Add at rest encryption to Meterpreter payloads

[sjanusz-r7]

This PR is requires rapid7/metasploit-payloads#679
The change to client_core.rb is necessary to support encrypted stdapi libraries.
Other changes were made to support encrypted file contents being added to zip/jar/archive files by using ::MetasploitPayloads.read which will handle encrypted/plain-text payloads as #add_files cannot take in a buffer, and we have to resort to calling #add_file for each file with the contents instead.

Verification

• Start msfconsole
• Use a payload, for example use payload/python/meterpreter/reverse_tcp
• Ensure you can get get a session as usual
• When in irb, check that the payload being read is stored as an encrypted file on disk
  • path = ::MetasploitPayloads.path('meterpreter', 'meterpreter.py')
  • raw_contents = ::File.binread(path); nil
  • raw_contents[0..5] == "msf\x01\x01\x01"
  • decrypted_contents = ::MetasploitPayloads.read('meterpreter', 'meterpreter.py')

[sempervictus]

Is there some place where we can find a bit of conext for the intended use of this functionality

[adfoster-r7]

The title was a bit of a misnomer. We're just adding at rest encryption for the Meterpreter payloads on disk, the changes should be transparent for the average user

[sempervictus]

Pesky AVs not appreciating users having the files on their machines?
Might be interesting to have runtime decryption on the other side for conditional loading/clearing of extensions when not in use to an encrypted read-only "inert code cave" (or just from remote again) for reloading on-demand. Might help reduce our runtime fingerprints in-memory.

[sempervictus]

Is there a facility for dumping the raw DLLs? Other tools might want access to them...

[adfoster-r7]

Suggested change

	encrypted_dll = ::File.binread(library_path)
	dll = ::MetasploitPayloads::Crypto.decrypt(ciphertext: encrypted_dll)
	dll = ::File.binread(library_path)

[adfoster-r7]

Release Notes

Add at rest encryption to Meterpreter payloads on the Metasploit host machine's file system