Raw RSA binary string and public key 'detection'

Been looking at the Telnic (dev.telnic.org) effort.

In essence; NAPTR dns records which contain private details such as a
phone number. These are encrypted against the public keys of your
friends (so if you have 20 friends and 3 phone numbers visible to all
friends - you need 20 subdomains x 3 NAPTR entries under your 'master').

Aside from the practicality of this - given a raw RSA encrypted block
and a list of public keys - is there any risk that someone could
establish which of those public keys may have been used to create that
block ? I.e. something which would be done in bulk for large
populations; so the use of large tables and what not is quite warranted.

Thanks,

Dw

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