Secure Hash

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the secret may be a one time key shared by both the sender and the recipient

this guarentees the identity of the sender

as the neither the hashed document  nor the encyption key that should be  replicated are known .

as, being used once,  the one time key used for hash encyption can not be guessed , even with a quantum computer. 

 will lead to an odd of  1/ 8 500  000   000 for a forged document not  to be  detected as such.

a  1Gbyte card can fit  8bn  33bit keys, enough to power transactions with or e-cash system for  150 million devices.

the process can be used to secure emails between the client and the mail server.

Files can be stored and their integrity checked .

Web pages integrity ,  including their embeded  pictures,  can be checked while being downloaded.

  to be quantum safe, the encryption key needs to be made of Qbits as otherwise, a quantum computer may compute at once all the encryptions of a document with all possible keys, save for one bit that remains set for instance to zero , store them in a quantum memory, and then check whether the official document encryption is comprised in the quantum memory. It can thus find out all the bits of the encryption key one by one, whereas, with a quantum key, this mechanism is not possible. 

  However an  encryption algorithm that would need considerably more Qbits than present quantum computers have to recompute the secret keys could be considered as momentarily quantum safe.  

 Traditionally, signatures are made of encrypted hash of a file. This method would need to apply an encryption of the hash with a quantum key to be quantum safe.