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A Java implemented key collision attack on the Data Encryption Standard (DES)

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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A Java implemented key collision attack on the Data Encryption Standard (DES)

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  1. A Java implemented key collision attack on the Data Encryption Standard (DES) John Loughran, Tom Dowling NUI, Maynooth, Co. Kildare, Ireland PPPJ ‘03

  2. Java Cryptography • Used Java Cryptographic Extension (JCE) based on Java Cryptographic Architecture (JCA) • Contains java.security and javax.crypto packages • Used Austrian IAIK version of the JCE • Contains classes which simplify encryption process • E.g. factory methods to return instances of a class as: • Cipher cryptObject = Cipher.getInstance(“DES”); Java implemented DES attack

  3. 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 Java implemented DES attack

  4. 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 • For a set with n (256 for DES) possible keys • In two subsets of randomly generated keys of size sqrt(n) (228 for DES) • Probability of a match is 0.63 • [Stallings, 2003] Java implemented DES attack

  5. 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 Java implemented DES attack

  6. Implementing the Attack • Used the JCE to generate the keys as: • KeyGenerator keyGen = KeyGenerator.getInstance(“DES”); • keyGen.init(new SecureRandom()); • Key key = keyGen.generateKey(); • Stored (ciphertext, key) pairs in Hashtables • Searching complexity of O(1) Java implemented DES attack

  7. Memory Considerations • Unable to keep a Hashtable with 228 pairs in memory • 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: • C:\>java –Xms 1640m –Xmx 1640m CrackDES Java implemented DES attack

  8. 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 Java implemented DES attack

  9. Java implemented DES attack

  10. 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: • 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 Java implemented DES attack

  11. Java implemented DES attack

  12. Time Considerations • To speed up the search process the “incoming” ciphertexts were split up into arrays such that • 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! Java implemented DES attack

  13. 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 Java implemented DES attack

  14. 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 Java implemented DES attack

  15. Website • This presentation and the full text of the thesis including references and appendices can be downloaded from my website:- • http://homepage.eircom.net/~johnloughran Java implemented DES attack

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