Self-Synchronising Timestamp-Keyed Encryption for LSB Image Steganography with In-Image Key Transport

Abstract

Background: Hybrid schemes that combine lightweight encryption with image steganography are attractive for resource-constrained environments, but time-derived keys introduce an operational problem that is rarely addressed: sender and receiver must agree on the exact key material, and clock drift of a single second causes decryption to fail. Objective: This study proposes a timestamp-keyed substitution cipher whose key material travels inside the stego image itself, removing the need for clock synchronisation or a separate key channel while retaining session-dependent ciphertext variability. Methods: A position-dependent shift sequence is derived from timestamp components combined with a pre-shared key. The timestamp is written into a reserved header region of the cover image together with the ciphertext length, so that the receiver recovers the shift sequence directly from the stego image. Embedding uses single-bit least-significant-bit substitution on the 24-bit RGB pixel data of a BMP cover. The two key components serve distinct roles: the timestamp provides inter-session variability and is transmitted openly, whereas the pre-shared key provides confidentiality and is never transmitted. Results: Across five 512 × 512 USC-SIPI cover images, the scheme achieves a mean PSNR of 77.8575 ± 0.0699 dB and an NPCR of 0.106506 ± 0.00172% at a payload of 199 characters (0.002116 bpp), with message-level BER of zero across all tested payloads from 50 to 2000 characters, confirming lossless recovery. Ciphertext variability across different transmission times reaches 60.30%, compared with 0.00% for a fixed-shift Caesar baseline, and the byte-frequency entropy rises from 4.2417 to 5.8035. Chi-square and RS analysis do not separate the stego images from their covers, the RM and R−M curves differing by 0.071 percentage points. The complete pipeline executes in 3.606 ± 0.283 ms per image. Originality/Value: The contribution is the in-image transport of time-derived key material, which makes a time-keyed cipher deployable without synchronised clocks. The roles of the transmitted timestamp and the withheld pre-shared key are separated explicitly, and the security claims are bounded accordingly. Conclusion: The scheme offers a reproducible, self-contained mechanism for lightweight confidential data hiding. Its limitations, including the periodicity of the cipher and the absence of validation against learning-based steganalysis, are stated explicitly.