URL applications|URL]]-safe Modification of Base64 used in [[YUI Library]] (Y64)<ref>{{cite web|url=http://www.yuiblog.com/blog/2010/07/06/in-the-yui-3-gallery-base64-and-y64-encoding/ |title=YUIBlog |publisher=YUIBlog |date= |accessdate=2012-06-21}}</ref>
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| | <code>.</code> || <code>_</code> || <code>-</code> || No (variable) || ''(application-dependent)'' || ''(none)'' || Forbidden || ''(none)''
| |
| |-
| |
| ! Modified Base64 for [[#XML|XML]] name tokens (''Nmtoken'')
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| | <code>.</code> || <code>-</code> || ''(none)'' || No (variable) || ''(XML parser-dependent)'' || ''(none)'' || Forbidden || ''(none)''
| |
| |-
| |
| ! Modified Base64 for [[#XML|XML]] identifiers (''Name'')
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| | <code>_</code> || <code>:</code> || ''(none)'' || No (variable) || ''(XML parser-dependent)'' || ''(none)'' || Forbidden || ''(none)''
| |
| |-
| |
| ! Modified Base64 for [[#Program identifiers|Program identifiers]] (variant 1, non standard)
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| | <code>_</code> || <code>-</code> || ''(none)'' || No (variable) || ''(language/system-dependent)'' || ''(none)'' || Forbidden || ''(none)''
| |
| |-
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| ! Modified Base64 for [[#Program identifiers|Program identifiers]] (variant 2, non standard)
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| | <code>.</code> || <code>_</code> || ''(none)'' || No (variable) || ''(language/system-dependent)'' || ''(none)'' || Forbidden || ''(none)''
| |
| |-
| |
| ! Modified Base64 for [[#Regular expressions|Regular expressions]] (non standard)
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| | <code>!</code> || <code>-</code> || ''(none)'' || No (variable) || ''(application-dependent)'' || ''(none)'' || Forbidden || ''(none)''
| |
| |}
| |
| | |
| ===Privacy-enhanced mail===
| |
| The first known standardized use of the encoding now called MIME Base64 was in the [[Privacy-enhanced Electronic Mail]] (PEM) protocol, proposed by RFC 989 in 1987. PEM defines a "printable encoding" scheme that uses Base64 encoding to transform an arbitrary sequence of [[octet (computing)|octets]] to a format that can be expressed in short lines of 6-bit characters, as required by transfer protocols such as [[SMTP]].<ref>{{cite IETF |title=Privacy Enhancement for Internet Electronic Mail |rfc=989 |year=1987 |month=February |publisher=[[Internet Engineering Task Force|IETF]] |accessdate=March 18, 2010}}</ref>
| |
| | |
| The current version of PEM (specified in RFC 1421) uses a 64-character alphabet consisting of upper- and lower-case [[Roman letters]] (<code>A</code>–<code>Z</code>, <code>a</code>–<code>z</code>), the numerals (<code>0</code>–<code>9</code>), and the "<code>+</code>" and "<code>/</code>" symbols. The "<code>=</code>" symbol is also used as a special suffix code.<ref>{{cite IETF |title=Privacy Enhancement for Internet Electronic Mail: Part I: Message Encryption and Authentication Procedures |rfc=1421 |year=1993 |month=February |publisher=[[Internet Engineering Task Force|IETF]] |accessdate=March 18, 2010}}</ref> The original specification, RFC 989, additionally used the "<code>*</code>" symbol to delimit encoded but unencrypted data within the output stream. | |
| | |
| To convert data to PEM printable encoding, the first byte is placed in the [[Most significant bit|most significant]] eight bits of a 24-bit [[data buffer|buffer]], the next in the middle eight, and the third in the [[Least significant bit|least significant]] eight bits. If there are fewer than three bytes left to encode (or in total), the remaining buffer bits will be zero. The buffer is then used, six bits at a time, most significant first, as indices into the string: "<code>ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/</code>", and the indicated character is output.
