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Backup of private key

 

 

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brokenn at gmail

Nov 25, 2009, 11:20 AM

Post #1 of 8 (1375 views)
Permalink
Backup of private key

Hi All,

This is a complete n00b question, but I still need to get an opinion on
this.

I've created myself a public/private key and got a bit concerned that if my
harddrive fails, I lost the key and all data I've ever encrypted using it.
Advice I find around the net suggest saving it to a floppy (what's that?),
storing it on cd-rom/usb in a safe location or printing it out.

All of these make sense to me, but aren't compatible with my ability to lose
physical things. So, what would the risks be of me using symmetrical
encryption with a long passphrase to encrypt my private key, and storing
that in an online email account (gmail/yahoo/etc)? If we consider the
symmetric encryption to be (practically) unbreakable, is this safe?

Looking forward to any comments.

Brian


ciprian.craciun at gmail

Nov 25, 2009, 1:06 PM

Post #2 of 8 (1314 views)
Permalink
Re: Backup of private key [In reply to]

On Wed, Nov 25, 2009 at 9:20 PM, Brian O'Kennedy <brokenn [at] gmail> wrote:
> Hi All,
> This is a complete n00b question, but I still need to get an opinion on
> this.
> I've created myself a public/private key and got a bit concerned that if my
> harddrive fails, I lost the key and all data I've ever encrypted using it.
>  Advice I find around the net suggest saving it to a floppy (what's that?),
> storing it on cd-rom/usb in a safe location or printing it out.
> All of these make sense to me, but aren't compatible with my ability to lose
> physical things.  So, what would the risks be of me using symmetrical
> encryption with a long passphrase to encrypt my private key, and storing
> that in an online email account (gmail/yahoo/etc)?  If we consider the
> symmetric encryption to be (practically) unbreakable, is this safe?
> Looking forward to any comments.
> Brian


:) I kind of had the same problem. My solution to this one was (in
order of commodity, but not reliability):

1) backup the files from ~/.gnupg to an off-line storage medium
(USB stick, CD/DVD-ROM, another computer, etc.) (usable mainly for
full-restore); (the offline-store should be as secure as your desktop
/ laptop;)

2a) export the secret keys and store them off-line (usable for
re-import in case the binary databases from ~/.gnupg break) (as
security the same as above):
gpg --armor --export-secret-keys
or
gpg --armor --export-secret-keys {id}

2b) the same as above but protected by an extra password (the keys
are already protected by a password) (for more vulnerable offline
stores, like Brian said):
gpg --armor --export-secret-keys {id} | gpg --armor --sign
--symmetric --force-mdc

4a) by using paperkey [1] I can create a ASCII file that is quite
small and contains only the needed secret bits of the secret key; the
idea with this one is that nothing outlives paper, and thus if
something happens with my previous backups I could just feed a scanned
version of the printed file (only one page) through an OCR; (or worse
I could just enter by hand the data, because it's quite small); (the
file must be stored somewhere quite safe;)

4b) I use the same idea as the previous one, but instead of
printing the text file I feed it (or a part of it) through a QRcode
[2] encoder and print the resulting image. Thus restoration implies
only photographing the printed page, and feeding it through a decoder;
(I've tried this one and works flawlessly.)

Please someone correct me if I'm doing something wrong
somewhere... My keys depend on it. :)

Ciprian.

[1] http://www.jabberwocky.com/software/paperkey/
[2] http://en.wikipedia.org/wiki/QRcode

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rjh at sixdemonbag

Nov 25, 2009, 1:13 PM

Post #3 of 8 (1315 views)
Permalink
Re: Backup of private key [In reply to]

Brian O'Kennedy wrote:
> This is a complete n00b question, but I still need to get an opinion on
> this.

We were all new once. :) Welcome to the list!

> All of these make sense to me, but aren't compatible with my ability to
> lose physical things. So, what would the risks be of me using
> symmetrical encryption with a long passphrase to encrypt my private key,
> and storing that in an online email account (gmail/yahoo/etc)? If we
> consider the symmetric encryption to be (practically) unbreakable, is
> this safe?

The good news is your private key is already encrypted with a symmetric
cipher. The passphrase you type to use your key is really the
passphrase needed to decrypt it.

If you are sure that no one will ever guess your passphrase, then you
could safely publish your private key in the _New York Times_. That
would be a really extreme case, but you could do it.

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brokenn at gmail

Nov 25, 2009, 1:16 PM

Post #4 of 8 (1315 views)
Permalink
Re: Backup of private key [In reply to]

Thanks for the useful tips - I think I'll go the encrypt-upload-to-email
route plus an additional paperkey option stored at relatives house in case
of email service going down.

