Three-dimensional image encryption method based on structured light illumination and iterative layered angular spectrum algorithm

Abstract

The application discloses a three-dimensional image encryption method based on structured light illumination and an iterative layered angular spectrum algorithm, which comprises an encryption step: using the iterative layered angular spectrum algorithm under the structured light illumination to encrypt a three-dimensional image into a phase-type hologram ciphertext; and a decryption step: using a decryption digital key to construct a decryption structured phase mask, and then generating a decryption structured light for illuminating the phase-type hologram ciphertext; after the decryption structured light is phase-modulated by the phase-type hologram ciphertext, the decryption structured light is forward diffracted and propagated, so that a decrypted three-dimensional image is formed. The decryption step can be completed by using a holographic imaging device composed of a laser light source, a polarization modulation device, a microscopic objective lens, a pinhole, a collimating lens and two phase modulation devices. The method has the advantages of large key space, high security, high decryption quality and simple optical decryption device structure.

Description

Technical Field

[0001] This invention relates to the field of information security technology, specifically to a three-dimensional image encryption method based on structured light illumination and iterative hierarchical angular spectrum algorithm. Background Technology

[0002] With the rapid development of information technology, image transmission and interaction are becoming increasingly frequent, but they also face serious security problems such as being spied on, stolen, and tampered with. Therefore, image encryption technology has become a research hotspot in the field of information security. Optical image encryption technology has advantages such as high speed, high parallelism, and high degree of freedom, showing broad application prospects in the field of image encryption. Since Refregier and Javidi first proposed the double random phase coding method in 1995, it has received widespread attention from researchers both domestically and internationally. In recent years, various cutting-edge technologies such as ghost imaging, metasurfaces, and compressed sensing have been successively introduced into optical image encryption schemes, promoting the rapid development of optical image encryption technology. However, current optical image encryption technology mainly focuses on two-dimensional images.

[0003] Compared to two-dimensional images, three-dimensional images have a larger information capacity and richer content, and they are also superior in expressing the true depth, position, and spatial relationships of target objects. Therefore, as the application of three-dimensional images becomes more and more widespread, protecting the information security of three-dimensional images has become increasingly important. Current research on three-dimensional image encryption mainly focuses on methods such as integrated imaging, diffraction imaging, digital holography, and computational holography. Among these, computational holography has attracted much attention due to its advantages such as flexible operation, ease of storage, and simple reconstruction. For example, Shi Yishi et al. used phase extraction-based three-dimensional information encoding and Fresnel domain double random phase encoding to achieve three-dimensional image encryption, but the decrypted three-dimensional information suffered from severe distortion and poor decryption quality. Piao et al. proposed a three-dimensional image encryption method based on cascaded Fresnel domain and fractional Fourier domain phase retrieval algorithms, but its security was insufficient due to the low sensitivity of some keys. In addition, Kong Dezhao et al. proposed a three-dimensional information encryption method based on computational holography and random phase encoding. Subsequently, they proposed a three-dimensional image encryption method based on Fourier domain computational holography and vector decomposition algorithms, and a three-dimensional scene layered encryption method based on layered angular spectrum algorithms and chaotic random phase masks. Subsequently, Han et al. proposed a 3D image encryption method based on the layered angular spectrum algorithm and iterative phase retrieval algorithm. Furthermore, Wang et al. proposed a 3D image encryption method combining the layered angular spectrum algorithm, the dual-phase method, and chaotic random phase masks. However, the above methods all involve phase keys with high complexity, which are difficult for users to remember or carry. Moreover, the types and number of digital keys are limited, leading to problems such as limited key space and insufficient security. To address this, this invention proposes a 3D image encryption method based on structured light illumination and the iterative layered angular spectrum algorithm, aiming to achieve 3D image encryption with a large key space, high security, and high decryption quality. Summary of the Invention

[0004] The purpose of this invention is to provide a three-dimensional image encryption method based on structured light illumination and an iterative layered angular spectrum algorithm. This invention utilizes an iterative layered angular spectrum algorithm under structured light illumination to encrypt a three-dimensional image into a phase-type holographic ciphertext, offering advantages such as a large key space, high security, and high decryption quality.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: a three-dimensional image encryption method based on structured light illumination and iterative hierarchical angular spectrum algorithm, characterized by comprising encryption steps:

[0006] S1: 3D Image Layering Processing: Using computer graphics software, the 3D image to be encrypted is sliced ​​into n parallel layers according to different depth information, where i = 1, 2, ..., n are layer numbers; the amplitude information of each layer is extracted, and the amplitude information of the i-th layer is denoted as the plaintext image L. i(x i ,y i Then, perform encryption step S2.

