Original text:
The following metadata items are defined, all of which are OPTIONAL: <del> A URI identifying the current terms of service. An HTTP or HTTPS URL locating a website providing more information about the ACME server. </del>  <del> Each string in this array MUST be a lowercase hostname which the ACME server recognises as referring to itself for the purposes of CAA record validation as defined in RFC6844.  This allows clients to determine the correct issuer domain name to use when configuring CAA record. </del>  <del> An HTTP or HTTPS URI indicating a website where a user can perform account-managment functions.  For example, this website might allow a user to associate a new account key pair to an account by proving possession of a registered contact, in the event that the user loses the account private key. </del>  Clients access the directory by sending a GET request to the directory URI. 6.3.2.  If a client no longer wishes to have an account key registered with the server, it may request that the server delete its account by sending a POST request to the account URI containing the "delete" 10.3.  As a protocol run over HTTPS, standard considerations for TCP-based and HTTP-based DoS mitigation also apply to ACME. or the rate at which validation can be requested within a given subtree of the DNS. <del> 10.4. </del>  The controls on issuance enabled by ACME are focused on validating that a certificate applicant controls the identifier he claims.

Reviewer comments:
This reverts commit 5ea8bb3afe51fa14b34593341b895e0262b454ca, reversing changes made to 278c6e8ab49e3013708cbf544c76df868973635f.

Revised text:
The following metadata items are defined, all of which are OPTIONAL:  <ins> A string containing a URI identifying the current terms of service. </ins>  <ins> String containing an HTTP or HTTPS URL locating a website providing more information about the ACME server. </ins>  <ins> If present, MUST be an array of strings.  Each string MUST be a lowercase hostname which the ACME server recognises as referring to itself for the purposes of CAA record validation as defined in RFC6844.  This allows clients to determine the correct issuer domain name to use when configuring CAA record. </ins> Clients access the directory by sending a GET request to the directory URI. 6.3.2. <ins> Once a client has created an account with an ACME server, it is possible that the private key for the account will be lost.  The recovery contacts included in the registration allows the client to recover from this situation, as long as it still has access to these contacts. By "recovery", we mean that the information associated with an old account key is bound to a new account key.  When a recovery process succeeds, the server provides the client with a new registration whose contents are the same as base registration object - except for the "key" field, which is set to the new account key.  The server reassigns resources associated with the base registration to the new registration (e.g., authorizations and certificates).  The server SHOULD delete the old registration resource after it has been used as a base for recovery. In addition to the recovery mechanisms defined by ACME, individual client implementations may also offer implementation-specific recovery mechanisms.  For example, if a client creates account keys deterministically from a seed value, then this seed could be used to recover the account key by re-generating it.  Or an implementation could escrow an encrypted copy of the account key with a cloud storage provider, and give the encryption key to the user as a recovery value. The client requests recovery by asking that the server send a message to one of the contact URIs registered for the account.  That message indicates some action that the server requires the client's user to perform, e.g., clicking a link in an email.  If the user successfully completes the server's required actions, then the server will bind the account to the new account key. (Note that this process is almost entirely out of band with respect to ACME.  ACME only allows the client to initiate the process, and the server to indicate the result.) To initiate recovery, the client sends a POST request to the server's recover-registration URI, with a body specifying which registration is to be recovered.  The body of the request MUST be signed by the client's new account key pair. The string "contact" The URI for the registration to be recovered. If the server agrees to attempt recovery, then it creates a new registration resource containing a stub registration object.  The stub registration has the client's new account key and contacts, but no authorizations or certificates associated.  The server returns the stub contact in a 201 (Created) response, along with a Location header field indicating the URI for the new registration resource (which will be the registration URI if the recovery succeeds). After recovery has been initiated, the server follows its chosen recovery process, out-of-band to ACME.  While the recovery process is ongoing, the client may poll the registration resource's URI for status, by sending a POST request with a trivial body ({"resource":"reg"}).  If the recovery process is still pending, the server sends a 202 (Accepted) status code, and a Retry-After header field.  If the recovery process has failed, the server sends an error code (e.g., 404), and SHOULD delete the stub registration resource. If the recovery process has succeeded, then the server will send a 200 (OK) response, containing the full registration object, with any necessary information copied from the old registration).  The client may now use this in the same way as if he had gotten it from a new- registration transaction. 6.3.3. </ins> If a client no longer wishes to have an account key registered with the server, it may request that the server delete its account by sending a POST request to the account URI containing the "delete" 10.3. <ins> The account recovery processes described in account-recovery allow authorization to be transferred from one account key to another, in case the former account key pair's private key is lost.  ACME needs to prevent these processes from being exploited by an attacker to hijack the authorizations attached to one key and assign them to a key of the attacker's choosing. Recovery takes place in two steps: 1.  Provisioning recovery information (contact or recovery key) 2.  Using recovery information to recover an account The provisioning process needs to ensure that only the account key holder ends up with information that is useful for recovery.  The recovery process needs to assure that only the (now former) account key holder can successfully execute recovery, i.e., that this entity is the only one that can choose the new account key that receives the capabilities held by the account being recovered. Account recovery uses both the ACME channel and the contact channel. The provisioning process is only visible to an ACME MitM, and even then, the MitM can only observe the contact information provided.  If the ACME attacker does not also have access to the contact channel, there is no risk. The security of the recovery process is entirely dependent on the security of the contact channel.  The details of this will depend on the specific out-of-band technique used by the server.  For example: If the server requires a user to click a link in a message sent to a contact address, then the contact channel will need to ensure that the message is only available to the legitimate owner of the contact address.  Otherwise, a passive attacker could see the link and click it first, or an active attacker could redirect the message. If the server requires a user to respond to a message sent to a contact address containing a secret value, then the contact channel will need to ensure that an attacker cannot observe the secret value and spoof a message from the contact address. In practice, many contact channels that can be used to reach many clients do not provide strong assurances of the types noted above. In designing and deploying recovery schemes, ACME servers operators will need to find an appropriate balance between using contact channels that can reach many clients and using contact-based recovery schemes that achieve an appropriate level of risk using those contact channels. 10.4. </ins> As a protocol run over HTTPS, standard considerations for TCP-based and HTTP-based DoS mitigation also apply to ACME. or the rate at which validation can be requested within a given subtree of the DNS.  <ins> 10.5. </ins> The controls on issuance enabled by ACME are focused on validating that a certificate applicant controls the identifier he claims.