When a file is too long to be processed directly by a digital signature algorith

Question

When a file is too long to be processed directly by a digital signature algorithm, a hash of the document is computed and then signed. Which properties do you require from this hash function to prevent an attacker from forging signatures? Discuss the differences (if any) where the attacker only knows the messages signed by the victim versus where the attacker can choose the messages that the victim will sign, Discuss the differences (if any) where the attacker has control over the content of the forged message versus where the attacker has no such control. Experts disagree on the relative strength of proxy servers and packet filtering firewalls. What do you think, each sides argument is? Search the Internet, Examine the arguments and justify your own verdict on this disagreement. Research and find two breech notification information (via web, blog, etc.) issued from actual breeches. Discuss the similarities (if any) and the differences (if any) between the two.

Explanation / Answer

Digital signatures are based on asymetric cryptography. Now in hashing value of hash remains unique which is based on to the hashed data.Therefore even a slight change in data will produce a different hash. Therefore even if attacker has control over the content of forged message, he wont be able to access it since the data is forged and hash value will change automatically.

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