View source for Base64

{{Short description| Encoding for a sequence of byte values using 64 printable characters}}

'''Base64''' is a [[binary-to-text encoding]] that uses 64 [[Graphic character|printable characters]] to represent each 6-bit segment of a sequence of byte<ref>technically [[octet (computing)|octet]]</ref> values. As for all binary-to-text encodings, Base64 encoding enables [[data transmission|transmitting]] [[binary data]] on a [[communication channel]] that only supports text.

When comparing the original data to the resulting encoded data, Base64 encoding increases the size by 33% plus about 4% additional if inserting line breaks for typical line length.

The earliest uses of this encoding were for dial-up communication between systems running the same [[operating system]] – for example, [[uuencoding|uuencode]] for [[UNIX]] and [[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.<ref name="rfc 1421">{{cite IETF |title= Privacy Enhancement for InternetElectronic Mail: Part I: Message Encryption and Authentication Procedures |rfc= 1421 |date=February 1993 |publisher=[[Internet Engineering Task Force|IETF]] |access-date= March 18, 2010}}</ref><ref name="rfc 2045">{{cite IETF |title= Multipurpose Internet Mail Extensions: (MIME) Part One: Format of Internet Message Bodies |rfc= 2045 |date=November 1996 |publisher=[[Internet Engineering Task Force|IETF]] |access-date= March 18, 2010}}</ref><ref name="rfc 3548">{{cite IETF |title= The Base16, Base32, and Base64 Data Encodings |rfc= 3548 |date=July 2003 |publisher=[[Internet Engineering Task Force|IETF]] |access-date= March 18, 2010}}</ref><ref name="autogenerated2006"/>

==Applications==
[[File:35_mm_angle_of_view_vs_focal_length.svg|thumb|link={{filepath:35_mm_angle_of_view_vs_focal_length.svg}}|Example of an SVG file containing embedded JPEG images encoded in Base64<ref>&lt;image xlink:href="data:image/jpeg;base64,<code>JPEG contents encoded in Base64</code>" ... /&gt;</ref>]]
Notable applications of Base64:

; Web pages: Encoding as Base64 is prevalent on the [[World Wide Web]]<ref>{{cite web |title= Base64 encoding and decoding – Web APIs |url= https://developer.mozilla.org/en-US/docs/Web/API/WindowBase64/Base64_encoding_and_decoding |publisher= MDN Web Docs |archive-url= https://web.archive.org/web/20141111151440/https://developer.mozilla.org/en-US/docs/Web/API/WindowBase64/Base64_encoding_and_decoding |archive-date= 2014-11-11 |url-status=live}}</ref> where is it often used to embed binary data such as a digital image in text such as [[HTML]] and [[CSS]].<ref>{{cite web |title= When to base64 encode images (and when not to) |date= 28 August 2011 |url=https://www.davidbcalhoun.com/2011/when-to-base64-encode-images-and-when-not-to/ |archive-url= https://web.archive.org/web/20230829143759/https://www.davidbcalhoun.com/2011/when-to-base64-encode-images-and-when-not-to/ |archive-date=2023-08-29 |url-status=live}}</ref>

; E-mail attachment: Base64 is widely used for sending [[e-mail]] attachments, because [[Simple Mail Transfer Protocol|SMTP]]&nbsp;– in its original form&nbsp;– was designed to transport [[7-bit ASCII]] characters only. Encoding an attachment as Base64 before sending, and then decoding when received, assures older SMTP servers correctly transmit messages with attached binary information.

; Embed binary data in a text file: For example, to include the data of an image in a script to avoid depending on external files.

; Embed binary data in XML: 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 exported <code>bookmarks.html</code>.

; Embed PDF file: To embed a [[PDF]] file in an HTML page.<ref>{{Cite web |title=Encode PDF (Portable Document Format) File (.pdf) to Base64 Data |url=https://base64.online/encoders/encode-pdf-to-base64?utm_campaign=og |access-date=2024-03-21 |website=base64.online |language=en}}</ref>

; Embedded elements: Although not part of the official specification for the [[SVG]] format, some viewers can interpret Base64 when used for embedded elements, such as raster images inside SVG files.<ref>{{cite web|url=http://jsfiddle.net/MxHPq/|title=Edit fiddle |website=jsfiddle.net}}</ref>

; Prevent delimiter collision: To transmit and store text that might otherwise cause [[delimiter collision]].

