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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><html version="-//W3C//DTD HTML 3.2 Final//EN"><head><title>RFC 1341 Excerpt: Base64 Encoding</title><style type="text/css"> DIV.bodycopy { margin-left: 15%; margin-right: 10% }</style><meta name="keywords" content="Base64, RFC 1341, MIME"><meta name="description" content="RFC 1341 Excerpt: Base64 Encoding"><meta name="author" content="N. Borenstein, N. Freed"><meta name="robots" content="noindex"></head><body bgcolor="#FFFFFF"><center><table cols=2 width="65%"><tr><td align=left><font size="+1" face="Arial, Helvetica"><b>Network Working Group</b></font><td align=right><font size="+1" face="Arial, Helvetica"><b>N. Borenstein, Bellcore</b></font><tr><td align=left><font size="+1" face="Arial, Helvetica"><b>Request for Comments: 1341</b></font><td align=right><font size="+1" face="Arial, Helvetica"><b>N. Freed, Innosoft</b></font><tr><td align=left> <td align=right><font size="+1" face="Arial, Helvetica"><b>June 1992</b></font><tr><td colspan=2 align=center><font size="+1" face="Arial, Helvetica"><b>MIME (Multipurpose Internet Mail Extensions):<p>Mechanisms for Specifying and Describing <br>the Format of Internet Message Bodies</b></font></table><p><em>The following excerpt from RFC 1341 defines Base64 encoding.</em></center><div class="bodycopy"><h3>5.2 Base64 Content-Transfer-Encoding</h3>The Base64 Content-Transfer-Encoding is designed to representarbitrary sequences of octets in a form that is not humanlyreadable. The encoding and decoding algorithms are simple, butthe encoded data are consistently only about 33 percent largerthan the unencoded data. This encoding is based on the one usedin Privacy Enhanced Mail applications, as defined in RFC 1113.The base64 encoding is adapted from RFC 1113, with one change:base64 eliminates the "*" mechanism for embedded clear text.<p>A 65-character subset of US-ASCII is used, enabling 6 bits to berepresented per printable character. (The extra 65th character,"=", is used to signify a special processing function.)<p>NOTE: This subset has the important property that it isrepresented identically in all versions of ISO 646, including USASCII, and all characters in the subset are also representedidentically in all versions of EBCDIC. Other popular encodings,such as the encoding used by the UUENCODE utility and the base85encoding specified as part of Level 2 PostScript, do not sharethese properties, and thus do not fulfill the portabilityrequirements a binary transport encoding for mail must meet.<p>The encoding process represents 24-bit groups of input bits asoutput strings of 4 encoded characters. Proceeding from left toright, a 24-bit input group is formed by concatenating 3 8-bitinput groups. These 24 bits are then treated as 4 concatenated6-bit groups, each of which is translated into a single digit inthe base64 alphabet. When encoding a bit stream via the base64encoding, the bit stream must be presumed to be ordered with themost- significant-bit first. That is, the first bit in thestream will be the high-order bit in the first byte, and theeighth bit will be the low-order bit in the first byte, and soon.<p>Each 6-bit group is used as an index into an array of 64printable characters. The character referenced by the index isplaced in the output string. These characters, identified inTable 1, below, are selected so as to be universallyrepresentable, and the set excludes characters with particularsignificance to SMTP (e.g., ".", "CR", "LF") and to theencapsulation boundaries defined in this document (e.g., "-").<p><table align=center><caption><b>Table 1: The Base64 Alphabet</b></caption><tr><th> Value <th> Encoding <th> Value <th> Encoding <th> Value <th> Encoding <th> Value <th> Encoding<tr><td align=center> 0 <td align=center> A <td align=center>17 <td align=center>R <td align=center>34 <td align=center>i <td align=center>51 <td align=center>z<tr><td align=center> 1 <td align=center> B <td align=center> 18 <td align=center> S <td align=center> 35 <td align=center> j <td align=center> 52 <td align=center> 0<tr><td align=center> 2 <td align=center> C <td align=center> 19 <td align=center> T <td align=center> 36 <td align=center> k <td align=center> 53 <td align=center> 1<tr><td align=center> 3 <td align=center> D <td