There are many programs that demonstrate various concepts in cryptography and network security that can be written in C. Here are a few examples:

1). Caesar cipher: This is a simple encryption algorithm that shifts the letters of a message by a fixed number of positions in the alphabet. This program could take a message and a key as input, and output the encrypted message.

The Caesar cipher is a simple substitution cipher that shifts the letters of a message by a fixed number of positions in the alphabet. To encrypt a message using the Caesar cipher, each letter in the message is replaced with a letter that is a certain number of positions down the alphabet. For example, if the key is 3, every letter in the message is replaced with the letter that is 3 positions down the alphabet from it. So, the letter 'A' would be replaced with the letter 'D', the letter 'B' would be replaced with the letter 'E', and so on.

Here is an example of how the Caesar cipher could work in C:

#include <stdio.h>

#include <string.h>

int main() {

  char message[100];

  int i, key;

  printf("Enter a message to encrypt: ");

  scanf("%s", message);

  printf("Enter a key: ");

  scanf("%d", &key);

  for (i = 0; i < strlen(message); i++) {

    char c = message[i];

    if (c >= 'a' && c <= 'z') {

      c = c + key;

      if (c > 'z') {

        c = c - 26;

      }

      else if (c < 'a') {

        c = c + 26;

      }

      message[i] = c;

    }

    else if (c >= 'A' && c <= 'Z') {

      c = c + key;

      if (c > 'Z') {

        c = c - 26;

      }

      else if (c < 'A') {

        c = c + 26;

      }

      message[i] = c;

    }

  }

  printf("Encrypted message: %s\n", message);

  return 0;

}

To use this program, the user would enter a message to be encrypted and a key, and the program would output the encrypted message. For example, if the user enters the message "hello" and the key 3, the program would output "khoor".

2). Vigenère cipher: This is a more advanced encryption algorithm that uses a keyword to shift the letters of a message. This program could take a message and a key as input, and output the encrypted message.

The Vigenère cipher is a substitution cipher that uses a keyword to shift the letters of a message. To encrypt a message using the Vigenère cipher, a table of alphabets, known as the Vigenère square, is used. The keyword is written at the top of the table, and the letters of the alphabet are written in the rows below.

Here is an example of the Vigenère square:

  | A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

------------------------------------------------------------

A | A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

B | B C D E F G H I J K L M N O P Q R S T U V W X Y Z A

C | C D E F G H I J K L M N O P Q R S T U V W X Y Z A B

D | D E F G H I J K L M N O P Q R S T U V W X Y Z A B C

E | E F G H I J K L M N O P Q R S T U V W X Y Z A B C D

F | F G H I J K L M N O P Q R S T U V W X Y Z A B C D E

G | G H I J K L M N O P Q R S T U V W X Y Z A B C D E F

H | H I J K L M N O P Q R S T U V W X Y Z A B C D E F G

I | I J K L M N O P Q R S T U V W X Y Z A B C D E F G H

J | J K L M N O P Q R S T U V W X Y Z A B C D E F G H I

K | K L M N O P Q R S T U V W X Y Z A B C D E F G H I J

L | L M N O P Q R S T U V W X Y Z A B C D E F G H I J K

M | M N O P Q R S T U V W X Y Z A B C D E F G H I J K L

N | N O P Q R S T U V W X Y Z A B C D E F G H I J K L M

O | O P Q R S T U V W X Y Z A B C D E F G H I J K L M N

P | P Q R S T U V W X Y Z A B C D E F G H I J K L M N O

Q | Q R S T U V W X Y Z A B C D E F G H I J K L M N O P

R | R S T U V W X Y Z A B C D E F G H I J K L M N O P Q

S | S T U V W X Y Z A B C D E F G H I J K L M N O P Q R

T | T U V W X Y Z A B C D E F G H I J K L M N O P Q R S

U | U V W X Y Z A B C D E F G H I J K L M N O P Q R S T

V | V W X Y Z A B C D E F G H I J K L M N O P Q R S T U

W | W X

3). Hash function: A hash function is a mathematical function that takes an input (or 'message') and returns a fixed-size string of characters, which is typically a hexadecimal number. This program could take a message as input and output the hash value.

A hash function is a mathematical function that takes an input, or 'message', and returns a fixed-size string of characters, which is typically a hexadecimal number. This output is known as the hash value, or 'digest'. The main properties of a hash function are that it is deterministic, meaning that the same input will always produce the same output, and it is one-way, meaning that it is computationally infeasible to generate the original input from the hash value.

To use a hash function in a program, you would pass the message as input to the hash function, and it would return the hash value. The exact implementation of the hash function will depend on the specific algorithm being used.

Here is an example of how a hash function could work in C using the SHA-256 algorithm:

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <openssl/sha.h>

int main() {

  char message[100];

  unsigned char hash[SHA256_DIGEST_LENGTH];

  printf("Enter a message: ");

  scanf("%s", message);

  SHA256_CTX sha256;

  SHA256_Init(&sha256);

  SHA256_Update(&sha256, message, strlen(message));

  SHA256_Final(hash, &sha256);

  printf("Hash value: ");

  for (int i = 0; i < SHA256_DIGEST_LENGTH; i++) {

    printf("%02x", hash[i]);

  }

  printf("\n");

  return 0;

}

To use this program, the user would enter a message, and the program would output the hash value of the message using the SHA-256 algorithm. For example, if the user enters the message "hello", the program might output "2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824"

4). Digital signature: A digital signature is a way to ensure the authenticity and integrity of a message or document. This program could take a message as input, generate a digital signature for it, and verify the signature.

