The derivation algorithm chosen by the HOTP method is HMAC-SHA-1. The most famous OTP standards are the HMAC-based one-time password (HOTP 4) and the time-based one-time password (TOTP 5). The breach cost EMC $66.3 million, according to the company’s earnings. In the aftermath, RSA offered token replacements or free security monitoring services to its more than 30,000 SecurID customers. The letter did not disclose technical details, probably due to the concern of benefiting potential attackers, but it revealed that the attack had resulted in certain information specifically related to SecurID being extracted from RSA’s systems. In March 2011, the company issued an open letter 3 stating that its corporate security systems had identified an “extremely sophisticated cyber-attack” being mounted against it. RSA Security, the Security Division of EMC, designs and manufactures a well-known OTP token, SecurID. However, the server’s security hardenings are even more critical because if it is hacked, then likely seeds for all tokens are at risk. Traditionally, more attention has been paid to designing physically strong and tamper-resistant tokens. The security strength of an OTP system solely depends on the seed, so the seed must be reliably protected by both the server and the client from leakage. Synchronization: The same OTP value must be known by the server and the client at any given moment, without requiring synchronizations after initialization. The HMAC 2 ( hash-based message authentication code) algorithm is the most popular choice. Theoretically, any collision-free one-way cryptography function with a secret seed as input is qualified. Secrecy: With reasonable resources, adversaries shall not be able to calculate or guess OTP values. Figure 10-1 shows a real-time man-in-the-middle attack scenario.Īn OTP system has two aspects to consider: This makes the attack more complex and expensive. Instead, the phishing server has to be set up as a real-time man-in-the-middle, where it simultaneously establishes two connections, one with the victim’s client platform and the other with the real authentication server. Because the OTP is valid for only a small duration, the attacker cannot save the OTP and make use of it later. The attacker’s fake web site has to also collect the OTP entered by the user. The utilization of OTP significantly increases the difficulty of phishing attacks. To prove his ownership of the token to the server, the user types in the OTP value displayed on the token at the time of authentication to satisfy the requirement of second-factor authentication, supplementing other factors (for example, the username and password). The token (client) possessed by the user and the back-end authentication server are always in sync-they refresh the OTP by performing the same calculation at the same time with the same “derivation materials.” In other words, after initialization, the token and the server will both assume the same OTP at any given moment in the future. The OTP is usually updated at fixed internals, for example every 30 or 60 seconds, depending on security models of specific applications. A good OTP algorithm shall render it practically infeasible to predict future OTP values based on previous observations. Although the value of an OTP may seem random, it is not randomly generated, but cryptographically derived. In contrast to a regular password that is valid for an unlimited number of authentication sessions until it is reset, a one-time password (OTP) is a credential that is used only once. A more robust “something you have” is a hardware digital token or key fob that displays a one-time password. This solution is not ideal, because the same set of keys is repeatedly reused, and may be monitored and replayed by thieves. During authentication, the web site challenges the user with a randomly selected index, and the user looks up the matrix and enters the corresponding key to sign in. It can be as simple as a “key-card matrix” on which a fairly large number of index-key pairs are printed. “Something you have” refers to a physical object that belongs to you. A user can unlock the phone by scanning his fingerprint. For example, the iPhone 5s is equipped with a fingerprint identity sensor. “Something you are” refers to something that is part of you, commonly your biological characteristics, such as fingerprints. The username and password compromise the first factor-“something you know.” Two other types of credentials are the following: Multifactor schemes would mitigate phishing and key logging attacks by requiring additional credentials during the authentication process.
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