Authentication in .NET

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authentication. The basics of certificate-based mutual authentication procedure as described by NIST standard for entity authentication in Federal Information Processing Standard publication 196 ([FIPS196]). The message exchange defined by NIST is rather generic and does not include any specifics on the types of challenges, identifiers, or certificates used in the process, but provides the concept in a rather concise manner as depicted in Figure 2.1:
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In the first step, which is an optional step, the initiator (entity A) of the exchange sends an authentication request to the responder, with which the initiator wishes to engage in mutual authentication. The format of the request is not defined by NIST in order to allow a choice for a protocol that fits the exact deployment scenario. The responder (entity B) generates a random number (RB) as a challenge and sends it to entity A. This challenge is called TokenIR1 and must be used by entity A in a manner that fits the challenge/response protocol, being used. The responder also includes an optional token identifier (TokenID) along with the token in its response to initiator. The initiator receives the RB and creates a random challenge (RA) of its own and takes the two random numbers along with the name of entity B, and optionally other useful data as part of its own token (TokenRI). The token also includes a signature of all that information with the private key of the initiator. The initiator creates a message and includes its public key certificate (Cert I) to the message, so that the responder can verify the signature. The responder receives the message, verifies the signature. If the verification passes, it means the initiator is authenticated. The responder then creates a new token (TokenIR2) including RB, RA, and name of initiator and possibly other useful data and a signature over these data using the responder s private key. The responder sends a message back to the initiator and includes its own public key certificate (Cert R) to help the initiator with the verification of the signature and thereby authentication of responder to the initiator.
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2.1.3 Examples of Message Authentication Mechanisms
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Methods that protect the information from tampering by illegitimate parties are referred to data integrity protection methods. When information is carried through messages over communications channels, the integrity protection is typically provided by message authentication mechanisms. In order to provide data integrity protection for the message, the sender needs to provide a proof of authenticity for the message. In real life, all the legal documents are signed by the involved parties. The signatures not only provide proof of authenticity since the signature for each person is unique, but also prevent the documents from forgeries.
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Authentication request (optional) (optional TokenID), TokenIR1 Initiator (entity A) (optional TokenID), Cert I, TokenRI (optional TokenID), Cert R, Token IR2 Responder (entity B)
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Figure 2.1 Basic exchange for two-challenge mutual authentication based on certificates
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Of course, anybody who has asked a big sister to forge the parent signatures on a report card knows that there are artistic ways to get around the weakness of authentication methods. After that analogy, understanding message authentication in the world of digital communications becomes very simple: the sender of a message can provide proof of authenticity for the message by signing the message with a secret that is unknown to the outside world and adds the signature to the end of the message. However, it should be noted that in contrast to the personal signatures that always look the same and hence can be easily forged, the signatures in the digital world depend on the message content and take a different form (bit string) every time. It is as if your parents and the teacher had a secret agreement that your parent would sign your report card in a different way for every season. This way, if a man-in-the-middle (MITM) tries to change the message contents, without the knowledge of the secret, she cannot reproduce a signature that matches the content of the message. A typed legal document that has crossover and handwritings over it must again be signed by the involved parties, otherwise it has no legal bearings. To produce the digital signature, the sender needs to run the message through an algorithm that takes the secret (key) as a secondary input. However, since running these algorithms over entire messages are computationally expensive, the sender compresses the data using a so-called hash algorithm (H) and arrives at a digest value, which is typically called message authentication code (MAC). The MAC value is added by the sender to the end of the message sent to the receiver and is checked by the receiver. Hash algorithms are based on hash functions that are mathematical one-way functions, meaning that it is close to impossible (depending on the dispensable amount of time and computing power, of course) to guess the input of the hash function, given the output of the hash function. This is a very important characteristic of the hash function to be used for message authentication. The attacker should not be able to guess the input of the hash function from its output. Now an observant reader might say, when the data is simply sent in the clear, the attacker can easily read the message as well as the MAC. If both message and MAC value are readable, then both input and output of the hash function are exposed and there is no use for the hash function. Well, not quite, the sender and receiver also share a secret that they use as input to the hash function while calculating the hash value (MAC). The hash functions that can accept secret keys are often referred to as keyed (or secure) hash functions. An attacker that does not know the secret cannot tamper with the message data without being exposed, since she cannot re-calculate the hash value based on the altered data. However, secret hash algorithms have been put to test by hackers as well as cryptoanalysts, who attempt to break the existing algorithms as part of their day jobs. Experience and science have shown that in majority of cases increasing the size of key, i.e. the number of bits in the key, tends to make the keyed hash function more resilient to attacks. Over the years, many hash functions have been developed for message authentication. In the following section, we describe HMAC as a standardized mechanism for providing MACs.
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