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 Some public-key algorithms can be used to generate digital signatures. A digital signature is a small amount of data that was created using some secret key, and there is a public key that can be used to verify that the signature was really generated using the corresponding private key. The algorithm used to generate the signature must be such that without knowing the secret key it is not possible to create a signature that would verify as valid. Digital signatures are used to verify that a message really comes from the claimed sender (assuming only the sender knows the secret key corresponding to his/her public key). They can also be used to timestamp documents: a trusted party signs the document and its timestamp with his/her secret key, thus testifying that the document existed at the stated time. Digital signatures can also be used to testify (or certify) that a public key belongs to a particular person. This is done by signing the combination of the key and the information about its owner by a trusted key. The digital signature by a third party (owner of the trusted key), the public key and information about the owner of the public key are often called certificates. The reason for trusting that third party key may again be that it was signed by another trusted key. Eventually some key must be a root of the trust hierarchy (that is, it is not trusted because it was signed by somebody, but because you believe a priori that the key can be trusted). In a centralized key infrastructure there are very few roots in the trust network (e.g., trusted government agencies; such roots are also called certification authorities). In a distributed infrastructure there need not be any universally accepted roots, and each party may have different trusted roots (such of the party's own key and any keys signed by it). This is the web of trust concept used in e.g. PGP. A digital signature of an arbitrary document is typically created by computing a message digest from the document, and concatenating it with information about the signer, a timestamp, etc. The resulting string is then encrypted using the private key of the signer using a suitable algorithm. The resulting encrypted block of bits is the signature. It is often distributed together with information about the public key that was used to sign it. To verify a signature, the recipient first determines whether it trusts that the key belongs to the person it is supposed to belong to (using the web of trust or a priori knowledge), and then decrypts the signature using the public key of the person. If the signature decrypts properly and the information matches that of the message (proper message digest etc.), the signature is accepted as valid. Several methods for making and verifying digital signatures are freely available. The most widely known algorithm is RSA. |
