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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>John,<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Unless the file is small, the typical way to encrypt a file is
to use a symmetric key, like TDES or AES. Asymmetric key crypto is
usually too slow for bulk encryption; you use a symmetric key for that, and
store/export the symmetric key using asymmetric key protection.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>The block size of a public key is fixed by the length of the
key; in other words, if you generate a 2048 bit key, the block size is 2048
bits (256 bytes). Symmetric key block length is set by the algorithm: 8
bytes for TDES, 16 for AES. Any data you encrypt has to be padded to a
multiple of the block length.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>If you use RSA, there are a couple of rules for encrypting data,
one of which is that the data has to be numerically less than the public modulus.
This is usually accomplished by setting the left-most bit (MSB) of the data
block to zero; another reason why using a public key directly is not a good
choice for bulk encryption.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>HTH,<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Dean<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
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<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>
pycrypto-bounces@lists.dlitz.net [mailto:pycrypto-bounces@lists.dlitz.net] <b>On
Behalf Of </b>John Matthew<br>
<b>Sent:</b> Tuesday, November 08, 2011 11:59 AM<br>
<b>To:</b> pycrypto@lists.dlitz.net<br>
<b>Subject:</b> [pycrypto] Public Key encryption of files<o:p></o:p></span></p>
</div>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>First off, pycrypto is awesome! Thank you for creating
it!<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>I'd like to use Public Keys to encrypt files, is this
something that seems appropriate for file encryption?<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>I've noticed that the encrypt method for RSA keys is only
256 bytes, which seems rather small.<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>I know I could wrap that in a generator, but was hoping for
some feedback on another way or a configuration change to increase that number.<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>Thanks for your contribution, and help.<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>J<o:p></o:p></p>
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