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README.md

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+Diffie-Hellman key exchange example in python
+
+You can use as reference OpenSSL's dhparam module that can produce following output, e.g.
+
+```bash
+openssl dhparam 16 |openssl dhparam -noout -text
+Generating DH parameters, 16 bit long safe prime, generator 2
+This is going to take a long time
+.++*++*++*++*++*++*++*++*++*++*++*++*++*++*++*++*++*++*++*++*++*++*++*++*++*++*++*
+    PKCS#3 DH Parameters: (16 bit)
+        prime: 61547 (0xf06b)
+        generator: 2 (0x2)
+```

dhtest.py

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+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+#
+#                   -------------------------------------
+#    Diffie-Hellman key exchange example
+#    Copyright (C) 2014  Andrey Arapov
+#
+#    This program is free software: you can redistribute it and/or modify
+#    it under the terms of the GNU General Public License as published by
+#    the Free Software Foundation, either version 3 of the License, or
+#    (at your option) any later version.
+#
+#    This program is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU General Public License for more details.
+#
+#    You should have received a copy of the GNU General Public License
+#    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#                   -------------------------------------
+#
+#
+# Notes:
+#   - Based on the https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange
+#   - The parameters used here are artificially small
+#   - I've tried to apply KISS principle here
+#
+#
+
+import random
+from fractions import gcd
+
+class bcolors:
+        RED = '\033[91m'
+        DRED = '\033[31m'
+        GREEN = '\033[92m'
+        YELLOW = '\033[93m'
+        BLUE = '\033[94m'
+        PURPLE = '\033[95m'
+        CYAN = '\033[96m'
+        ENDC = '\033[0m'
+
+
+# Primality test
+# https://en.wikipedia.org/wiki/Primality_test#Python_implementation
+
+def is_prime(num):
+    if num <= 3:
+        if num <= 1:
+            return False
+        return True
+    if not num % 2 or not num % 3:
+        return False
+    for i in range(5, int(num ** 0.5) + 1, 6):
+        if not num % i or not num % (i + 2):
+            return False
+    return True
+
+print ":: Diffie-Hellman key exchange"
+print
+
+# Alice and Bob agree to use a prime number p and base random g.
+
+print "1. Alice and Bob agree to use a prime number"+bcolors.BLUE+" p"+bcolors.ENDC+\
+	 " and base random "+bcolors.YELLOW+"g."+bcolors.ENDC
+i = 0
+while i < 1:
+        rand = random.randint(100000, 999999)
+        if is_prime(rand):
+                p=rand
+                i += 1
+
+
+g = random.randint(1000000, 9999999)
+
+print bcolors.BLUE+"p"+bcolors.ENDC+" =", bcolors.BLUE, p, bcolors.ENDC, "\tprime?", is_prime(p)
+print bcolors.YELLOW+"g"+bcolors.ENDC+" =", bcolors.YELLOW, g, bcolors.ENDC
+print
+
+
+print "2. Alice chooses a secret integer "+bcolors.DRED+"a"+bcolors.ENDC+\
+	 ", then sends Bob "+bcolors.CYAN+"A"+bcolors.ENDC+" = "+bcolors.YELLOW+"g"+bcolors.ENDC+\
+	" ^ "+bcolors.DRED+"a"+bcolors.ENDC+" mod "+bcolors.BLUE+"p"+bcolors.ENDC
+a = random.randint(10000, 99999)
+print bcolors.DRED+"a"+bcolors.ENDC+" =", bcolors.DRED, a, bcolors.ENDC
+A = (g ** a) % p
+print bcolors.CYAN+"A"+bcolors.ENDC+" =", bcolors.YELLOW, g, bcolors.ENDC, "^", \
+	bcolors.DRED, a, bcolors.ENDC, "mod", bcolors.BLUE, p, bcolors.ENDC, "=",\
+	bcolors.CYAN, A, bcolors.ENDC
+print
+
+
+print "3. Bob chooses a secret integer "+bcolors.DRED+"b"+bcolors.ENDC+\
+	", then sends Alice "+bcolors.CYAN+"B"+bcolors.ENDC+" = "+bcolors.YELLOW+"g"+bcolors.ENDC+\
+        " ^ "+bcolors.DRED+"b"+bcolors.ENDC+" mod "+bcolors.BLUE+"p"+bcolors.ENDC
+b = random.randint(10000, 99999)
+print bcolors.DRED+"b"+bcolors.ENDC+" =", bcolors.DRED, b, bcolors.ENDC
+B = (g ** b) % p
+#print bcolors.CYAN+"B"+bcolors.ENDC+" =", bcolors.CYAN, B, bcolors.ENDC
+print bcolors.CYAN+"B"+bcolors.ENDC+" =", bcolors.YELLOW, g, bcolors.ENDC, "^", \
+	bcolors.DRED, b, bcolors.ENDC, "mod", bcolors.BLUE, p, bcolors.ENDC, "=",\
+	bcolors.CYAN, B, bcolors.ENDC
+print
+
+
+print "4. Alice computes her "+bcolors.RED+"s"+bcolors.ENDC+" = "+bcolors.CYAN+"B"+bcolors.ENDC+\
+	" ^ "+bcolors.DRED+"a"+bcolors.ENDC+" mod "+bcolors.BLUE+"p"+bcolors.ENDC
+s1 = (B ** a) % p
+print bcolors.RED+"s"+bcolors.ENDC+" =", bcolors.CYAN, B, bcolors.ENDC, "^", \
+	bcolors.DRED, a, bcolors.ENDC, "mod", bcolors.BLUE, p, bcolors.ENDC, "=", bcolors.RED, s1, bcolors.ENDC
+print
+
+
+print "5. Bob computes his "+bcolors.RED+"s"+bcolors.ENDC+" = "+bcolors.CYAN+"A"+bcolors.ENDC+\
+	" ^ "+bcolors.DRED+"b"+bcolors.ENDC+" mod "+bcolors.BLUE+"p"+bcolors.ENDC
+s2 = (A ** b) % p
+#print bcolors.RED+"s"+bcolors.ENDC+" =", bcolors.RED, s2, bcolors.ENDC
+print bcolors.RED+"s"+bcolors.ENDC+" =", bcolors.CYAN, A, bcolors.ENDC, "^", \
+	bcolors.DRED, b, bcolors.ENDC, "mod", bcolors.BLUE, p, bcolors.ENDC, "=", bcolors.RED, s2, bcolors.ENDC
+print
+
+
+print "6. Alice and Bob now share a secret ( the number"+bcolors.RED, s1, bcolors.ENDC+") which had never passed over the channel"
+print
+
+print "    The only "+bcolors.BLUE+"p",p,bcolors.ENDC+", "+bcolors.YELLOW+"g", g, bcolors.ENDC+", "+\
+	bcolors.CYAN+"A",A,bcolors.ENDC+" and "+bcolors.CYAN+"B",B,bcolors.ENDC+" have passed over the channel"
+print
+
+print "  ", bcolors.DRED, "a", a, bcolors.ENDC, "," , bcolors.DRED, "b", b, bcolors.ENDC, \
+	"and", bcolors.RED, "s", s1, bcolors.ENDC, "have never passed over the channel"
+print
+