[0007] S2: Encryption Condition Setting: Set the encryption parameters used in the encryption step, including the encryption incident light wavelength, the optical parameters in the encryption structured phase mask, the distance between the encryption structured phase mask and the phase hologram ciphertext, and the distance between the phase hologram ciphertext and the n-layer plaintext image generated after slicing the 3D image. Then, execute encryption step S3.

[0008] S3: Generation and Illumination of Encrypted Structured Light: Based on the optical parameters in the set encrypted structured phase mask, the phase distribution of the encrypted structured phase mask is first calculated using the phase calculation formula. Then, the encrypted structured light is generated by phase modulation of a collimated plane wave of unit amplitude using the encrypted structured phase mask; therefore, the complex amplitude distribution of the encrypted structured light is equal to the phase distribution of the encrypted structured phase mask. Subsequently, the encrypted structured light generated after phase modulation by the encrypted structured phase mask diffracts forward unconstrainedly to the front surface of the phase-type hologram ciphertext. The complex amplitude of the encrypted structured light propagating forward to the front surface of the phase-type hologram ciphertext is calculated using the angular spectrum diffraction algorithm. Then, encryption step S4 is executed.

[0009] S4: Forward Diffraction Propagation: After phase modulation of the phase-type holographic ciphertext, the encrypted structured light continues to diffract and propagate forward, sequentially reaching the n plaintext image planes generated after slicing the 3D image. These n plaintext image planes are denoted as Plane1, Plane2, ..., Planen. The complex amplitude U of the encrypted structured light propagating forward to Planen is calculated using the angular spectrum diffraction algorithm. i (x i ,y i ), where i = 1, 2, ..., n. Then, the amplitude A of the encrypted structured light propagated forward to Planei is extracted and saved. i (x i ,y i ) and phase Where i = 1, 2, ..., n. Then, encryption step S5 is executed.

[0010] S5: Convergence Condition Judgment: The correlation coefficient (CC) is used as the evaluation function to calculate the correlation between the decrypted image and the plaintext image, thereby verifying the convergence condition. The amplitude A of the encrypted structured light propagating forward to Planei is obtained. i (x i ,y i After that, it is used as the i-th layer decrypted image, and the correlation coefficient CC between the i-th layer decrypted image and the i-th layer plaintext image is calculated. iWhere i = 1, 2, ..., n. After calculating the correlation coefficients for all layers, extract the minimum value min{CC} among all correlation coefficients. i}, where i = 1, 2, ..., n. When min{CC i If the value exceeds the preset threshold σ, i.e., the convergence condition is met, then the encryption step S7 is executed; when min{CC} i If the convergence condition is not met, and the preset threshold σ is not exceeded, then the phase of the encrypted structured light propagated forward to Planei is retained. The amplitude L of the plaintext image in the i-th layer remains unchanged. i (x i ,y i The amplitude A of the structured light transmitted forward to Planei is replaced by the encrypted structured light. i (x i ,y i Amplitude constraints are implemented to generate new complex amplitudes on Planei. Where i = 1, 2, ..., n. Then, execute encryption step S6.

[0011] S6: Backward diffraction propagation: The encrypted structured light generates a new complex amplitude after being amplitude-constrained on the Planei beam. The structured light begins to diffract backwards and reaches the back surface of the phase hologram ciphertext, where i = 1, 2, ..., n. The complex amplitude of the structured light propagating backwards to the back surface of the phase hologram ciphertext is calculated using the angular spectrum diffraction algorithm. Subsequently, the complex amplitude of the encrypted structured light is back-propagated to the ciphertext back surface of the phase hologram. The phase is calculated from the complex amplitude U0(x0,y0) of the structured light propagating forward to the front surface of the ciphertext in the phase hologram. for: Where arg(·) represents the phase extraction operation, followed by the use of the phase Update the phase of the phase hologram ciphertext Right now: Then, the encryption step S4 is executed again.

[0012] S7: Output of Phase-Type Holographic Ciphertext: When the convergence condition is met, output the phase of the phase-type holographic ciphertext in this iteration. The output is the encrypted ciphertext of the 3D image.

[0013] This completes the encryption process.

[0014] Decryption steps:

[0015] D1: Decryption Condition Setting: Set the decryption parameters used in the decryption step, including the wavelength of the incident light, the optical parameters in the structured phase mask, the distance between the structured phase mask and the phase hologram ciphertext, and the distance between the phase hologram ciphertext and the n-layer decrypted image. Then, execute decryption step D2.

