one recv gets two or more send in python sockets

Question

this is a simple socket program which has a server and some clients. clients send their texts encrypted by a simple RSA cryptography then the server side decrypts the sentence and then sends the decrypted sentence back to the client.

server:

import socket
import sys
from thread import *
from math import *
from random import *
import random
HOST = ''   
PORT = 8888
size=2**16
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
print 'Socket created'

#Bind socket to local host and port
try:
    s.bind((HOST, PORT))
except socket.error , msg:
    print 'Bind failed. Error Code : ' + str(msg[0]) + ' Message ' + msg[1]
    sys.exit()

print 'Socket bind complete'

#Start listening on socket
s.listen(10)
print 'Socket now listening'

def decoder(codedString,d,n):
    breakcoded=[]
    coded=(codedString)
    #print coded;
    for i in range (len(coded)):
            breakcoded.append(chr(((int(coded[i])**d) % n)+48))
    stri= ""
    for i in range (len(breakcoded)):
            stri+=breakcoded[i]
    return stri

#Function for handling connections. This will be used to create threads
def clientthread(conn):
    #Sending message to connected client
    conn.send('Welcome to the server. Type something and hit enter\n') #send only takes string
    #infinite loop so that function do not terminate and thread do not end.
    while True:    
    #Receiving from client
        data = conn.recv(1024)
##        data=s.recv(size)
    l = int (data)
    #print l
    coded=[]
    i=0
    data1=conn.recv(size)
print 'Recieved n: ',data1
n = int (data1)
data2=conn.recv(size)
print 'Recieved d: ',data2
d = int (data2)
for i in range (l):
            data3=conn.recv(size)
            #print 'Recieved: ',data3
            print
            coded.append(data3)       
    print 'coded string has been recieved....'
    print ('coded string: ' , coded)
d= decoder(coded,d,n)
print d        
    reply = 'OK... your message decrypted as: ' + d
    if not d:
        break
    conn.sendall(reply)
#came out of loop
conn.close()
#now keep talking with the client
while 1:
    #wait to accept a connection - blocking call
    conn, addr = s.accept()
    print 'Connected with ' + addr[0] + ':' + str(addr[1])

    #start new thread takes 1st argument as a function name to be run, second is the tuple of     arguments to the function.
    start_new_thread(clientthread ,(conn,))

s.close()

client:

import socket
from math import *
from random import *
host='localhost'
port=8888
size=2**16
def declare():
        a = sample([5,3],2)
        return (a[0],a[1])
def coder(input_message):
        (p,q)=declare()
        for i in range (1):
                p=2**p-1
        for i in range (1):
                q=2**q-1
        #print p
        #print q
        #print ("n= ",p*q)
        #print a
        def gcd(a,b):
                if a%b==0:
                    return b
                elif a%b > 0:
                    s=a%b
                        return gcd(b,s)
                else:
                    raise ValueError
        n=p*q
        phi=(p-1)*(q-1)
        e=2
        while gcd(phi,e)!=1:
                e+=1
        d=1
        while ((e*d)%phi)!=1:
                d+=1
        public_key=(n,e)
        special_key=(n,d)
        ListOfAsciis=[]
        coded=[]
        breakcoded=[]
        for i in input_message:
                ListOfAsciis.append(ord(i)-48)
        for j in ListOfAsciis:
                coded.append((j**e)%n)
        #print ("e= ",e)
        #print ("d= ",d)
        #print ("coded= ",coded)
        for i in coded:
                breakcoded.append(chr(((i**d) % n)+48))
        #print ('n= ' , n)
        #print str(coded)
        #print coded
        return (d,n,str(coded[0]))
def decoder(codedString,d,n):
        #input_d= input("please enter your private key d: ")
        #input_n= input("please enter your private key n: ")
        #d = int (input_d)
        #n = int (input_n)
        breakcoded=[]
        coded=(codedString)
        print coded;
        for i in range (len(coded)):
                breakcoded.append(chr(((int(coded[i])**d) % n)+48))

        stri= ""
        for i in range (len(breakcoded)):
                stri+=breakcoded[i]
        return stri
s=socket.socket(socket.AF_INET,socket.SOCK_STREAM)
print 'socket created'
s.connect((host,port))
print 'connected'
data=s.recv(size)
print 'Recieved: ', data
while True:
    input_message= raw_input("please enter your message: ")
    message = list(input_message)
    s.send(str(len(message)))
##  (p,q)=declare()
    #n=p*q
    (d,n,c)=coder('i')

    n=str(n)
print "                  ",
print "                  ",
print "                  ",
print "                  ",
s.send(n)
print "                                                            ",
d=str(d)
s.send((d))
print "   ",
print "   ",
print "   ",
for i in range (len(message)):
            (d,n,c)=coder(input_message[i])
            print "                                                           ",
            print "   ",
            print "   ",
            s.send((c))
    print 'coded string has been sent to the server....'
    data=s.recv(size)
    print 'Recieved: ', data

now the problem is that the program sometimes works correctly and sometimes not! in false cases the server side gets two send items by the client by one recv. what should i

Answer 1

This is an inherent part of TCP. Stream sockets are byte streams, not message streams.

So, things are working exactly as they're supposed to. If you want to send a sequence of messages over a TCP stream, it's exactly the same problem as saving a sequence of objects to a file—you need some way of delimiting the messages. This could be as simple as using a stream of text, where newlines delimit the messages, or it could be a complex protocol, but it has to be something.

See this blog post for more detail.