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<li><a class="reference internal" href="#">base64 – Encode binary data into ASCII characters</a><ul>
<li><a class="reference internal" href="#base-64-encoding">Base 64 Encoding</a></li>
<li><a class="reference internal" href="#base-64-decoding">Base 64 Decoding</a></li>
<li><a class="reference internal" href="#url-safe-variations">URL-safe Variations</a></li>
<li><a class="reference internal" href="#other-encodings">Other Encodings</a></li>
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<div class="section" id="module-base64">
<span id="base64-encode-binary-data-into-ascii-characters"></span><h1>base64 – Encode binary data into ASCII characters<a class="headerlink" href="#module-base64" title="Permalink to this headline">¶</a></h1>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Purpose:</th><td class="field-body">The base64 module contains functions for translating binary data into a subset of ASCII suitable for transmission using plaintext protocols.</td>
</tr>
<tr class="field"><th class="field-name">Python Version:</th><td class="field-body">1.4 and later</td>
</tr>
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<p>The base64, base32, and base16 encodings convert 8 bit bytes to values with 6, 5, or 4 bits of useful data per byte, allowing non-ASCII bytes to be encoded as ASCII characters for transmission over protocols that require plain ASCII, such as SMTP. The <em>base</em> values correspond to the length of the alphabet used in each encoding. There are also URL-safe variations of the original encodings that use slightly different results.</p>
<div class="section" id="base-64-encoding">
<h2>Base 64 Encoding<a class="headerlink" href="#base-64-encoding" title="Permalink to this headline">¶</a></h2>
<p>A basic example of encoding some text looks like this:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">base64</span>
<span class="c"># Load this source file and strip the header.</span>
<span class="n">initial_data</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">__file__</span><span class="p">,</span> <span class="s">'rt'</span><span class="p">)</span><span class="o">.</span><span class="n">read</span><span class="p">()</span><span class="o">.</span><span class="n">split</span><span class="p">(</span><span class="s">'#end_pymotw_header'</span><span class="p">)[</span><span class="mi">1</span><span class="p">]</span>
<span class="n">encoded_data</span> <span class="o">=</span> <span class="n">base64</span><span class="o">.</span><span class="n">b64encode</span><span class="p">(</span><span class="n">initial_data</span><span class="p">)</span>
<span class="n">num_initial</span> <span class="o">=</span> <span class="nb">len</span><span class="p">(</span><span class="n">initial_data</span><span class="p">)</span>
<span class="n">padding</span> <span class="o">=</span> <span class="p">{</span> <span class="mi">0</span><span class="p">:</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">:</span><span class="mi">2</span><span class="p">,</span> <span class="mi">2</span><span class="p">:</span><span class="mi">1</span> <span class="p">}[</span><span class="n">num_initial</span> <span class="o">%</span> <span class="mi">3</span><span class="p">]</span>
<span class="k">print</span> <span class="s">'</span><span class="si">%d</span><span class="s"> bytes before encoding'</span> <span class="o">%</span> <span class="n">num_initial</span>
<span class="k">print</span> <span class="s">'Expect </span><span class="si">%d</span><span class="s"> padding bytes'</span> <span class="o">%</span> <span class="n">padding</span>
<span class="k">print</span> <span class="s">'</span><span class="si">%d</span><span class="s"> bytes after encoding'</span> <span class="o">%</span> <span class="nb">len</span><span class="p">(</span><span class="n">encoded_data</span><span class="p">)</span>
<span class="k">print</span>
<span class="c">#print encoded_data</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">xrange</span><span class="p">((</span><span class="nb">len</span><span class="p">(</span><span class="n">encoded_data</span><span class="p">)</span><span class="o">/</span><span class="mi">40</span><span class="p">)</span><span class="o">+</span><span class="mi">1</span><span class="p">):</span>
