Caesar cipher encryption/decryption

week1/krypto2.c

+#include <stdio.h>
+
+void encrypt ();
+void decrypt ();
+
+int main (void)
+{
+	char valik;
+
+	printf ("Do you wish to encrypt or decrypt?\n");
+	do
+	{
+		printf ("Please enter a valid option(e/d):");
+		scanf ("%c", &valik);
+	} 
+	while (valik != 'e' && valik != 'd');
+
+	if (valik == 'e')
+	{
+		encrypt ();
+	}
+	else if (valik == 'd')
+	{
+		decrypt ();
+	}
+	else
+	{
+		printf ("option does not exist");
+	}
+
+	return 0;
+}
+
+void encrypt ()
+{
+	char ni[80] = "sisend.txt", no[80] = "tul.txt";
+	char c;
+	int key;
+	int caesar;
+
+	FILE *fi, *fo;
+	fi = fopen (ni, "r");
+	fo = fopen (no, "w");
+
+	printf ("Enter the encryption key to encrypt: ");
+	scanf ("%d", &key);
+	do
+	{
+		c = fgetc(fi);
+		if (feof(fi))
+		{
+			break;
+		}
+
+		if (c >= 65 && c <= 90)
+		{ 
+			if ((c + key) > 90)
+			{
+				caesar = c - (26 - key);
+			}
+			else
+			{
+				caesar = (int)c + key;
+			}
+		}
+		else if (c >= 97 && c <= 122)
+		{
+			if ((c + key) > 122)
+			{
+				caesar = c - (26 - key);
+			}
+			else
+			{
+				caesar = (int)c + key;
+			}
+		}
+
+		printf ("%c", caesar);
+		fprintf (fo, "%c", caesar);
+	}
+	while (1);
+	printf (" \n");
+}
+
+void decrypt ()
+{
+	char ni[80] = "sisend.txt", no[80] = "tul.txt";
+	char c;
+	int key;
+	int caesar;
+
+	FILE *fi, *fo;
+	fi = fopen (ni, "r");
+	fo = fopen (no, "w");
+
+	printf("Enter the encryption key to decrypt: ");
+	scanf ("%d", &key);
+	do
+	{
+		c = fgetc(fi);
+		if (feof(fi))
+		{
+			break;
+		}
+
+		if (c >= 65 && c <= 90)
+		{ 
+			if ((c - key) < 65)
+			{
+				caesar = c + (26 - key);
+			}
+			else
+			{
+				caesar = (int)c - key;
+			}
+		}
+		else if (c >= 97 && c <= 122)
+		{
+			if ((c - key) < 97)
+			{
+				caesar = c + (26 - key);
+			}
+			else
+			{
+				caesar = (int)c - key;
+			}
+		}
+
+		printf ("%c", caesar);
+		fprintf (fo, "%c", caesar);
+	}
+	while (1);
+	printf (" \n");
+}
+