Example - 1

This code takes a password, concatenates it with a nonce, then encrypts it before sending over a network:

char *nonce = "bad";...char *data = (unsigned char*)malloc(20);int para_size = strlen(nonce) + strlen(password);char *paragraph = (char*)malloc(para_size);SHA1((const unsigned char*)paragraph,parsize,(unsigned char*)data);sendEncryptedData(data)void encryptAndSendPassword(char *password){}

Because the nonce used is always the same, an attacker can impersonate a trusted party by intercepting and resending the encrypted password. This attack avoids the need to learn the unencrypted password.

Example - 2

This code sends a command to a remote server, using an encrypted password and nonce to prove the command is from a trusted party:

String command = new String("some command to execute");MessageDigest nonce = MessageDigest.getInstance("SHA");nonce.update(String.valueOf("bad nonce"));byte[] nonce = nonce.digest();MessageDigest password = MessageDigest.getInstance("SHA");password.update(nonce + "secretPassword");byte[] digest = password.digest();sendCommand(digest, command)

Once again the nonce used is always the same. An attacker may be able to replay previous legitimate commands or execute new arbitrary commands.