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Encryption Decryption Interview Questions

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The McEliece cryptosystem is a public-key encryption algorithm based on algebraic coding theory. The system uses a class of error-correcting codes, known as the Goppa codes, for which fast decoding algorithms exist. The basic idea is to construct a Goppa code as the private key and disguise it as a general linear code, which is the public key. The general linear code cannot be easily decoded unless the corresponding private matrix is known. 

Merkle proposed a digital signature scheme that was based on both one-time signatures and a hash function and that provides an infinite tree of one-time signatures.

One-time signatures normally require the publishing of large amounts of data to authenticate many messages, since each signature can only be used once. Merkle's scheme solves the problem by implementing the signatures via a tree-like scheme. Each message to be signed corresponds a node in a tree, with each node consisting of the verification parameters that are used to sign a message and to authenticate the verification parameters of subsequent nodes. Although the number of messages that can be signed is limited by the size of the tree, the tree can be made arbitrarily large. Merkle's signature scheme is fairly efficient, since it requires only the application of hash functions. 

Probabilistic encryption, discovered by Goldwasser and Micali [GM84], is a design approach for encryption where a message is encrypted into one of many possible ciphertexts (not just a single ciphertext as in deterministic encryption), in such a way that it is provably as hard to obtain partial information about the message from the ciphertext, as it is to solve some hard problem. In previous approaches to encryption, even though it was not always known whether one could obtain such partial information, neither was it proved that one could not do so. 

The Rabin signature scheme is a variant of the RSA signature scheme. It has the advantage over RSA that finding the private key and forgery are both provably as hard as factoring. Verification is faster than signing, as with RSA signatures. In Rabin's scheme, the public key is an integer n where n = pq, and p and q are prime numbers which form the private key. The message to be signed must have a square root mod n; otherwise, it has to be modified slightly. Only about 1/4 of all possible messages have square roots mod n. 

RSA is combined with a secret-key cryptosystem, such as DES, to encrypt a message by means of an RSA digital envelope.

Suppose Alice wishes to send an encrypted message to Bob. She first encrypts the message with DES, using a randomly chosen DES key. Then she looks up Bob's public key and uses it to encrypt the DES key. The DES-encrypted message and the RSA-encrypted DES key together form the RSA digital envelope and are sent to Bob. Upon receiving the digital envelope, Bob decrypts the DES key with his private key, then uses the DES key to decrypt to message itself. This combines the high speed of DES with the key-management convenience of RSA. 

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