Face Search in Encrypted Domain

Abstract

Visual information of images and videos usually is encrypted for the purposes of security applications. Straightforward manipulations on the encrypted data without requiring any decryption have the advantage of speed over performing those operations in spatial, temporal, frequency or compressed domain. In this paper, we will investigate encrypted image search. More specifically, given a face image as the target object, we search it amongst encrypted images. We accomplish the search by using a novel method that extracts features and locates the face object region within the given encrypted image. We evaluate the search results by using precision and recall as well as F-measure. Our experiments reveal that there exists a trade-off between the quality of search and the quality of encryption, namely, stronger encryption leads to poorer search results.

1 Introduction

One of straightforward ways of securing digital image transmission is to encrypt the images [11, 23, 29]. In image encryption, traditional methods like those of asymmetric encryption in public key systems  [6] are often adopted, other methods including image scrambling using Hilberts Space-filling Curves (HSC) [17] as well as image sharing based on Chinese Remainder Theorem (CRT) [26] and Visual Cryptography(VC) [20], etc. have also been employed.

Since digital images have relatively huge volume of file size when compared to text, direct manipulations of encrypted images have been highly recommended for the sake of saving space and speeding up the computation  [1]. There already has been a slew of inaugural work in this direction. For example, digital image enhancement has been adopted in encrypted domain [14]. Visual features such as histogram and SIFT have been extracted from encrypted domain for a variety of applications [21]. The empirical methods such as clustering and classification on encrypted domain have been developed to group visual objects in categories [30]. Recently, face emotion recognition has been attempted in encrypted domain [22] which is often thought as an important biometric issue, the focus of relevant research work has been shifted from compressed domain to encrypted domain [7, 22].

Fig. 1.

Flowchart of search in encrypted domain

In this paper, the goal of our research is to accurately find a face within an encrypted image. We perform face object search in scrambled images since scrambling is deemed as a form of encryption. Motivated by face detection and recognition in computer vision [12], search and retrieval in encrypted domain [16, 28], and the mighty value of reuse of encrypted data [18], in this paper we will develop a face object search algorithm based on Hilberts Space-filling Curves (HSC) as shown as Fig. 1.

Given an image, we segment it into identical sized blocks and use DCT (Discrete Cosine Transform) transform to transfer the pixel values to frequency domain. The reason why we select DCT transform is that most of images and videos are stored in JPEG and MPEG formats nowadays which are based on DCT transform. Using DCT transform could greatly reduce our processing time.

We also employ the HSC curves based image scrambling for encrypting the images to make sure that the encrypted image is secure enough. [17] Given an encrypted image having face objects, we search for the encrypted face along scanline order from top-left to bottom-right. The precision and recall as well as F-measure for evaluating the search results are calculated.

Our contributions of this paper are listed below.

• (1) Encrypting digital images using the scrambling algorithm based on Hilberts space-filling curves. In this paper, we will utilize the HSC based image encryption [2, 17] and present our findings along with face object searching. The key for image encryption is a pseudo random number which is used for selecting different HSC curves so as to scramble an image.

• (2) Searching for the given face object hierarchically in the encrypted image and locate it. We conduct hierarchically search on given encrypted images in multi-resolution. The features are extracted based on mean, variance and histogram which preserve the invariance of the encrypted image. This approach allows the face object to be scaled, rotated or having various lighting conditions in encrypted domain before the search.

• (3) Evaluating performance of face object search in encrypted domain. In this paper, we take use of the Wild Face Dataset for our experiments. The precision and recall as well as F-measure will be taken into consideration for the search evaluations.

The challenges of this work are to find the given face in encrypted domain hierarchically, there may have many faces within a given image. Our goal in all the cases is to find each face and mark it using a rectangle. The rest of this paper is organized as follows. The related work will be introduced in Sect. 2, our contributions will be presented in Sects. 3 and 4 will provide the experimental results and analysis, conclusion and future work will be stated in Sect. 5.

2 Related Work

With regard to search in encrypted domain, usually lexical features and quantitative features as well as security-specific features are employed for the purposes of confidentiality [15], the domain of these search is usually limited to text encryption. In order to fully utilize the outcomes, homomorphic encryption [8], Yaos Garbled circuits (GC) [19] and reuse of encrypted values [18] have been adapted for data encryption recently [3, 11, 25].

