4. acme-tls/1 Protocol Definition

4. acme-tls/1 Protocol Definition

The acme-tls/1 protocol MUST only be used for validating ACME tls-alpn-01 challenges. The protocol consists of a TLS handshake in which the required validation information is transmitted. The acme-tls/1 protocol does not carry application data. Once the handshake is completed, the client MUST NOT exchange any further data with the server and MUST immediately close the connection. While this protocol uses X.509 certificates, it does not use the authentication method described in [RFC5280] and, as such, does not require a valid signature on the provided certificate nor require the TLS handshake to complete successfully. An ACME server may wish to use an off-the-shelf TLS stack where it is not simple to allow these divergences in the protocol as defined. Because of this, an ACME server MAY choose to withhold authorization if either the certificate signature is invalid or the handshake doesn't fully complete.

ACME servers that implement acme-tls/1 MUST only negotiate TLS 1.2 [RFC5246] or higher when connecting to clients for validation.