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Here B starts with a document that it is required to prove only could come from B
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This could be a piece of software that we wish to know comes from a reputable source
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We combine software with a "certificate" (a statement that B is Bob) and either encrypt this with dB or more normally encrypt a message digest (that depends on both message and signature) with dB
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This use of a message digest is done for performance as it is time consuming to use public key encryption on full message
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Note this signature cannot be forged either by A or any other person pretending to be B.
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In secret key version A shares B's secret key and can forge messages that purport to be from B
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