; LDAP Data Interchange Format: To encode character strings in [[LDAP Data Interchange Format]] files.

; Data URI scheme: The [[data URI scheme]] can use Base64 to represent file contents. For instance, background images and fonts can be specified in a [[CSS]] stylesheet file as <code>data:</code> URIs, instead of being supplied in separate files.

; Leverage clipboard: To store/transmit relatively small amounts of binary data via a computer's text [[Clipboard (computing)|clipboard]] functionality, especially in cases where the information doesn't warrant being permanently saved or when information must be quickly sent between a wide variety of different, potentially incompatible programs. An example is the representation of the public keys of [[cryptocurrency]] recipients as Base64 encoded text strings, which can be easily copied and pasted into users' [[Cryptocurrency wallet|wallet software]].

; Support human verification: Binary data that must be quickly verified by humans as a safety mechanism, such as [[Checksum|file checksums]] or [[Public key fingerprint|key fingerprints]], is often represented in Base64 for easy checking, sometimes with additional formatting, such as separating each group of four characters in the representation of a [[Pretty Good Privacy|PGP]] key fingerprint with a space.

; QR code encoding: A [[QR code]], which contains binary data, is sometimes stored as Base64 since it is more likely that a QR code reader accurately decodes text than binary data. Also, some devices more readily save text from a QR code than potentially malicious binary data.

==Alphabet==
The set of characters used to represent the values for each base-64 digit (value from 0 to 63) differs slightly between the variations of Base64. The general strategy is to use printable characters that are common to most [[character encoding]]s. This tends to result in data remaining unchanged as it moves 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 |date=October 2006 |publisher=[[Internet Engineering Task Force|IETF]] |access-date= March 18, 2010}}</ref> Typically, an encoding 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. Many variants use <code>+</code> and <code>/</code> for the last two.

<span id="Base64table">Per [https://datatracker.ietf.org/doc/html/rfc4648#section-4 RFC 4648 §4], the following table lists the characters used for each numeric value.</span> To indicate padding, <code>=</code> is used.

{|class="wikitable" style="text-align:center"
|+ Base64 alphabet
!scope="col"| value !!scope="col"| {{abbr|char|Character}}
|rowspan="17"|
!scope="col"| value !!scope="col"| {{abbr|char|Character}}
|rowspan="17"|
!scope="col"| value !!scope="col"| {{abbr|char|Character}}
|rowspan="17"|
!scope="col"| value !!scope="col"| {{abbr|char|Character}}
|-
|  0 || <code>A</code> || 16 || <code>Q</code> || 32 || <code>g</code> || 48 || <code>w</code>
|-
|  1 || <code>B</code> || 17 || <code>R</code> || 33 || <code>h</code> || 49 || <code>x</code>
|-
|  2 || <code>C</code> || 18 || <code>S</code> || 34 || <code>i</code> || 50 || <code>y</code>
|-
|  3 || <code>D</code> || 19 || <code>T</code> || 35 || <code>j</code> || 51 || <code>z</code>
|-
|  4 || <code>E</code> || 20 || <code>U</code> || 36 || <code>k</code> || 52 || <code>0</code>
|-
|  5 || <code>F</code> || 21 || <code>V</code> || 37 || <code>l</code> || 53 || <code>1</code>
|-
|  6 || <code>G</code> || 22 || <code>W</code> || 38 || <code>m</code> || 54 || <code>2</code>
|-
|  7 || <code>H</code> || 23 || <code>X</code> || 39 || <code>n</code> || 55 || <code>3</code>
|-
|  8 || <code>I</code> || 24 || <code>Y</code> || 40 || <code>o</code> || 56 || <code>4</code>
|-
|  9 || <code>J</code> || 25 || <code>Z</code> || 41 || <code>p</code> || 57 || <code>5</code>
|-
| 10 || <code>K</code> || 26 || <code>a</code> || 42 || <code>q</code> || 58 || <code>6</code>
|-
| 11 || <code>L</code> || 27 || <code>b</code> || 43 || <code>r</code> || 59 || <code>7</code>
|-
| 12 || <code>M</code> || 28 || <code>c</code> || 44 || <code>s</code> || 60 || <code>8</code>
|-
| 13 || <code>N</code> || 29 || <code>d</code> || 45 || <code>t</code> || 61 || <code>9</code>
|-
| 14 || <code>O</code> || 30 || <code>e</code> || 46 || <code>u</code> || 62 || <code>+</code>
|-
| 15 || <code>P</code> || 31 || <code>f</code> || 47 || <code>v</code> || 63 || <code>/</code>
|}