<span class="k">print</span> <span class="n">encoded_data</span><span class="p">[</span><span class="n">i</span><span class="o">*</span><span class="mi">40</span><span class="p">:(</span><span class="n">i</span><span class="o">+</span><span class="mi">1</span><span class="p">)</span><span class="o">*</span><span class="mi">40</span><span class="p">]</span>
</pre></div>
</div>
<p>The output shows the 558 bytes of the original source expand to 744 bytes after being encoded.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">There are no carriage returns in the output produced by the library, so I have broken the encoded data up artificially to make it fit better on the page.</p>
</div>
<div class="highlight-python"><pre>$ python base64_b64encode.py
113 bytes before encoding
Expect 1 padding bytes
152 bytes after encoding
CgppbXBvcnQgYmFzZTY0CgojIExvYWQgdGhpcyBz
b3VyY2UgZmlsZSBhbmQgc3RyaXAgdGhlIGhlYWRl
ci4KaW5pdGlhbF9kYXRhID0gb3BlbihfX2ZpbGVf
XywgJ3J0JykucmVhZCgpLnNwbGl0KCc=</pre>
</div>
</div>
<div class="section" id="base-64-decoding">
<h2>Base 64 Decoding<a class="headerlink" href="#base-64-decoding" title="Permalink to this headline">¶</a></h2>
<p>The encoded string can be converted back to the original form by taking 4 bytes and converting them to the original 3, using a reverse lookup. The <tt class="docutils literal"><span class="pre">b64decode()</span></tt> function does that for you.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">base64</span>
<span class="n">original_string</span> <span class="o">=</span> <span class="s">'This is the data, in the clear.'</span>
<span class="k">print</span> <span class="s">'Original:'</span><span class="p">,</span> <span class="n">original_string</span>
<span class="n">encoded_string</span> <span class="o">=</span> <span class="n">base64</span><span class="o">.</span><span class="n">b64encode</span><span class="p">(</span><span class="n">original_string</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Encoded :'</span><span class="p">,</span> <span class="n">encoded_string</span>
<span class="n">decoded_string</span> <span class="o">=</span> <span class="n">base64</span><span class="o">.</span><span class="n">b64decode</span><span class="p">(</span><span class="n">encoded_string</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Decoded :'</span><span class="p">,</span> <span class="n">decoded_string</span>
</pre></div>
</div>
<p>The encoding process looks at each sequence of 24 bits in the input (3 bytes) and encodes those same 24 bits spread over 4 bytes in the output. The last two characters, the <tt class="docutils literal"><span class="pre">==</span></tt>, are padding because the number of bits in the original string was not evenly divisible by 24 in this example.</p>
<div class="highlight-python"><pre>$ python base64_b64decode.py
Original: This is the data, in the clear.
Encoded : VGhpcyBpcyB0aGUgZGF0YSwgaW4gdGhlIGNsZWFyLg==
Decoded : This is the data, in the clear.</pre>
</div>
</div>
<div class="section" id="url-safe-variations">
<h2>URL-safe Variations<a class="headerlink" href="#url-safe-variations" title="Permalink to this headline">¶</a></h2>
<p>Because the default base64 alphabet may use <tt class="docutils literal"><span class="pre">+</span></tt> and <tt class="docutils literal"><span class="pre">/</span></tt>, and those two characters are used in URLs, it became necessary to specify an alternate encoding with substitutes for those characters. The <tt class="docutils literal"><span class="pre">+</span></tt> is replaced with a <tt class="docutils literal"><span class="pre">-</span></tt>, and <tt class="docutils literal"><span class="pre">/</span></tt> is replaced with underscore (<tt class="docutils literal"><span class="pre">_</span></tt>). Otherwise, the alphabet is the same.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">base64</span>
<span class="k">for</span> <span class="n">original</span> <span class="ow">in</span> <span class="p">[</span> <span class="nb">chr</span><span class="p">(</span><span class="mi">251</span><span class="p">)</span> <span class="o">+</span> <span class="nb">chr</span><span class="p">(</span><span class="mi">239</span><span class="p">),</span> <span class="nb">chr</span><span class="p">(</span><span class="mi">255</span><span class="p">)</span> <span class="o">*</span> <span class="mi">2</span> <span class="p">]:</span>