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| The process is repeated on the remaining data until fewer than four octets remain. If three octets remain, they are processed normally. If fewer than three octets (24 bits) are remaining to encode, the input data is right-padded with zero bits to form an integral multiple of six bits.
| |
| | |
| After encoding the non-padded data, if two octets of the 24-bit buffer are padded-zeros, two "<code>=</code>" characters are appended to the output; if one octet of the 24-bit buffer is filled with padded-zeros, one "<code>=</code>" character is appended. This signals the decoder that the zero bits added due to padding should be excluded from the reconstructed data. This also guarantees that the encoded output length is a multiple of 4 bytes.
| |
| | |
| PEM requires that all encoded lines consist of exactly 64 printable characters, with the exception of the last line, which may contain fewer printable characters. Lines are delimited by whitespace characters according to local (platform-specific) conventions.
| |
| | |
| ===MIME===
| |
| {{main|MIME}}
| |
| The [[MIME]] (Multipurpose Internet Mail Extensions) specification lists Base64 as one of two [[binary-to-text encoding]] schemes (the other being [[quoted-printable]]).<ref>{{cite IETF |title=Multipurpose Internet Mail Extensions: (MIME) Part One: Format of Internet Message Bodies |rfc=2045 |year=1996 |month=November |publisher=[[Internet Engineering Task Force|IETF]] |accessdate=March 18, 2010}}</ref> MIME's Base64 encoding is based on that of the RFC 1421 version of PEM: it uses the same 64-character alphabet and encoding mechanism as PEM, and uses the "<code>=</code>" symbol for output padding in the same way, as described at RFC 1521.
| |
| | |
| MIME does not specify a fixed length for Base64-encoded lines, but it does specify a maximum line length of 76 characters. Additionally it specifies that any extra-alphabetic characters must be ignored by a compliant decoder, although most implementations use a CR/LF [[newline]] pair to delimit encoded lines.
| |
| | |
| Thus, the actual length of MIME-compliant Base64-encoded binary data is usually about 137% of the original data length, though for very short messages the overhead can be much higher due to the overhead of the headers. Very roughly, the final size of Base64-encoded binary data is equal to 1.37 times the original data size + 814 bytes (for headers). The size of the decoded data can be approximated with this formula:
| |
| bytes = (string_length(encoded_string) - 814) / 1.37
| |
| | |
| ===UTF-7===
| |
| {{main|UTF-7}}
| |
| [[UTF-7]], described first in RFC 1642, which was later superseded by RFC 2152, introduced a system called ''modified Base64''. This data encoding scheme is used to encode [[UTF-16]] as [[ASCII]] characters for use in 7-bit transports such as [[SMTP]]. It is a variant of the Base64 encoding used in MIME.<ref>{{cite IETF |title=UTF-7 A Mail-Safe Transformation Format of Unicode |rfc=1642 |year=1994 |month=July |publisher=[[Internet Engineering Task Force|IETF]] |accessdate=March 18, 2010}}</ref><ref>{{cite IETF |title=UTF-7 A Mail-Safe Transformation Format of Unicode |rfc=2152 |year=1997 |month=May |publisher=[[Internet Engineering Task Force|IETF]] |accessdate=March 18, 2010}}</ref>
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| The "Modified Base64" alphabet consists of the MIME Base64 alphabet, but does not use the "<code>=</code>" padding character. UTF-7 is intended for use in mail headers (defined in RFC 2047), and the "<code>=</code>" character is reserved in that context as the escape character for "quoted-printable" encoding. Modified Base64 simply omits the padding and ends immediately after the last Base64 digit containing useful bits leaving up to three unused bits in the last Base64 digit.
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| ===OpenPGP===
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| {{main|OpenPGP}}
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| [[OpenPGP]], described in RFC 4880, describes '''Radix-64''' encoding, also known as "ASCII Armor". Radix-64 is identical to the "Base64" encoding described from MIME, with the addition of an optional 24-bit [[cyclic redundancy check|CRC]]. The [[checksum]] is calculated on the input data before encoding; the checksum is then encoded with the same Base64 algorithm and, using an additional "<code>=</code>" symbol as separator, appended to the encoded output data.<ref>{{cite IETF |title=OpenPGP Message Format |rfc=4880 |year=2007 |month=November |publisher=[[Internet Engineering Task Force|IETF]] |accessdate=March 18, 2010}}</ref>
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| ===RFC 3548===
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| RFC 3548, entitled ''The Base16, Base32, and Base64 Data Encodings'', is an informational (non-normative) memo that attempts to unify the RFC 1421 and RFC 2045 specifications of Base64 encodings, alternative-alphabet encodings, and the seldom-used Base32 and Base16 encodings.
