In computing

Computing

Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...

, the Challenge-Handshake Authentication Protocol (CHAP) authenticates

Authentication

Authentication is the act of confirming the truth of an attribute of a datum or entity...

 a user or network host to an authenticating entity. That entity may be, for example, an Internet service provider

Internet service provider

An Internet service provider is a company that provides access to the Internet. Access ISPs directly connect customers to the Internet using copper wires, wireless or fiber-optic connections. Hosting ISPs lease server space for smaller businesses and host other people servers...

. CHAP is specified in RFC 1994.

CHAP provides protection against playback attack by the peer through the use of an incrementally changing identifier and of a variable challenge-value. CHAP requires that both the client and server know the plaintext of the secret, although it is never sent over the network. The MS-CHAP

MS-CHAP

MS-CHAP is the Microsoft version of the Challenge-handshake authentication protocol, CHAP. The protocol exists in two versions, MS-CHAPv1 and MS-CHAPv2...

 variant does not require either peer to know the plaintext, but has other drawbacks.

Working Cycle

CHAP is an authentication scheme used by Point to Point Protocol (PPP) servers to validate the identity of remote clients. CHAP periodically verifies the identity of the client

Client (computing)

A client is an application or system that accesses a service made available by a server. The server is often on another computer system, in which case the client accesses the service by way of a network....

 by using a three-way handshake

Handshaking

In information technology, telecommunications, and related fields, handshaking is an automated process of negotiation that dynamically sets parameters of a communications channel established between two entities before normal communication over the channel begins...

. This happens at the time of establishing the initial link (LCP)

Link Control Protocol

In computing, the Link Control Protocol forms part of the Point-to-Point Protocol. In setting up PPP communications, both the sending and receiving devices send out LCP packets to determine the standards of the ensuing data transmission...

, and may happen again at any time afterwards. The verification is based on a shared secret

Shared secret

In cryptography, a shared secret is a piece of data, known only to the parties involved, in a secure communication. The shared secret can be a password, a passphrase, a big number or an array of randomly chosen bytes....

 (such as the client user's password).

1. After the completion of the link establishment phase, the authenticator sends a "challenge" message to the peer.
2. The peer responds with a value calculated using a one-way hash function
   One-way function
   In computer science, a one-way function is a function that is easy to compute on every input, but hard to invert given the image of a random input. Here "easy" and "hard" are to be understood in the sense of computational complexity theory, specifically the theory of polynomial time problems...
   
    on the challenge and the secret combined.
3. The authenticator checks the response against its own calculation of the expected hash value. If the values match, the authenticator acknowledges the authentication; otherwise it should terminate the connection.
4. At random intervals the authenticator sends a new challenge to the peer and repeats steps 1 through 3.

CHAP Packets

Description	1 byte	1 byte	2 bytes	1 byte	Variable	variable
Challenge	Code = 1	ID	Length	Challenge length	Challenge value	Name
Response	Code = 2	ID	Length	Response Length	Response value	Name
Success	Code = 4	ID	Length		Message
Failure	Code = 4	ID	Length		Message

The ID chosen for the random challenge is also used in the corresponding response, success, and failure packets. A new challenge with a new ID must be different from the last challenge with another ID. If the success or failure is lost the same response can be sent again, and triggers the same success or failure indication. For MD5

MD5

The MD5 Message-Digest Algorithm is a widely used cryptographic hash function that produces a 128-bit hash value. Specified in RFC 1321, MD5 has been employed in a wide variety of security applications, and is also commonly used to check data integrity...

 as hash the response value is MD5(ID||secret||challenge), the MD5 for the concatenation of ID, secret, and challenge.

See also

• List of authentication protocols
• Password Authentication Protocol
  Password authentication protocol
  A password authentication protocol is an authentication protocol that uses a password.PAP is used by Point to Point Protocol to validate users before allowing them access to server resources. Almost all network operating system remote servers support PAP.PAP transmits unencrypted ASCII passwords...
  
  
• Challenge-response test
• Cryptographic hash function
  Cryptographic hash function
  A cryptographic hash function is a deterministic procedure that takes an arbitrary block of data and returns a fixed-size bit string, the hash value, such that an accidental or intentional change to the data will change the hash value...