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The  public-  key infrastructure (PKI) and certificate authorities (CAs)

In order for digital signatures and  public-  key encryption to be effective it is necessary to be sure 

that the public key intended for decryption of a document actually belongs to the person you 

believe is sending you the document. The developing solution to this problem is the issuance 

by a trusted third party (TTP) of a message containing owner identification information and a 

copy of the public key of that person. The TTPs are usually referred to as