A digital signature is a way to ensure the authenticity and integrity of a message or document. It works by using a combination of a hash function and a private key to create a unique signature for the message, and a public key to verify the signature.

To create a digital signature, the message is first passed through a hash function to produce a hash value. The hash value is then encrypted with the sender's private key to create the digital signature. The message and the digital signature are then sent to the recipient.

To verify the digital signature, the recipient uses the sender's public key to decrypt the digital signature and obtain the hash value of the message. The recipient then calculates the hash value of the received message using the same hash function and compares it to the decrypted hash value. If the two hash values match, the recipient can be confident that the message is authentic and has not been tampered with.

Here is an example of how a digital signature could work in C using the RSA algorithm:

#include <stdio.h>

#include <string.h>

#include <openssl/rsa.h>

#include <openssl/pem.h>

#include <openssl/err.h>

#define KEY_LENGTH 2048

#define PUB_EXP 3

#define PRINT_KEYS 0

#define WRITE_TO_FILE 0

int main() {

  // Generate key pair

  printf("Generating RSA (%d bits) key pair...", KEY_LENGTH);

  fflush(stdout);

  RSA *keypair = RSA_generate_key(KEY_LENGTH, PUB_EXP, NULL, NULL);

  printf("done\n");

  // Convert to PEM

  BIO *pri = BIO_new(BIO_s_mem());

  BIO *pub = BIO_new(BIO_s_mem());

  PEM_write_bio_RSAPrivateKey(pri, keypair, NULL, NULL, 0, NULL, NULL);

  PEM_write_bio_RSAPublicKey(pub, keypair);

  // Get sizes of PEM

  size_t pri_len = BIO_pending(pri);

  size_t pub_len = BIO_pending(pub);

  // Allocate memory for PEM

  char *pri_key = malloc(pri_len + 1);

  char *pub_key = malloc(pub_len + 1);

  // Read PEM into memory

  BIO_read(pri, pri_key, pri_len);

  BIO_read(pub, pub_key, pub_len);

  // Add null terminator to end of string

  pri_key[pri_len] = '\0';

  pub_key[pub_len] = '\0';

  #if PRINT_KEYS

  printf("Private key:\n%s\n", pri_key);

  printf("Public key:\n%s\n", pub_key);

  #endif

  #if WRITE_TO_FILE

  // Write keys to files

  FILE *pri_file = fopen("private.pem", "w");

  FILE *pub_file = fopen("public.pem", "w");

  fprintf(pri_file, "%s", pri_key);

  fprintf(pub_file, "%s", pub

5). Network scanner: This program could scan a network and identify the devices connected to it. It could also gather information about those devices, such as their IP addresses and MAC addresses.

. A network scanner is a program that can scan a network and identify the devices connected to it. It can also gather information about those devices, such as their IP addresses and MAC addresses.

.There are many ways to implement a network scanner in C, but one approach is to use the pcap library to capture network traffic and process it. The pcap library allows you to open a network interface and capture packets that pass through it.

.To scan a network with this approach, the program would need to do the following:

1). Open a network interface using pcap_open_live().

2). Set a filter using pcap_compile() and pcap_setfilter() to capture only the packets of interest (e.g. ARP packets).

3). Use pcap_loop() to capture packets and process them in a callback function.

4). In the callback function, extract the relevant information from the packet (e.g. the IP and MAC addresses of the device).

5).Store the information in a data structure (e.g. an array or linked list).

Here is an example of how a network scanner might work in C using the pcap library:

#include <stdio.h>

#include <pcap.h>

#include <arpa/inet.h>

#define MAX_DEVICES 100

struct Device {

  uint32_t ip_address;

  uint8_t mac_address[6];

};

struct Device devices[MAX_DEVICES];

int num_devices = 0;

void callback(u_char *args, const struct pcap_pkthdr *header, const u_char *packet) {

  // Extract the IP and MAC addresses from the packet

  struct Device device;

  device.ip_address = *((uint32_t *)(packet + 28));

  memcpy(device.mac_address, packet + 22, 6);

  // Check if the device is already in the list

  int i;

  for (i = 0; i < num_devices; i++) {

    if (devices[i].ip_address == device.ip_address) {

      break;

    }

  }

  // If the device is not in the list, add it

  if (i == num_devices) {

    devices[num_devices++] = device;

  }

}

int main() {

  char *dev;

  char errbuf[PCAP_ERRBUF_SIZE];

  // Find a device to use

  dev = pcap_lookupdev(errbuf);

  if (dev == NULL) {

    fprintf(stderr, "Couldn't find default device: %s\n", errbuf);

    return 1;

  }

  // Open the device

  pcap_t *handle = pcap_open_live(dev, BUFSIZ, 1, 1000, errbuf);

  if (handle == NULL) {

    fprintf(stderr, "Couldn't open device %s: %s\n", dev, errbuf);

    return 1;

  }

  // Set a filter to only capture ARP packets

  struct bpf_program fp;

  char filter_exp[] = "arp";

  if (pcap_compile(handle, &)

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