Reusing a Nonce, Key Pair in Encryption

Background

Nonces are often bundled with a key in a communication exchange to produce a new session key for each exchange.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

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

void encryptAndSendPassword(char *password){
  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)
}

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 Two

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.

See Also