Note that Base64URL encoding replaces '+' with '-' and '/' with '_' to make the encoded string HTTP-safe and avoid the need for escaping.

==Examples==
To simplify explanation, the example below uses [[ASCII]] text for input even though this is not a typical use. More commonly, input is [[binary data]], such as an image, and the result then represents binary data in a printable text format.

For the input data:

 Many hands make light work.

The typical Base64 represented is:

 TWFueSBoYW5kcyBtYWtlIGxpZ2h0IHdvcmsu

===Encoding when no padding needed===

Each input sequence of 6 bits (which can encode 2<sup>6</sup>&nbsp;=&nbsp;64 values) is mapped to a Base64 alphabet letter. Therefore, Base64 encoding results in four characters for each three input bytes. Assuming the input is ASCII or similar, the byte-data for the first three characters 'M', 'a', 'n' are values <code>77</code>, <code>97</code>, and <code>110</code> which in 8-bit binary representation are <code>01001101</code>, <code>01100001</code>, and <code>01101110</code>. Joining these representations and splitting into 6-bit groups gives:

 010011 010110 000101 101110

Which encodes the string {{code|TWFu}} (per ASCII or similar).

The following table shows how input is encoded. For example, the letter 'M' has the value 77 (per ASCII and similar). The first 6 bits of the value is <code>010011</code> or 19 decimal which maps to Base64 letter 'T' which has a value 84 (per ASCII and similar).

{| class="wikitable" style="text-align:center;"
|+ Encoding 'M', 'a', 'n' as Base64
|- style="font-weight:bold;"
! rowspan=2 scope="row" | input <br> (ASCII)
! scope="row" | letter (ASCII)
| colspan="8" | M
| colspan="8" | a
| colspan="8" | n
|-
! scope="row" | 8-bit <br> decimal value
| colspan="8" | 77
| colspan="8" | 97
| colspan="8" | 110
|-
! colspan=2 scope="row" | bits
| 0 || 1 || 0 || 0 || 1 || 1 || 0 || 1
| 0 || 1 || 1 || 0 || 0 || 0 || 0 || 1
| 0 || 1 || 1 || 0 || 1 || 1 || 1 || 0
|-
! rowspan=3 scope="row" | encoded <br> (Base64)
! scope="row" | 6-bit <br> decimal value
| colspan="6" | 19
| colspan="6" | 22
| colspan="6" | 5
| colspan="6" | 46
|- style="font-weight:bold;"
! scope="row" | letter <br> (Base64 alphabet)
| colspan="6" | T
| colspan="6" | W
| colspan="6" | F
| colspan="6" | u
|-
! scope="row" | byte
| colspan="6" | 84
| colspan="6" | 87
| colspan="6" | 70
| colspan="6" | 117
|}

===Encoding with one padding character===

If the input consists of a number of bytes that is 2 more than a multiple of 3 (e.g. 'M', 'a'), then the last 2 bytes (16 bits) are encoded in 3 Base64 digits (18 bits). The two [[least significant bit]]s of the last content-bearing 6-bit block are treated as zero for encoding and discarded for decoding (along with the trailing <code>=</code> padding character).

