Web of Trust: What is "signatory trust"?

Question

I am learning about the Web of Trust and how to use it. However, I have some issues to fully grasp the concept of Signatory Trust.

What is an easily understandable definition of signatory trust? An example would be very helpful.

Answer 1

Signatory Trust

Signatory trust is used to describe the situation of transitive certifications. Consider Alice signed Bob Alice -> Bob, and also puts owner trust on Bob (owner trust means, Alice puts faith in certification Bob issues). Now, if Bob signs Carol Bob -> Carol (but Alice doesn't), Alice cannot directly verify Carol's key (she didn't certify it herself), but she can build a trust path:

Alice -> Bob -> Carol

Remember that these transient trust steps always require three components, a certification from a trusted and already valid key. PGP/GnuPG use a more advanced trust model with marginal and full trust, but the basic principle is the same.

Trust Categories in the Example

In this situation, we have different kinds of trust:

• trust in identity (Alice is sure about Bob's key ownership, and Bob is sure about Carol's key ownership)
• owner trust: do I trust the key's owner to certify other keys? (Alice trusts Bob's certifications)
• key validity (the German Wikipedia uses "legitimacy" here): Is there a trust path to the other key, based on the first two kinds of trust?
  • trust path from Alice to herself is trivial (same for Bob to Bob and Carol to Carol)
  • direct trust from Alice to Bob, and Bob to Carol
  • signatory trust: transient trust path from Alice to Carol, over Bob's trusted and valid key
  • Bob cannot put trust on Alice' key, Carol can neither trust Alice nor Bob (no trust paths)