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applied cryptography - protocols, algorithms, and source code in c

# Since the bank already cashed a check from alice with

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Unformatted text preview: e is any alteration to the document, the signature can no longer be verified with Alice’s public key. 5. The signature cannot be repudiated. Bob doesn’t need Alice’s help to verify her signature. Signing Documents and Timestamps Actually, Bob can cheat Alice in certain circumstances. He can reuse the document and signature together. This is no problem if Alice signed a contract (what’s another copy of the same contract, more or less?), but it can be very exciting if Alice signed a digital check. Let’s say Alice sends Bob a signed digital check for \$100. Bob takes the check to the bank, which verifies the signature and moves the money from one account to the other. Bob, who is an unscrupulous character, saves a copy of the digital check. The following week, he again takes it to the bank (or maybe to a different bank). The bank verifies the signature and moves the money from one account to the other. If Alice never balances her checkbook, Bob can keep this up for years. Consequently, digital signatures often include timestamps. The date and time of the signature are attached to the message and signed along with the rest of the message. The bank stores this timestamp in a database. Now, when Bob tries to cash Alice’s check a second time, the bank checks the timestamp against its database. Since the bank already cashed a check from Alice with the same timestamp, the bank calls the police. Bob then spends 15 years in Leavenworth prison reading up on cryptographic protocols. Signing Documents with Public-Key Cryptography and One-Way Hash Functions In practical implementations, public-key algorithms are often too inefficient to sign long documents. To save time, digital signature protocols are often implemented with one-way hash functions [432,433]. Instead of signing a document, Alice signs the hash of the document. In this protocol, both the one-way hash function and the digital signature algorithm are agreed upon beforehand. (1) Alice produces a one-way hash of a document. (2)...
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