align=center> 20 <td align=center> U <td align=center> 37 <td align=center> l <td align=center> 54 <td align=center> 2<tr><td align=center> 4 <td align=center> E <td align=center> 21 <td align=center> V <td align=center> 38 <td align=center> m <td align=center> 55 <td align=center> 3<tr><td align=center> 5 <td align=center> F <td align=center> 22 <td align=center> W <td align=center> 39 <td align=center> n <td align=center> 56 <td align=center> 4<tr><td align=center> 6 <td align=center> G <td align=center> 23 <td align=center> X <td align=center> 40 <td align=center> o <td align=center> 57 <td align=center> 5<tr><td align=center> 7 <td align=center> H <td align=center> 24 <td align=center> Y <td align=center> 41 <td align=center> p <td align=center> 58 <td align=center> 6<tr><td align=center> 8 <td align=center> I <td align=center> 25 <td align=center> Z <td align=center> 42 <td align=center> q <td align=center> 59 <td align=center> 7<tr><td align=center> 9 <td align=center> J <td align=center> 26 <td align=center> a <td align=center> 43 <td align=center> r <td align=center> 60 <td align=center> 8<tr><td align=center> 10 <td align=center> K <td align=center> 27 <td align=center> b <td align=center> 44 <td align=center> s <td align=center> 61 <td align=center> 9<tr><td align=center> 11 <td align=center> L <td align=center> 28 <td align=center> c <td align=center> 45 <td align=center> t <td align=center> 62 <td align=center> +<tr><td align=center> 12 <td align=center> M <td align=center> 29 <td align=center> d <td align=center> 46 <td align=center> u <td align=center> 63 <td align=center> /<tr><td align=center> 13 <td align=center> N <td align=center> 30 <td align=center> e <td align=center> 47 <td align=center> v<tr><td align=center> 14 <td align=center> O <td align=center> 31 <td align=center> f <td align=center> 48 <td align=center> w <td align=center> (pad) <td align=center> =<tr><td align=center> 15 <td align=center> P <td align=center> 32 <td align=center> g <td align=center> 49 <td align=center> x<tr><td align=center> 16 <td align=center> Q <td align=center> 33 <td align=center> h <td align=center> 50 <td align=center> y</table><p>The output stream (encoded bytes) must be represented in linesof no more than 76 characters each. All line breaks or othercharacters not found in Table 1 must be ignored by decodingsoftware. In base64 data, characters other than those in Table1, line breaks, and other white space probably indicate atransmission error, about which a warning message or even amessage rejection might be appropriate under some circumstances.<p>Special processing is performed if fewer than 24 bits areavailable at the end of the data being encoded. A full encodingquantum is always completed at the end of a body. When fewerthan 24 input bits are available in an input group, zero bitsare added (on the right) to form an integral number of 6-bitgroups. Output character positions which are not required torepresent actual input data are set to the character "=". Sinceall base64 input is an integral number of octets, only thefollowing cases can arise: (1) the final quantum of encodinginput is an integral multiple of 24 bits; here, the final unitof encoded output will be an integral multiple of 4 characterswith no "=" padding, (2) the final quantum of encoding input isexactly 8 bits; here, the final unit of encoded output will betwo characters followed by two "=" padding characters, or (3)the final quantum of encoding input is exactly 16 bits; here,the final unit of encoded output will be three charactersfollowed by one "=" padding character.<p>Care must be taken to use the proper octets for line breaks ifbase64 encoding is applied directly to text material that hasnot been converted to canonical form. In particular, text linebreaks should be converted into CRLF sequences prior to base64encoding. The important thing to note is that this may be donedirectly by the encoder rather than in a prior canonicalizationstep in some implementations.<p>NOTE: There is no need to worry about quoting apparentencapsulation boundaries within base64-encoded parts ofmultipart entities because no hyphen characters are used in thebase64 encoding.</div><h3><a href="./">Back to <cite>Base64</cite></a></h3></body></html>⌨️ 快捷键说明
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