What we're talking about here are trust roots and one of the things people usually get wrong when they try to draw a diagram of how this works is they draw certificates as nodes and then just connect those nodes together with abstract lines.
Actually the correct mental model is a graph of public keys and the lines between them (joining two nodes directionally) are the certificates.
So although we traditionally handle the root trust set as a bunch of self-signed certificates, those certificates are largely unimportant, what's vital is the public keys baked inside them.
As a result what makes older roots obsolete is not a signature algorithm, but the type and size of key chosen when they were created, that key is in a very real sense the root.
This AddTrust root for example was 2048-bit RSA. You can use that size of RSA key today for your funny cat video site, no problem, but it's clearly an inadequate choice for a CA root. Fortunately roots like this are gradually expiring.
If I figure out how to build a machine that can break 2048-bit RSA keys for $10M per key it makes no sense to target your cat videos. But a CA root is an attractive target. So we'd like to have more margin for the roots not the same or less.
Some years ago Mozilla finally prohibited 1024-bit RSA keys in roots. Some of the oldest roots in the business were 1024-bit RSA, which today would not be considered acceptable even on your cat video site, but when those roots were created 1024-bit RSA seemed safe enough.
In 5-10 years you'd probably want as much as possible for roots to be the more compact elliptic curve public keys, maybe there will be some better (more secure) curves in use by then, maybe not.
Actually the correct mental model is a graph of public keys and the lines between them (joining two nodes directionally) are the certificates.
So although we traditionally handle the root trust set as a bunch of self-signed certificates, those certificates are largely unimportant, what's vital is the public keys baked inside them.
As a result what makes older roots obsolete is not a signature algorithm, but the type and size of key chosen when they were created, that key is in a very real sense the root.
This AddTrust root for example was 2048-bit RSA. You can use that size of RSA key today for your funny cat video site, no problem, but it's clearly an inadequate choice for a CA root. Fortunately roots like this are gradually expiring.
If I figure out how to build a machine that can break 2048-bit RSA keys for $10M per key it makes no sense to target your cat videos. But a CA root is an attractive target. So we'd like to have more margin for the roots not the same or less.
Some years ago Mozilla finally prohibited 1024-bit RSA keys in roots. Some of the oldest roots in the business were 1024-bit RSA, which today would not be considered acceptable even on your cat video site, but when those roots were created 1024-bit RSA seemed safe enough.
In 5-10 years you'd probably want as much as possible for roots to be the more compact elliptic curve public keys, maybe there will be some better (more secure) curves in use by then, maybe not.