There are plenty of articles all over the Internet talking about how most people are dangerously bad at crypto, and most people new to the field often fall victim to common pitfalls.
I'm new to crypto, but I've spent hours researching this and I cannot for the life of of me seem to come up with something that could be insecure about the following implementation (barring a bug in the operating system, the programming language, etc.)
What's insecure about this?
Use Case: Storing files in a cloud environment and preventing anyone who gets access to those files from viewing them. I do not care about people knowing how many files I have, or what their sizes are, etc. I only want to protect the contents.
Implementation:
- For each file, generate a random AES key and IV.
- Encrypt the file using AES-CTR, and rename the file to a randomly generated filename.
- Store the encrypted files in cloud environment.
- Store a plaintext file locally that has an index of filenames, keys, and IVs.
Now, as long as nobody ever gets a copy of my local plaintext file, can I assume that the data stored in the cloud is secure?
Example Code
require 'openssl'
# Do the following for each file...
infile = "...input filename..."
outfile = "~/Dropbox/randomly_generated_name"
cipher = OpenSSL::Cipher::AES.new(128, :CTR)
cipher.encrypt
key = cipher.random_key
iv = cipher.random_iv
input = File.open(infile, 'rb')
output = File.open(outfile, 'wb')
while !input.eof? do
chunk = input.read(100000)
encrypted = cipher.update(chunk)
output.write(encrypted)
end
input.close
output.close
store_locally_only = {
infile: infile,
outfile: outfile,
key: key,
iv: iv
}
File.open('~/local_only/private.txt', 'a') do |f|
f.write(store_locally_only.to_json)
end
Now, clearly if my private.txt became compromised, this would give an attacker access to everything that was in my Dropbox. But, assuming that this file as appropriately kept private, can I assume that the data that is stored in Dropbox is secure?
To be clear, I don't mean this post to be an "audit" of my simple code above. Rather, I am trying to understand something fundamental about crypto here. A lot of people say that it is "dangerously hard" to implement crypto. But this implementation seems so simple, that I am unable to think of what would compromise the data.
