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Abram Hunchback
Abram Hunchback

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This article lists the certificate trust policies for watchOS, and is updated when changes are made to the certificate list. It lists the certificates for watchOS Trust Store version 2016102100, which is current for watchOS 3 and later.


If the program contains tests or examplesand no main function, the service runs the tests.Benchmarks will likely not be supported since the program runs in a sandboxedenvironment with limited resources.

The playground service is used by more than just the official Go project(Go by Example is one other instance)and we are happy for you to use it on your own site.All we ask is that youcontact us first (note this is a public mailing list),that you use a unique user agent in your requests (so we can identify you),and that your service is of benefit to the Go community.

The GIF format is an excellent format for compressing images which have a limited range of colors. This is fine for simple images, but it does not suit the compression of photographs, thus the JPEG standard was developed by the Joint Photographic Expert Group (JPEG), which is a subcommittee of the ISO/IEC. It was based on research into compression ratios on images and their resulting image quality. The standards produced can be summarized as follows:

JPEG is an excellent compression technique which produces lossy compression (although in one mode it is lossless). As seen from the previous section it has excellent compression ratios when applied to a color image. The main steps are:

Unfortunately, compared with GIF, TIFF and PCX, the compression process is relatively slow. It is also lossy in that some information is lost in the compression process. This information is perceived to have little effect on the decoded image. GIF files typically use 24-bit color information (8 bits for red, 8 bits for green and 8 bits for blue) and convert it into an 8-bit color palette (thus reducing the number of bits stored to approximately one-third of the original). It then uses LZW compression to further reduce the storage. JPEG operates differently in that it stores changes in color. As the eye is very sensitive to brightness changes it is particularly sensitive to changes in brightness. If these changes are similar to the original then the eye will perceive the recovered image as similar to the original.Color conversion and subsampling

In the first part of the JPEG compression, each color component (red, green and blue) is separated in luminance (brightness) and chrominance (color information). JPEG allows more losses on the chromi-nance and less on the luminance. This is because the human eye is less sensitive to color changes than to brightness changes. In an RGB image, all three channels carry some brightness information but the green component has a stronger effect on brightness than the blue component.A typical scheme for converting RGB into luminance and color is known as CCIR 601, which con-verts the components into Y (can be equated to brightness), Cb (blueness) and Cr (redness). The Y component can be used as a black and white version of the image. The components are computed from the RGB components:

For the brightness, it can be seen that green has the most effect and blue has the least. For the redness, the red color (of course) has the most effect and green the least, and for the blueness, the blue color has the most effect and green the least. Note that the YCbCr components are often known as YUV. A subsampling process is then conducted which samples the Cb and Cr components at a lower rate than the Y component. A typical sampling rate is four samples of the Y component to a single sample on the Cb and Cr component. This sampling rate is normally set with the compression parameters. The lower the sampling, the smaller the compressed data and the shorter the compression time. All the required information on how to decode the JPEG data is contained in the JPEG header.

The DCT (discrete cosine transform) converts intensity data into frequency data, which can be used to tell how fast the intensities vary. In JPEG coding the image is segmented into 88 pixel rectangles, as illustrated in Figure B2.8. If the image contains several components (such as Y,Cb,Cr or R,G,B), then each of the components in the pixel blocks is operated on separately. If an image is subsampled, there will be more blocks of some components than of others. For example, for 22 sampling there will be four blocks of Y data for each block of Cb or Cr data.The data points in the 8x8 pixel array start at the upper right at (0,0) and finish at the lower right at (7,7). At the point (x,y) the data value is f(x,y). The DCT produces a new 88 block (uv) of trans-formed data using the formula:

This results in an array of space frequency F(u,v) which gives the rate of change at a given point. These are normally 12-bit values which give a range of 0 to 4095. Each component specifies the degree to which the image changes over the sampled block. For example:

The coefficients are equivalent to representing changes of frequency within the data block. The value in the upper left block (0,0) is the DC or average value. The values to the right of a row have increasing horizontal frequency and the values to the bottom of a column have increasing vertical frequency. Many of the bands end up having zero or almost zero terms, which is useful when we compress long runs of zero values.