[0016] D2: Generation and Illumination of Decrypted Structured Light: A decrypted structured light beam is generated by phase modulation of a collimated plane wave of unit amplitude using a decrypted structured phase mask. The generated decrypted structured light, after phase modulation by the decrypted structured phase mask, is then diffracted forward without constraint to illuminate the ciphertext of the phase-type hologram. The complex amplitude of the decrypted structured light propagating forward to the front surface of the phase-type hologram ciphertext is calculated using an angular spectrum diffraction algorithm. Next, perform decryption step D3.

[0017] D3: Forward Propagation Decryption: The decrypted structured light, after phase modulation of the ciphertext in the phase-type hologram, continues to propagate forward through diffraction. The complex amplitude of the decrypted structured light propagating forward to the Planeti decryption image plane of the i-th layer is calculated using the angular spectrum diffraction algorithm. Where i = 1, 2, ..., n. Then, the amplitude of the decrypted structured light propagated forward to Planei is extracted and saved. This represents the decryption result of the i-th layer, where i = 1, 2, ..., n. After obtaining the decryption result of each layer, the decryption results of all layers together constitute the decrypted 3D image.

[0018] This completes the decryption process.

[0019] The aforementioned three-dimensional image encryption method based on structured light illumination and iterative hierarchical angular spectrum algorithm includes at least one editable optical parameter in the phase distribution calculation formula of the encrypted structured phase mask in encryption step S3, which is used as a digital key.

[0020] The above-mentioned three-dimensional image encryption method based on structured light illumination and iterative hierarchical angular spectrum algorithm can be completed in decryption steps D2-D3 by either digital means or optical means.

[0021] Based on the above method, the present invention provides an optical decryption device, specifically a holographic imaging device based on structured light illumination, comprising a laser source 1 for providing incident light, and, sequentially arranged on the optical axis, a polarization modulation device 2 for modulating the polarization state of the incident light, a microscope objective 3 for converging the beam, a pinhole 4 for spatial filtering, a collimating lens 5 for collimating the beam, a phase modulation device 6 for carrying the phase distribution of the decryption structured phase mask, and a phase modulation device 7 for carrying the phase distribution of the ciphertext of the phase-type hologram. The polarization modulation device 2 is a polarizer or a half-wave plate; the pinhole 4 is placed at the focal point of the microscope objective 3, and the focal point of the microscope objective 3 coincides with the object-side focal point of the lens 5; the phase modulation devices 6 and 7 are phase masks or phase-type spatial light modulators.

[0022] The beneficial effects of this invention are as follows: (1) This invention uses the optical parameters of the structured phase mask as a digital key, which increases the types and number of digital keys, expands the key space, and improves the security of three-dimensional image encryption; (2) This invention introduces structured light illumination technology into the layered angular spectrum algorithm, which enhances the unreadableness and security of the ciphertext of the phase hologram and overcomes the common problem of plaintext outline appearance in three-dimensional image holograms calculated by the layered angular spectrum algorithm; (3) This invention uses the iterative layered angular spectrum algorithm under structured light illumination to encode three-dimensional image information into the ciphertext of the phase hologram, avoiding the problems caused by paraxial approximation. The calculation error effectively suppresses the speckle noise of the decrypted 3D image when it is correctly decrypted, and improves the quality of the decrypted 3D image; (4) The algorithm complexity of the 3D image encryption method proposed in this invention is only related to the number of layers of the 3D image and is not related to the complexity of the 3D image, so it can realize the encryption of complex 3D images; (5) The encryption step in this invention is completed digitally, while the decryption step can be completed digitally or optically. The holographic imaging device used for optical decryption has a simple optical path and is easy to build. The decrypted 3D image can be observed directly by the human eye or recorded by a camera. Attached Figure Description

[0023] Figure 1 This is a flowchart of the encryption steps in a 3D image encryption method based on structured light illumination and iterative hierarchical angular spectrum algorithm.

[0024] Figure 2 To aid in describing the iterative flowchart of the encryption steps, ESPM represents the encrypted structured phase mask, POHC represents the phase-type holographic ciphertext, ASD represents angular spectral diffraction, AC represents amplitude constraint, and PU represents phase update.

[0025] Figure 3 This is a flowchart of the decryption steps in a 3D image encryption method based on structured light illumination and iterative hierarchical angular spectrum algorithm.

[0026] Figure 4 The diagram shows an optical decryption device to aid in describing the decryption steps. In the diagram, 1 represents a laser source; 2 represents a polarization modulation device; 3 represents a microscope objective; 4 represents a pinhole; 5 represents a collimating lens; and 6 and 7 represent phase modulation devices.

[0027] Figure 5 (a) is the intensity map of the 3D train model; (b) is the depth map of the 3D train model; (c) is the encrypted structured phase mask; (d) is the ciphertext of the phase hologram.