{|class="wikitable" style="text-align:center;"
|- style="font-weight:bold;"
! rowspan=2 scope="row"   | input <br> (ASCII)
! scope="row"             | letter (ASCII)
| colspan="8"             | M
| colspan="8"             | a
| colspan="8" rowspan="2"   {{n/a|}}
|-
! scope="row" | 8-bit <br> decimal value
| colspan="8" | 77
| colspan="8" | 97
|-
! colspan=2 scope="row" | bits
| 0 || 1 || 0 || 0 || 1 || 1
| 0 || 1 || 0 || 1 || 1 || 0
| 0 || 0 || 0 || 1
| style="background-color:lightblue;" | 0
| style="background-color:lightblue;" | 0
| {{n/a|{{fsp}}}}
| {{n/a|{{fsp}}}}
| {{n/a|{{fsp}}}}
| {{n/a|{{fsp}}}}
| {{n/a|{{fsp}}}}
| {{n/a|{{fsp}}}}
|-
! rowspan=3 scope="row" | encoded <br> (Base64)
! scope="row" | 6-bit <br> decimal value
| colspan="6" | 19
| colspan="6" | 22
| colspan="6" | 4
| colspan="6"   {{n/a|Padding}}
|- style="font-weight:bold;"
! scope="row" | letter <br> (Base64 alphabet)
| colspan="6" | T
| colspan="6" | W
| colspan="6" | E
| colspan="6" | =
|-
! scope="row" | byte
| colspan="6" | 84
| colspan="6" | 87
| colspan="6" | 69
| colspan="6" | 61
|}

===Encoding with two padding characters===

If the input consists of a number of bytes that is 1 more than a multiple of 3 (e.g. 'M'), then the last 8 bits are represented in 2 Base64 digits (12 bits). The four [[least significant bit]]s of the last content-bearing 6-bit block are treated as zero for encoding and discarded for decoding (along with the trailing two <code>=</code> padding characters):

{| class="wikitable" style="text-align:center;"
|- style="font-weight:bold;"
! rowspan=2 scope="row"    | input <br> (ASCII)
! scope="row"              | letter (ASCII)
| colspan="8"              | M
| colspan="16" rowspan="2"   {{n/a|}}
|-
! scope="row" | 8-bit <br> decimal value
| colspan="8" | 77
|-
! colspan=2 scope="row" | bits
| 0 || 1 || 0 || 0 || 1 || 1
| 0 || 1
| style="background-color:lightblue;" | 0
| style="background-color:lightblue;" | 0
| style="background-color:lightblue;" | 0
| style="background-color:lightblue;" | 0

| {{n/a|{{fsp}}}}
| {{n/a|{{fsp}}}}
| {{n/a|{{fsp}}}}
| {{n/a|{{fsp}}}}
| {{n/a|{{fsp}}}}
| {{n/a|{{fsp}}}}

| {{n/a|{{fsp}}}}
| {{n/a|{{fsp}}}}
| {{n/a|{{fsp}}}}
| {{n/a|{{fsp}}}}
| {{n/a|{{fsp}}}}
| {{n/a|{{fsp}}}}
|-
! rowspan=3 scope="row" | encoded <br> (Base64)
! scope="row" | 6-bit <br> decimal value
| colspan="6" | 19
| colspan="6" | 16
| colspan="6"   {{n/a|Padding}}
| colspan="6"   {{n/a|Padding}}
|- style="font-weight:bold;"
! scope="row" | letter <br> (Base64 alphabet)
| colspan="6" | T
| colspan="6" | Q
| colspan="6" | =
| colspan="6" | =
|-
! scope="row" | byte
| colspan="6" | 84
| colspan="6" | 81
| colspan="6" | 61
| colspan="6" | 61
|}

===Decoding with padding===
When decoding, each sequence of four encoded characters is converted to three output bytes, but with a single padding character the final 4 characters decode to only two bytes, or with two padding characters, the final 4 characters decode to a single byte. For example:

{| class="wikitable" style="text-align:center;"
! Encoded !! Padding !! Length !! Decoded
|-
| {{mono|1=bGlnaHQg{{bg|lightgrey|2=dw==}}}}
| <code>==</code> || 1
| ''light {{bg|lightgrey|w}}''
|-
| {{mono|1=bGlnaHQg{{bg|lightgrey|2=d28=}}}}
| <code>=</code> || 2
| ''light {{bg|lightgrey|wo}}''
|-
| {{mono|1=bGlnaHQg{{bg|lightgrey|d29y}}}}
| {{CNone|None}} || 3
| ''light {{bg|lightgrey|wor}}''
|}
Another way to interpret the padding character is to consider it as an instruction to discard 2 trailing bits from the bit string each time a <code>=</code> is encountered. For example, when {{mono|1=bGlnaHQg{{bg|lightgrey|2=dw==}}}} is decoded, we convert each character (except the trailing occurrences of <code>=</code>) into their corresponding 6-bit representation, and then discard 2 trailing bits for the first <code>=</code> and another 2 trailing bits for the other <code>=</code>. In this instance, we would get 6 bits from the <code>d</code>, and another 6 bits from the <code>w</code> for a bit string of length 12, but since we remove 2 bits for each <code>=</code> (for a total of 4 bits), the <code>dw==</code> ends up producing 8 bits (1 byte) when decoded.

