Would it be hard to support MD5E keys that omit the -sSIZE part, the way this is allowed for URL keys? I have a use case where I have the MD5 hashes and filenames of files stored in the cloud, but not their sizes, and want to construct keys for these files to use with setpresentkey and registerurl. I could construct URL keys, but then I lose the error-checking and have to set annex.security.allow-unverified-downloads . Or maybe, extend URL keys to permit an -hMD5 hash to be part of the key?

Another (and more generally useful) solution would be alternate keys for same content. Then can start with a URL-based key but then attach an MD5 to it as metadata, and have the key treated as a checksum-containing key, without needing to migrate the contents to a new key.