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# -*- coding: binary -*-
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module Rex
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  module Exploit
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    class ViewState
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      class Error < Rex::RuntimeError
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      end
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      def self.decode_viewstate(encoded_viewstate, algo: 'sha1')
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        viewstate = Rex::Text.decode_base64(encoded_viewstate)
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        unless Rex::Text.encode_base64(viewstate) == encoded_viewstate
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          raise Error.new('Could not decode ViewState')
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        end
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        hmac_len = OpenSSL::Digest.new(algo).digest_length
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        if (data = viewstate[0...-hmac_len]).empty?
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          data = nil
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        end
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        hmac = viewstate[-hmac_len..-1]
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        unless hmac&.length == hmac_len
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          raise Error.new('Could not decode ViewState')
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        end
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        { data: data, hmac: hmac }
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      end
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      def self.generate_viewstate(data, extra: '', algo: 'sha1', key: '')
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        # Generate ViewState HMAC from known values and validation key
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        hmac = generate_viewstate_hmac(data + extra, algo: algo, key: key)
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        # Append HMAC to provided data and Base64-encode the whole shebang
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        Rex::Text.encode_base64(data + hmac)
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      end
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      def self.generate_viewstate_hmac(data, algo: 'sha1', key: '')
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        OpenSSL::HMAC.digest(algo, key, data)
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      end
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      def self.is_viewstate_valid?(encoded_viewstate, extra: '', algo: 'sha1', key: '')
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        viewstate = decode_viewstate(encoded_viewstate)
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        unless viewstate[:data]
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          raise Error.new('Could not retrieve ViewState data')
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        end
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        unless (their_hmac = viewstate[:hmac])
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          raise Error.new('Could not retrieve ViewState HMAC')
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        end
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        our_hmac = generate_viewstate_hmac(
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          viewstate[:data] + extra,
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          algo: algo,
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          key: key
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        )
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        # Do we have what it takes?
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        our_hmac == their_hmac
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      end
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      class << self
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        alias_method :can_sign_viewstate?, :is_viewstate_valid?
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      end
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    end
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  end
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end
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