===Decoding without padding===
Use of the padding character in encoded text is ''not'' essential for decoding. The number of missing bytes can be inferred from the length of the encoded text. In some variants, the padding character is mandatory, while for others it is not used. Notably, when [[string concatenation|concatenating]] Base64 encoded strings, then use of padding characters is required.

Without padding, after decoding each sequence of 4 encoded characters, there may be 2 or 3 encoded characters left over. A single remaining encoded character is not possible because a single Base64 character only contains 6 bits, and 8 bits are required to create a byte. The first character contributes 6 bits, and the second character contributes its first 2 bits. The following table demonstrates decoding encoded strings that have 2, 3 or no left-over characters.

{| class="wikitable" style="text-align:center;"
! Encoded !! Length <br> of last group !! Decoded !! Decoded length <br> of last group
|-
| {{mono|1=bGlnaHQg{{bg|lightgrey|dw}}}} || 2
| ''light {{bg|lightgrey|w}}'' || 1
|-
| {{mono|1=bGlnaHQg{{bg|lightgrey|d28}}}} || 3
| ''light {{bg|lightgrey|wo}}'' || 2
|-
| {{mono|1=bGlnaHQg{{bg|lightgrey|d29y}}}} || 4
| ''light {{bg|lightgrey|wor}}'' || 3
|}

Decoding without padding is not performed consistently among decoders{{clarify|date=October 2025}}. In addition, allowing padless decoding by definition allows multiple strings to decode into the same set of bytes{{clarify|date=October 2025}}, which can be a security risk.<ref>{{cite conference |last1=Chalkias |first1=Konstantinos |last2=Chatzigiannis |first2=Panagiotis |title=Base64 Malleability in Practice |conference=ASIA CCS '22: 2022 ACM on Asia Conference on Computer and Communications Security |date=30 May 2022 |pages=1219–1221 |doi=10.1145/3488932.3527284 |url=https://eprint.iacr.org/2022/361.pdf}}</ref>

==Variants==
Variations of Base64 differ in the alphabet used and structural aspects like maximum line length. The most commonly used alphabet is that described by RFC 4648 and most variations only differ in the last two letters used. The following table describes more commonly used encodings that are specified by an [[Request for Comments|RFC]].

