Aggregated transaction data from a transaction data provider may be encrypted and exchanged with a content item selection system using commutative encryption algorithms. The transaction data provider and content item selection system may utilize a set of common identifiers that are each encrypted using a respective commutative encryption algorithm of the transaction data provider or content item selection system. The other of the transaction data provider or content item selection system encrypts the single-encrypted common identifier using a respective commutative encryption algorithm to generate double encrypted common identifiers. The double encrypted common identifiers may be used to match a set of common identifiers with transaction data. The transaction data may be encrypted and/or may include random offset values.