Day 12 - Elfcryption

Date: 02, June, 2021

Author: Dhilip Sanjay S

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GPG

• GPG uses the AES algorithm to encrypt your files

• To encrypt a file:

  • gpg -c data.txt

• To decrypt a file:

  • gpg -d data.txt

• Decrypting requires the password

Open SSL

• To generate a private key we use the following command (8912 creates the key 8912 bits long):

  • openssl genrsa -aes256 -out private.key 8912

• To generate a public key we use our previously generated private key:

  • openssl rsa -in private.key -pubout -out public.key

• Lets now encrypt a file (plaintext.txt) using our public key:

  • openssl rsautl -encrypt -pubin -inkey public.key -in plaintext.txt -out encrypted.txt

• Now, if we use our private key, we can decrypt the file and get the original message:

  • openssl rsautl -decrypt -inkey private.key -in encrypted.txt -out plaintext.txt

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Questions

What is the md5 hashsum of the encrypted note1 file?

• Answer: 24cf615e2a4f42718f2ff36b35614f8f

• Steps to Reproduce:

md5sum note1.txt.gpg 
24cf615e2a4f42718f2ff36b35614f8f  note1.txt.gpg

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Where was elf Bob told to meet Alice?

• Answer:

• Steps to Reproduce:

  • The gpg key is 25daysofchristmas

gpg -d note1.txt.gpg 
gpg: AES.CFB encrypted data
gpg: encrypted with 1 passphrase
I will meet you outside Santa's Grotto at 5pm!

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Decrypt note2 and obtain the flag!

• Answer:

• Steps to Reproduce:

  • The passphrase for private.key is hello

$ openssl rsautl -decrypt -inkey private.key -in note2_encrypted.txt -out note2.txt 
Enter pass phrase for private.key:

$ cat note2.txt 
THM{ed9ccb6802c5d0f905ea747a310bba23}

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