"That only works if I own the domain. Most people get their email from a higher level provider, which also own the DNS record. It is easy to redirect selected queries." Semantically email@example.com carries the same global meaning in connection with working relationships, name delegation and a potential certificate chain as joe.example.com, though I accept that the syntax differs. "And even if I own the domain, a simple change at my domain registrar would suffice to redirect queries to another server. Or at a root server; as you say, you have to trust the certificate chain down to the root. How do you defend against that?" If you trust the .com root key signing certificate, and .com registry has signed the example.com certificate, to assert that this key genuinely belongs to the owner of example.com, and the example.com key has similarly been used to sign the joe.example.com or even the firstname.lastname@example.org certificate, then a verifiable chain of trust exists. If you don't like or trust the .com key-signing certificate, you can register example.co.uk or example.de . In fact any community that wants to establish a certification tree can root this at any DNS domain or subdomain that they elect to use for this purpose. As Joe, you defend against example.com reallocating joe.example.com to someone else in the same way that you prevent .com reallocating example.com to someone else if you are example.com . If you don't trust the next label up the chain reallocating your identity in breach of contract, find a domain parent you do trust to keep to its contract. If the standards agreed for this purpose place references to locations for certificates at regular and known places within the DNS then a chain enabling a certificate to be located and verfied based on knowledge of the the location of the root and trust in the root certificate, will exist so long as all parent domains to yours up to the root server operate in accordance with the agreed standard for achieving this. Relocation is no more of an issue than it already is if the IP address of a DNS server containing delegation records for your domain is changed - the next level up has to be updated to contain the new address. If a domain's DNS is spoofed e.g. through a cache poisoning attack, this will create the possibility of a denial of service attack in the sense that the spoofed domain location will not contain a certificate for the domain certificated by its parent, making the genuine domain uncontactable until the redirection is resolved. Generally domain parents don't do the reputation of their own domain any good by playing fast and loose with identities or breaching contracts with customers, but if a domain registration contract has been allowed to lapse, there is nothing to prevent the identities concerned from being reallocated, just as there is nothing to prevent someone else from having the same name as me or someone given the family name: McDonald at birth from opening a restaurant under their own name. Cryptography will always be seen to fail if the standard required for success is for it to be seen as solving all of the world's honesty, security and identity problems. This doesn't prevent crypto from being used more universally and easily than it now is. Personally I can see some good reasons for the set of working relationships we know as the DNS to be extended in this manner. I'm not interested in evaluating this against the assumption that this has to give "perfect security" in all situations as all proposals will fail based on such a criterion. I am interested in whether this enables us achieve better security than what we now have in practice more readily than other possible approaches.
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