CHAP Protocol | Features of CHAP | Chap Packet | Advantages and Disadvantages of CHAP

The IETF(Internet Engineering Task Force) developed the challenge handshake authentication mechanism (CHAP), which is a point-to-point protocol (PPP) authentication mechanism. When the link first starts up, it is utilized.

Features of CHAP

  • CHAP is achieved through a challenge-response mechanism.
  • It is more secure than static password-based authentication.
  • It periodically authenticates to see if communication is occurring with the same device or not.
  • It uses a way to hash a function called MD5.
  • CHAP offers flexibility in selecting the level of security by a variety of authentication techniques.
  • Since the password is not communicated in plaintext, CHAP is resistant to eavesdropping attempts since attackers are unable to quickly intercept and decode the password.
  • CHAP is a popular option for protecting PPP connections.
  • The CHAP challenge has a counter which helps prevent a replay attack.

Chap Protocol Packet

Challenge packet

At the beginning of the CHAP3 way handshake the authenticator sends a packet to the peer known as a  challenge packet. To check if the connection has not changed, challenge packets are frequently issued as well. It includes an identifier value, a value field with a random value, and a name field with the authenticator's name. The name field is used to search for passwords. A one-way hash valye is constructed using the name field and the MD5 hash generator.

Response packet

The client responds to the challenge by hashing the challenge value and its secret password or key using a one-watch hash function. A response packet carrying the outcome is delivered back to the server.

Success or failure packet

The response packet was downloaded by the server from the client. Using its copy of the shared secret and the challenge it has been given, it determines its own expected response. Authentication is successful if the calculated answer and the one from the client agree. when the client has been successfully authenticated and access to the network or resources has been allowed, the NAS then sends a success packet. The server sends or packet and denies access if the replies do not match.

Advantages of CHAP

Enhance security

CHAP doesn't send passwords in plaintext, it is safer than the outdated password authentication protocol(PAP)


CHAP is simple to install and use in a variety of network situations. 

Protection against replay attacks

CHAP prevents replay attacks when an attacker intercepts and reuses authentication data by using a different challenge for every authentication attempt.


CHAP is capable of supporting the large number of users and devices on the network.

Support for strong hash functions

CHAP allows the calculation of answers using strong cryptographic hash functions like MD5 or SHA1.

Widespread support

CHAP is a possible option for many network authentication scenarios since it is widely supported by a variety of networking technologies including PPP and VPNs.

Prevent replay attacks

CHAP is able to prevent replay attacks by making sure that the same challenge can never be used twice.

Disadvantages of CHAP

Requires a pre-shared key

CHAP needs a pre-shared key, which can be challenging to handle in large-scale network systems.

No initial encryption

CHAP's main function is authentication, it does not however encrypt the data being sent.

Shared secret management

CHAP relies on shared secret passwords between the client and the server, just like many password-based authentication techniques.

Lack of mutual authentication

Because CHAP does not support mutual authentication, the user cannot be authenticated by the network in the same way that the network is authenticated by the user.

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