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Text of the page (random words):
last input group contains only two octets all 16 bits will be captured in the first three base64 digits 18 bits the two least significant bits of the last content bearing 6 bit block will turn out to be zero and discarded on decoding along with the succeeding padding character source text ascii m a octets 77 0x4d 97 0x61 bits 0 1 0 0 1 1 0 1 0 1 1 0 0 0 0 1 0 0 base64 encoded sextets 19 22 4 padding character t w e octets 84 0x54 87 0x57 69 0x45 61 0x3d if there is only one significant input octet e g m or when the last input group contains only one octet all 8 bits will be captured in the first two base64 digits 12 bits the four least significant bits of the last content bearing 6 bit block will turn out to be zero and discarded on decoding along with the succeeding two padding characters source text ascii m octets 77 0x4d bits 0 1 0 0 1 1 0 1 0 0 0 0 base64 encoded sextets 19 16 padding padding character t q octets 84 0x54 81 0x51 61 0x3d 61 0x3d output padding edit because base64 is a six bit encoding and because the decoded values are divided into 8 bit octets every four characters of base64 encoded text 4 sextets 4 6 24 bits represents three octets of unencoded text or data 3 octets 3 8 24 bits this means that when the length of the unencoded input is not a multiple of three the encoded output must have padding added so that its length is a multiple of four the padding character is which indicates that no further bits are needed to fully encode the input this is different from a which means that the remaining bits are all zeros the example below illustrates how truncating the input of the above quote changes the output padding input output padding text length text length light wor k 11 bglnahqg d29y ay4 16 1 light wor k 10 bglnahqg d29y aw 16 2 light wor 9 bglnahqg d29y 12 0 light wo 8 bglnahqg d28 12 1 light w 7 bglnahqg dw 12 2 the padding character is not essential for decoding since the number of missing bytes can be inferred from the length of the encoded text in some implementations the padding character is mandatory while for others it is not used an exception in which padding characters are required is when multiple base64 encoded files have been concatenated decoding base64 with padding edit when decoding base64 text four characters are typically converted back to three bytes the only exceptions are when padding characters exist a single indicates that the four characters will decode to only two bytes while indicates that the four characters will decode to only a single byte for example encoded padding length decoded bglnahqg dw 1 light w bglnahqg d28 2 light wo bglnahqg d29y none 3 light wor decoding base64 without padding edit without padding after normal decoding of four characters to three bytes over and over again fewer than four encoded characters may remain in this situation only two or three characters can remain 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 so a minimum of two base64 characters are required the first character contributes 6 bits and the second character contributes its first 2 bits for example length encoded length decoded 2 bglnahqg dw 1 light w 3 bglnahqg d28 2 light wo 4 bglnahqg d29y 3 light wor implementations and history edit variants summary table edit implementations may have some constraints on the alphabet used for representing some bit patterns this notably concerns the last two characters used in the alphabet at positions 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 provides links to the subsections below encoding encoding characters separate encoding of lines decoding non encoding characters 62nd 63rd pad separators length checksum rfc 1421 base64 for privacy enhanced mail deprecated mandatory cr lf 64 or lower for the last line no no rfc 2045 base64 transfer encoding for mime mandatory cr lf at most 76 no discarded rfc 2152 base64 for utf 7 no no no rfc 3501 base64 encoding for imap mailbox names no no no rfc 4648 4 base64 standard a optional no no rfc 4648 5 base64url url and filename safe standard a _ optional no no rfc 4880 radix 64 for openpgp mandatory cr lf at most 76 radix 64 encoded 24 bit crc no other variations see applications not compatible with rfc 4648 base64 below a b it is important to note that this variant is intended to provide common features where they are not desired to be specialised by implementations ensuring robust engineering this is particularly in light of separate line encodings and restrictions which have not been considered when previous standards have been co opted for use elsewhere thus the features indicated here may be over ridden privacy enhanced mail edit 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 octets to a format that can be expressed in short lines of 6 bit characters as required by transfer protocols such as smtp 7 the current version of pem specified in rfc 1421 uses a 64 character alphabet consisting of upper and lower case roman letters a z a z the numerals 0 9 and the and symbols the symbol is also used as a padding suffix 4 the original specification rfc 989 additionally used the 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 eight bits of a 24 bit buffer the next in the middle eight and the third in the 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 abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz0123456789 and the indicated character is output 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 characters are appended to the output if one octet of the 24 bit buffer is filled with padded zeros one 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 edit main article mime the mime multipurpose internet mail extensions specification lists base64 as one of two binary to text encoding schemes the other being quoted printable 5 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 symbol for output padding in the same way as described at 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 4 3 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 utf 7 edit main article 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 8 9 the modified base64 alphabet consists of the mime base64 alphabet but does not use the padding character utf 7 is intended for use in mail headers defined in rfc 2047 and the 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 edit further information pretty good privacy openpgp 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 crc the checksum is calculated on the input data before encoding the checksum is then encoded with the same base64 algorithm and prefixed by symbol as separator appended to the encoded output data 10 rfc 3548 edit 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 base32 which is seldom used and base16 encodings unless implementations are written to a specification that refers to rfc 3548 and specifically requires otherwise rfc 3548 forbids implementations from generating messages containing characters outside the encoding alphabet or without padding and it also declares that decoder implementations must reject data that contain characters outside the encoding alphabet 6 rfc 4648 edit this rfc obsoletes rfc 3548 and focuses on base64 32 16 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 url applications edit base64 encoding can be helpful when fairly lengthy identifying information is used in an http environment for example a database persistence framework for java objects might use base64 encoding to encode a relatively large unique id generally 128 bit uuids into a string for use as an http parameter in http forms or http get 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 using standard base64 in url requires encoding of and characters into special percent encoded hexadecimal sequences becomes 2b becomes 2f and becomes 3d which makes the string unnecessarily longer for this reason modified base64 for url variants exist such as base64url in rfc 4648 where the and characters of standard base64 are respectively replaced by and _ 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 11 some variants allow or require omitting the padding signs to avoid them being confused with field separators or require that any such padding be percent encoded some libraries which will encode to potentially exposing applications to relative path attacks when a folder name is encoded from user data citation needed html edit the atob and btoa javascript methods defined in the html5 draft specification 12 provide base64 encoding and decoding functionality to web pages the btoa method outputs padding characters but these are optional in the input of the atob method other applications edit example of an svg containing embedded jpeg images encoded in base64 13 base64 can be used in a variety of contexts base64 can be used to transmit and store text that might otherwise cause delimiter collision base64 is used to encode character strings in ldap data interchange format files base64 is often used to embed binary data in an xml file using a syntax similar to data encoding base64 data e g favicons in firefox s exported bookmarks html base64 is used to encode binary files such as images within scripts to avoid depending on external files 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 data uris instead of being supplied in separate files although not part of the official specification for svg some viewers can interpret base64 when used for embedded elements such as images inside svg 14 base64 can be used to store transmit relatively small amounts of binary data via a computer s text 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 wallet software binary data that must be quickly verified by humans as a safety mechanism such as file checksums or 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 pgp key fingerprint with a space qr codes which contain binary data will sometimes store it encoded in base64 rather than simply storing the raw binary data as there is a stronger guarantee that all qr code readers will accurately decode text as well as the fact that some devices will more readily save text from a qr code than potentially malicious binary data applications not compatible with rfc 4648 base64 edit some application use a base64 alphabet that is significantly different from the alphabets used in the most common base64 variants see variants summary table above the uuencoding alphabet includes no lower case characters instead using ascii codes 32 space through 95 _ consecutively uuencoding uses the alphabet 0123456789 abcdefghijklmnopqrstuvwxyz _ 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 som...
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