[0028] Figure 6 (a1)-(a3) represent the decryption results at 1000mm, 1005mm, and 1010mm from the phase hologram ciphertext when all keys are correct; (b1)-(b3) represent the decryption results at 1000mm, 1005mm, and 1010mm from the phase hologram ciphertext when the decryption wavelength key λ deviates by 1nm and all other keys are correct; (c1)-(c3) represent the decryption results at 1mm from the phase hologram ciphertext when the decryption distance key z0 deviates by 1mm and all other keys are correct. The decryption results at 1000mm, 1005mm, and 1010mm of the ciphertext are shown in (d1)-(d3), respectively, when the decryption focal length key f deviates by 1mm and all other keys are correct, at 1000mm, 1005mm, and 1010mm of the ciphertext of the phase hologram. The decryption results at 1000mm, 1005mm, and 1010mm of the ciphertext of the phase hologram are shown in (e1)-(e3), respectively, when the decryption topological charge key P deviates by 1 and all other keys are correct.

[0029] Figure 7 (a) is the sensitivity curve for the wavelength key λ; (b) is the sensitivity curve for the distance key z0; (c) is the sensitivity curve for the focal length key f; (d) is the sensitivity curve for the topological charge key P. Detailed Implementation

[0030] The present invention will be further described below with reference to the embodiments and accompanying drawings, but this should not be construed as limiting the present invention.

[0031] Example: A 3D image encryption method based on structured light illumination and iterative hierarchical angular spectrum algorithm, including, for example... Figure 1 The encryption steps shown are appended to aid in the description of the encryption steps. Figure 2 (Iterative flowchart shown):

[0032] S1: 3D Image Layering Processing: Using computer graphics software, the 3D image to be encrypted is sliced ​​into n parallel layers according to different depth information, where i = 1, 2, ..., n are layer numbers; the amplitude information of each layer is extracted, and the amplitude information of the i-th layer is denoted as the plaintext image L. i (x i ,y i Then, perform encryption step S2.

[0033] S2: Encryption Condition Setting: Set the encryption parameters used in the encryption step, including the encryption incident light wavelength, the optical parameters in the encryption structured phase mask, the distance between the encryption structured phase mask and the phase hologram ciphertext, and the distance between the phase hologram ciphertext and the n-layer plaintext image generated after slicing the 3D image. Then, execute encryption step S3.

[0034] S3: Generation and Illumination of Encrypted Structured Light: Based on the optical parameters in the set encrypted structured phase mask, the phase distribution ESPM(ξ,η) of the encrypted structured phase mask is first calculated using the phase calculation formula of the structured phase mask. In this embodiment, the encrypted structured phase mask consists of a Fresnel zone plate and a radial Hilbert mask, and its phase calculation formula is as follows:

[0035]

[0036] In this context, arg(·) represents the phase extraction operation, j is the imaginary unit, λ is the wavelength of the encrypted incident light, r is the radius of the Fresnel zone plate, f is the focal length of the Fresnel zone plate, P is the topological charge number of the radial Hilbert mask, and θ is the spatial azimuth angle of the radial Hilbert mask. Then, the encrypted structured phase mask is used to phase-modulate a collimated plane wave of unit amplitude, thereby generating encrypted structured light. Therefore, the complex amplitude distribution of the encrypted structured light is equal to the phase distribution of the encrypted structured phase mask.

[0037] Subsequently, the encrypted structured light generated after phase modulation by the encrypted structured phase mask propagates forward without constraint to the front surface of the phase hologram ciphertext. The complex amplitude U0(x0,y0) of the encrypted structured light propagating forward to the front surface of the phase hologram ciphertext is:

[0038]

[0039] in, This represents the angular spectrum diffraction operation with wavelength λ and diffraction distance z0. FT(·) and IFT(·) represent the Fourier transform and inverse Fourier transform operations, respectively. k = 2π / λ is the wavenumber, z0 is the distance from the encrypted structured phase mask to the ciphertext of the phase hologram, and f x and f yThese are the spatial frequency coordinates. Next, encryption step S4 is executed.

