Add Technical/picoCTF/t1/q4.md

Technical/picoCTF/t1/q4.md

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+# Question: Verify
+### Key: picoCTF{trust_but_verify_2cdcb2de}
+
+**Core concept: SHA256 Hash**
+
+We're given a directory full of files, a checksum.txt file, and a ./decrypt.sh. 
+
+First, I checked the contents of the checksum.txt file. It contained a SHA256 hash of some file in the directory. 
+Since all the files were in the same directory, I just generaterdid the SHA256 hashes of the entire directory using `sha256sum files/*`.
+
+I copied the hash from the checksum.txt file and pasted it alongside the sha256sum command, to filter the contents using `grep`.
+
+```bash
+ctf-player@pico-chall$ sha256sum files/* | grep 55b983afdd9d10718f1db3983459efc5cc3f5a66841e2651041e25dec3efd46a
+55b983afdd9d10718f1db3983459efc5cc3f5a66841e2651041e25dec3efd46a  files/2cdcb2de
+```
+Then, I ran the ./decrypt.sh script with the file I found in the checksum.txt file.
+
+```bash
+ctf-player@pico-chall$ ./decrypt.sh files/2cdcb2de
+picoCTF{trust_but_verify_2cdcb2de}
+```
+
+Voila! Found the flag!
+
+Output: picoCTF{trust_but_verify_2cdcb2de}