Premise:
"digital signature and public-key infrastructure"
Hypothesis: The signature is digital.

yes

Premise:
"You saw all of this? asked San'doro."
Hypothesis: San'doro wants to know if you saw the crime take place?

it is not possible to tell

Premise:
"We've had a letter from young Beresford, said Mr. Carter, coming to the point at once."
Hypothesis: A letter from Beresford is here according to Mr. Carter.
yes