{|class="wikitable" style="text-align:center"
! rowspan=2 | Encoding<ref>Some specifications describe a Base64 encoding without naming it. This column identifies Base64 encodings in a descriptive way if no particular name is specified.</ref>
! rowspan=2 | Specification
! colspan=3 | Alphabet
! colspan=3 | Lines
|-
! 62nd
! 63rd
! pad
! Separators
! Length
! Checksum
|-
! {{rh}} | Base 64 Encoding
| {{rh}} | {{nowrap|[https://datatracker.ietf.org/doc/html/rfc4648#section-4 RFC 4648 §4]}}
| <code>+</code> || <code>/</code>
| <code>=</code> || {{No}} || || {{No}}
|-
! {{rh}} | Base 64 Encoding with URL and Filename Safe Alphabet
| {{rh}} | {{nowrap|[https://datatracker.ietf.org/doc/html/rfc4648#section-5 RFC 4648 §5]}}
| <code>-</code> || <code>_</code>
| <code>=</code><br><small>optional</small> || {{No}} || || {{No}}
|-
! {{rh}} | for [[#MIME|MIME]]
| {{rh}} | {{nowrap|[https://datatracker.ietf.org/doc/html/rfc2045 RFC 2045]}}
| <code>+</code> || <code>/</code>
| <code>=</code> || {{Yes}} || 76 || {{No}}
|-
! {{rh}} | for [[#Privacy-enhanced mail|Privacy-Enhanced Mail]] (deprecated)
| {{rh}} | {{nowrap|[https://datatracker.ietf.org/doc/html/rfc1421 RFC 1421]}}
| <code>+</code> || <code>/</code> 
| <code>=</code> || {{Yes}} || 64 || {{Yes}}, in PEM CRC
|-
! {{rh}} | for [[#UTF-7|UTF-7]]
| {{rh}} | {{nowrap|[https://datatracker.ietf.org/doc/html/rfc2152 RFC 2152]}}
| <code>+</code> || <code>/</code>
|  || {{No}} || || {{No}}
|-
! {{rh}} | for IMAP mailbox names
| {{rh}} | {{nowrap|[https://datatracker.ietf.org/doc/html/rfc3501#section-5.1.3 RFC 3501]}}
| <code>+</code> || <code>,</code>
|  || {{No}} || || {{No}}
|-
! {{rh}} | Textual Encodings of PKIX, PKCS, and CMS Structures
| {{rh}} | {{nowrap|[https://datatracker.ietf.org/doc/html/rfc7468 RFC 7468]}}
| <code>+</code> || <code>/</code> 
| <code>=</code> || {{Yes}} || 64 || {{No}}
|-
! {{rh}} | ASCII armor for [[#OpenPGP|OpenPGP]]
| {{rh}} | {{nowrap|[https://datatracker.ietf.org/doc/html/rfc9580 RFC 9580]}}
| <code>+</code> || <code>/</code> 
| <code>=</code> || {{Yes}} || 76 || {{Yes}}, (CRC24)
|}

===RFC 4648===
{{IETF RFC|4648}} describes a various encodings including Base64, and it discusses the use of line feeds in encoded data, the use of padding in encoded data, the use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. The variant that it calls ''Base 64 Encoding'' and ''base64'' is intended for general-use.

The RFC also specifies a second Base64 encoding that is calls ''Base 64 Encoding with URL and Filename Safe Alphabet'' that is intended for representing relatively long identifying information. 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) as a string for use as an HTTP parameter in an HTTP form or an HTTP GET [[URL]]. Also, many [[application software|applications]] need to encode binary data in a way that is convenient for inclusion in a URL, including in hidden web form fields, and Base64 is a convenient encoding to render them in a compact way.

Using standard Base64 in a [[URL]] requires encoding the <code>+</code>, <code>/</code> and <code>=</code> characters as special [[percent-encoding|percent-encoded]] hexadecimal sequences (<code>+</code> becomes <code>%2B</code>, <code>/</code> becomes <code>%2F</code> and <code>=</code> becomes <code>%3D</code>), which makes the string longer and harder to read. Using a different alphabet allows for encoding as Base64 without requiring this extra markup. Typically, <code>+</code> and <code>/</code> are replaced by <code>-</code> and <code>_</code>, respectively, so that using URL encoders/decoders is no longer necessary and has no effect 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. A popular site to make use of such is [[YouTube#Uploading|YouTube]].<ref>{{cite web |title=Here's Why YouTube Will Practically Never Run Out of Unique Video IDs |url=https://www.mentalfloss.com/article/77598/heres-why-youtube-will-never-run-out-unique-video-ids |website=www.mentalfloss.com |access-date=27 December 2021 |language=en |date=23 March 2016}}</ref> 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 {{which|date=December 2020}} encode <code>=</code> as <code>.</code>, potentially exposing applications to relative path attacks when a folder name is encoded from user data.{{Citation needed|date=June 2022}}

===RFC 3548===
{{IETF RFC|3548}}, entitled ''The Base16, Base32, and Base64 Data Encodings'', is an informational (non-normative) memo that attempts to unify the {{IETF RFC|1421}} and {{IETF RFC|2045}} specifications of Base64 encodings, alternative-alphabet encodings, and the Base32 (which is seldom used) and Base16 encodings. RFC 4648 obsoletes RFC 3548.

Unless an encoder is written to a specification that refers to {{IETF RFC|3548}} and specifically requires otherwise{{clarify|date=October 2025}}, RFC 3548 forbids an encoder from generating messages containing characters outside the encoding alphabet or without padding, and it also declares that a decoder must reject data that contain characters other than the encoding alphabet.<ref name="rfc 3548" />

===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 name="rfc 2045"/> MIME's Base64 encoding is based on that of the {{IETF 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 {{IETF RFC|2045}}.

