As the ultimate goal of steganalysis, secret message extraction plays a decisive role in obtaining secret communication evidence and cracking down on criminal activities. For STC (Syndrome-Trellis Codes)-based adaptive steganography, existing pioneering work on secret message extraction: the method based on run test under plaintext embedding may misjudge incorrect stego key as correct stego key, resulting in the failure of extraction. To avoid such a situation, this manuscript proposed a secret message extraction method based on optimal hypothesis test with 100% accuracy under plaintext embedding. First, it is proved that there is a probability distribution difference between the sub-sequence extracted by correct and incorrect stego key. Then, based on the difference, an optimal hypothesis test model is designed to recover the correct stego key. Finally, given the probability of type I and II errors, the sample size and threshold in the hypothesis test are derived. Classic adaptive steganography such as HUGO (Highly Undetectable Steganography) and J-UNIWARD (JPEG Universal Wavelet Relative Distortion) have been conducted experiment, showing that the proposed method can extract message with 100% accuracy and 44 bits sample size, which verifies the correctness of the theorem and the effectiveness of the method.

Compared with traditional steganography, adaptive steganography based on STC (Syndrome-Trellis Codes) has extremely high antidetection ability and has been a mainstream and hot research direction in the field of information hiding over the past decades. However, it is noted, in specific scenarios, that a small number of methods can extract data from STC-based adaptive steganography, indicating security risks related to such algorithms. In this manuscript, the cryptographic secrecy of this kind of steganography is analyzed, on condition of two common attacks: stego-only attack and known-cover attack, respectively, from three perspectives: steganographic key equivocation, message equivocation, and unicity distance of the steganographic key. Focusing on the special layout characteristics of the parity-check matrix of STC, under the two attack conditions, the theoretical boundaries of the steganographic key equivocation function, the message equivocation function, and the unicity distance of the steganographic key are separately obtained, showing the impact of the three elements: the submatrix size, the randomness of the data, and the cover object on the cryptographic secrecy of the STC-based adaptive steganography, resulting in a theoretical reference to accurately judge the cryptographic secrecy of such steganography and design more secure steganography methods.