The Needham–Schroeder protocol is one of the two key transport protocols intended for use over an insecure network, both proposed by Roger Needham and Michael Schroeder. These are:
The Needham–Schroeder Public-Key Protocol, based on public-key cryptography. This protocol is intended to provide mutual authentication between two parties communicating on a network, but in its proposed form is insecure.
The symmetric protocol
Here, Alice initiates the communication to Bob. S is a server trusted by both parties. In the communication:
NA and NB are nonces generated by A and B respectively
KAB is a symmetric, generated key, which will be the session key of the session between A and B
The protocol can be specified as follows in security protocol notation:
Attacks on the protocol
The protocol is vulnerable to a replay attack. If an attacker uses an older, compromised value for KAB, he can then replay the message to Bob, who will accept it, being unable to tell that the key is not fresh.
This flaw is fixed in the Kerberos protocol by the inclusion of a timestamp. It can also be fixed with the use of nonces as described below. At the beginning of the protocol: The protocol then continues as described through the final three steps as described in the original protocol [|above]. Note that is a different nonce from.The inclusion of this new nonce prevents the replaying of a compromised version of since such a message would need to be of the form which the attacker can't forge since she does not have.
The public-key protocol
This assumes the use of a public-key encryption algorithm. Here, Alice and Bob use a trusted server to distribute public keys on request. These keys are:
KPA and KSA, respectively public and private halves of an encryption key-pair belonging to A
KPB and KSB, similar belonging to B
KPS and KSS, similar belonging to S.
The protocol runs as follows: At the end of the protocol, A and B know each other's identities, and know both NA and NB. These nonces are not known to eavesdroppers.
An attack on the protocol
Unfortunately, this protocol is vulnerable to a man-in-the-middle attack. If an impostor can persuade to initiate a session with them, they can relay the messages to and convince that he is communicating with. Ignoring the traffic to and from S, which is unchanged, the attack runs as follows: At the end of the attack, B falsely believes that A is communicating with him, and that NA and NB are known only to A and B. The following example illustrates the attack. Alice would like to contact her bank. We assume that an impostor successfully convinces A that they are the bank. As a consequence A uses the public key of I instead of using the public key of B to encrypt the messages she intends to send to her bank. Therefore, A sends I her nonce encrypted with the public key of I. I decrypts the message using their private key and contacts B sending it the nonce of A encrypted with the public key of B. B has no way to know that this message was actually sent by I. B responds with their own nonce and encrypts the message with the public key of A. Since I is not in possession of the private key of A they have to relay the message to A without knowing the content. A decrypts the message with her private key and respond with the nonce of B encrypted with the public key of I. I decrypts the message using their private key and is now in possession of nonce A and B. Therefore they can now impersonate the bank and the client respectively.
Fixing the man-in-the-middle attack
The attack was first described in a 1995 paper by Gavin Lowe. The paper also describes a fixed version of the scheme, referred to as the Needham–Schroeder–Lowe protocol. The fix involves the modification of message six to include the responder's identity, that is we replace: with the fixed version: and the intruder cannot successfully replay the message because A is expecting a message containing the identity of I whereas the message will have identity of B.