Homomorphic encryption is a form of encryption that allows computations to be carried out on ciphertexts. A cryptosystem is said to be homomorphic with respect to an operation $\star$, if another operation $\circ$ exists such that, given two plaintexts $m_1$ and $m_2$,

$$\begin{aligned} D(E[m_1] \circ E[m_2]) = m_1 \star m_2 \end{aligned}$$

(1)

where D and E indicate the decryption and encryption operators respectively. If the function of operation $\star$ is identical to that of the operation $\circ$, the homomorphic encryption satisfies,

$$\begin{aligned} D(f(E[m])) = D(E(f[m]))= f[m] \end{aligned}$$

(2)

From functional viewpoint, we have,

$$\begin{aligned} D(f(E[\cdot ])) = D(E(f[\cdot ]))= f[\cdot ] \end{aligned}$$

(3)

Namely,

$$\begin{aligned} D(f(E)) = D(E(f))= f \end{aligned}$$

(4)

This indicates the operators f and E are commutative in the encrypted domain.

The homomorphic encryption is possible to be propagated to other media such as image or picture, even audio and video. Based on the prevalent image encryption methods such as Hilberts Space-filling Curves (HSC) based scrambling, the encryptions are able to be iteratively applied to the encrypted images by following the same type of encryption while properties of the homomorphic encryption are still persevered. In this paper, we will take advantage of the HSC based image scrambling as the encryption algorithm and assert the validity of homomorphic encryption in image encrypted domain.

The HSC is with a fractal structure generated by a recursive production rule which has the property of self-similarity and satisfies IFS system, its dimension is a fraction and it is supported by the fixed-point theory. After several rounds of recursions started from the fractal generator, the curve will fill up a given space recursively. If the curve space comprises of raster grids, the curve is utilized for re-ordering each pixel within the discrete space along the pixel order on the curve.

Fig. 2.

Generating a HSC curve (a) Fractal generator (b) Generated HSC curve using recursion at resolution $8\times 8$ (c) Generated HSC curve at resolution of $32\times 32$ (d) Scrambled image based on the HSC curve ($512\times 512$) in spatial domain.

Generating procedure of a HSC curve is described as Fig. 2(a). In Fig. 2(a), all the pixels on this plane along the HSC curve have been numbered. Figure 2(b) and (c) show the curves at resolutions $8\times 8$ and $32\times 32$ respectively. Figure 2(d) shows a scrambled image of the Lena ($512\times 512$) in spatial domain using the $512\times 512$ HSC curve. In a scrambled image, only the pixel locations have been re-ordered, the pixel color information still holds.

Our contribution in this paper is to search for a given face object in encrypted domain. Amongst the existing work, most of them are related to keyword search, this is based on plaintext encryption and ciphertext decryption in the encrypted domain which was derivative from the traditional cryptography.

In face recognition, eigenvalues and eigenvectors are calculated directly from the encrypted images [7] which is based on traditional PCA algorithm. Eigenfaces based recognition algorithm and a combination of known cryptographic techniques, in particular Homomorphic Encryption and Garbled Circuits (GC) have been employed to improve the computational complexity and server client communications [25].

Holomorphic properties of Paillier Cryptosystem specially for Euclidean distance has been used to calculate the distance between two feature vectors. In Paillier Cryptosystem,

$$\begin{aligned}{}[|m_1 + m_2|]=[|m_1|][|m_2|] \end{aligned}$$

(5)

$$\begin{aligned}{}[|\alpha \cdot m|]=[|m|]^\alpha \end{aligned}$$

(6)

Different from those existing work, in this paper our focus is on face object search in encrypted domain. The novelty of this paper lies in that the encryption of digital images is conducted via image scrambling based on the HSC curves; we select mean, variance and histogram as our features and compose them into a feature vector, the distance between feature vectors is calculated by using inner or dot product. We search for the given object in the encrypted domain and evaluate our search results by using precision and recall as well as F-measure.

3 Our Contributions

In image encryption and decryption as well as visual object search, we transform an image from spatial domain to DCT domain first and utilize the DCT coefficients for encryption and decryption. After these operations, we commit inverse DCT / IDCT transforms and transfer the image blocks back to spatial domain for the purpose of displaying. Therefore, in this paper we deal with the DCT transform as our pre-processing, our encryption and decryption are block based scrambling and descrambling. The steps of image search in encryption domain based on HSC curves scrambling are described as below,

Algorithm. Search a given face object in encrypted domain

3.1 Image Encryptions

In this section, we elucidate how digital images are encrypted using HSC curves based image scrambling, we manage the algorithm to serve image encryption and decryption well by adaptively tuning the parameters.

