# File lib/net/ssh/transport/packet_stream.rb, line 185
185:       def poll_next_packet
186:         if @packet.nil?
187:           minimum = server.block_size < 4 ? 4 : server.block_size
188:           return nil if available < minimum
189:           data = read_available(minimum)
190: 
191:           # decipher it
192:           @packet = Net::SSH::Buffer.new(server.update_cipher(data))
193:           @packet_length = @packet.read_long
194:         end
195: 
196:         need = @packet_length + 4 - server.block_size
197:         raise Net::SSH::Exception, "padding error, need #{need} block #{server.block_size}" if need % server.block_size != 0
198: 
199:         return nil if available < need + server.hmac.mac_length
200: 
201:         if need > 0
202:           # read the remainder of the packet and decrypt it.
203:           data = read_available(need)
204:           @packet.append(server.update_cipher(data))
205:         end
206: 
207:         # get the hmac from the tail of the packet (if one exists), and
208:         # then validate it.
209:         real_hmac = read_available(server.hmac.mac_length) || ""
210: 
211:         @packet.append(server.final_cipher)
212:         padding_length = @packet.read_byte
213: 
214:         payload = @packet.read(@packet_length - padding_length - 1)
215:         padding = @packet.read(padding_length) if padding_length > 0
216: 
217:         my_computed_hmac = server.hmac.digest([server.sequence_number, @packet.content].pack("NA*"))
218:         raise Net::SSH::Exception, "corrupted mac detected" if real_hmac != my_computed_hmac
219: 
220:         # try to decompress the payload, in case compression is active
221:         payload = server.decompress(payload)
222: 
223:         debug { "received packet nr #{server.sequence_number} type #{payload.getbyte(0)} len #{@packet_length}" }
224: 
225:         server.increment(@packet_length)
226:         @packet = nil
227: 
228:         return Packet.new(payload)
229:       end