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| RFC 3548 forbids implementations from generating messages containing characters outside the encoding alphabet or without padding, unless they are written to a specification that refers to RFC 3548 and specifically requires otherwise; it also declares that decoder implementations must reject data that contain characters outside the encoding alphabet, unless they are written to a specification that refers to RFC 3548 and specifically requires otherwise.<ref>{{cite IETF |title=The Base16, Base32, and Base64 Data Encodings |rfc=3548 |year=2003 |month=July |publisher=[[Internet Engineering Task Force|IETF]] |accessdate=March 18, 2010}}</ref>
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| ===RFC 4648===
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| This RFC obsoletes RFC 3548 and focuses on Base64/32/16:
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| : ''This document describes the commonly used Base64, Base32, and Base16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings.''
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| ===Filenames===
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| Another variant called '''modified Base64 for filename''' uses '<code>-</code>' instead of '<code>/</code>', because Unix and Windows filenames cannot contain '<code>/</code>'.
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| It could be recommended to use the ''modified Base64 for URL'' instead, since then the filenames could be used in URLs also.
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| ===URL applications===
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| Base64 encoding can be helpful when fairly lengthy identifying information is used in an HTTP environment. For example, a database persistence framework for [[Java (programming language)|Java]] objects might use Base64 encoding to encode a relatively large unique id (generally 128-bit [[UUID]]s) into a string for use as an HTTP parameter in HTTP forms or HTTP GET [[Uniform Resource Locator|URLs]]. Also, many applications need to encode binary data in a way that is convenient for inclusion in URLs, including in hidden web form fields, and Base64 is a convenient encoding to render them in a compact way.
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| Using standard Base64 in [[URL]] requires encoding of '<code>+</code>', '<code>/</code>' and '<code>=</code>' characters into special [[Percent-encoding|percent-encoded]] hexadecimal sequences ('<code>+</code>' = '<code>%2B</code>', '<code>/</code>' = '<code>%2F</code>' and '<code>=</code>' = '<code>%3D</code>'), which makes the string unnecessarily longer.
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| For this reason, '''modified Base64 for URL''' variants exist, where the '<code>+</code>' and '<code>/</code>' characters of standard Base64 are respectively replaced by '<code>-</code>' and '<code>_</code>', so that using [[Percent-encoding|URL encoders/decoders]] are no longer necessary and have no impact on the length of the encoded value, leaving the same encoded form intact for use in relational databases, web forms, and object identifiers in general. Some variants allow or require omitting the padding '<code>=</code>' signs to avoid them being confused with field separators, or require that any such padding be percent-encoded. Some libraries (like <code>org.bouncycastle.util.encoders.UrlBase64Encoder</code>) will encode '<code>=</code>' to '<code>.</code>'.
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| ===Program identifiers===
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| There are other variants that use '<code>_-</code>' or '<code>._</code>' when the Base64 variant string must be used within valid identifiers for programs.
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| ===XML===
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| [[XML]] identifiers and name tokens are encoded using two variants:
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| * '<code>.-</code>' for use in [[XML]] name tokens (''Nmtoken''), or even
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| * '<code>_:</code>' for use in more restricted XML identifiers (''Name'').
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| ===HTML===
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| The <tt>atob()</tt> and <tt>btoa()</tt> JavaScript methods, defined in the HTML5 draft specification,<ref>[http://dev.w3.org/html5/spec/Overview.html#atob Base64 utility methods], HTML5 Editor's Draft, section 6.2, introduced by [http://html5.org/tools/web-apps-tracker?from=5813&to=5814 changeset 5814], 2011-02-01</ref> provide Base64 encoding and decoding functionality to web pages. The <tt>atob()</tt> method is unusual in that it does not ignore whitespace or new lines, throwing an <tt>INVALID_CHARACTER_ERR</tt> instead. The <tt>btoa()</tt> method outputs padding characters, but these are optional in the input of the <tt>atob()</tt> method.
