security of key if someone gets original and encrypted file

Hello all,
accidentally I forgot to delete the unencrypted original after
encrypting it to myself with my own public key, so the unencrypted
original and the encrypted file stayed in the same folder on my hard
disk for some while.

Now if some "third party" gets both the unencrypted and the encrypted
file, would it be possible for them to reconstruct the key, or could the
security of the key be impaired?


--
Wilfried Hennings
please reply in the newsgroup, the e-mail address is invalid

In message <lobdl3t3i3d7v0v2dbs74vlk58nt7rqnvn@4ax.com>, Wilfried
Hennings <nospam@fz-juelich.de> writes[color=blue]
>Hello all,
>accidentally I forgot to delete the unencrypted original after
>encrypting it to myself with my own public key, so the unencrypted
>original and the encrypted file stayed in the same folder on my hard
>disk for some while.
>
>Now if some "third party" gets both the unencrypted and the encrypted
>file, would it be possible for them to reconstruct the key, or could the
>security of the key be impaired?[/color]

Happily, no.

The file is actually encrypted with a one time session key. Only that
session key is compromised by possession of the plain & the encrypted.
That session key was made randomly.

The random session key was then encrypted with your private key, so that
anyone with your public key would be able to get the session key & so
decrypt the file.

<https://supportimg.pgp.com/guides/Introduction_To_Cryptography_eng.pdf>

Regards
--
Dave English Senior Software & Systems Engineer
Internet Platform Development, Thus plc

On Wed, 05 Dec 2007 09:06:28 -0500, Wilfried Hennings <nospam@fz-juelich.de> wrote:
[color=blue]
> Now if some "third party" gets both the unencrypted and the encrypted
> file, would it be possible for them to reconstruct the key, or could the
> security of the key be impaired?[/color]

That's called a "known plaintext attack.

PGP/GPG encrypts the file using a randomly generated key and the symmetric
cipher you've chosen. It then encrypts a copy of the randomly generated key,
for each public key you've specified, using the specified public key cipher.

Having both, known plaintext, and the encrypted message doesn't help much.
The plaintext tells the attacker nothing about the the private key, that
they didn't already know. All it does, is give them extra information,
for trying to determine the randomly generated key, which, since it's only
used for the one message, doesn't help them anyway.

I'm using gpg, which lists the following for the supported ciphers ...
Symmetric - IDEA, 3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH
Public key -RSA, RSA-E, RSA-S, ELG-E, DSA

In addition, before you ask :), if you sign the message, pgp/gpg calculates
a hash of the message, and then encrypts the hash using the private key.
Anyone verifying the sig uses the public key to decrypt the hash, and
compares it to a newly calculated hash of the signed file, to see if they
are the same.

The version of gpg I'm using lists the following hash algorithms ...
MD5, SHA1, RIPEMD160, SHA256, SHA384, SHA512, SHA224

The biggest risk, is allowing an attacker to have access to your computer, to
get a copy of your secret key, and install a keylogger to capture your
passprhase, or using a weak passphrase, that they can easily guess.

Hope this helps. You may want to bookmark
[url]http://www.mccune.cc/PGPpage2.htm[/url]

Regards, Dave Hodgins

--
Change nomail.afraid.org to ody.ca to reply by email.
(nomail.afraid.org has been set up specifically for
use in usenet. Feel free to use it yourself.)

On 12/5/2007 6:47 AM, Dave English wrote:[color=blue]
> In message <lobdl3t3i3d7v0v2dbs74vlk58nt7rqnvn@4ax.com>, Wilfried
> Hennings <nospam@fz-juelich.de> writes[color=green]
>> Hello all,
>> accidentally I forgot to delete the unencrypted original after
>> encrypting it to myself with my own public key, so the unencrypted
>> original and the encrypted file stayed in the same folder on my hard
>> disk for some while.
>>
>> Now if some "third party" gets both the unencrypted and the encrypted
>> file, would it be possible for them to reconstruct the key, or could the
>> security of the key be impaired?[/color]
>
> Happily, no.
>
> The file is actually encrypted with a one time session key. Only that
> session key is compromised by possession of the plain & the encrypted.
> That session key was made randomly.
>
> The random session key was then encrypted with your private key, so that
> anyone with your public key would be able to get the session key & so
> decrypt the file.
>
> <https://supportimg.pgp.com/guides/Introduction_To_Cryptography_eng.pdf>
>
> Regards[/color]

You have it backwards.

The session key is encrypted using a PUBLIC key (your own, the
recipient's, or both). The private key -- with its passphrase -- is
used to decrypt the session key.

Theoretically, everyone has your public key. That's why it's called a
"public key".

--
David E. Ross
<http://www.rossde.com/>

Natural foods can be harmful: Look at all the
people who die of natural causes.

In message <trCdnYvyDYogd8vanZ2dnUVZ_gidnZ2d@softcom.net>, David E. Ross
<nobody@nowhere.not> writes[color=blue]
>On 12/5/2007 6:47 AM, Dave English wrote:[/color]
....[color=blue][color=green]
>> The random session key was then encrypted with your private key, so that
>> anyone with your public key would be able to get the session key & so
>> decrypt the file.[/color][/color]
....[color=blue]
>You have it backwards.
>
>The session key is encrypted using a PUBLIC key (your own, the
>recipient's, or both). The private key -- with its passphrase -- is
>used to decrypt the session key.[/color]

You are quite right, of course. At that point I confused the encryption
process with signing.

Thanks
--
Dave English Senior Software & Systems Engineer
Internet Platform Development, Thus plc