[0040] S4: Forward diffraction propagation: The complex amplitude of the encrypted structured light on the back surface of the phase-type holographic ciphertext after phase modulation. for:

[0041]

[0042] in, For the phase of the ciphertext in a phase-type hologram, The initial value is a computer-generated random phase with element values ​​distributed between [0, 2π], and it is continuously updated in subsequent iterations. Afterwards, the encrypted structured light continues to diffract and propagate forward, sequentially reaching the n plaintext image planes generated after slicing the three-dimensional image. Let the n plaintext image planes be denoted as Plane1, Plane2, ..., Planen, then the complex amplitude U of the encrypted structured light propagating forward to Planen is... i (x i ,y i )for:

[0043]

[0044] Among them, z i Let A be the distance from the phase-type hologram ciphertext to Planei. Then, the amplitude A of the encrypted structured light propagating forward to Planei is extracted and saved. i (x i ,y i ) and phase They are respectively:

[0045] A i (x i ,y i )=|U i (x i ,y i )|

[0046]

[0047] Where |·| represents the amplitude extraction operation. Then, encryption step S5 is executed.

[0048] S5: Convergence Condition Judgment: The correlation coefficient (CC) is used as the evaluation function to calculate the correlation between the decrypted image and the plaintext image, thereby verifying the convergence condition. The amplitude A of the encrypted structured light propagating forward to Planei is obtained. i (x i ,y i After that, it is used as the decrypted image of the i-th layer. Then, the correlation coefficient CC between the decrypted image of the i-th layer and the plaintext image of the i-th layer is calculated.i for:

[0049]

[0050] in, and Let be the average pixel grayscale values ​​of the decrypted image and the plaintext image at layer i, respectively, and M and N be the horizontal and vertical pixel counts of the image, respectively. After calculating the correlation coefficients for all layers, extract the minimum value min{CC} among all correlation coefficients. i}, where i = 1, 2, ..., n. When min{CC i If the value exceeds the preset threshold σ, i.e., the convergence condition is met, then the encryption step S7 is executed; when min{CC} i If the convergence condition is not met, and the preset threshold σ is not exceeded, then the phase of the encrypted structured light propagated forward to Planei is retained. The amplitude L of the plaintext image in the i-th layer remains unchanged. i (x i ,y i The amplitude A of the structured light transmitted forward to Planei is replaced by the encrypted structured light. i (x i ,y i Amplitude constraints are implemented to generate new complex amplitudes on Planei. Right now:

[0051]

[0052] Next, perform encryption step S6.

[0053] S6: Backward diffraction propagation: The encrypted structured light generates a new complex amplitude after being amplitude-constrained on the Planei beam. The structured light begins to diffract backwards and reaches the back surface of the phase hologram ciphertext, where i = 1, 2, ..., n. The complex amplitude of the structured light propagating backwards to the back surface of the phase hologram ciphertext is... for:

[0054]

[0055] Subsequently, the complex amplitude of the encrypted structured light is back-propagated to the ciphertext back surface of the phase hologram. The phase is calculated from the complex amplitude U0(x0,y0) of the structured light propagating forward to the front surface of the ciphertext in the phase hologram. for:

[0056]

[0057] Then use phase Update the phase of the phase hologram ciphertext Right now: Then, the encryption step S4 is executed again.

[0058] S7: Output of Phase-Type Holographic Ciphertext: When the convergence condition is met, output the phase of the phase-type holographic ciphertext in this iteration. The output is the encrypted ciphertext of the 3D image.

[0059] This completes the encryption process.

[0060] like Figure 3 The decryption steps shown are as follows:

[0061] D1: Decryption Condition Setting: Set the decryption parameters used in the decryption step, including the wavelength of the incident light, the optical parameters in the decryption structured phase mask, the distance between the decryption structured phase mask and the phase hologram ciphertext, and the distance between the phase hologram ciphertext and the n-layer decryption image. Then, execute decryption step D2. D2: Generation and Illumination of Decryption Structured Light: Use the decryption structured phase mask to phase-modulate a collimated plane light wave of unit amplitude, thereby generating decryption structured light. The decryption structured light generated after phase modulation by the decryption structured phase mask is diffracted forward without constraint to illuminate the phase hologram ciphertext. The complex amplitude of the decryption structured light propagating forward to the front surface of the phase hologram ciphertext... for:

[0062]

[0063] in, To decipher the phase distribution of a structured phase mask, To decipher the wavelength of the incident light, To decrypt the distance between the structured phase mask and the ciphertext of the phase-type hologram, decryption step D3 is then performed. D3: Forward propagation decryption: The decrypted structured light, after phase modulation of the ciphertext of the phase-type hologram, continues to diffract and propagate forward. The decrypted structured light then propagates forward to the complex amplitude of the Planeti decryption image plane at the i-th layer. for:

[0064] Subsequently, the amplitude of the decrypted structured light transmitted forward to Planei was extracted and saved. for:

[0065]

[0066] This is the decryption result of the i-th layer. After obtaining the decryption result of each layer, the decryption results of all layers together constitute the decrypted 3D image.

[0067] This completes the decryption process.

