Base64
Dan Tobias (Talk | contribs) |
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[[BinHex]] uses a similar encoding scheme (except for really early versions which used hexadecimal as implied by the name), but with a different set of 64 characters and some other format differences. | [[BinHex]] uses a similar encoding scheme (except for really early versions which used hexadecimal as implied by the name), but with a different set of 64 characters and some other format differences. | ||
| − | == | + | == Specs == |
| − | + | ||
| − | + | ||
* RFC 3548 | * RFC 3548 | ||
* RFC 4648 | * RFC 4648 | ||
| + | |||
| + | == Programming libraries/modules == | ||
| + | * [http://search.cpan.org/~gaas/MIME-Base64-Perl-1.00/lib/MIME/Base64/Perl.pm Base64 module for Perl] | ||
| + | |||
| + | == Other links == | ||
| + | * [http://en.wikipedia.org/wiki/MIME#Content-Transfer-Encoding Content Transfer Encoding (Wikipedia)] | ||
| + | * [http://en.wikipedia.org/wiki/Base64 Base64 (Wikipedia)] | ||
Revision as of 01:54, 24 January 2014
Base64 is one of the two transfer encodings used in MIME e-mail messages to encode binary data entirely in characters of the 7-bit ASCII range so that it could be safely transmitted even through systems not supporting anything else (which was more of an issue in the 1970s when e-mail protocols were defined than at present). The other encoding is Quoted-printable, which is best suited for data that is mostly ASCII text but may have a few "unsafe" characters needing encoding. Base64 is best suited for binary data where all byte values from 0 to 255 are likely to occur, and encodes them efficiently (but not easily readably by the "naked eye").
A Base64-encoded message part is indicated in its MIME headers (following the Content-type header giving the type of data) with the line:
Content-Transfer-Encoding: Base64
The data is encoded as a sequence of base-64 digits, consisting of a character from a set of 64 characters, which (starting at the character representing zero) goes in the order:
- Capital letters from A to Z
- Lowercase letters from a to z
- Digits from 0 to 9
- The characters + and /
A base-64 digit (fitting in one byte of the encoded data) encodes six bits of the original data. Since a byte has eight bits, three bytes of the original file (24 bits) correspond to four base-64 digits. Thus, the encoding method requires you to consider each group of three bytes as a number (big-endian), and express it as four digits in the base-64 system. The padding character = is used to fill out a group of four characters if not needed to encode the end of the original data.
BinHex uses a similar encoding scheme (except for really early versions which used hexadecimal as implied by the name), but with a different set of 64 characters and some other format differences.