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 character outside the standard set of 64 encoding characters (for example CRLF sequences), 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 ({{fract|4|3}}×{{fract|78|76}}), 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

===Privacy-enhanced mail===
The first known standardized use of the encoding now called MIME Base64 was in the [[Privacy-Enhanced Mail]] (PEM) protocol, proposed by {{IETF RFC|989}} in 1987. PEM defines a "printable encoding" scheme that uses Base64 encoding to transform an arbitrary sequence of bytes 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 |date=February 1987 |publisher=[[Internet Engineering Task Force|IETF]] |access-date=March 18, 2010}}</ref>

The current version of PEM (specified in {{IETF 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 padding suffix.<ref name="rfc 1421"/> The original specification, {{IETF 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.

The process is repeated on the remaining data until fewer than four bytes remain. If three bytes remain, they are processed normally. If fewer than three bytes (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 bytes of the 24-bit buffer are padded-zeros, two <code>=</code> characters are appended to the output; if one byte 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.

===UTF-7===
[[UTF-7]], described first in {{IETF RFC|1642}}, which was later superseded by {{IETF 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 |date=July 1994 |publisher=[[Internet Engineering Task Force|IETF]] |access-date=March 18, 2010}}</ref><ref>{{cite IETF |title=UTF-7 A Mail-Safe Transformation Format of Unicode |rfc=2152 |date=May 1997 |publisher=[[Internet Engineering Task Force|IETF]] |access-date=March 18, 2010}}</ref>

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 {{IETF 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.

===OpenPGP===
{{further|Pretty Good Privacy#OpenPGP}}
[[OpenPGP]], described in {{IETF RFC|9580}}, specifies "[[ASCII armor]]", which is identical to the "Base64" encoding described by 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, prefixed by the "<code>=</code>" symbol as the separator, appended to the encoded output data.<ref>{{cite IETF |title=OpenPGP Message Format |rfc=9580 |date=July 2024 |publisher=[[Internet Engineering Task Force|IETF]] |access-date=February 13, 2025}}</ref>

===Javascript (DOM Web API) ===
The <code>atob()</code> and <code>btoa()</code> JavaScript methods, defined in the HTML5 draft specification,<ref>{{cite web|title=7.3. Base64 utility methods|url=https://w3c.github.io/html/webappapis.html#atob|website=HTML 5.2 Editor's Draft|publisher=[[World Wide Web Consortium]]|access-date=2 January 2018}} Introduced by [http://html5.org/tools/web-apps-tracker?from=5813&to=5814 changeset 5814] {{Webarchive|url=https://web.archive.org/web/20140222225511/http://html5.org/tools/web-apps-tracker?from=5813&to=5814 |date=2014-02-22 }}, 2021-02-01.</ref><ref>{{cite web|title=Window: btoa() method|date=24 June 2025 |url=https://developer.mozilla.org/en-US/docs/Web/API/Window/btoa|access-date=2025-07-31}}</ref> provide Base64 encoding and decoding functionality to web pages. The <code>btoa()</code> method outputs padding characters, but these are optional in the input of the <code>atob()</code> method.<br />
Example: Encoding of the beginning of a GIF file: <code>btoa("GIF89a")</code> ↦ <code>"R0lGODlh"</code>.

===With atypical alphabet order===
Several variants use alphabets similar to the common variants, but in a different order.

; Unix password: Unix stores password hashes computed with [[crypt (C)|'''crypt''']] in the [[passwd#Password file|<code>/etc/passwd</code> file]] using an encoding called <span id="B64">B64</span>. crypt's alphabet puts the punctuation <code>.</code> and <code>/</code> before the alphanumeric characters. crypt uses the alphabet "<code>./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz</code>" without padding. An advantage over RFC 4648 is that sorting encoded ASCII data results in the same order as sorting the plain ASCII data.

