# File lib/net/ssh/transport/packet_stream.rb, line 188
188:       def poll_next_packet
189:         if @packet.nil?
190:           minimum = server.block_size < 4 ? 4 : server.block_size
191:           return nil if available < minimum
192:           data = read_available(minimum)
193: 
194:           # decipher it
195:           @packet = Net::SSH::Buffer.new(server.update_cipher(data))
196:           @packet_length = @packet.read_long
197:         end
198: 
199:         need = @packet_length + 4 - server.block_size
200:         raise Net::SSH::Exception, "padding error, need #{need} block #{server.block_size}" if need % server.block_size != 0
201: 
202:         return nil if available < need + server.hmac.mac_length
203: 
204:         if need > 0
205:           # read the remainder of the packet and decrypt it.
206:           data = read_available(need)
207:           @packet.append(server.update_cipher(data))
208:         end
209: 
210:         # get the hmac from the tail of the packet (if one exists), and
211:         # then validate it.
212:         real_hmac = read_available(server.hmac.mac_length) || ""
213: 
214:         @packet.append(server.final_cipher)
215:         padding_length = @packet.read_byte
216: 
217:         payload = @packet.read(@packet_length - padding_length - 1)
218:         padding = @packet.read(padding_length) if padding_length > 0
219: 
220:         my_computed_hmac = server.hmac.digest([server.sequence_number, @packet.content].pack("NA*"))
221:         raise Net::SSH::Exception, "corrupted mac detected" if real_hmac != my_computed_hmac
222: 
223:         # try to decompress the payload, in case compression is active
224:         payload = server.decompress(payload)
225: 
226:         debug { "received packet nr #{server.sequence_number} type #{payload.getbyte(0)} len #{@packet_length}" }
227: 
228:         server.increment(@packet_length)
229:         @packet = nil
230: 
231:         return Packet.new(payload)
232:       end