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Re: BugTraq: EFS Win 2000 flaw
From: Russ <Russ.Cooper () RC ON CA>
Date: Tue, 23 Jan 2001 09:51:22 -0500

In case anyone's interested, here's a summary of the responses I received to
my incorrect assertions;

I should say that I was under the honest belief that companies, such as
OnTrack, made available services which could recover overwritten data at a
reasonable price. I called them this morning and asked, they responded that
if the data was overwritten then it was basically not possible to recover.
They wouldn't say whether they did make such a service available, but the
implication is clearly that its not as trivial, or inexpensive, as I
believed it to be. Thanks to Ryan Russell for setting me straight on that.
Frank Knobbe <FKnobbe () KnobbeITS com> pointed out that PCGuardian's
Encryption Plus Hard Disk software works well on Windows 2000 and does
complete disk encryption (enter password at boot to decrypted system files),
solving the EFS issues posed by Rickard.

Kris Kennaway <kris () FreeBSD org> was succinct; "Don't be silly. If the file
was overwritten even once then it can't be recovered in software. Not many
people have access to expensive scanning equipment which can pick up
residual magnetisation of the storage medium."

Camillo Särs <Camillo.Sars () F-Secure com> said; "F-Secure FileCrypto does a
secure delete, that is overwrite, of the original when doing an initial
encryption. Nevertheless, any files created after encryption comes into
effect are immediately written to disk in encrypted form, without any
intermediate steps of writing temporary plaintext to disk."

Roman Fischer <roman.fischer () ubs com> said; "PGPDisk creates one large file.
On this file, it reads/writes the data. Thus it overwrites the same parts of
the file all the time, not leaving any temp files behind (other than maybe
in swap space or memory)."

Its probably also interesting to note that Microsoft makes significant
mention of EFS' ability to encrypt temporary files created by applications
(e.g. Word), thereby protecting encrypted data from leakage, in their EFS
White Paper;


"EFS is integrated with the operating system so that it stops the leaking of
key information to page files and ensures that all temporary copies of an
encrypted file are encrypted."

Note they mention "that all temporary copies of an encrypted file are
encrypted", which doesn't address Rickard's observations of the plaintext
copy of a file being encrypted. They also make no mention of the temporary
file being created in their graphic "Figure 1 File Encryption Process" on
that page.

Bottom line is that my assertion was wrong that it was naive to believe that
over-writing was a resolution to the problems observed by Rickard. While not
assuring files couldn't be obtained, it does offer significant resistance to
attack (Dan Kaminsky's phrase.)

Russ - Surgeon General of TruSecure Corporation/NTBugtraq Editor

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