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+<tt>

+<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40">

+

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+

+<div class=Section1>

+

+<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";

+color:#1F497D'>John,<o:p></o:p>

+

+<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";

+color:#1F497D'><o:p>&nbsp;</o: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.&nbsp; 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>

+

+<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";

+color:#1F497D'><o:p>&nbsp;</o: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).&nbsp; Symmetric key block length is set by the algorithm: 8

+bytes for TDES, 16 for AES.&nbsp; Any data you encrypt has to be padded to a

+multiple of the block length.<o:p></o:p>

+

+<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";

+color:#1F497D'><o:p>&nbsp;</o: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.&nbsp;

+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>

+

+<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";

+color:#1F497D'><o:p>&nbsp;</o:p>

+

+<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";

+color:#1F497D'>HTH,<o:p></o:p>

+

+<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";

+color:#1F497D'>Dean<o:p></o:p>

+

+<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";

+color:#1F497D'><o:p>&nbsp;</o:p>

+

+<div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'>

+

+<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:<span

+style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>

+pycrypto-bounces@lists.dlitz.net [mailto:pycrypto-bounces@lists.dlitz.net] On

+Behalf Of John Matthew

+Sent: Tuesday, November 08, 2011 11:59 AM

+To: pycrypto@lists.dlitz.net

+Subject: [pycrypto] Public Key encryption of files<o:p></o:p>

+

+</div>

+

+<p class=MsoNormal><o:p>&nbsp;</o:p>

+

+<p class=MsoNormal>First off, pycrypto is awesome! &nbsp;Thank you for creating

+it!<o:p></o:p>

+

+<div>

+

+<p class=MsoNormal><o:p>&nbsp;</o:p>

+

+</div>

+

+<div>

+

+<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>

+

+</div>

+

+<div>

+

+<p class=MsoNormal><o:p>&nbsp;</o:p>

+

+</div>

+

+<div>

+

+<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>

+

+</div>

+

+<div>

+

+<p class=MsoNormal><o:p>&nbsp;</o:p>

+

+</div>

+

+<div>

+

+<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>

+

+</div>

+

+<div>

+

+<p class=MsoNormal><o:p>&nbsp;</o:p>

+

+</div>

+

+<div>

+

+<p class=MsoNormal>Thanks for your contribution, and help.<o:p></o:p>

+

+</div>

+

+<div>

+

+<p class=MsoNormal><o:p>&nbsp;</o:p>

+

+</div>

+

+<div>

+

+<p class=MsoNormal>J<o:p></o:p>

+

+</div>

+

+</div>

+

+</body>

+

+</html>

+</tt>