In this paper, a novel scheme of scalable encryption for image data compressed by CCSDS recommended algorithm is proposed. The scheme can ensure end-to-end security by supporting low-complexity transparent transcoding directly on ciphertext bitstream without decryption and re-encryption. A structure of packet header is designed to hint the truncation points facilitating transcoding and store the initialization vector for encryption. Experiment results show that the transcoding can be operated with near rate-distortion optimal performance and relatively low overhead.