Move encryption/decryption into a separate Crypto module

gem/Rakefile

@@ -1,5 +1,6 @@
 require "bundler/gem_tasks"
 require 'openssl'
+require 'metasploit-payloads/crypto'
 
 c_source = "../c/meterpreter/"
 java_source = "../java"
@@ -46,19 +47,6 @@ platform_config = {
   }
 }
 
-def chacha20_encrypt(contents: '')
-  cipher = ::OpenSSL::Cipher.new('chacha20')
-  cipher.encrypt
-  cipher.iv = 'EncryptedPayload'
-  cipher.key = 'Rapid7MetasploitEncryptedPayload'
-
-  encrypted_contents = 'encrypted_payload_chacha20_v1'
-  encrypted_contents << cipher.update(contents)
-  encrypted_contents << cipher.final
-
-  encrypted_contents
-end
-
 def copy_files(cnf, meterpreter_dest)
   cnf[:sources].each do |f|
     cnf[:extensions].each do |ext|
@@ -66,7 +54,7 @@ def copy_files(cnf, meterpreter_dest)
         target = File.join(meterpreter_dest, File.basename(bin))
         print("Copying: #{bin} -> #{target}\n")
         contents = ::File.binread(::File.expand_path(bin))
-        encrypted_contents = chacha20_encrypt(contents: contents)
+        encrypted_contents = ::MetasploitPayloads::Crypto.encrypt(plaintext: contents)
         ::File.binwrite(::File.expand_path(target), encrypted_contents)
       end
     end

gem/lib/metasploit-payloads.rb

@@ -3,6 +3,7 @@
 require 'openssl' unless defined? OpenSSL::Digest
 require 'metasploit-payloads/version' unless defined? MetasploitPayloads::VERSION
 require 'metasploit-payloads/error' unless defined? MetasploitPayloads::Error
+require 'metasploit-payloads/crypto' unless defined? MetasploitPayloads::Crypto
 
 #
 # This module dispenses Metasploit payload binary files
@@ -12,10 +13,6 @@ module MetasploitPayloads
   METERPRETER_SUBFOLDER = 'meterpreter'
   USER_DATA_SUBFOLDER   = 'payloads'
 
-  ENCRYPTED_PAYLOAD_HEADER = 'encrypted_payload_chacha20_v1'
-  CHACHA20_IV = 'EncryptedPayload' # 16 bytes
-  CHACHA20_KEY = 'Rapid7MetasploitEncryptedPayload' # 32 bytes
-
   #
   # @return [Array<Hash<String, Symbol>>] An array of filenames with warnings. Provides a file name and error.
   #  Empty if all needed Meterpreter files exist and have the correct hash.
@@ -157,26 +154,10 @@ module MetasploitPayloads
       raise e
     end
 
-    encrypted_file = file_contents.start_with?(ENCRYPTED_PAYLOAD_HEADER)
+    encrypted_file = file_contents.start_with?(Crypto::ENCRYPTED_PAYLOAD_HEADER)
     return file_contents unless encrypted_file
 
-    self.decrypt_payload(payload: file_contents)
-  end
-
-  def self.decrypt_payload(payload: '')
-    return payload unless payload.start_with?(ENCRYPTED_PAYLOAD_HEADER)
-
-    # Remove the header from the file.
-    encrypted_contents = payload.sub(ENCRYPTED_PAYLOAD_HEADER, '')
-    cipher = ::OpenSSL::Cipher.new('chacha20')
-    cipher.decrypt # Call before using .key
-    cipher.iv = CHACHA20_IV
-    cipher.key = CHACHA20_KEY
-
-    decrypted_contents = cipher.update(encrypted_contents)
-    decrypted_contents << cipher.final
-
-    decrypted_contents
+    Crypto.decrypt(ciphertext: file_contents)
   end
 
   #

gem/lib/metasploit-payloads/crypto.rb

@@ -0,0 +1,44 @@
+require 'openssl'
+
+module MetasploitPayloads
+  module Crypto
+    CIPHER_NAME = 'chacha20'.freeze
+    IV = 'EncryptedPayload'.freeze # 16 bytes
+    KEY = 'Rapid7MetasploitEncryptedPayload'.freeze # 32 bytes
+    ENCRYPTED_PAYLOAD_HEADER = 'encrypted_payload_chacha20_v1'.freeze
+
+    def self.encrypt(plaintext: '')
+      raise ::ArgumentError, 'Unable to encrypt plaintext: ' << plaintext, caller unless plaintext.to_s
+
+      cipher = ::OpenSSL::Cipher.new(CIPHER_NAME)
+
+      cipher.encrypt
+      cipher.iv = IV
+      cipher.key = KEY
+
+      output = ENCRYPTED_PAYLOAD_HEADER.dup
+      output << cipher.update(plaintext)
+      output << cipher.final
+
+      output
+    end
+
+    def self.decrypt(ciphertext: '')
+      raise ::ArgumentError, 'Unable to decrypt ciphertext: ' << ciphertext, caller unless ciphertext.to_s
+
+      cipher = ::OpenSSL::Cipher.new(CIPHER_NAME)
+
+      cipher.decrypt
+      cipher.iv = IV
+      cipher.key = KEY
+
+      # Remove encrypted header if present
+      ciphertext = ciphertext.sub(ENCRYPTED_PAYLOAD_HEADER, '')
+
+      output = cipher.update(ciphertext)
+      output << cipher.final
+
+      output
+    end
+  end
+end

gem/spec/metasploit_payloads/crypto_spec.rb

@@ -0,0 +1,22 @@
+require 'spec_helper'
+require 'metasploit-payloads'
+
+RSpec.describe ::MetasploitPayloads::Crypto do
+  describe '#encrypt' do
+    let(:encrypted_header) { "encrypted_payload_chacha20_v1".b }
+    let(:plaintext) { "Hello World!".b }
+    let(:ciphertext) { encrypted_header + "\\c\xB6N\x95\xE58\x8D\xDF\xBF4c".b }
+
+    it 'can encrypt plaintext' do
+      expect(described_class.encrypt(plaintext: plaintext)).to eq ciphertext
+    end
+
+    it 'can decrypt ciphertext' do
+      expect(described_class.decrypt(ciphertext: ciphertext)).to eq plaintext
+    end
+
+    it 'is idempotent' do
+      expect(described_class.decrypt(ciphertext: described_class.encrypt(plaintext: plaintext))).to eq plaintext
+    end
+  end
+end