Cryptanalysis of malware encrypted output files. Nelson Uto CPqD
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1
2 Cryptanalysis of malware encrypted output files Nelson Uto CPqD
3 Agenda Introduction. Cryptanalysis of File #1. Cryptanalysis of File #2. Cryptanalysis of File #3.
4 Introduction CPqD was hired by a big Brazilian company to find out which information had been stolen by three different malwares, that infected its environment. Each one of them stored information in encrypted form using different mechanisms. We did only have access to the encrypted files and the malware binaries, meaning we could not use the special purpose hardware targeted by them. Due to the sensitivity of the stolen data and signed NDA, this talk will not use the real information we retrieved from those files.
5 Covered topics Detection of weak cryptosystems. Cryptanalysis of classical algorithms. Block ciphers. DES. Modes of operation. Searching key in malware binary or in memory. Worst scenario.
6 File #1 Sample
7 File #1 Histogram
8 File #1 Important facts File#1 is pretty redundant. This means a weak cryptosystem was used. The distance between occurrences of the string robin@hoo is always multiple of its length. Most of the bytes has values between 80 and 180.
9 File #1 Hypothesis Hypothesis #1: a constant number is added to each byte modulo 256 and a given string is repeated several times in the plain text. Not likely, but it should be tested. How? Hypothesis #2: a Vigenère cipher over an alphabet of 256 elements and period equals 9 was used. Candidate key: robin@hoo
10 File #1 First attempt
11 File #1 Correction
12 File #1 Description of cipher Alphabet of definition: A = {0, 1, 2, 3,, 255} Plain text: M = m 0 m 1 m 2 m t-1, m i ϵ A Cipher text: C = c 0 c 1 c 2 c t-1, c i ϵ A Key: K = k 0 k 1 k 2 k 3 k 4 k 5 k 6 k 7 k 8 = 0x52 4f e f 4f Encryption function: c i = m i + k (i mod 9) mod 256 Decryption function: m i = c i k (i mod 9) mod 256
13 File #2 Sample
14 File #2 Base64 decoded
15 File #2 Redundancy check
16 File #2 Base64 review Added bits octet Encoded 1 Encoded 2 = = Padding 1 st octet 2 nd octet Added bits 0 0 Encoded 1 Encoded 2 Encoded 3 = Padding
17 File #2 Block size? 1) Length = 56 Base64 chars. 2) Ends with ==. 3) Therefore input length equals 40 bytes. 4) Possible block size: 64 bits.
18 File #2 Candidate ciphers DES. 2TDES. 3TDES. FEAL. IDEA. SAFER. RC5. LOKI. Blowfish.
19 File #2 String search
20 File #2 Narrowing the options LbCipher is a library for Delphi. It implements the following algorithms from our list: Blowfish (ECB, CBC). DES (ECB, CBC). 2TDES (ECB, CBC). 3TDES (ECB, CBC).
21 File #2 Starting with DES DES is a 64-bit block cipher. The cipher employs a 64-bit key of which only 56 bits are effective. Based on a Feistel network. It is possible to search the entire key space using special purpose hardware 1, which was first built in 1998.
22 File #2 Inside DES (1) Figure: DES rounds. Source: [2] HAC.
23 File #2 Inside DES (2) Figure: DES initial permutation and inverse. Source: [2] HAC.
24 File #2 Inside DES (3) Figure: DES round function expansion E and permutation P. Source: [2] HAC.
25 File #2 Inside DES (4) Figure: DES key schedule bit selections. Source: [2] HAC.
26 File #2 From LbCipher procedure InitEncryptDES(const Key : TKey64; var Context : TDESContext; Encrypt : Boolean); const PC1 : array [0..55] of Byte = (56, 48, 40, 32, 24, 16, 8, 0, 57, 49, 41, 33, 25, 17, 9, 1, 58, 50, 42, 34, 26, 18, 10, 2, 59, 51, 43, 35, 62, 54, 46, 38, 30, 22, 14, 6, 61, 53, 45, 37, 29, 21, 13, 5, 60, 52, 44, 36, 28, 20, 12, 4, 27, 19, 11, 3); PC2 : array [0..47] of Byte = (13, 16, 10, 23, 0, 4, 2, 27, 14, 5, 20, 9, 22, 18, 11, 3, 25, 7, 15, 6, 26, 19, 12, 1, 40, 51, 30, 36, 46, 54, 29, 39, 50, 44, 32, 47, 43, 48, 38, 55, 33, 52, 45, 41, 49, 35, 28, 31);
27 File #2 Next steps Load the malware in OllyDbg. Search for PC1 and use it to locate the address of InitEncryptDES, if present. Set a breakpoint in that address. Run the malware. Extract the key from the first parameter.
28 File #2 Finding PC1 (1)
29 File #2 Finding PC1 (2)
30 File #2 References To find references to PC1, we need to select its first byte (0x38) and press Ctrl+R.
31 File #2 - Beginning of the function Beginning of procedure InitEncryptDES
32 File #2 Running the malware
33 File #2 Which parameter? Remember the procedure signature is as follows: procedure InitEncryptDES( const Key : TKey64; var Context : TDESContext; Encrypt : Boolean); TKey64 definition: TKey64 = array [0..7] of Byte; A TKey64 value can not be stored by a single register in a 32-bit architecture.
34 File #2 Calling convention Delphi s calling convention (left-to-right): 1st parameter: EAX. 2nd parameter: EDX. 3rd parameter: ECX. Remaining parameters: stack.
35 File #2 Key address
36 File #2 Key value
37 File #2 Description of cipher Encryption algorithm: DES. Mode of operation: ECB. Key: K = 0xc24fa010744eb153
38 Alternative for finding keys A properly generated key is entropic. Information, on the other hand, is structured. Based on those facts, in 1999, Shamir and Someren 3 proposed a way of finding stored keys. The basic idea is to traverse memory and identify the region with more entropy. One way of doing that is to set a window size and count the number of different elements on each window.
39 File #3 Sample 50E96823#0851CDA207333E St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
40 File #3 Description of cipher Alphabet of definition: A = {0, 1, 2, 3,, 255} Plain text: M = m 0 m 1 m 2 m t-1, m i ϵ A Cipher text: C = c 0 c 1 c 2 c t-1, c i ϵ A Key: K = k 0 k 1 k 2 k 3 k 4 k 5 k 6 k 7 k 8 k 9 k 10 Encryption function: c i = m i + k (i mod 11) mod 256 Decryption function: m i = c i k (i mod 11) mod 256
41 References [1] Electronic Frontier Foundation, Cracking Des: Secrets of Encryption Research, Wiretap Politics & Chip Design, O Reilly Media, [2] Menezes, A., van Oorschot, P, and Vanstone, S., Handbook of Applied Cryptography, CRC Press, [3] Shamir, A. and van Someren,N., Playing Hide and Seek with Stored Keys, in FC 99 Proc. of the 3 rd Intl. Conference on Financial Cryptography, 1999.
42 Thank you for listening! Questions? Nelson Uto
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