Acest fișier conține informații suplimentare, introduse probabil de aparatul fotografic digital sau scannerul care l-a generat. Dacă fișierul a fost modificat între timp, este posibil ca unele detalii să nu mai fie valabile.

This article is part two of three covering encryption concepts and the Internet public key infrastructure (PKI). The first article in this series introduced symmetric and public key (asymmetric) encryption in cryptography. If you're not familiar with the basic concept of public-key encryption, you should read part one before you go ahead with this one.

In this part, I show you the basics of Transport Layer Security and Secure Socket Layer (TLS/SSL), how the Internet PKI works, and OpenSSL, the Swiss Army knife for TLS/SSL tasks. I cover how to use OpenSSL to create key-pairs and to generate a certificate signing request (CSR) to send to your certificate authority (CA) for signing. After that, I discuss some weaknesses of the Internet PKI you should be aware of.

Assume that you're a sysadmin like me and one of your tasks is to manage a webserver. Because your users care about authenticity, integrity, and privacy, you'd like to secure your web application with some kind of encryption.* You don't know in advance who's using your site, so symmetric encryption is off the table because of its key distribution problem. I use public-key encryption in the following sections instead.

The acronyms for Transport Layer Security and Secure Socket Layer are TLS and SSL. They are used interchangeably most of the time, and that's OK. While the old SSL protocol versions are deprecated, you'll usually find TLSv1.2 and TLSv1.3 on the web these days. TLS is used in HTTPS connections between some clients and some web servers. The following image shows a simple example of an HTTPS handshake.

First, the well-known TCP handshake happens between client and server. Then the client starts the HTTPS handshake by sending the ClientHello. In this step, the client transmits information about the server name it requests and the supported cipher suites. The server responds with the ServerHello, transmits a selected cipher suite, connection parameters, and sends information for calculating a symmetric key for the ongoing connection. Last but not least, it sends its certificate to authenticate itself to the client.

Focus on the certificate the server has transmitted to the client. It contains the server's public key, which the client uses to encrypt data before sending it to the server. A trusted Certificate Authority (CA) signed the public key in the certificate. Today, every operating system and web browser comes with a store containing the public keys of many different trusted CAs. These public keys are then used to verify the signatures in server certificates like the one discussed here. This way, the client can check the server's authenticity and that it is the correct host the client wants to connect to.

Be aware that public key encryption is used only to establish the HTTPS connections and calculate a symmetric session key used for further communication. That's because symmetric encryption is much faster than asymmetric.

So now the question is, how do you get the key-pair for the webserver? As stated earlier, OpenSSL is the Swiss Army knife for SSL and TLS tasks. Since its documentation has left some room for improvement, I suggest that you read the free book, OpenSSL Cookbook by Ivan Ristic to get all the details. My article focuses on creating a key-pair and a Certificate Signing Request (CSR) for a single domain name and a CSR that includes multiple domain names.

The CSR is needed to send the public key and other information about your domain to a CA for signing. The signed public key you'll get back is your certificate which you will install on your web server along with the corresponding private key.

You'll find both files in your current working directory. The passphrase you entered during the creation process protects your private key file. If you open them in some kind of text viewer, you'll only recognize that these are a private key and a CSR but won't find further plain text information:

Attention: Never share your private key(s) with anyone. They are yours and yours only. Keep them secret, safe, and sound. Follow your organization's policies for handling private keys, CSRs, and certificates. The security of your organization (and your job) may depend on it.

The first section [ req ] specifies that a private RSA key with 2048 bits is to be generated and stored as test_privatekey.pem. Also, the section contains information about finding the bits that you entered interactively in the earlier section of this article (in the section [ req_distinguished_name ]. In [ v3_req ], you'll find some constraints on keyUsage but more importantly, for this article, the parameter subjectAltName where the common name and all additional names are specified. Save it as openssl.cnf and run it with the following command to create a private key and CSR: 041b061a72


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