English [Sharif University CTF 2016] [Reverse 50 – dMd] Write Up

Description

Flag is : The valid input

The file is a: dMd: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.24, BuildID[sha1]=2643fecd383362fe9593ef8605a9ce882a85a38a, not stripped

Resolution

Launching the binary ask for a validation code.

We jump right into IDA directly to see if we can get something interesting from the decompiled code and we find that:

dMd

We convert that into an ascii string which is: 780438d5b6e29db0898bc4f0225935c0, we send try to valid with it, unsuccessfull.
Reverting the MD5 value and we get another hash which is: b781cbb29054db12f88f08c6e161c199, this time it validates the binary and we have the flag!

Flag is: b781cbb29054db12f88f08c6e161c199

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