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+---- Thread: Help Decrypt / Bruteforce DES Ciphertext (/showthread.php?tid=5999324)
Help Decrypt / Bruteforce DES Ciphertext - Yell0w - Aug 15, 2019
I've been given ciphertext which has been encrypted with DES.
Ciphertext:
Code
0x ce 12 6d 2d df 2d 1e 64I have also been given a hint on what the final section of the DES encrypt key is:
Code
0x ZZ ZZ ZZ ZZ ZZ e5 f6 66Where ZZ are the components in which I need to calculate / find out for the correct key.
I am also told that the plaintext is all capital letters (8 capital letters, 2 4 letter words with no space)
I have written a basic program in C# which attempts to decrypt the cipher and then check if the decryption is only capital letters but it is seems to be relatively slow. (I used a C# DES Encrypt/Decrypt String [altered to reference my hex cipher text] using DES example from CodeProject to build my solution)[cannot post links as I do not have 50 posts]
I was wondering if anyone has a better or faster approach into helping me to acquire the correct key.
Many thanks!
RE: Help Decrypt / Bruteforce DES Ciphertext - AbakBarama - Aug 15, 2019
(Aug 15, 2019 07:28 AM)Yell0w Wrote: I was wondering if anyone has a better or faster approach into helping me to acquire the correct key.
I suppose you'll find a lot of keys, not single key.
Full number of 8-bytes combinations: 18 446 744 073 709 551 616.
Number of combinations with capital letters only: 208 827 064 576.
Approx. 1 of 88 335 025 combinations will contain capital letters only.
5 unknown bytes from key, 7 unknown bits in each byte --> 34 359 738 368 attempts are needed.
Brutforcing all keyspace will give approx. 389 different keys with valid plaintexts.
--------------
Average speed on CPU is about 1e6..2e6 keys per sec.
Estimated time for your task will be 10-20 hours.
RE: Help Decrypt / Bruteforce DES Ciphertext - Yell0w - Aug 15, 2019
(Aug 15, 2019 09:31 AM)AbakBarama Wrote: I suppose you'll find a lot of keys, not single key.
Full number of 8-bytes combinations: 18 446 744 073 709 551 616.
Number of combinations with capital letters only: 208 827 064 576.
Approx. 1 of 88 335 025 combinations will contain capital letters only.
5 unknown bytes from key, 7 unknown bits in each byte --> 34 359 738 368 attempts are needed.
Brutforcing all keyspace will give approx. 389 different keys with valid plaintexts.
--------------
Average speed on CPU is about 1e6..2e6 keys per sec.
Estimated time for your task will be 10-20 hours.
I'm getting an exception thrown when I decrypt with the wrong key, so I'm using a try catch approach, with nothing inside the catch to pass this.
I believe the try catch is what's making my code operate slow as I've been running it for over 1 day.
I'm not sure how to brute force in an efficient / quick manner.
RE: Help Decrypt / Bruteforce DES Ciphertext - 991z3r0 - Aug 15, 2019
(Aug 15, 2019 05:44 PM)Yell0w Wrote: I'm getting an exception thrown when I decrypt with the wrong key, so I'm using a try catch approach, with nothing inside the catch to pass this.
I believe the try catch is what's making my code operate slow as I've been running it for over 1 day.
I'm not sure how to brute force in an efficient / quick manner.
Split the work up into many threads, that should help with processing speed.
RE: Help Decrypt / Bruteforce DES Ciphertext - AbakBarama - Aug 16, 2019
(Aug 15, 2019 05:44 PM)Yell0w Wrote: I'm not sure how to brute force in an efficient / quick manner.
I've tried to make bruteforcer on C#, it works 50-100 times slower then C program. I don't explain such a huge difference.
If you want to write your own C/C++ bruteforcer then you can easily find DES C sources, for example GitHub - schwabe/polarssl: A portable, easy to use, readable and flexible SSL library (dual-licensed)
Some valid keys and plaintexts:
00 00 B0 56 82 E5 F6 66 --> LIFOBHJU
00 B8 7E 28 8A E5 F6 66 --> NVAZLXVC
00 BC 7E 34 18 E5 F6 66 --> HRVYUICA
00 C2 EE 6E 5C E5 F6 66 --> KISSVGKM
00 EE C0 4A EC E5 F6 66 --> TBELFEAL
Estimated number of valid results is about 400.
RE: Help Decrypt / Bruteforce DES Ciphertext - Yell0w - Aug 16, 2019
(Aug 16, 2019 06:23 AM)AbakBarama Wrote: I've tried to make bruteforcer on C#, it works 50-100 times slower then C program. I don't explain such a huge difference.
If you want to write your own C/C++ bruteforcer then you can easily find DES C sources, for example
Some valid keys and plaintexts:
00 00 B0 56 82 E5 F6 66 --> LIFOBHJU
00 B8 7E 28 8A E5 F6 66 --> NVAZLXVC
00 BC 7E 34 18 E5 F6 66 --> HRVYUICA
00 C2 EE 6E 5C E5 F6 66 --> KISSVGKM
00 EE C0 4A EC E5 F6 66 --> TBELFEAL
Estimated number of valid results is about 400.
I've also been told that the plaintext is 2 4 letter words. Eg BLUETREE. I can't send you a PM as I don't reach the post requirements, I was wondering if you could send me your bruteforcer on C# as I've noticed you've gotten some valid results (ie could you PM me etc?)
RE: Help Decrypt / Bruteforce DES Ciphertext - AbakBarama - Aug 17, 2019
(Aug 16, 2019 09:59 PM)Yell0w Wrote: could you PM me etc?
PM sent.