[0068] The optical decryption method used in this invention will be described in detail below. This optical decryption requires the optical decryption device provided by this invention, specifically a holographic imaging device based on structured light illumination, such as... Figure 4 As shown, the device includes a laser source 1 for providing incident light, and sequentially arranged on the optical axis are a polarization modulation device 2 for modulating the polarization state of the incident light, a microscope objective 3 for converging the beam, a pinhole 4 for spatial filtering, a collimating lens 5 for collimating the beam, a phase modulation device 6 for carrying the phase distribution of the decrypted structured phase mask, and a phase modulation device 7 for carrying the phase distribution of the ciphertext of a phase-type hologram. The device is characterized in that: the polarization modulation device 2 is a polarizer or a half-wave plate; the pinhole 4 is placed at the focal point of the microscope objective 3, and the focal point of the microscope objective 3 coincides with the object-side focal point of the collimating lens 5; the phase modulation devices 6 and 7 are phase masks or phase-type spatial light modulators.

[0069] The specific optical decryption process is as follows: The laser emitted by the laser source 1 first passes through the polarization modulation device 2 to form polarized light. Then, the polarized light is incident on the microscope objective 3, which converges the incident light to the focal point. The pinhole 4 at the focal point further spatially filters the converged spherical light wave formed by the microscope objective 3. After being spatially filtered by the pinhole 4, the divergent spherical light wave is incident on the collimating lens 5. Since the pinhole 4 is placed at the object-side focal point of the collimating lens 5, the divergent spherical light wave is modulated into a collimated plane light wave after passing through the collimating lens 5. Subsequently, the collimated plane light wave is phase-modulated by the phase modulation device 6, which carries the phase distribution of the decryption structured phase mask, to generate decryption structured light. Then, the decryption structured light diffracts forward until it reaches the phase modulation device 7, which carries the phase distribution of the phase-type holographic ciphertext. After being phase-modulated by the phase modulation device 7, the decryption structured light continues to diffract forward, thereby forming a decryption three-dimensional image in space. Thus, the optical decryption is completed. The characteristic is that the decryption three-dimensional image can be directly observed by the human eye or recorded by a camera.

[0070] The invention will be further explained below with reference to specific embodiments and accompanying drawings.

[0071] First, a 3D train model is modeled in computer graphics software as the 3D image to be encrypted. The maximum depth of this 3D image is set to 10mm, and the shortest and longest distances between this 3D image and the phase hologram ciphertext are 1000mm and 1010mm, respectively. After computer rendering, the intensity map and depth map of this 3D image are obtained, as shown below. Figure 5As shown in (a) and 5(b). Furthermore, the wavelength λ of the encrypted incident light is set to 632.8 nm, the distance z0 between the encrypted structured phase mask and the ciphertext of the phase-type hologram is 200 mm, the preset threshold σ of CC is 0.99, and the encrypted structured phase mask consists of a Fresnel zone plate with a focal length f of 30 mm and a radial Hilbert mask with a topological charge P of 6. Its phase distribution is as follows: Figure 5 As shown in (c). Following... Figure 2 After the iterative process shown, the generated phase-type hologram ciphertext is as follows: Figure 5 As shown in (d). At this point, the encryption step is complete. The key includes the light wavelength λ, diffraction distance z0, and optical parameters (focal length f and topological charge P) used in the encryption structured phase mask.

[0072] Next, the validity and security of the keys in this invention were tested. To demonstrate the stereoscopic effect of decrypting the 3D image, the decryption results at the train's front (1000mm from the phase hologram ciphertext), train body (1005mm from the phase hologram ciphertext), and train rear (1010mm from the phase hologram ciphertext) were recorded and displayed. When all keys were correct, the decryption results at distances of 1000mm, 1005mm, and 1010mm from the phase hologram ciphertext were as follows: Figure 6 As shown in (a1)-(a3); when the decryption wavelength key λ deviates by 1 nm and all other keys are correct, the decryption results at distances of 1000 mm, 1005 mm, and 1010 mm from the phase hologram ciphertext are respectively as follows: Figure 6 As shown in (b1)-(b3); when the decryption distance key z0 deviates by 1 mm and all other keys are correct, the decryption results at distances of 1000 mm, 1005 mm, and 1010 mm from the phase hologram ciphertext are respectively as follows: Figure 6 As shown in (c1)-(c3); when the decryption focal length key f deviates by 1 mm and all other keys are correct, the decryption results at distances of 1000 mm, 1005 mm, and 1010 mm from the phase hologram ciphertext are respectively as follows: Figure 6 As shown in (d1)-(d3); when the decryption topological charge key P deviates by 1 and all other keys are correct, the decryption results at distances of 1000mm, 1005mm, and 1010mm from the phase hologram ciphertext are respectively as follows: Figure 6 As shown in (e1)-(e3). From Figure 6 The decryption results show that when all keys are correct, the original 3D plaintext image can be reconstructed with high quality. However, when any key is incorrect, the original 3D plaintext image cannot be correctly recovered. In this case, the CC between the decryption result and the original plaintext image is close to 0, showing obvious noise distribution, thus proving the effectiveness and security of the keys in this invention.

