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I choose “Compress drive to save disc space” under drive properties.

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If compression is applied after encryption, then certainly not, if compression is applied before encryption, I could see it swinging either way - hopefully somebody who knows how this works can help? –  Phoshi Nov 8 '10 at 11:03
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up vote 13 down vote accepted

The compression should have no effect on the security offered by TrueCrypt.

If the compression is applied before the encryption, i.e. at the filesystem level hosted within an encrypted file/partition then the data will still be as well encrypted as it would be uncompressed. No one would know it was compressed data until they'd managed to decrypt some of it anyway.

If the compression is applied after the encryption then it will have little or no effect either on security or space used - in fact it would most likely back-fire and result in wasting space - so you'll just be wasting CPU cycles. Data encrypted with cryptographically safe algorithms should not have enough redundancy for the compression algorithm to find.

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Usual implementation is to set compression on the filesystem which is within the TrueCrypt container. For example if you are running NTFS filesystem, then you can use the NTFS compression tools. This will make a difference to the size of data within the TrueCrypt volume, so you can fit more data in it. It obviously will not change the size of the volume itself, but you can always do that manually.

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Truecrypt WON'T work with a file on a compressed drive and the compression doesn't achieve much either (as the data is too random to compress).

I made the mistake of copying a 500G file container to a compressed drive as a backup - It still used up 500 GB & truecrypt would not open it until I copied it to an uncompressed drive. You don't want to be copying 500G files more often than you need!

Create truecrypt volume (as device or file eg G: or G:\tc.vol) then apply compression to the volume truecrypt mounts it as (eg T:)

It's prob best to backup the contents of the TC volume rather than the whole volume - do it to a TC file container with a grow option & again compress the TC volume if you need to save space.

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We're taking about encrypting entire partition, not TrueCrypt containers. –  gronostaj Dec 3 '13 at 17:56
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