Digital Signature - The above scheme...
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The above scheme has some problems in terms of processing and communications overhead costs. Encryption and decryption have to be applied to the entire message contents and the volume of data sent is at least double the original message size. To overcome this problem, a hash function, which is a one-way function that maps values from a large domain into a comparatively small range, is used to generate from the content of the message to be signed, a fixed – length data item known as a message digest. The digest has the property that, in general, any change to the message will result in a different digest.
With this scheme, the originating system applies the hash function to obtain the digest, then RSA encrypts the digest to give the signature which is transmitted with the message, upon receiving the message, the recipient’s system re-computes the digest, and also RSA decrypts the signature.
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