thanks,
Brian

2009/11/25 Ciprian Dorin, Craciun <ciprian.craciun [at] gmail>

> On Wed, Nov 25, 2009 at 9:20 PM, Brian O'Kennedy <brokenn [at] gmail>
> wrote:
> > Hi All,
> > This is a complete n00b question, but I still need to get an opinion on
> > this.
> > I've created myself a public/private key and got a bit concerned that if
> my
> > harddrive fails, I lost the key and all data I've ever encrypted using
> it.
> > Advice I find around the net suggest saving it to a floppy (what's
> that?),
> > storing it on cd-rom/usb in a safe location or printing it out.
> > All of these make sense to me, but aren't compatible with my ability to
> lose
> > physical things. So, what would the risks be of me using symmetrical
> > encryption with a long passphrase to encrypt my private key, and storing
> > that in an online email account (gmail/yahoo/etc)? If we consider the
> > symmetric encryption to be (practically) unbreakable, is this safe?
> > Looking forward to any comments.
> > Brian
>
>
> :) I kind of had the same problem. My solution to this one was (in
> order of commodity, but not reliability):
>
> 1) backup the files from ~/.gnupg to an off-line storage medium
> (USB stick, CD/DVD-ROM, another computer, etc.) (usable mainly for
> full-restore); (the offline-store should be as secure as your desktop
> / laptop;)
>
> 2a) export the secret keys and store them off-line (usable for
> re-import in case the binary databases from ~/.gnupg break) (as
> security the same as above):
> gpg --armor --export-secret-keys
> or
> gpg --armor --export-secret-keys {id}
>
> 2b) the same as above but protected by an extra password (the keys
> are already protected by a password) (for more vulnerable offline
> stores, like Brian said):
> gpg --armor --export-secret-keys {id} | gpg --armor --sign
> --symmetric --force-mdc
>
> 4a) by using paperkey [1] I can create a ASCII file that is quite
> small and contains only the needed secret bits of the secret key; the
> idea with this one is that nothing outlives paper, and thus if
> something happens with my previous backups I could just feed a scanned
> version of the printed file (only one page) through an OCR; (or worse
> I could just enter by hand the data, because it's quite small); (the
> file must be stored somewhere quite safe;)
>
> 4b) I use the same idea as the previous one, but instead of
> printing the text file I feed it (or a part of it) through a QRcode
> [2] encoder and print the resulting image. Thus restoration implies
> only photographing the printed page, and feeding it through a decoder;
> (I've tried this one and works flawlessly.)
>
> Please someone correct me if I'm doing something wrong
> somewhere... My keys depend on it. :)
>
> Ciprian.
>
> [1] http://www.jabberwocky.com/software/paperkey/
> [2] http://en.wikipedia.org/wiki/QRcode
>


rjh at sixdemonbag

Nov 25, 2009, 2:10 PM

Post #5 of 8 (1330 views)
Permalink
Re: Backup of private key [In reply to]

Brian O'Kennedy wrote:
> So this implies that I could safely upload my ascii-armored private
> key to an email server without fear (assuming of course that my
> passphrase is secure and large).

Correct. You just have to make *absolutely certain* your passphrase is
unguessable. If someone is able to grab your private key and your
passphrase, then you're in a world of hurt.

> What symmetric encryption is typically used on the key itself? I'm
> assuming that this level of encryption is secure enough to not worry
> about it being broken?

*All* of the ciphers used in GnuPG are secure enough to not worry about
them being broken. :) Some of the algorithms GnuPG uses are even rated
by the United States government as being suitable protection for
classified material.

That said, I think the particular algorithm used is CAST. I might be
mistaken. It used to be CAST, but it may be AES now.


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yaverot at nerdshack

Nov 27, 2009, 8:59 PM

Post #6 of 8 (1289 views)
Permalink
Re: Backup of private key [In reply to]

Robert J. Hansen wrote:
> If you are sure that no one will ever guess your passphrase, then you
> could safely publish your private key in the _New York Times_. That
> would be a really extreme case, but you could do it.

But what if you publish it in a paper people actually _read_? :)

While I understand the intent of the statement, and been wanting to
question it for some time (about 3 months). I do believe it to the
limits of my understanding of modern cryptography. But I want to make
sure I'm not missing something by the example using such a weak means of
distribution. I can't say that I've _never_ seen a NYT, but I know I
didn't read the copy that appeared in my elementary school in the 80s.
They've had a good 20 years with which to have folded, or to have
dropped down to a "oh, they still publish?" distribution.

I'll try this modern bent to the question:

If I had a sufficiently good passphrase, would Google returning my
secret key as the first hit result for every search for a day still be
secure?

With my understanding, the answer is _still_ yes.

I am under no delusions that my passphrase is that good. Not that I
have the friends or enemies at Google with which to test it.


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rjh at sixdemonbag

Nov 27, 2009, 10:50 PM

Post #7 of 8 (1287 views)
Permalink
Re: Backup of private key [In reply to]

Matt wrote:
> If I had a sufficiently good passphrase, would Google returning my
> secret key as the first hit result for every search for a day still be
> secure?

"Secure" is not a very good word to use. It means so many different
things to so many different people. "Secure" really means "in
accordance with my security policies" -- the use of the word is
inherently subjective.

Let me try giving you an answer that doesn't involve the word "secure,"
but will still hopefully answer your question.




"For any symmetric cipher used in GnuPG, for any purpose supported by
GnuPG, there is *no* effective way for someone who has the ciphertext
and *only* the ciphertext to recover the plaintext without knowing the
passphrase."



The qualifiers are very important. For clarity's sake, I'll restate
them here, very directly:

* I am only talking about GnuPG
* I am excluding gratuitously stupid things you can do by abusing
the "--expert" flag
* We are assuming the adversary has *only* the ciphertext
* The adversary has *no* ability to execute side-channel attacks
against you


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chaz at chaz6

Nov 28, 2009, 8:33 AM

Post #8 of 8 (1292 views)
Permalink
Re: Backup of private key [In reply to]

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Hash: SHA1

There is thread in the archives with the subject "TPK Archival" that may
be useful.

http://lists.gnupg.org/pipermail/gnupg-users/2009-March/035996.html

Regards,

Chris
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