Procedure. As shown in the algorithm, in image encryption we import images in spatial domain and segment them into blocks having identical size, for each block we recursively generate a corresponding HSC curve using the generator presented in Fig. 2(a), the curve starts from very beginning shown in red color, its mouth points in upward.

In the second iteration, for each turning point at start or end, we generate the same shape however the size and orientation will be changed. At the starting point, we rotate the generator for 90 degrees toward the left (anti-clockwise); at the end point, we turn the generator to right for 90 degrees (clockwise), at the other two turning points, the orientations of the generators are the same. We link these generated shapes together and yield the blue curve in Fig. 2(a).

We repeat this step and acquire the black curve shown in Fig. 2(b). If the end resolution is $32\times 32$, we take use of the same way to generate Fig. 2(c). The procedure is described as Eq. (7),

$$\begin{aligned} p_{n+1}(x,y)=HSC (p_n(x,y)), n=1,2,\cdots \end{aligned}$$

(7)

where HSC($\cdot$) is the iterative function, p(x, y) is the turning point on the curve, the stop condition for this recursion is the final resolution reached so as to fully fill the given plane, the Hausdorff dimension of this fractal curve is 2.00. [17] The image scrambling procedure is described as,

$$\begin{aligned} I^\prime =HSC(I)~mod~W \end{aligned}$$

(8)

where I is the previous image without scrambling, $I^\prime$ is the scrambled image, W is the image width. After generated this HSC curve, we sort the pixel order according to the pixel sequence on the HSC curve shown as Eq. (8). The Eq. (8) first converts points on 2D plane to 1D curve order, then the 1D sequence will be used to fill up the image space line by line from top to bottom, consequently the image is fully scrambled shown as Fig. 2(d) which is exported as the encrypted image.

Figure 3 shows the scrambled images of Lena in different resolutions of the HSC curves. In Fig. 3(a), we scramble each $16\times 16$ image blocks in DCT domain using the generated $16\times 16$ HSC curve, Fig. 3(b) is based on $32\times 32$ blocks meanwhile the Fig. 3(c) is based on $64\times 64$ blocks of image scrambling. From our observations, we find that Lena’s faces are gradually becoming tougher to be perceived from left to right. From the trade-off perspective, this means that it will be much harder to find a face from Fig. 3(c) than from Fig. 3(a).

Fig. 3.

Image encryption for Lena ($512\times 512$) in DCT domain using HSC for the blocks with different size (a) block size: $16\times 16$ (b) block size: $32\times 32$ (c) block size: $64\times 64$

The encryption using HSC based image scrambling in DCT domain changes the pixel sequence spatially, but does not alter color information of image pixels. Figure 3 shows one of results of image scrambling using the HSC curves based image scrambling in DCT domain with different resolutions.

Security. The security of this encryption is ensured by the key and the scrambling algorithm. This is because the HSC generator has multiple choices, and could be rotated along the clockwise and anti-clockwise directions, the generator has four orientations. Based on different generators, the HSC curves will be completely different. Meanwhile, the image block has multiple choices with various resolutions, the scrambling based on different block sizes will lead to image encryption with different strengths. Larger the bock size, stronger the encryption. Therefore, which HSC curve will be selected at what resolution will be the unique key of the encryption algorithm.

The HSC based image scrambling is different from the traditional encryption algorithms such as RSA, ECC and secret sharing, etc. The reason is that the scrambling completely destroys the order of pixel locations in the image, therefore the pixel neighborhood operations such as edge extraction, SIFT and others, are not possible anymore, especially in the DCT domain. However these geometric information could be detected from those encrypted images using RSA or ECC algorithms sometimes.

3.2 Face Object Search

In this section, we search for a given face object in encrypted domain. Our goal is to find the matched coefficients of the given face object in DCT domain after encryption. Therefore, we seek the face object using a sliding window. We keep the encrypted image at its given size but vary the face object size in a hierarchical multi-resolution search.

The window is initially defined for one face image size. For each window, we traverse the input image completely in scanline order from top to bottom and left to right, calculate the distance between feature vectors of the face image and regions of the encrypted image. When arriving the right-bottom corner of the image, we modify the face image size and search it starting from the left-up corner again, till scanned all sizes of encrypted face image, the procedure is shown as Fig. 4.

