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Some people choose to upload their public key, with others' signatures, to "key servers", which help to automate the process of exchanging keys. If a person chooses to "marginally" trust a key, then that key can be used to help establish the authenticity of a public key that is has been used to sign; if three marginally trusted keys have been used to sign a single key, that key is considered verified by the web of trust (but this is only allowed up to two levels).