[0073] The key sensitivity in this invention will now be examined. Figure 7 The relationship between the CC (correlation coefficient) and the original plaintext image (CC) and the bias of each key is given, which is the sensitivity curve of each key. Figure 7 (a)-(d) represent the sensitivity curves for the wavelength key λ, distance key z0, focal length key f, and topological charge key P, respectively. Figure 7 As can be seen from the sensitivity curve, when any key has a slight deviation, the CC drops sharply to near 0. At this point, the original three-dimensional plaintext image cannot be correctly decrypted, thus proving that the key in this invention has high sensitivity.

[0074] In summary, this invention discloses a three-dimensional image encryption method based on structured light illumination and an iterative layered angular spectrum algorithm. In this invention, the encryption process utilizes an iterative layered angular spectrum algorithm under structured light illumination to encrypt the three-dimensional image into a phase-type holographic ciphertext. This effectively suppresses speckle noise in the decrypted three-dimensional image during correct decryption, improving the quality of the decrypted three-dimensional image. Furthermore, the optical parameters in the structured phase mask are all used as security keys, thereby increasing the types and number of digital keys, significantly expanding the key space, and improving the security of the three-dimensional image encryption. The decryption process can be completed digitally or optically. The holographic imaging device used for optical decryption has a simple optical path and is easy to build. The decrypted three-dimensional image can be observed directly by the human eye or recorded by a camera.

