> For CSAM matches, the cryptographic header in the voucher combines with the server-side blinding secret (that was used to blind the known CSAM database at setup time) to successfully decrypt the outer layer of encryption.
In the text you referenced, it specifically says that the blinding key would be needed to decrypt vouchers which are CSAM matches. This is because Apple set up their CSAM database in a blinded manner. Therefore to access a hash from the database from which to derive a decryption key, Apple would need the blinding key to first decrypt that hash value.
However, and attacker would be generating their own (presumably unblinded) database, and therefore wouldn't need to access Apple's blinding key.