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| ===Other applications===
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| Base64 can be used in a variety of contexts:
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| * Base64 can be used to transmit and store text that might otherwise cause [[delimiter collision]]
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| * [[Spam (electronic)|Spammers]] use Base64 to evade basic [[Anti-spam techniques (e-mail)|anti-spamming]] tools, which often do not decode Base64 and therefore cannot detect keywords in encoded messages.
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| * Base64 is used for [[PHP]] [[Obfuscated code|obfuscation]].
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| * Base64 is used to encode character strings in [[LDIF]] files
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| * Base64 is often used to embed binary data in an [[XML]] file, using a syntax similar to <code><nowiki><data encoding="base64">…</data></nowiki></code> e.g. [[favicon]]s in [[Firefox]]'s <tt>bookmarks.html</tt>.
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| * Base64 is used to encode binary files such as images within scripts, to avoid depending on external files.
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| * The [[data URI scheme]] can use Base64 to represent file contents. For instance, background images and fonts can be specified in a [[Cascading Style Sheets|CSS]] stylesheet file as <code>data:</code> URIs, instead of being supplied in separate files.
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| * The FreeSWAN ipsec implementation precedes Base64 strings with <tt>0s</tt>, so they can be distinguished from text or hexadecimal strings.
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| ===Radix 64 applications not compatible with Base64===
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| * A base 64 encoding is used to store password hashes computed with [[Crypt (3)#Library Function|crypt]] in the <tt>[[passwd (file)|/etc/passwd]]</tt>. Its alphabet starts with '.' for zero, then '/' for one, followed by 0-9, A-Z and a-z. Padding is not used.
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| * The [[GEDCOM]] 5.5 standard for Genealogical data interchange uses a concept similar to Base64 to encode multimedia files in its text-line hierarchical file format. The choice of extra characters are '<code>.</code>' and '<code>/</code>' with a different assignment of characters for the 64 6-bit values, that is <code>.</code>, <code>/</code>, <code>0</code>–<code>9</code>, <code>A</code>–<code>Z</code>, <code>a</code>–<code>z</code> for values 0–63.<ref>{{cite web|url=http://homepages.rootsweb.ancestry.com/~pmcbride/gedcom/55gctoc.htm |title=The GEDCOM Standard Release 5.5 |publisher=Homepages.rootsweb.ancestry.com |date= |accessdate=2012-06-21}}</ref>
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| * [[Uuencoding]] uses a system with base 64 for binary data, but with a very different set of characters in the encoding. It uses many punctuation characters but no lower-case letters.
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| * [[BinHex]], which was used within the [[Mac OS]], has an encoding system with 64 as a base but with different characters from Base64. It uses punctuation characters, digits, upper and lower case letters but does not use some visually confusable characters like '<code>7</code>', '<code>O</code>', '<code>g</code>' and '<code>o</code>'.
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| * 6PACK, used with some [[terminal node controller]]s, encodes 3 raw octets into 4 characters, but with a different set of 64 characters.<ref>
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| {{cite web|url=http://private.freepage.de/cgi-bin/feets/freepage_ext/41030x030A/rewrite/alexs/xfr/flexnet/6pack_en/6pack.htm|title=6PACK a "real time" PC to TNC protocol|accessdate=2013-05-19}}</ref>
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| ==See also==
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| * [[Ascii85]] encoding scheme
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| * [[Base32]] encoding scheme
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| * [[8BITMIME]]
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| * [[Uniform Resource Locator|URL]]
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| * [[Binary-to-text encoding]] for a comparison of various encoding algorithms
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| * [[Binary number]]
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| ==References==
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| {{reflist|2}}
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| ==External links==
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| <!-- Please do NOT add any more base 64 encoders or decoders. There are plenty of examples already.
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| See [[WP:EL]] (http://en.wikipedia.org/wiki/Wikipedia:EL) for an explanation of why external links are routinely vigorously eliminated from articles.
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| -->
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| {{Wikibooks|Algorithm implementation|Miscellaneous/Base64|Base64}}
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| [[Category:Usenet]]
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| [[Category:Email]]
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| [[Category:Internet Standards]]
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| [[Category:Binary-to-text encoding formats]]
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| [[Category:Data serialization formats]]
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