<span class="k">print</span> <span class="s">'Original :'</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="n">original</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Standard encoding:'</span><span class="p">,</span> <span class="n">base64</span><span class="o">.</span><span class="n">standard_b64encode</span><span class="p">(</span><span class="n">original</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'URL-safe encoding:'</span><span class="p">,</span> <span class="n">base64</span><span class="o">.</span><span class="n">urlsafe_b64encode</span><span class="p">(</span><span class="n">original</span><span class="p">)</span>
<span class="k">print</span>
</pre></div>
</div>
<div class="highlight-python"><pre>$ python base64_urlsafe.py
Original : '\xfb\xef'
Standard encoding: ++8=
URL-safe encoding: --8=
Original : '\xff\xff'
Standard encoding: //8=
URL-safe encoding: __8=</pre>
</div>
</div>
<div class="section" id="other-encodings">
<h2>Other Encodings<a class="headerlink" href="#other-encodings" title="Permalink to this headline">¶</a></h2>
<p>Besides base 64, the module provides functions for working with base 32 and base 16 (hex) encoded data.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">base64</span>
<span class="n">original_string</span> <span class="o">=</span> <span class="s">'This is the data, in the clear.'</span>
<span class="k">print</span> <span class="s">'Original:'</span><span class="p">,</span> <span class="n">original_string</span>
<span class="n">encoded_string</span> <span class="o">=</span> <span class="n">base64</span><span class="o">.</span><span class="n">b32encode</span><span class="p">(</span><span class="n">original_string</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Encoded :'</span><span class="p">,</span> <span class="n">encoded_string</span>
<span class="n">decoded_string</span> <span class="o">=</span> <span class="n">base64</span><span class="o">.</span><span class="n">b32decode</span><span class="p">(</span><span class="n">encoded_string</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Decoded :'</span><span class="p">,</span> <span class="n">decoded_string</span>
</pre></div>
</div>
<div class="highlight-python"><pre>$ python base64_base32.py
Original: This is the data, in the clear.
Encoded : KRUGS4ZANFZSA5DIMUQGIYLUMEWCA2LOEB2GQZJAMNWGKYLSFY======
Decoded : This is the data, in the clear.</pre>
</div>
<p>The base 16 functions work with the hexadecimal alphabet.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">base64</span>
<span class="n">original_string</span> <span class="o">=</span> <span class="s">'This is the data, in the clear.'</span>
<span class="k">print</span> <span class="s">'Original:'</span><span class="p">,</span> <span class="n">original_string</span>
<span class="n">encoded_string</span> <span class="o">=</span> <span class="n">base64</span><span class="o">.</span><span class="n">b16encode</span><span class="p">(</span><span class="n">original_string</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Encoded :'</span><span class="p">,</span> <span class="n">encoded_string</span>
<span class="n">decoded_string</span> <span class="o">=</span> <span class="n">base64</span><span class="o">.</span><span class="n">b16decode</span><span class="p">(</span><span class="n">encoded_string</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Decoded :'</span><span class="p">,</span> <span class="n">decoded_string</span>
</pre></div>
</div>
<div class="highlight-python"><pre>$ python base64_base16.py
Original: This is the data, in the clear.
Encoded : 546869732069732074686520646174612C20696E2074686520636C6561722E
Decoded : This is the data, in the clear.</pre>
</div>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt><a class="reference external" href="http://docs.python.org/library/base64.html">base64</a></dt>
<dd>The standard library documentation for this module.</dd>
<dt><span class="target" id="index-0"></span><a class="rfc reference external" href="http://tools.ietf.org/html/rfc3548.html"><strong>RFC 3548</strong></a></dt>
<dd>The Base16, Base32, and Base64 Data Encodings</dd>
</dl>
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