Claims

1. A three-dimensional image encryption method based on structured light illumination and iterative hierarchical angular spectrum algorithm, characterized in that: Including encryption steps: S1: 3D Image Layering Processing: Using computer graphics software, the 3D image to be encrypted is sliced ​​into parallel layers based on different depth information. layer, It is the layer number; extract the amplitude information of each layer, where the first layer is the layer number. The amplitude information of the layer is recorded as plaintext image. Then, perform encryption step S2; S2: Encryption Condition Settings: Set the encryption parameters used in the encryption steps, including the encryption incident light wavelength, the optical parameters in the encrypted structured phase mask, the distance between the encrypted structured phase mask and the phase hologram ciphertext, and the distance between the phase hologram ciphertext and the 3D image slice generated. The distance between the plaintext images in each layer; then, the encryption step S3 is performed; S3: Generation and Illumination of Encrypted Structured Light: Based on the optical parameters in the set encrypted structured phase mask, the phase distribution of the encrypted structured phase mask is first calculated using the phase calculation formula of the structured phase mask. Then, the phase of a collimated plane light wave with a unit amplitude is modulated using the encrypted structured phase mask to generate encrypted structured light. Therefore, the complex amplitude distribution of the encrypted structured light is equal to the phase distribution of the encrypted structured phase mask. Subsequently, the encrypted structured light generated after phase modulation by the encrypted structured phase mask diffracts forward without constraint and reaches the front surface of the phase hologram ciphertext. The complex amplitude of the encrypted structured light propagating forward to the front surface of the phase hologram ciphertext is calculated according to the angular spectrum diffraction algorithm. Next, perform encryption step S4; S4: Forward Diffraction Propagation: After phase modulation of the phase-type holographic ciphertext, the encrypted structured light continues to diffract forward, sequentially reaching the 3D image slices generated... Plain text image plane; will The plaintext image planes are respectively denoted as , , ..., The forward propagation of the encrypted structured light is calculated based on the angular spectrum diffraction algorithm. complex amplitude ,in Subsequently, the encrypted structured light was extracted and saved and forward-transmitted to... amplitude and phase ,in Then, perform encryption step S5; S5: Convergence condition judgment: The correlation coefficient (CC) is used as the evaluation function to calculate the correlation between the decrypted image and the plaintext image, thereby verifying the convergence condition; After receiving the encrypted structured light, it is transmitted forward. amplitude Then, it was taken as the first Decrypt the image layer by layer and calculate the first layer. Layer decryption image and the first Correlation coefficient between plaintext images ,in After calculating the correlation coefficients for all layers, extract the minimum value among all correlation coefficients. ,in ;when Exceeding the preset threshold When the convergence condition is met, encryption step S7 is executed; when Not exceeding the preset threshold That is, if the convergence condition is not met, the encrypted structured light is retained and forward-propagated to... phase Unchanged, and utilizing the first Amplitude of plaintext image Replaces encrypted structured light forward transmission amplitude Achieve amplitude constraint, thereby in Generate new complex amplitude ,in Then, perform encryption step S6; S6: Backward diffraction propagation: Encrypted structured light in After being constrained by amplitude, a new complex amplitude is generated. The diffraction propagation begins to spread backward and reaches the back surface of the phase hologram ciphertext, where... The complex amplitude of the encrypted structured light propagating backward to the ciphertext surface of the phase hologram is calculated using the angular spectrum diffraction algorithm. Subsequently, the complex amplitude of the structured light back-propagated to the ciphertext surface of the phase hologram is utilized. The complex amplitude of the encrypted structured light propagating forward to the front surface of the phase hologram ciphertext. Calculate phase for: ,in This indicates a phase extraction operation, followed by the use of phase. Update the phase of the phase hologram ciphertext ,Right now: Then, the encryption step S4 is executed again; S7: Output of Phase-Type Holographic Ciphertext: When the convergence condition is met, output the phase of the phase-type holographic ciphertext in this iteration. The output is the encrypted ciphertext of the 3D image; This completes the encryption process; Decryption steps: D1: Decryption Condition Setting: Set the decryption parameters used in the decryption steps, including the incident light wavelength, the optical parameters in the structured phase mask, the distance between the structured phase mask and the phase hologram ciphertext, and the distance between the phase hologram ciphertext and... Decode the distance between the layers of images, and then perform decryption step D2; D2: Generation and Illumination of Decrypted Structured Light: A decrypted structured light beam is generated by phase modulation of a collimated plane wave of unit amplitude using a decrypted structured phase mask. The generated decrypted structured light, after phase modulation by the decrypted structured phase mask, is then diffracted forward without constraint to illuminate the ciphertext of the phase-type hologram. The complex amplitude of the decrypted structured light propagating forward to the front surface of the phase-type hologram ciphertext is calculated using an angular spectrum diffraction algorithm. Then, perform decryption step D3. D3: Forward Propagation Decryption: After phase modulation of the ciphertext in the phase-type hologram, the decrypted structured light continues to propagate forward through diffraction. The forward propagation time of the decrypted structured light is calculated using the angular spectrum diffraction algorithm. Layer decryption image plane complex amplitude ,in Subsequently, the decrypted structured light was extracted and saved, and then forward-transmitted to... amplitude That is, the first The decryption results of the layer, among which After obtaining the decryption results of each layer, the decryption results of all layers together constitute the decrypted 3D image. This completes the decryption process.

2. The three-dimensional image encryption method based on structured light illumination and iterative hierarchical angular spectrum algorithm according to claim 1, characterized in that: The phase distribution calculation formula for the encrypted structured phase mask in encryption step S3 includes at least one editable optical parameter, which is used as a digital key.

3. The three-dimensional image encryption method based on structured light illumination and iterative hierarchical angular spectrum algorithm according to claim 1, characterized in that: The decryption steps D2-D3 are completed digitally or optically.

4. The three-dimensional image encryption method based on structured light illumination and iterative hierarchical angular spectrum algorithm according to claim 1, characterized in that: The device for optical decryption is a holographic imaging device based on structured light illumination, including a laser source 1 for providing incident light, and a polarization modulation device 2 for modulating the polarization state of the incident light, a microscope objective 3 for converging the beam, a pinhole 4 for spatial filtering, a collimating lens 5 for collimating the beam, a phase modulation device 6 for carrying the phase distribution of the decryption structured phase mask, and a phase modulation device 7 for carrying the phase distribution of the ciphertext of the phase-type hologram.

5. The three-dimensional image encryption method based on structured light illumination and iterative hierarchical angular spectrum algorithm according to claim 1, characterized in that: The polarization modulation device 2 in the structured light illumination-based holographic imaging device is a polarizer or a half-wave plate.

6. The three-dimensional image encryption method based on structured light illumination and iterative hierarchical angular spectrum algorithm according to claim 1, characterized in that: In the holographic imaging device based on structured light illumination, the pinhole 4 is placed on the focal point of the microscope objective 3, and the focal point of the microscope objective 3 coincides with the object-side focal point of the lens 5.

7. The three-dimensional image encryption method based on structured light illumination and iterative hierarchical angular spectrum algorithm according to claim 1, characterized in that: The phase modulation device 6 and phase modulation device 7 in the structured light illumination-based holographic imaging device are phase masks or phase-type spatial light modulators.

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