Class: Origami::Encryption::RC4

Inherits:
Object
  • Object
show all
Defined in:
lib/origami/encryption.rb

Overview

Class wrapper for the RC4 algorithm.

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(key) ⇒ RC4

Creates and initialises a new RC4 generator using given key



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# File 'lib/origami/encryption.rb', line 538

def initialize(key)
  @key = key
end

Class Method Details

.decrypt(key, data) ⇒ Object

Decrypts data using the given key



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# File 'lib/origami/encryption.rb', line 531

def self.decrypt(key, data)
  RC4.new(key).decrypt(data)
end

.encrypt(key, data) ⇒ Object

Encrypts data using the given key



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# File 'lib/origami/encryption.rb', line 524

def self.encrypt(key, data)
  RC4.new(key).encrypt(data)
end

Instance Method Details

#cipher(data) ⇒ Object Also known as: encrypt, decrypt

Encrypt/decrypt data with the RC4 encryption algorithm RC4 is part of the PDF specification, so we need to support it



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# File 'lib/origami/encryption.rb', line 546

def cipher(data)
  return +'' if data.empty?

  # Enable RC4 cipher in OpenSSL security level
  # This is needed for PDF compatibility as per PDF 1.7 specification
  begin
    # Try to lower security level temporarily to enable legacy ciphers
    original_security_level = begin
      OpenSSL::SSL.send(:security_level)
    rescue
      nil
    end
    if original_security_level
      begin
        OpenSSL::SSL.send(:security_level=, 0)
      rescue
        nil
      end
    end

    # Now try to use RC4
    rc4 = OpenSSL::Cipher.new('rc4').encrypt
    rc4.key_len = @key.length
    rc4.key = @key
    result = rc4.update(data) + rc4.final

    # Restore security level
    if original_security_level
      begin
        OpenSSL::SSL.send(:security_level=, original_security_level)
      rescue
        nil
      end
    end

    return result
  rescue
    # If we can't use OpenSSL's RC4 for any reason, silently fall back to pure Ruby implementation
  end

  # Pure Ruby RC4 implementation as fallback
  s = (0..255).to_a
  key_bytes = @key.bytes
  key_length = key_bytes.length
  j = 0

  # Key-scheduling algorithm (KSA)
  256.times do |i|
    j = (j + s[i] + key_bytes[i % key_length]) % 256
    s[i], s[j] = s[j], s[i]
  end

  # Pseudo-random generation algorithm (PRGA)
  result = +""
  i = j = 0
  data.each_byte do |byte|
    i = (i + 1) % 256
    j = (j + s[i]) % 256
    s[i], s[j] = s[j], s[i]
    k = s[(s[i] + s[j]) % 256]
    result << (byte ^ k).chr
  end

  result
end