Unformatted text preview: Alice encrypts the hash with her private key, thereby signing the document. (3) Alice sends the document and the signed hash to Bob. (4) Bob produces a one-way hash of the document that Alice sent. He then, using the digital signature algorithm, decrypts the signed hash with Alice’s public key. If the signed hash matches the hash he generated, the signature is valid. Speed increases drastically and, since the chances of two different documents having the same 160-bit hash are only one in 2160, anyone can safely equate a signature of the hash with a signature of the document. If a non-one-way hash function were used, it would be an easy matter to create multiple documents that hashed to the same value, so that anyone signing a particular document would be duped into signing a multitude of documents. This protocol has other benefits. First, the signature can be kept separate from the document. Second, the recipient’s storage requirements for the document and signature are much smaller. An archival system can use this type of protocol to verify the existence of documents without storing their contents. The central database could just store the hashes of files. It doesn’t have to see the files at all; users submit their hashes to the database, and the database timestamps the submissions and stores them. If there is any disagreement in the future about who created a document and when, the database could resolve it by finding the hash in its files. This system has vast implications concerning privacy: Alice could copyright a document but still keep the document secret. Only if she wished to prove her copyright would she have to make the document public. (See Section 4.1). Algorithms and Terminology
There are many digital signature algorithms. All of them are public-key algorithms with secret information to sign documents and public information to verify signatures. Sometimes the signing process is called encrypting with a private key and the verification process is called decrypting with a public key....
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips