Class: Rex::Encoder::Alpha2::Generic

Inherits:
Object
  • Object
show all
Defined in:
lib/rex/encoder/alpha2/generic.rb

Direct Known Subclasses

AlphaMixed, AlphaUpper, UnicodeMixed, UnicodeUpper

Class Method Summary collapse

Class Method Details

.add_terminator ⇒ Object

‘A’ signifies the end of the encoded shellcode



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# File 'lib/rex/encoder/alpha2/generic.rb', line 86

def Generic.add_terminator()
	'AA'
end

.default_accepted_chars ⇒ Object

Note: ‘A’ is presumed to be accepted, but excluded from the accepted characters, because it serves as the terminator



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# File 'lib/rex/encoder/alpha2/generic.rb', line 13

def Generic.default_accepted_chars ; ('a' .. 'z').to_a + ('B' .. 'Z').to_a + ('0' .. '9').to_a ; end

.encode(buf, reg, offset, badchars = '') ⇒ Object 



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# File 'lib/rex/encoder/alpha2/generic.rb', line 71

def Generic.encode(buf, reg, offset, badchars = '')
	encoded = gen_decoder(reg, offset)

	buf.each_byte {
		|block|

		encoded << encode_byte(block, badchars)
	}

	encoded << add_terminator()

	return encoded
end

.encode_byte(block, badchars) ⇒ Object 



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# File 'lib/rex/encoder/alpha2/generic.rb', line 32

def Generic.encode_byte(block, badchars)
	accepted_chars = default_accepted_chars.dup
	
	badchars.each_char {|c| accepted_chars.delete(c) } if badchars
	
	# No, not nipple.
	nibble_chars = Array.new(0x10) {[]}
	accepted_chars.each {|c| nibble_chars[c.unpack('C')[0] & 0x0F].push(c) }
	
	poss_encodings = []
	
	block_low_nibble = block & 0x0F
	block_high_nibble = block >> 4
	
	# Get list of chars suitable for expressing lower part of byte
	first_chars = nibble_chars[block_low_nibble]
	
	# Build a list of possible encodings
	first_chars.each do |first_char|
		first_high_nibble = first_char.unpack('C')[0] >> 4
	
		# In the decoding process, the low nibble of the second char gets combined
		# (either ADDed or XORed depending on the encoder) with the high nibble of the first char,
		# and we want the high nibble of our input byte to result
		second_low_nibble = gen_second(block_high_nibble, first_high_nibble) & 0x0F
		
		# Find valid second chars for this first char and add each combination to our possible encodings
		second_chars = nibble_chars[second_low_nibble]
		second_chars.each {|second_char| poss_encodings.push(second_char + first_char) }
	end
	
	if poss_encodings.empty?
		raise RuntimeError, "No encoding of #{"0x%.2X" % block} possible with limited character set"
	end
	
	# Return a random encoding
	poss_encodings[rand(poss_encodings.length)]
end

.gen_decoder(reg, offset) ⇒ Object 



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# File 'lib/rex/encoder/alpha2/generic.rb', line 22

def Generic.gen_decoder(reg, offset)
	# same as above
	return ''
end

.gen_decoder_prefix(reg, offset) ⇒ Object 



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# File 'lib/rex/encoder/alpha2/generic.rb', line 15

def Generic.gen_decoder_prefix(reg, offset)
	# Should never happen - have to pick a specifc
	# encoding:
	# alphamixed, alphaupper, unicodemixed, unicodeupper
	''
end

.gen_second(block, base) ⇒ Object 



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# File 'lib/rex/encoder/alpha2/generic.rb', line 27

def Generic.gen_second(block, base)
	# XOR encoder for ascii - unicode uses additive
	(block^base)
end