Replay Attack is one of the form of #Modification. It works like this: the intruder will capture data unit while being the middle of the man, and then send a modified version of the data to the user which could contain malicious information.
Replay Attack could be prevented using Nonce#, where the machines will use an arbitrary number (random) embedded to the communication Protocol to recognise each other.
Nonce is a random number generated to used just once during a communication session. The main purpose of it is to prevent Replay Attack#.
Modification is a form of #Security Attack aims at gaining access to the system and altering the information. It undermines the integrity# of the system. Replay Attack# and modification is one of the example of Modification attack.
However, Version 4 is not without flaws. There is a double encryption on the tickets provided to the client (\(\text{Ticket}_{tgs}\) and \(\text{Ticket}_{V}\)). The encryption scheme used by V4 is not secure at all (PCBC, which is a non-standard Data Encryption Standard (DES)#). It is vulnerable to an attack which involves the interchange of the ciphertext blocks. (Version 5 prevents it by providing Cipher Block Chaining (CBC)# mode for encryption). In addition, the same ticket could be used by the client to access a server which is at risk of potential Replay Attack using an old session key. (V5 allows the use of a subsession key, which will be invalid after one connection)
Note: The lifetime of the session key should not be too long or too short. If the lifetime is too long, it is vulnerable to Replay Attack# which will be used by the intruder to access the services. The lifetime should not be too as to avoid asking password too frequently.
Integrity is a measure on whether how complete (remained unaltered) the received message itself is, as it would be the same or highly similar as the one that being sent. By further utilising encryption#, Digital Signature# and Data Integrity#, it could be used to counter Replay Attack# and Modification#
The data encapsulation is done by two extension headers (append to the IP header): Authentication Header (AH) and Encapsulating Security Payload (ESP), defined by RFC 4303 and RFC 4302 respectively. AH provides Access Control#, Authentication# (including the IP Address#) and Connectionless Data Integrity# services using Message Authentication Code (MAC# /HMAC) where both parties must share secret key. It guards against address spoofing attacks and Replay Attack. ESP provides Confidentiality# services over message contents and traffic flow (but limited) and optionally support Access Control# services like in AH using MAC. It can also guard against Replay Attack#. The users can design which kind of ciphers, modes, and padding to be used in ESP. The IPsec could be set up to have AH only or ESP only or both.
Authenticity is a quality of how true or authentic the entity is the one as claimed. There are several threats to Authenticity such as physical access to authenticated device, impersonation of authenticated device by masking its Media Access Control Address (MAC) (Masquerade), and gaining entrance to server or disrupting operations with Replay Attack. This could be avoid with cryptographic techniques.