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As an exercise we need to reverse a SHA-1 to get the 'original' message.

All we know is that the lowercase SHA-1 of the code gives:

B39ECFBC2C64ADBB7C7A9292EEE31794D28FE224

And, the SHA-1 of the case sensitive code should be:

0D353038908AD0FC8C51A5312BB3E2FEE1CDDF83

And, the broken original message (letters can be upper or lowercase):

HV15-G.UJ-1.Q7-DYC2-WLRE-6..J

So, my question is, how can I reverse the hashes to get the full original message?

Any tips for me?

4
  • probably brute force Dec 9, 2015 at 15:29
  • I think there needs to be an other way :/
    – Ihara
    Dec 9, 2015 at 15:33
  • 1
    This question should be asked on Crypto SE... You should have more accurate answers.
    – perror
    Dec 9, 2015 at 15:38
  • 1
    If there are only 4 missing letters (in the dots) then you can brute force the lowercase one with at max (36^4 = ) 1.6 million guesses Dec 9, 2015 at 15:38

1 Answer 1

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By design, SHA-1 can't be reversed.

The best approach would be to brute-force the lowercase version, and then once you've found the missing characters, use them to brute-force the case-sensitive version.

However, according to oclhashcat, there's no solution to this. See below:

C:\>oclHashcat64.exe -m 100 -a 3 -1 ?l?d B39ECFBC2C64ADBB7C7A9292EEE31794D28FE224 hv15-g?1uj-1?1q7-dyc2-wlre-6?1?1j
oclHashcat v2.00 starting...

Device #1: Hawaii, 4096MB, 947Mhz, 40MCU

Hashes: 1 hashes; 1 unique digests, 1 unique salts
Bitmaps: 16 bits, 65536 entries, 0x0000ffff mask, 262144 bytes, 5/13 rotates
Applicable Optimizers:
* Zero-Byte
* Precompute-Init
* Precompute-Merkle-Demgard
* Early-Skip
* Not-Salted
* Not-Iterated
* Single-Hash
* Single-Salt
* Brute-Force
* Scalar-Mode
* Raw-Hash
Watchdog: Temperature abort trigger set to 90c
Watchdog: Temperature retain trigger set to 80c
Device #1: Kernel ./kernels/4098/m00100_a3.Hawaii_1912.5_1912.5 (VM)_1449240640.kernel (196520 bytes)
Device #1: Kernel ./kernels/4098/markov_be_v1.Hawaii_1912.5_1912.5 (VM)_1449240640.kernel (36168 bytes)


INFO: approaching final keyspace, workload adjusted


Session.Name...: oclHashcat
Status.........: Exhausted
Input.Mode.....: Mask (hv15-g?1uj-1?1q7-dyc2-wlre-6?1?1j) [29]
Hash.Target....: b39ecfbc2c64adbb7c7a9292eee31794d28fe224
Hash.Type......: SHA1
Time.Started...: 0 secs
Time.Estimated.: 0 secs
Speed.GPU.#1...:   106.5 MH/s
Recovered......: 0/1 (0.00%) Digests, 0/1 (0.00%) Salts
Progress.......: 1679616/1679616 (100.00%)
Rejected.......: 0/1679616 (0.00%)
HWMon.GPU.#1...: 19% Util, 53c Temp, 40% Fan

Started: Wed Dec 09 10:18:50 2015
Stopped: Wed Dec 09 10:18:52 2015
4
  • is there a way to give oclhashcat the original code in order to use it for the bruteforce?
    – Ihara
    Dec 9, 2015 at 15:44
  • Yes, see my updated answer above. Dec 9, 2015 at 16:20
  • hm strange, maybe I messed something up with the original message, since we got it as an audio
    – Ihara
    Dec 9, 2015 at 16:32
  • On the bright side, now you know how to crack it, and you also see how fast brute-force can be for this scenario -- less than 2 seconds of processing! :) Dec 9, 2015 at 16:35

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