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Day 12 - ElfCryption

Room Level

Easy

Challenge description ​

You think the Christmas Monster is intercepting and reading your messages! Elf Alice has sent you an encrypted message. Its your job to go and decrypt it!

Challenge Overview ​

We are provided with three files:

  • note1.txt.gpg
  • note2_encrypted.txt
  • private.key

Our objective is to answer the following questions based on these files.

Task 1: What is the MD5 hash of the encrypted note1 file? ​

To retrieve the MD5 hash of the note1.txt.gpg file, we can use the md5sum command as shown below:

bash
md5sum note1.txt.gpg

md5sum output

Task 2: Where was elf Bob told to meet Alice? ​

Hint: The GPG key passphrase is 25daysofchristmas.

To decrypt the note1.txt.gpg file, we use the gpg command and supply the provided passphrase:

bash
gpg --batch --passphrase 25daysofchristmas --output note1.txt --decrypt note1.txt.gpg

After decryption, the contents of note1.txt reveal the meeting location.

gpg decryption output

Task 3: Decrypt note2 and obtain the flag ​

Hint: The private key password is hello.

Using the private.key file, we can decrypt note2_encrypted.txt with the openssl command. The decryption process is as follows:

bash
openssl rsautl -decrypt -inkey private.key -in note2_encrypted.txt -out note2.txt

When prompted, we enter the passphrase hello. Upon successful decryption, the flag can be found in the note2.txt file.

openssl decryption output