Fig. 4.

Sliding windows search for a face object

Feature Selection. Since the images have been encrypted based on DCT domain, the features are used for searching in encryption domain including mean, variance, entropy, etc. The visual features therefore are combined such as Eqs. (9) and (10),

$$\begin{aligned} V_O=[f_{O1}~~f_{O2}~~\cdots ~~f_{Om}] \end{aligned}$$

(9)

$$\begin{aligned} V_\varOmega =[f_{\varOmega 1}~~f_{\varOmega 2}~~\cdots ~~f_{\varOmega m}] \end{aligned}$$

(10)

where $V_O$ and $V_\varOmega$ are the relevant features from face object O and sliding window $\varOmega$.

Distance Calculation. The feature vectors are employed for the face object search, equation for calculating the distance between feature vectors is shown in Eqs. (11) and (12) which are applied to the cases having invariance in encrypted domain such as rotating, scaling and filliping. After histogram equalization, it is able to be applied to the images having various lighting conditions,

$$\begin{aligned} \varOmega = \arg \min \limits _\varOmega \{|V_\varOmega (f_1,f_2,\cdots ,f_m) - V_O(f_1,f_2,\cdots ,f_m)|\} \end{aligned}$$

(11)

where $I_O$ is the image having face object, $V_O$ is its feature vector; $I_\varOmega$ is sliding window of the encrypted image for searching, its feature vector is $V_\varOmega$. What we like to emphasize in this paper is that the histograms, means and variances are all normalized.

$$\begin{aligned} \varOmega =\arg \max \limits _\varOmega \frac{V_O(f_1,f_2,\cdots ,f_m)\cdot V_\varOmega (f_1,f_2,\cdots ,f_m)}{|V_O(f_1,f_2,\cdots ,f_m)|\cdot |V_\varOmega (f_1,f_2,\cdots ,f_m)|} \end{aligned}$$

(12)

For an example, we search the given object in encrypted domain by using the inner or dot product between two feature vectors first shown as Eq. (12); later we calculate the EMD (Earth Movers Distance) shown as Eq. (13), which is used to refine the image distance for the search in encrypted domain [24].

$$\begin{aligned} \varOmega =\arg \min \limits _\varOmega EMD(O,\varOmega )= \arg \min \limits _\varOmega \frac{\varSigma _i\varSigma _j d_O (i,j)d_\varOmega (i,j)}{\min (\varSigma _i w_{pi}, \varSigma _j w_{qj})} \end{aligned}$$

(13)

where $w_{pi} \ge \varSigma f(i, j) (m \ge i \ge 1)$ and $w_{qj} \ge \varSigma f(i, j) (n \ge j \ge 1)$, $f(i, j) \ge 0$, $d(i, j) \ge 0$.

Multi-resolution Search. While conducting face search in encrypted domain, we have three down-sampling operations: original, half and quarter. The search results ‘1’ or ‘0’ will be merged together using ‘or’ operations. The finally found region will be generated by merging the detected regions in different resolution together.

3.3 Search Evaluations

In this section, we will detail on how to evaluate search results. Suppose an encrypted face object has been traversed from a large number of encrypted pictures. From the results, we need find true positive tp, true negative tn; false positive fp and false negative fn. Based on these parameters, we objectively evaluate our search by using precision, recall and F-measure.

In the context of this paper, when a face object is found, we usually refer to search region A and the image region B having intersection, $A\cap B\ne \emptyset$. The corresponding mathematical description is shown as Eq. (14).

$$\begin{aligned} p=\frac{\varLambda (A\cap B)}{\varLambda (A\cup B)}\cdot 100\,\% \end{aligned}$$

(14)

where $\varLambda (\cdot )$ is the area of the specific regions, $p\in [0,1]$. Equation (14) shows how many percent of the face image has been found in the search. If $A\cap B = \emptyset$, that means the search is a failure, we could not get the face object from this image, so $p=0$.

After we have received the search results, we calculate the recall and precision as well as F-measure utilizing our ground truth. The ground truth tells us whether an image has the designated face or not, it is ‘0’ or ‘1’. Our results reveal from multi-resolution viewpoint whether we have successfully found the face or not.

