Compression and Encryption • WinZip creates two records for each file – Main file record – Central directory record • Each Zip archive contains (in order):
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Analysis of the WinZip Encryption
Method
Paper by: Tadayoshi Kohno
Presented by: Ken, Mike, Jeremy & Paul
The popular compression utility fo
rMicrosoft Windows computers
Easy-to-use AES encryption"
A dvanced Encryption version two (AE-2) D erives AES and HMAC-SHA1 keys from user's passphrase• E ncrypts compression output with with AES-CTR A uthenticates resulting ciphertext with HMAC-SHA1A Secure Implementation?
Proven
secure MAC: -H M A C S H A 1Proven
secure Encryption: AES in counter mode
Proven
secure combination: E ncrypt-then-MACBut...
S ecurity products must be evaluated as awhole, and the security of a whole product may not follow as a simple corollary of the security of the underlying components."
Compression and Encryption
WinZip creates two records for each file
M ain file record C entral directory record E ach Zip archive contains (in order): T he main file records concatenated together T he central directory records concatenated together A n End-of-Archive record Note:A WinZip archive can contain
multiple files. Each f ile is compressed/encrypted independentlyArchive Contents
The Main Fil
eRecord
File Record IndicatorExtraction Version NeededGeneral Purpose Bit FlagCompression MethodLast Modified TimeLast Modified Date32-Bit CRCCompressed SizeUn
compressed S i zeFilename LengthExtra Field LengthFilenameEx
tra FieldFile Data
The Central Directory Record
Directory Re
cord IndicatorVersion Made ByExtraction Version NeededGeneral Purpose Bit FlagCompression MethodLast Modified TimeLast Modified Date32-Bit CRCCompressed SizeUn
compressed S i ze Filename LengthExtra Field LengthFile Comment LengthDisk Number Star tInternal/External File Attr
ibutesRelative Header OffsetFilenameEx
tra FieldFile Co
mmentArchive Contents
The Main Fil
eRecord
File Record IndicatorExtraction Version NeededGeneral Purpose Bit FlagCompre ssion MethodLast Modified TimeLast Modif
i ed Date32-Bit
CRC Compressed SizeUncompressed SizeFilename LengthExtra Field LengthFilenameExtra FieldFile DataThe Central Directory Record
Directory Re
cord Indicator Version Made ByVersion Needed to ExtractGeneral Purpose Bit FlagCompre ssion MethodLast Modif
i ed TimeLast Modif
i ed Date32-Bit
CRC Compressed SizeUncompressed SizeFilename LengthExtra Field LengthFile Comment LengthDisk Number Star tInternal/External File Attr
ibutesRelative Header OffsetFilenameExtra FieldFile Co
mmentImportant Archive Contents
The Main Fil
eRecord
Compre
ssion MethodLast Modified TimeLast Modif
i ed Date32-Bit
CRCUncompressed SizeFilenameExtra FieldFile Data
The Central Directory Record
Compre
ssion MethodLast Modif
i ed TimeLast Modif
i ed Date32-Bit
CRCUncompressed SizeFilenameExtra Field
With AE-2 Encryption Enabled
the Extra Fields Contain:Extra Fields Header IDData SizeVersion Numbe
rVendor IDEncryption StrengthActual Compre
ssion MethodWith AE-2 Encryption Enabled
the File Data Field C ontains:SaltPassword Verif
i cation Value Encry p ted File DataAuthentication Code
Plaintext-1
Plaintext-1
C o mpressedPassphrase
Salt-n
Enc r y p tedCiphertext-1
Reco rd 1Ciphertext
M ACAES in CT
R Mod e (Counter=0)HMAC-SHA1
Ciphertext-2
Reco rd 2Ciphertext
M ACCiphertext-3
Reco rd 3Ciphertext
M ACCiphertext-n
Reco rd nCiphertext
M AC MAC nCiphertext-n
File Record n
Pas s w o r d Ver i f i c a t i on Va lueMetadata-n
Salt-n
Metadata-1
File Encryption and Authentication
Code Process
FCounter Mode AES Encryption
X 0 CTR-0 M 0 C 0 Xor F X 1 CTR-1 C 1 Xor F X 2 CTR-2 Xor F X n CTR-n Xor M 2 M n M 1 C 2 C nWinZip Security Problems:
Interactions between compression and the AE-2 encryption method.The names of files and their interpretations
Information leakage from encrypted files' metadata Interactions with AE-1 and a chosen-protocol attack Archives with both encrypted and unencrypted files.Key collisions and repeated keystreams
Exploiting the Interaction Between
Compression and Encryption
F.zip Alice Bob F .zip Mall ory changes the comp ressio n metho d o f F.zip to cre a te F .zipRecall that the metadata is
not Authenticated , therefore Mallory can change these values without voiding the HMAC-SHA1 tag. W hen Bob attempts to decrypt F .zip (with the wrong compression method), the contents will be garbage.Create Encrypted Zip Archive
Using 128-bit AES
Encryption
Change Compression Method Values:08 00Compressed00 00Not CompressedDecryption of the Modified Archive -
WinZip 9.0
Garbage...
•If Mallory obtains this garbage, he can reconstruct F.zip.•Is it practical for Mallory to obtain this garbage?