; GEDCOM: The '''[[GEDCOM]]''' 5.5 standard for genealogical data interchange encodes multimedia files in its text-line hierarchical file format. GEDCOM uses the same alphabet as crypt, which is <span class="nowrap">"<code>./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz</code>"</span>.<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 |access-date=2012-06-21}}</ref>

; bcrypt: '''[[bcrypt]]''' hashes are designed to be used in the same way as traditional crypt(3) hashes, but bcrypt's alphabet is in a different order than crypt's. bcrypt uses the alphabet <span class="nowrap">"<code>./ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789</code>"</span>.<ref>{{cite web|url=https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/lib/libc/crypt/bcrypt.c?rev=1.1&content-type=text/x-cvsweb-markup|title=src/lib/libc/crypt/bcrypt.c r1.1|author-link=Niels Provos|first=Niels|last=Provos|date=1997-02-13|access-date=2018-05-18}}</ref>

; Xxencoding: '''[[Xxencoding]]''' uses a mostly-alphanumeric character set similar to crypt, but using <code>+</code> and <code>-</code> rather than <code>.</code> and <code>/</code>. Xxencoding uses the alphabet <span class="nowrap">"<code>+-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz</code>"</span>.

; 6PACK: Used with some [[terminal node controller]]s, uses an alphabet from 0x00 to 0x3f.<ref>{{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|access-date=2013-05-19|archive-date=2012-02-24|archive-url=https://web.archive.org/web/20120224051938/http://private.freepage.de/cgi-bin/feets/freepage_ext/41030x030A/rewrite/alexs/xfr/flexnet/6pack_en/6pack.htm|url-status=dead}}</ref>

; Bash: [[Bash (Unix shell)|'''Bash''']] supports numeric literals in Base64. Bash uses the alphabet <span class="nowrap">"<code>0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ@_</code>"</span>.<ref>{{cite web |title=Shell Arithmetic |url=https://www.gnu.org/software/bash/manual/html_node/Shell-Arithmetic.html |website=Bash Reference Manual |access-date=8 April 2020 |quote=Otherwise, numbers take the form [base#]n, where the optional base is a decimal number between 2 and 64 representing the arithmetic base, and n is a number in that base.}}</ref>

===With atypical alphabet===
Some variants use a Base64 alphabet that is significantly different from the alphabets used in the most common Base64 variants (like RFC 4648).

; Uuencoding: The '''[[Uuencoding]]''' alphabet includes no lowercase characters, instead using ASCII codes 32 ("<code>&nbsp;</code>" (space)) through 95 ("<code>_</code>"), consecutively. Uuencoding uses the alphabet <span class="nowrap">"<code>&nbsp;!"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_</code>"</span>. Avoiding all lower-case letters was helpful, because many older printers only printed uppercase. Using consecutive ASCII characters saved computing power, because it was only necessary to add 32, without requiring a lookup table. Its use of most punctuation characters and the space character may limit its usefulness in some applications, such as those that use these characters as syntax.{{citation needed|date=April 2016}}

; BinHex: [[BinHex|'''BinHex 4''']] (HQX), which was used within the [[classic Mac OS]], excludes some visually confusable characters like '<code>7</code>', '<code>O</code>', '<code>g</code>' and '<code>o</code>'. Its alphabet includes additional punctuation characters. It uses the alphabet <span class="nowrap">"<code><nowiki>!"#$%&'()*+,-012345689@ABCDEFGHIJKLMNPQRSTUVXYZ[`abcdefhijklmpqr</nowiki></code>"</span>.

; UTF-8: A [[UTF-8]] environment can use non-synchronized continuation bytes as base64: <code>0b10<b>xxxxxx</b></code>. See [[UTF-8#Comparison with other encodings|UTF-8#Self-synchronization]].

==See also==
* [[8BITMIME]] {{endash}} 8-bit data transmission for SMTP
* {{Annotated link |Ascii85}}
* [[Base16]] {{endash}} Encoding for a sequence of byte values using hexadecimal
* {{Annotated link |Base32}}
* {{Annotated link |Base36}}
* {{Annotated link |Base62}}
* {{Annotated link |Binary number}}

==References==
{{Reflist|2}}

{{Data Exchange}}

[[Category:Usenet]]
[[Category:Email]]
[[Category:Internet Standards]]
[[Category:Binary-to-text encoding formats]]
[[Category:Data serialization formats]]
[[Category:Power-of-two numeral systems]]