Some basic encryption algorithms

shiftcipher.c

#include <stdio.h>
//STUDYING MATERIAL
//shift encryption is not secure


char* encrypt(int k, char* message){
	char* temp=malloc(strlen(message)+1);
	int i;
	for(i=0;i<strlen(message);i++){
		temp[i]=(message[i]+k+26-'a')%26+'a';
	}
	temp[i]=NULL;
	return temp;
}

char* decrypt(int k, char* cipher){
	char* temp=malloc(strlen(cipher)+1);
	int i;
	for(i=0;i<strlen(cipher);i++){
		temp[i]=(cipher[i]-'a'+26-k)%26+'a';
	}
	temp[i]=NULL;
	return temp;
}


int main(){
	char* message = "shiftcipherisnotsecure";
	int key = 25;
	printf("Shift encryption of %s with key %d: ",message,key);
	char* cipher = encrypt(key,message);
	printf(cipher);
	printf("\n\nDecrypting cipher: %s \n",cipher);
	message = decrypt(key,cipher);
	printf("Result is %s\n",message);
}

vigenerecipher.c

#include <stdio.h>
//STUDYING MATERIAL
//vigenere encryption is not secure too

//encription function
char* encrypt(char* message, char* key){
	char* result;
	result= malloc(strlen(message)+1);
	int keyLength = strlen(key);

	int i;		//message index
	int k=0;	//key index
	
	for(i=0;i<strlen(message);i++){
		result[i]=(message[i]+key[k]-2*'a')%26+'a';
		if(++k==keyLength)
			k=0;
	}
	result[i] = message[i];
	return result;
}

char* decrypt(char* cipher, char* key){
	char* result;
	result= malloc(strlen(cipher)+1);
	int keyLength = strlen(key);

	int i;		//cipher index
	int k=0;	//key index
	
	for(i=0;i<strlen(cipher);i++){
		result[i]=(cipher[i]-key[k]+2*26)%26+'a';
		if(++k==keyLength)
			k=0;
	}
	//filling with last nullterminated symbol
	result[i] = cipher[i];
	return result;	
}


int main(){
	
	char* message = "thegreaestday";
	char* key = "security";

	char* cipher = encrypt(message,key);
	printf("%s\n",cipher);
	printf("%s\n",decrypt(cipher,key));

	return 0;
}