[RD (OP)]

Is there free PDF-reader software, other than Adobe reader, which can decrypt 256-bit encrypted PDFs ?

Free Non-Adobe PDF readers can deal with 128-bit encryption, but here doesn't seem to be a free one which can decrypt a PDF which has been encrypted using the latest 256-bit AES.

[syhprum]

I have Adobe acrobat 9 pro will that do it ? I have never tried it is not free but trial versions can be downloaded , put up an encrypted one plus key to make a test.

[RD (OP)]

I have Adobe acrobat 9 pro will that do it ?

I was looking for something other than Adobe products, (something more lightweight, and preferably multi-platform).

... put up an encrypted one plus key to make a test.

Currently I don't have the ability to produce a 256-bit test PDF,

and I don't fancy posting the genuine encrypted document in question on the internet with the password (key).

[CliffordK]

I'm seeing several commercial packages that claim to create or decode the 256 bit encryption.

Did you try Lizzard Secure PDF viewer?

http://www.locklizard.com/pdf_security_viewer.htm

I'm not sure of all of its capabilities, but it would be worth trying.

[RD (OP)]

I'm seeing several commercial packages that claim to create or decode the 256 bit encryption.

Currently I don't need to create 256-bit encrypted PDFs , I'm just interested in reading them using a free PDF reader, other than Adobe reader.

Did you try Lizzard Secure PDF viewer?

The locklizard PDF software seems to use their own unique encryption methods ...

If you seek unprecedented PDF protection for your valuable information, want total control over how it is used, wish to scale up from password protecting documents, which is neither comprehensive nor fool-proof, then Safeguard PDF Security is for you.

http://www.locklizard.com/pdf_security.htm

[ and locklizard is not free ]

I think I have it : the latest version of FireFox browser [#19] has a PDF reader built-in which can read encrypted PDFs ... http://support.mozilla.org/en-US/kb/view-pdf-files-firefox-without-downloading-them

[syhprum]

I have produced an encrypted file for test with Adobe Acrobat X but I cannot post it as an attachment but will email it to any interested parties.
It will not decrypt with Lizard secure but I will try Firefox and post results.
I have loaded Firefox/Thunderbird but I cannot se anyway that they help with encrypted files
 

[RD (OP)]

I have produced an encrypted file for test with Adobe Acrobat X but I cannot post it as an attachment

PDFs can be attached to posts if they are less than 128kB in size,
one is attached to this post, the password is "thenakedscientists" (don't include the quotes).

[syhprum]

There is a simple Adobe Acrobat reader 11.1 that will open both 128 and 256 encrypted files, free download only it foists Chrome on you.

[RD (OP)]

There is a simple Adobe Acrobat reader 11.1 that will open both 128 and 256 encrypted files, free download only it foists Chrome on you.

But the brief is ...

Topic: Is there a free non-Adobe PDF-reader which can decrypt 256-bit encrypted PDFs

[syhprum]

There may or may not be a non adobe PDF reader but I am not going to hunt for it when there is a small ,Free adobe program to do the job
Here is a test file encrypted with 256 bit encryption to test the highly original password is password

[CliffordK]

Ok,
Both Okular & "Document Viewer" in Linux can open both "Test Card 2" &  "* 128-bit encryption, key is ''thenakedscientists'' (no quotes).pdf"

So far I am unable to open the doc sent by PM from Syhprum using the PW specified.

[syhprum]

I made a check the 361Kb encrypted file opens with both Acrobat reader and Acrobat X pro no problem with the key "openme"

[CliffordK]

I wonder...
Do we truly need 256 bit encryption?

2¹²⁸ = 3 x 10³⁸

Most of the current PCs run on the order of a giga operations per second, or a billion ops per second.
Multiply that by a thousand, and one gets teraflops, or a trillion ops per second, or 10¹² ops per second.

So, at a teraflop, a brute force decryption would still take about 10²⁶ seconds.
And one still has a good number of years to do the calculation.

On the other hand, if one uses a lower case alpha password that is 10 characters long, one ends up with 26¹⁰ = 1 x 10¹⁴, or so possibilities.  At 1 teraflop, it would take about a minute to decode.

Use, say a 100 character set for a 10 letter password, and one gets 100¹⁰ possibilities, or about 1 x 10²⁰.  And, it takes about a year to decode at 1 teraflop.  A few teraflops and it would decode much quicker. 

However, If one restricted the search to, say 7 lower case characters, and 3 "other" characters, it would again reduce the possibilities greatly.

If you restrict the search to words in a dictionary, various tenses, abbreviations, and names.   One comes up with less than a million or so possibilities.  Even a rather mediocre computer should be able to crack the password rather quickly.

The key, of course, is good heuristics, and the ability to guess the PW.

[RD (OP)]

... Here is a test file encrypted with 256 bit encryption ... Test Card 2.pdf (72.75 kB

Your "Test Card 2.pdf" has 128-bit encryption, not 256-bit  ...

 [ Invalid Attachment ]

[RD (OP)]

I wonder...
Do we truly need 256 bit encryption?

2¹²⁸ = 3 x 10³⁸

Most of the current PCs run on the order of a giga operations per second, or a billion ops per second.
Multiply that by a thousand, and one gets teraflops, or a trillion ops per second, or 10¹² ops per second.

So, at a teraflop, a brute force decryption would still take about 10²⁶ seconds.
And one still has a good number of years to do the calculation.

I think the 256-bit is overkill at the moment , an attempt to future-proof ... http://en.wikipedia.org/wiki/Moore%27s_law

... If you restrict the search to words in a dictionary, various tenses, abbreviations, and names.   One comes up with less than a million or so possibilities.  Even a rather mediocre computer should be able to crack the password rather quickly.

The password is the weakest link. Some password encryption methods apply PBKDF with thousands of iterations to delay dictionary-type cracking.

[syhprum]

Indeed I may have accidentally created some with only 128 bit encryption I took care with this one it should definitely be 256 key "openme".

[RD (OP)]

... I took care with this one it should definitely be 256 key "openme"...  P Marchese 2007.pdf (27.16 kB

No joy with Adobe reader 9 ...
 [ Invalid Attachment ]

Nor with FireFox 19 ...
 [ Invalid Attachment ]

FireFox 19 CAN read 128-bit ...
 [ Invalid Attachment ]

[syhprum]

there is an AES 256 bit encryption system available but it does not seem to be compatible with the Adobe Acrobat system.

http://www.integralmemory.com/faq/what-aes-256-bit-hardware-based-encryption

[RD (OP)]

there is an AES 256 bit encryption system available but it does not seem to be compatible with the Adobe Acrobat system.

256-bit AES is not a proprietary (patented) encryption, it's used widely, and is just an extension of the 128-bit AES encryption that the non-Adobe PDF readers can currently cope with ... http://en.wikipedia.org/wiki/Advanced_Encryption_Standard#Description_of_the_cipher

Unless Adobe have added a secret tweak to their implementation of 256-bit AES, then I don't see why Non-Adobe PDF readers could not be modified to decrypt 256-bit PDFs.

[syhprum]

It seems a bit of a pointless exorcise hunting for a non adobe decrypter when a free adobe one is available, so far I have tried four but none work !!, if you have one please publish it.