Download Attacking DES Using Java

A Java implemented key collision
attack on the Data Encryption
Standard (DES)
John Loughran, Tom Dowling
NUI, Maynooth, Co. Kildare,
Ireland
PPPJ ‘03
Java Cryptography
• Used Java Cryptographic Extension (JCE)
based on Java Cryptographic Architecture
(JCA)
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Contains java.security and javax.crypto packages
Used Austrian IAIK version of the JCE
• Contains classes which simplify encryption
process
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E.g. factory methods to return instances of a class
as:
Cipher cryptObject = Cipher.getInstance(“DES”);
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Data Encryption Standard
(DES)
• DES is a symmetric block cipher
• Same key for encryption and decryption
• Works on blocks of fixed length
• DES has 256 different keys
• = 70 000 000 000 000 000
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Birthday Paradox
• Brute force attack: Try all 256 possible keys
• 70 000 000 000 000
• Birthday attack: Reduces complexity of attack
to 228 i.e. sqrt(256) ~ 200 000 000
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For a set with n (256 for DES) possible keys
In two subsets of randomly generated keys of size
sqrt(n) (228 for DES)
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Probability of a match is 0.63
[Stallings, 2003]
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The Biham Algorithm
• A known plaintext header is used
E.g. Postcript file header: “%!PS-Ado”
This plaintext header is encrypted using 228 different
random DES keys
Resulting (ciphertext, key) pairs stored in a table
Compare 228 incoming ciphertexts of the same
header whose keys are unknown with table
When a match is found the corresponding key is
returned
The key can be used to decrypt the message or even
substitute a favourable message
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Implementing the Attack
• Used the JCE to generate the keys as:
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KeyGenerator keyGen =
KeyGenerator.getInstance(“DES”);
keyGen.init(new SecureRandom());
Key key = keyGen.generateKey();
• Stored (ciphertext, key) pairs in
Hashtables
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Searching complexity of O(1)
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Memory Considerations
• Unable to keep a Hashtable with 228
pairs in memory
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Even using a Pentium 4, 2 GHz, 1 GB RAM,
60 GB HD
With virtual memory set to 4 GB
Despite increasing Java heap size to max
using switch at runtime:
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C:\>java –Xms 1640m –Xmx 1640m CrackDES
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Multiple Hashtable Approach
• 214 Hashtables each containing 214
(ciphertext, key) pairs were generated
• During generations pairs were allocated to a
particular Hashtable based on the mod 16394
of the hashCode() of the ciphertext
• Hashtables were finally stored on disk as files
bightable000000.ser to bightable016383.ser
after various merging operations
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Finding a Key
• Arrays of ciphertexts of the same plaintext
header were generated to simulate incoming
ciphertexts with unknown keys
• Each ciphertext was examined as follows:
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Its hashCode() mod 16384 was obtained
The corresponding Hashtable was read in and
searched
If a matching ciphertext was found the
corresponding key was returned
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Time Considerations
• To speed up the search process the
“incoming” ciphertexts were split up into
arrays such that
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Each array contained ciphertexts whose
hashCode() mod 16384 were the same
Thus only one Hashtable needs to be read in for
each array
• This reduced the estimated time needed to
find a key from 6.2 years to 2.3 hours!
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Space Optimisation
• Storing only the key value as a Long
object rather than a Key object reduced
the space required for the Hashtables
from an estimated 55 GB to 8 GB
• This necessitates creating a Key object
from the returned key value using
SecretKey methods from the JCE
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Conclusions
• Biham algorithm implemented successfully in
Java
• Use of JCE in cryptanalysis demonstrated
• Space and time optimisations mean that DES
can now be cracked in 2 hours on a single PC
using Java
• Possible future work on implementation of a
parallel search using a distributed system
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Website
• This presentation and the full text of
the thesis including references and
appendices can be downloaded from
my website:•
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http://homepage.eircom.net/~johnloughran
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