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Doing it in memory is by far the easiest thing. It is also simple with one line calls to decrypt/encrypt and compress/decompress. This uses Crypt::Blowfish but you could use your favourite block cipher. The order is VITAL. plaintext->compress->crypt->file is the only way to go (reverse on the way out of course).

By definition a decent encryption algorithm will turn '0'x1000000 into random (and thus totally uncompressable noise). On the other hand a compression algorithm will compress the hell out of the plaintext due to the repeated pattern. Then you can crypt that. For interest reverse the order of crypt<->compress and you will see no compression at all. This is a simple measure of the quality of the encryption - the compression algorithm can no longer see any patterns (and we know the plaintext has patterns). Crap encryption WILL compress. Try ROT13, or bongo crypt ;-)

$|++; use Compress::Zlib ; use Crypt::CBC; my $cipher = Crypt::CBC->new( {'key' => 'secret key', 'cipher' => 'Blo +wfish', }); my $file = 'c:/text.txt'; write_file( $file, $cipher, "abcbefghijklmnopqrstuvwxyz\n"x1000 ); print "\n\nDecrypting and decompressing\n\n"; my $data = read_file( $file, $cipher ); printf "Got back %d bytes\n%s\n[snip]", length($data), substr($data,0, +64); sub write_file { my ( $file, $cipher, $data ) = @_; printf "Before compression %d bytes\n", length($data); $data = compress($data); printf "After compression %d bytes\n", length($data); $data = $cipher->encrypt($data); printf "After encryption %d bytes\n", length($data); open my $fh, '>', $file or die $!; print $fh $data; close $fh; print map{ s/[^\040-\177]//g;$_ }$data; } sub read_file { my ( $file, $cipher ) = @_; open my $fh, $file or die $!; local $/; my $data = <$fh>; close $fh; $data = $cipher->decrypt($data); $data = uncompress($data); return $data; } __DATA__ Before compression 27000 bytes After compression 116 bytes After encryption 136 bytes RandomIV#vTP;\\-Ij{Et@t--sC7SWy{<6P~)}y9p6_r$0bB(d1$k8:6, Decrypting and decompressing Got back 27000 bytes abcbefghijklmnopqrstuvwxyz abcbefghijklmnopqrstuvwxyz abcbefghij [snip]



In reply to Re: Compressing and Encrypting files on Windows by tachyon
in thread Compressing and Encrypting files on Windows by hawtin

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