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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>All blocks need to be 16 bytes. So for example, if the
file is 2,000,005 bytes, you'd encrypt 125,000 16-byte blocks; you then have 5
bytes left over, to which you'd add 11 padding bytes, which can be any binary
value and then encrypt that last block.<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'>Some schemes add a header to the beginning of the file with the cleartext
length, so the decrypter can easily discard the padding. Also, consider
using CBC mode with a non-zero IV, which has some security advantages.<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> Thursday, November 10, 2011 12:10 PM<br>
<b>To:</b> PyCrypto discussion list<br>
<b>Subject:</b> Re: [pycrypto] Public Key encryption of files<o:p></o:p></span></p>
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<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Dean, thanks for the reply<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>Ah, I see. If I want to encrypt a 2mb file with
a Symmetric Key, it just has to have a "16 byte padding"
for AES, or do all the chunks need to be 16 bytes?<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal style='margin-bottom:12.0pt'>J<o:p></o:p></p>
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<p class=MsoNormal>On Thu, Nov 10, 2011 at 5:17 AM, Dean Macinskas <<a
href="mailto:dmacinskas@geobridge.net">dmacinskas@geobridge.net</a>> wrote:<o:p></o:p></p>
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<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:11.0pt;color:#1F497D'>John,</span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:11.0pt;color:#1F497D'> </span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:11.0pt;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.</span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:11.0pt;color:#1F497D'> </span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:11.0pt;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.</span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:11.0pt;color:#1F497D'> </span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:11.0pt;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.</span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:11.0pt;color:#1F497D'> </span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:11.0pt;color:#1F497D'>HTH,</span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:11.0pt;color:#1F497D'>Dean</span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:11.0pt;color:#1F497D'> </span><o:p></o:p></p>
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<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><b><span
style='font-size:10.0pt'>From:</span></b><span style='font-size:10.0pt'> <a
href="mailto:pycrypto-bounces@lists.dlitz.net" target="_blank">pycrypto-bounces@lists.dlitz.net</a>
[mailto:<a href="mailto:pycrypto-bounces@lists.dlitz.net" target="_blank">pycrypto-bounces@lists.dlitz.net</a>]
<b>On Behalf Of </b>John Matthew<br>
<b>Sent:</b> Tuesday, November 08, 2011 11:59 AM<br>
<b>To:</b> <a href="mailto:pycrypto@lists.dlitz.net" target="_blank">pycrypto@lists.dlitz.net</a><br>
<b>Subject:</b> [pycrypto] Public Key encryption of files</span><o:p></o:p></p>
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<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>First
off, pycrypto is awesome! Thank you for creating it!<o:p></o:p></p>
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<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
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<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>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 style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
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<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>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 style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
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<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>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 style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
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<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Thanks
for your contribution, and help.<o:p></o:p></p>
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<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
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<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>J<o:p></o:p></p>
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<p class=MsoNormal style='margin-bottom:12.0pt'><br>
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<p class=MsoNormal><o:p> </o:p></p>
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