[pycrypto] Public Key encryption of files

Imran Azad Imran.Azad at bthft.nhs.uk
Fri Nov 11 07:54:34 CST 2011

Please note my new email address Imran.Azad at bthft.nhs.uk


I don't mean to steal your thunder, I noticed you asked a question about the suitability of public key cryptography for file encryption and the use of padding. Unless one is an expert in cryptography I don't think it's a good idea to encrypt sensitive data using your own scheme as cryptography is very easy to get wrong with catastrophic consequences, even 'experts' make mistakes. Rather it would be safer to use an established scheme, you also need to think about the integrity of what you are encrypting - although you may have taken this into account. I am not an expert in cryptography and I appreciate and admire the work that has gone into PyCrypto. I am speaking from past experiences and I thought I would share these with you.

You may find this link useful: http://security.stackexchange.com/questions/8571/rsa-4096-with-aes-256-encryption-process-using-pycrypto

Kind Regards

From: pycrypto-bounces at lists.dlitz.net [mailto:pycrypto-bounces at lists.dlitz.net] On Behalf Of John Matthew
Sent: 10 November 2011 18:32
To: PyCrypto discussion list
Subject: Re: [pycrypto] Public Key encryption of files

All good stuff, thanks Dave
On Thu, Nov 10, 2011 at 10:26 AM, Dave Pawson <dave.pawson at gmail.com<mailto:dave.pawson at gmail.com>> wrote:
On 10 November 2011 18:18, Dean Macinskas <dmacinskas at geobridge.net<mailto:dmacinskas at geobridge.net>> wrote:
> 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.

def _lazysecret(secret, blocksize=32, padding='}'):
   """pads secret if not legal AES block size (16, 24, 32)"""
   if not len(secret) in (16, 24, 32):
       return secret + (blocksize - len(secret)) * padding
   return secret

Python makes it quite easy.
 The reason I started to use it!
Thanks to
src http://www.turnkeylinux.org/blog/python-symmetric-encryption

great piece of software.


Dave Pawson
Docbook FAQ.
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Imran Azad
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