If the search results are known, we have,

$$\begin{aligned} Pr=\frac{Tp}{Tp+Fp} \end{aligned}$$

(15)

$$\begin{aligned} Rc=\frac{Tp}{Tp+Fn} \end{aligned}$$

(16)

where Pr And Rc refer to precision and recall, respectively. Tp, Fp, Fn, and Tn are the true positive, false positive, false negative and true negative in the search. The Tp, Fp, Fn, and Tn show amongst the search results how many search results reflect the ground truth exactly. Furthermore, F-measure is calculated by, Eq. (17),

$$\begin{aligned} F_m=2\cdot \frac{Pr\cdot Rc}{Pr+Rc} \end{aligned}$$

(17)

4 Results and Analysis

We implement our search algorithm using Matlab platform and encrypted images in DCT domain. Our search results are shown in Figs. 5, 6 and 7 marked with red rectangles.

Fig. 5.

Searching Lena’s face on the scrambled images by using the HSC curve based scrambling (a) Lena’s face ($164\times 164$) in encrypted domain using HSC scrambling after DCT transform; (b) Image Lena ($512\times 512$) in encrypted domain using HSC scrambling after DCT transform, the red rectangle shows the found region of the visual object (Color figure online).

Fig. 6.

Searching Mona Lisa’s face on the scrambled images by using the HSC curve based scrambling (a) Mona Lisas face ($164\times 164$) in encrypted domain using HSC scrambling after DCT transform; (b) Image Mona lisa ($512\times 512$) in encrypted domain using HSC scrambling after DCT transform, the red rectangle shows the found region of the visual object (Color figure online).

Fig. 7.

Searching Nicole’s face on the scrambled images by using the HSC curve based scrambling (a) Nicoles face ($164\times 164$) in encrypted domain using HSC scrambling after DCT transform; (b) Nicole ($724\times 314$) in encrypted domain using HSC scrambling after DCT transform, the red rectangle shows the found region of the visual object (c) Nicoles face on the original color image (Color figure online).

Table 1. Comparisons of HSC encryption in encrypted domain

In Table 1, we encrypt the Lena $512\times 512$ image in blocks using 4 resolutions of HSC curves (resolution 1: $8\times 8$, resolution 2: $16\times 16$, resolution 3: $32\times 32$, resolution 4: $64\times 64$). Table 1 shows that there is a trade-off between the quality of image encryption algorithm and the quality of search. The higher encryption that means the search is more difficult since the visual information has been scrambled using the HSC curves.

Table 2. Comparisons of search results after transforms

Table 2 shows search results that we use the Lena face images ($512\times 512$) after 4 Affine transformations (scaling, rotating, horizontal filliping, and vertical flipping). It discovers the scaling and flipping transforms did affect the search quality, but the rotating does not affect the result too much, this may be related to the fact that the image size is not changed too much.

Table 3. Comparisons of search results of various samples

Table 3 demonstrates what are the differences between the given human faces and found regions. We calculate the differences using the metrics SSIM(Structural similarity ), DSSIM(Structural Dissimilarity), NCC (Normalized Cross Correlation) and EMD (The Earth Mover’s Distanc).

Table 4. Face search in encrypted domain using the Wild dataset

Fig. 8.

Precision of face search in encrypted domain: 82.5 % (Putin)

Table 4 shows the corpus of our search related to famous figures. We search the given faces within the encrypted domain, and compare the found face location and the ground truth. From the results, we calculate precision and recall as well as F-measure.

We adopted the LFW Face Database as corpus for searching human faces in encrypted domain, the database of face photographs was designed for studying the problem of unconstrained face recognition. We select 3 figures with a total of 115 images for the algorithm testing (Putin: 40, Agassi: 36, Clinton: 29). The results are shown in Table 4. The precisions of our search are acceptable.

The corresponding images are shown in Figs. 8, 9 and 10.

Fig. 9.

Precision of face search in encrypted domain: 72.22 % (Agassi)

Fig. 10.

Precision of face search in encrypted domain: 69 % (Clinton)

5 Conclusion

In this paper, we search for face objects in encrypted domain. The main purpose is to reuse the encrypted data and save the computation time by directly manipulating on the encrypted data. Our results show the superiority of face object search in encrypted domain. Our contributions are: (1) Image encryption using HSC based image scrambling; (2) Face object search within the given encrypted images; (3) Search evaluation in encrypted domain. In future, we will further investigate the relative issues in encrypted domain, especially for the security and privacy preservation problems in big data associated with social media.