Many Wireless Sensor Networks (WSNs) are used to collect and process confidential information. Confidentiality must be ensured at all times and, for example, solutions for confidential communication, processing or storage are required. To date, the research community has addressed mainly the issue of confidential communication. Efficient solutions for cryptographically secured communication and associated key exchange in WSNs exist. Many WSN applications, however, rely heavily on available on-node storage space and therefore it is essential to ensure the confidentiality of stored data as well. In this paper we present Codo, a confidential data storage solution which balances platform, performance and security requirements. We implement Codo for the Contiki WSN operating system and evaluate its performance.