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  1. #include <stdio.h> 
  2. #include <stdlib.h> 
  3. #include <string.h> 
  4. #define SIZE 30 
  5.  
  6. // Convert all the characters 
  7. // of a string to lowercase 
  8. void toLowerCase(char plain[], int ps) 
  9. { 
  10. 	int i; 
  11. 	for (i = 0; i < ps; i++) { 
  12. 		if (plain[i] > 64 && plain[i] < 91) 
  13. 			plain[i] += 32; 
  14. 	} 
  15. } 
  16.  
  17. // Remove all spaces in a string 
  18. // can be extended to remove punctuation 
  19. int removeSpaces(char* plain, int ps) 
  20. { 
  21. 	int i, count = 0; 
  22. 	for (i = 0; i < ps; i++) 
  23. 		if (plain[i] != ' ') 
  24. 			plain[count++] = plain[i]; 
  25. 	plain[count] = '\0'; 
  26. 	return count; 
  27. } 
  28.  
  29. // generates the 5x5 key square 
  30. void generateKeyTable(char key[], int ks, 
  31. 					char keyT[5][5]) 
  32. { 
  33. 	int i, j, k, flag = 0, *dicty; 
  34.  
  35. 	// a 26 character hashmap 
  36. 	// to store count of the alphabet 
  37. 	dicty = (int*)calloc(26, sizeof(int)); 
  38.  
  39. 	for (i = 0; i < ks; i++) { 
  40. 		if (key[i] != 'j') 
  41. 			dicty[key[i] - 97] = 2; 
  42. 	} 
  43. 	dicty['j' - 97] = 1; 
  44.  
  45. 	i = 0; 
  46. 	j = 0; 
  47. 	for (k = 0; k < ks; k++) { 
  48. 		if (dicty[key[k] - 97] == 2) { 
  49. 			dicty[key[k] - 97] -= 1; 
  50. 			keyT[i][j] = key[k]; 
  51. 			j++; 
  52. 			if (j == 5) { 
  53. 				i++; 
  54. 				j = 0; 
  55. 			} 
  56. 		} 
  57. 	} 
  58. 	for (k = 0; k < 26; k++) { 
  59. 		if (dicty[k] == 0) { 
  60. 			keyT[i][j] = (char)(k + 97); 
  61. 			j++; 
  62. 			if (j == 5) { 
  63. 				i++; 
  64. 				j = 0; 
  65. 			} 
  66. 		} 
  67. 	} 
  68. } 
  69.  
  70. // Search for the characters of a digraph 
  71. // in the key square and return their position 
  72. void search(char keyT[5][5], char a, 
  73. 			char b, int arr[]) 
  74. { 
  75. 	int i, j; 
  76.  
  77. 	if (a == 'j') 
  78. 		a = 'i'; 
  79. 	else if (b == 'j') 
  80. 		b = 'i'; 
  81.  
  82. 	for (i = 0; i < 5; i++) { 
  83. 		for (j = 0; j < 5; j++) { 
  84. 			if (keyT[i][j] == a) { 
  85. 				arr[0] = i; 
  86. 				arr[1] = j; 
  87. 			} 
  88. 			else if (keyT[i][j] == b) { 
  89. 				arr[2] = i; 
  90. 				arr[3] = j; 
  91. 			} 
  92. 		} 
  93. 	} 
  94. } 
  95.  
  96. // Function to find the modulus with 5 
  97. int mod5(int a) 
  98. { 
  99. 	return (a % 5); 
  100. } 
  101.  
  102. // Function to decrypt 
  103. void decrypt(char str[], char keyT[5][5], int ps) 
  104. { 
  105. 	int i, a[4]; 
  106. 	for (i = 0; i < ps; i += 2) { 
  107. 		search(keyT, str[i], str[i + 1], a); 
  108. 		if (a[0] == a[2]) { 
  109. 			str[i] = keyT[a[0]][mod5(a[1] - 1)]; 
  110. 			str[i + 1] = keyT[a[0]][mod5(a[3] - 1)]; 
  111. 		} 
  112. 		else if (a[1] == a[3]) { 
  113. 			str[i] = keyT[mod5(a[0] - 1)][a[1]]; 
  114. 			str[i + 1] = keyT[mod5(a[2] - 1)][a[1]]; 
  115. 		} 
  116. 		else { 
  117. 			str[i] = keyT[a[0]][a[3]]; 
  118. 			str[i + 1] = keyT[a[2]][a[1]]; 
  119. 		} 
  120. 	} 
  121. } 
  122.  
  123. // Function to call decrypt 
  124. void decryptByPlayfairCipher(char str[], char key[]) 
  125. { 
  126. 	char ps, ks, keyT[5][5]; 
  127.  
  128. 	// Key 
  129. 	ks = strlen(key); 
  130. 	ks = removeSpaces(key, ks); 
  131. 	toLowerCase(key, ks); 
  132.  
  133. 	// ciphertext 
  134. 	ps = strlen(str); 
  135. 	toLowerCase(str, ps); 
  136. 	ps = removeSpaces(str, ps); 
  137.  
  138. 	generateKeyTable(key, ks, keyT); 
  139.  
  140. 	decrypt(str, keyT, ps); 
  141. } 
  142.  
  143. // Driver code 
  144. int main() 
  145. { 
  146. 	char str[SIZE], key[SIZE]; 
  147.  
  148. 	// Key to be encrypted 
  149. 	strcpy(key, "cheater"); 
  150. 	printf("Key text: %s\n", key); 
  151.  
  152. 	// Ciphertext to be decrypted 
  153. 	strcpy(str, "XENPZTVNTX"); 
  154. 	printf("Plain text: %s\n", str); 
  155.  
  156. 	// encrypt using Playfair Cipher 
  157. 	decryptByPlayfairCipher(str, key); 
  158.  
  159. 	printf("Deciphered text: %s\n", str); 
  160.  
  161. 	return 0; 
  162. } 
  163.  
  164. // This code is contributed by AbhayBhat 
  165.  
Success #stdin #stdout 0s 5432KB
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Key text: cheater
Plain text: XENPZTVNTX
Deciphered text: qUkuu
https://ideone.com/eyM0Oj
language:
C (gcc 8.3)
created:
1 year ago
visibility:
public

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