Mutables' RSA keys are spec'd at 2048 bits

Some code existed to allow tests to shorten this and it's
conceptually possible a modified client produced mutables
with different key-sizes. However, the spec says that they
must be 2048 bits. If you happen to have a capability with
a key-size different from 2048 you may use 1.17.1 or earlier
to read the content.