concept PKIApplication
  purpose secure communication through the use of Public Key Infrastructure (PKI) principles.

  state
    users: set User
    keys: User -> set KeyPair
    approvedKeys: User -> set KeyPair

  actions
    createUser(u: User)
      add u to users
      // Additional initialization steps if needed

    generateKeyPair(u: User)
      pre u in users
      let newKeyPair = generateDisposableKeyPair()
      add newKeyPair to keys[u]

    approveKey(u: User, keyPair: KeyPair)
      pre u in users and keyPair in keys[u]
      add keyPair to approvedKeys[u]

    signMessage(u: User, message: Message)
      pre u in users
      let keyPair = selectKeyPair(u)
      let signature = sign(message, keyPair)
      // Additional steps to handle the signature

    verifySignature(message: Message)
      pre message is signed
      let signer = extractSigner(message)
      let signature = extractSignature(message)
      assert signature is valid for signer's approved keys

    buildWebOfTrust(u: User)
      pre u in users
      let userKeys = keys[u]
      let webOfTrustTree = buildHashTree(userKeys)
      // Additional steps to manage the web of trust structure

  operational principle
    after approveKey(u, keyPair), keyPair in approvedKeys[u]
    after buildWebOfTrust(u), u's web of trust is established
