Challenge–response authentication

For the spam-filtering technique, see Challenge–response spam filtering.

In computer security, challenge-response authentication is a family of protocols in which one party presents a question ("challenge") and another party must provide a valid answer ("response") to be authenticated.

The simplest example of a challenge-response protocol is password authentication, where the challenge is asking for the password and the valid response is the correct password.

Clearly an adversary who can eavesdrop on a password authentication can then authenticate itself in the same way. One solution is to issue multiple passwords, each of them marked with an identifier. The verifier can ask for any of the passwords, and the prover must have that correct password for that identifier. Assuming that the passwords are chosen independently, an adversary who intercepts one challenge-response message pair has no clues to help with a different challenge at a different time.

For example, when other communications security methods are unavailable, the U.S. military uses the AKAC-1553 TRIAD numeral cipher to authenticate and encrypt some communications. TRIAD includes a list of three-letter challenge codes, which the verifier is supposed to choose randomly from, and random three-letter responses to them. For added security, each set of codes is only valid for a particular time period which is ordinarily 24 hours.

A more interesting challenge-response technique works as follows: Say "Bob" is controlling access to some resource. Alice comes along seeking entry. Bob issues a challenge, perhaps "52w72y". Alice must respond with the one string of characters which "fits" the challenge Bob issued. The "fit" is determined by an algorithm "known" to Bob and Alice. (The correct response MIGHT be as simple as "63x83z" (each character of response one more than that of challenge)... but in the real world, the "rules" would be much more complex.) Bob issues a different challenge each time, and thus knowing a previous correct response... even if it isn't "hidden" by the means of communication used between Alice and Bob... is of no use. A part of Alice's response might convey that it is Alice (or the specific dongle she was supposed to be keeping secure) who is seeking authentication.

Software in the 1980s and 1990s often used a similar method for copy protection: challenges would be questions like "What is the second word in the third paragraph on page 418 of the manual?". The security assumption was that copying the manual was more difficult than copying the software disk. Sometimes the manual would be printed in such a way that contemporary photocopy machines couldn't duplicate the pages.

Other non-cryptographic protocols

Challenge-response protocols are also used to assert things other than knowledge of a secret value. CAPTCHAs, for example, are a sort of variant on the Turing test, meant to determine whether a viewer of a Web application is a real person. The challenge sent to the viewer is a distorted image of some text, and the viewer responds by typing in that text. The distortion is designed to make automated optical character recognition (OCR) difficult and preventing a computer program from passing as a human.

Cryptographic techniques

Non-cryptographic authentication were generally adequate in the days before the Internet, when the user could be sure that the system asking for the password was really the system they were trying to access, and that nobody was likely to be eavesdropping on the communication channel to observe the password being entered. To address the insecure channel problem, a more sophisticated approach is necessary. Many cryptographic solutions involve two-way authentication, where both the user and the system must each convince the other that they know the shared secret (the password), without this secret ever being transmitted in the clear over the communication channel, where eavesdroppers might be lurking.

One way this is done involves using the password as the encryption key to transmit some randomly generated information as the challenge, whereupon the other end must return as its response a similarly encrypted value which is some predetermined function of the originally offered information, thus proving that it was able to decrypt the challenge. For instance, in Kerberos, the challenge is an encrypted integer N, while the response is the encrypted integer N + 1, proving that the other end was able to decrypt the integer N. In other variations, a hash function operates on a password and a random challenge value to create a response value.

Such encrypted or hashed exchanges do not directly reveal the password to an eavesdropper. However, they may supply enough information to allow an eavesdropper to deduce what the password is, using a dictionary attack or brute-force attack. The use of information which is randomly generated on each exchange (and where the response is different from the challenge) guards against the possibility of a replay attack, where a malicious intermediary simply records the exchanged data and retransmits it at a later time to fool one end into thinking it has authenticated a new connection attempt from the other.

Authentication protocols usually employ a cryptographic nonce as the challenge to ensure that every challenge-response sequence is unique. This protects against a man-in-the-middle attack and subsequent replay attack. If it is impractical to implement a true nonce, a strong cryptographically secure pseudorandom number generator and cryptographic hash function can generate challenges that are highly unlikely to occur more than once. It is important not to use time-based nonces, as these can weaken servers in different time zones and servers with inaccurate clocks.

Mutual authentication is performed using a challenge-response handshake in both directions; the server ensures that the client knows the secret, and the client also ensures that the server knows the secret, which protects against a rogue server impersonating the real server.

Challenge–response authentication can help solve the problem of exchanging session keys for encryption. Using a key derivation function, the challenge value and the secret may be combined to generate an unpredictable encryption key for the session. This is particularly effective against a man-in-the-middle attack, because the attacker will not be able to derive the session key from the challenge without knowing the secret, and therefore will not be able to decrypt the data stream.

Simple example mutual authentication sequence


Password storage

To avoid storage of passwords, some operating systems (e.g. Unix-type) store a hash of the password rather than storing the password itself. During authentication, the system need only verify that the hash of the password entered matches the hash stored in the password database. This makes it more difficult for an intruder to get the passwords, since the password itself is not stored, and it is very difficult to determine a password that matches a given hash. However, this presents a problem for many (but not all) challenge-response algorithms, which require both the client and the server to have a shared secret. Since the password itself is not stored, a challenge-response algorithm will usually have to use the hash of the password as the secret instead of the password itself. In this case, an intruder can use the actual hash, rather than the password, which makes the stored hashes just as sensitive as the actual passwords. SCRAM is a challenge-response algorithm that avoids this problem.


Examples of more sophisticated challenge-response algorithms are zero-knowledge password proof and key agreement systems (such as Secure Remote Password (SRP)), Challenge-Handshake Authentication Protocol (CHAP) (RFC 1994), CRAM-MD5, OCRA: OATH Challenge-Response Algorithm (RFC 6287), Salted Challenge Response Authentication Mechanism (SCRAM) (RFC 5802), and ssh's challenge-response system based on RSA .

Some people consider a CAPTCHA a kind of challenge-response authentication that blocks spambots.[1]

See also

Wikibooks has a book on the topic of: Internet Technologies/Email spam


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