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<H1>[pycrypto] Public Key encryption of files</H1>
<B>John Matthew</B>
<A HREF="mailto:pycrypto%40lists.dlitz.net?Subject=%5Bpycrypto%5D%20Public%20Key%20encryption%20of%20files&In-Reply-To=8129C8D99AA86D4A9A8BEA3E7DCC806A36B9BBF048%40exch-mail.bridge.net"
TITLE="[pycrypto] Public Key encryption of files">john at compunique.com
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<I>Thu Nov 10 11:10:10 CST 2011</I>
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<PRE>Dean, thanks for the reply
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?
J
On Thu, Nov 10, 2011 at 5:17 AM, Dean Macinskas <<A HREF="http://lists.dlitz.net/cgi-bin/mailman/listinfo/pycrypto">dmacinskas at geobridge.net</A>>wrote:
><i> John,****
</I>><i>
</I>><i> ** **
</I>><i>
</I>><i> Unless the file is small, the typical way to encrypt a file is to use a
</I>><i> symmetric key, like TDES or AES. Asymmetric key crypto is usually too slow
</I>><i> for bulk encryption; you use a symmetric key for that, and store/export the
</I>><i> symmetric key using asymmetric key protection.****
</I>><i>
</I>><i> ** **
</I>><i>
</I>><i> The block size of a public key is fixed by the length of the key; in other
</I>><i> words, if you generate a 2048 bit key, the block size is 2048 bits (256
</I>><i> bytes). Symmetric key block length is set by the algorithm: 8 bytes for
</I>><i> TDES, 16 for AES. Any data you encrypt has to be padded to a multiple of
</I>><i> the block length.****
</I>><i>
</I>><i> ** **
</I>><i>
</I>><i> If you use RSA, there are a couple of rules for encrypting data, one of
</I>><i> which is that the data has to be numerically less than the public modulus.
</I>><i> This is usually accomplished by setting the left-most bit (MSB) of the data
</I>><i> block to zero; another reason why using a public key directly is not a good
</I>><i> choice for bulk encryption.****
</I>><i>
</I>><i> ** **
</I>><i>
</I>><i> HTH,****
</I>><i>
</I>><i> Dean****
</I>><i>
</I>><i> ** **
</I>><i>
</I>><i> *From:* <A HREF="http://lists.dlitz.net/cgi-bin/mailman/listinfo/pycrypto">pycrypto-bounces at lists.dlitz.net</A> [mailto:
</I>><i> <A HREF="http://lists.dlitz.net/cgi-bin/mailman/listinfo/pycrypto">pycrypto-bounces at lists.dlitz.net</A>] *On Behalf Of *John Matthew
</I>><i> *Sent:* Tuesday, November 08, 2011 11:59 AM
</I>><i> *To:* <A HREF="http://lists.dlitz.net/cgi-bin/mailman/listinfo/pycrypto">pycrypto at lists.dlitz.net</A>
</I>><i> *Subject:* [pycrypto] Public Key encryption of files****
</I>><i>
</I>><i> ** **
</I>><i>
</I>><i> First off, pycrypto is awesome! Thank you for creating it!****
</I>><i>
</I>><i> ** **
</I>><i>
</I>><i> I'd like to use Public Keys to encrypt files, is this something that seems
</I>><i> appropriate for file encryption?****
</I>><i>
</I>><i> ** **
</I>><i>
</I>><i> I've noticed that the encrypt method for RSA keys is only 256 bytes, which
</I>><i> seems rather small.****
</I>><i>
</I>><i> ** **
</I>><i>
</I>><i> I know I could wrap that in a generator, but was hoping for some feedback
</I>><i> on another way or a configuration change to increase that number.****
</I>><i>
</I>><i> ** **
</I>><i>
</I>><i> Thanks for your contribution, and help.****
</I>><i>
</I>><i> ** **
</I>><i>
</I>><i> J****
</I>><i>
</I>><i> _______________________________________________
</I>><i> pycrypto mailing list
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