How to decrypt a '.enc' file that has been encrypted with RSA using a public key? While solving a CTF challenge online, I came across a situation where I needed to decrypt a '.enc' file that has been encrypted using someone's public key. The corresponding public key '.pem' file was provided to me. I solved almost all other CTF challenges except this one and it has been bugging me since. I solved the first part: I have been able to decode a hint that says: "The file file.enc could be decrypted with a private key. Using raw algorithm, every 64 bits chunk of data was encrypted at a time. You can do the same thing to decrypt using RSA."

Here's what I have tried:

Read many posts like this one online and tried to decrypt the '.enc' file: http://www.czeskis.com/random/openssl-encrypt-file.html

I tried the following command for decryption: openssl rsautl -decrypt -in public_key.pem -out key.bin. I got the this error:

 no keyfile specified
 unable to load Private Key


Yes, but that's because private key is not provided to me. If I try to use the public_key.pem as infile, I get this error:

unable to load Private Key
139914348455592:error:0906D06C:PEM routines:PEM_read_bio:no start line:pem_lib.c:703:Expecting: ANY PRIVATE KEY


Tried the tool 'openssl' in Linux in several ways to try to decrypt the file.

Here is the 3 files zipped in tarball, if anyone wants to analyse them: http://www.filedropper.com/chaltar

Could someone please advise me on how to decrypt the '.enc' file before I give up on this challenge? I have worked long and broken the first part of the challenge, now I feel I'm so close. 

Facts:


I know the tool 'openssl' will be used to solve this challenge.
Encryption: RSA 
Files: Only three files, public_key.pem, file.enc,
and a random file that I decrypted that provided the hint I wrote
above.
openssl rsa -pubin -inform PEM -text -noout &lt; public_key.pem
Public-Key: (64 bit)
Modulus: 16513720463601767803 (0xe52c8544a915157b)
Exponent: 65537 (0x10001)


The modulus is small enough that you can easily factor it

After finding the prime factors, you can calculate the private exponent

After you have the private exponent, you raise each 64-bit block of ciphertext to the private exponent mod the modulus.