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Cryptography General Interview Questions

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The authenticated Diffie-Hellman key agreement protocol, or Station-to-Station (STS) protocol, was developed by Diffie, van Oorschot, and Wiener in 1992 [DVW92] to defeat the middleperson attack on the Diffie-Hellman key agreement protocol. The immunity is achieved by allowing the two parties to authenticate themselves to each other by the use of digital signatures and public-key certificates. 

The Digital Signature Algorithm (DSA) was published by the National Institute of Standards and Technology (NIST) in the Digital Signature Standard (DSS), which is a part of the U.S. government's Capstone project. DSS was selected by NIST, in cooperation with the NSA, to be the digital authentication standard of the U.S. government. The standard was issued on May 19, 1994.

DSA is based on the discrete logarithm problem and derives from cryptosystems proposed by Schnorr and ElGamal. It is for authentication only. For a detailed description of DSA.

In DSA, signature generation is faster than signature verification, whereas in RSA, signature verification is faster than signature generation (if the public and private exponents, respectively, are chosen for this property, which is the usual case). NIST claims that it is an advantage of DSA that signing is faster, but many people in cryptography think that it is better for verification to be the faster operation. 

DSA is based on the difficulty of computing discrete logarithm. The algorithm is generally considered secure when the key size is large enough. DSS was originally proposed by NIST with a fixed 512-bit key size. After much criticism that this is not secure enough especially for long-term security, NIST revised DSS to allow key sizes up to 1024 bits.

The particular form of the discrete logarithm problem used in DSA is to compute discrete logarithms in certain subgroups in the finite field GF(p) for some prime p. The problem was first proposed for cryptographic use in 1989. Even though no attacks have been reported on this form of the discrete logarithm problem, further analysis is necessary to fully understand the difficulty of the problem. 

David Kravitz, former member of the NSA, holds a patent on DSA. Claus P. Schnorr has asserted that his patent covers certain implementations of DSA. 

Since the time Diffie and Hellman introduced the concept of digital signatures, many signature schemes have been proposed in cryptographic literature. These schemes can be categorized as either conventional digital signature schemes (e.g., RSA, DSA) or special signature schemes depending on their security features.

In a conventional signature scheme (the original model defined by Diffie and Hellman), we generally assume the following situation:

The signer knows the contents of the message that he has signed.
Anyone who knows the public key of the signer can verify the correctness of the signature at any time without any consent or input from the signer. (Digital signature schemes with this property are called self-authenticating signature schemes.)

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