Recipes ======= We've written some recipes that demonstrate how to solve common problems with Okio. Read through them to learn about how everything works together. Cut-and-paste these examples freely; that's what they're for. These recipes work on all platforms: Java, Android, Kotlin/Native, and Kotlin/JS. See [java.io Recipes](java_io_recipes.md) for samples that integrate Java APIs. Read a text file line-by-line ([Java][ReadFileLineByLine]/[Kotlin][ReadFileLineByLineKt]) ----------------------------------------------------------------------------------------- Use `FileSystem.source(Path)` to open a source stream to read a file. The returned `Source` interface is very small and has limited uses. Instead we wrap the source with a buffer. This has two benefits: * **It makes the API more powerful.** Instead of the basic methods offered by `Source`, `BufferedSource` has dozens of methods to address most common problems concisely. * **It makes your program run faster.** Buffering allows Okio to get more done with fewer I/O operations. Each `Source` that is opened needs to be closed. The code that opens the stream is responsible for making sure it is closed. === "Java" Here we use Java's `try` blocks to close our sources automatically. ```java public void readLines(Path path) throws IOException { try (Source fileSource = FileSystem.SYSTEM.source(path); BufferedSource bufferedFileSource = Okio.buffer(fileSource)) { while (true) { String line = bufferedFileSource.readUtf8Line(); if (line == null) break; if (line.contains("square")) { System.out.println(line); } } } } ``` === "Kotlin" This uses `use` to automatically close the streams. This prevents resource leaks, even if an exception is thrown. ```kotlin fun readLines(path: Path) { FileSystem.SYSTEM.source(path).use { fileSource -> fileSource.buffer().use { bufferedFileSource -> while (true) { val line = bufferedFileSource.readUtf8Line() ?: break if ("square" in line) { println(line) } } } } } ``` The `readUtf8Line()` API reads all of the data until the next line delimiter – either `\n`, `\r\n`, or the end of the file. It returns that data as a string, omitting the delimiter at the end. When it encounters empty lines the method will return an empty string. If there isn’t any more data to read it will return null. === "Java" The above Java program can be written more compactly by inlining the `fileSource` variable and by using a fancy `for` loop instead of a `while`: ```java public void readLines(Path path) throws IOException { try (BufferedSource source = Okio.buffer(FileSystem.SYSTEM.source(path))) { for (String line; (line = source.readUtf8Line()) != null; ) { if (line.contains("square")) { System.out.println(line); } } } } ``` === "Kotlin" In Kotlin, we can use `FileSystem.read()` to buffer the source before our block and close the source afterwards. In the body of the block, `this` is a `BufferedSource`. ```kotlin @Throws(IOException::class) fun readLines(path: Path) { FileSystem.SYSTEM.read(path) { while (true) { val line = readUtf8Line() ?: break if ("square" in line) { println(line) } } } } ``` The `readUtf8Line()` method is suitable for parsing most files. For certain use-cases you may also consider `readUtf8LineStrict()`. It is similar but it requires that each line is terminated by `\n` or `\r\n`. If it encounters the end of the file before that it will throw an `EOFException`. The strict variant also permits a byte limit to defend against malformed input. === "Java" ```java public void readLines(Path path) throws IOException { try (BufferedSource source = Okio.buffer(FileSystem.SYSTEM.source(path))) { while (!source.exhausted()) { String line = source.readUtf8LineStrict(1024L); if (line.contains("square")) { System.out.println(line); } } } } ``` === "Kotlin" ```kotlin @Throws(IOException::class) fun readLines(path: Path) { FileSystem.SYSTEM.read(path) { while (!source.exhausted()) { val line = source.readUtf8LineStrict(1024) if ("square" in line) { println(line) } } } } ``` Write a text file ([Java][WriteFile]/[Kotlin][WriteFileKt]) ----------------------------------------------------------- Above we used a `Source` and a `BufferedSource` to read a file. To write, we use a `Sink` and a `BufferedSink`. The advantages of buffering are the same: a more capable API and better performance. ```java public void writeEnv(Path path) throws IOException { try (Sink fileSink = FileSystem.SYSTEM.sink(path); BufferedSink bufferedSink = Okio.buffer(fileSink)) { for (Map.Entry entry : System.getenv().entrySet()) { bufferedSink.writeUtf8(entry.getKey()); bufferedSink.writeUtf8("="); bufferedSink.writeUtf8(entry.getValue()); bufferedSink.writeUtf8("\n"); } } } ``` There isn’t an API to write a line of input; instead we manually insert our own newline character. Most programs should hardcode `"\n"` as the newline character. In rare situations you may use `System.lineSeparator()` instead of `"\n"`: it returns `"\r\n"` on Windows and `"\n"` everywhere else. === "Java" We can write the above program more compactly by inlining the `fileSink` variable and by taking advantage of method chaining: ```java public void writeEnv(Path path) throws IOException { try (BufferedSink sink = Okio.buffer(FileSystem.SYSTEM.sink(path))) { for (Map.Entry entry : System.getenv().entrySet()) { sink.writeUtf8(entry.getKey()) .writeUtf8("=") .writeUtf8(entry.getValue()) .writeUtf8("\n"); } } } ``` === "Kotlin" In Kotlin, we can use `FileSystem.write()` to buffer the sink before our block and close the sink afterwards. In the body of the block, `this` is a `BufferedSink`. ```kotlin @Throws(IOException::class) fun writeEnv(path: Path) { FileSystem.SYSTEM.write(path) { for ((key, value) in System.getenv()) { writeUtf8(key) writeUtf8("=") writeUtf8(value) writeUtf8("\n") } } } ``` In the above code we make four calls to `writeUtf8()`. Making four calls is more efficient than the code below because the VM doesn’t have to create and garbage collect a temporary string. ```java sink.writeUtf8(entry.getKey() + "=" + entry.getValue() + "\n"); // Slower! ``` UTF-8 ([Java][ExploreCharsets]/[Kotlin][ExploreCharsetsKt]) ----------------------------------------------------------- In the above APIs you can see that Okio really likes UTF-8. Early computer systems suffered many incompatible character encodings: ISO-8859-1, ShiftJIS, ASCII, EBCDIC, etc. Writing software to support multiple character sets was awful and we didn’t even have emoji! Today we're lucky that the world has standardized on UTF-8 everywhere, with some rare uses of other charsets in legacy systems. If you need another character set, `readString()` and `writeString()` are there for you. These methods require that you specify a character set. Otherwise you may accidentally create data that is only readable by the local computer. Most programs should use the UTF-8 methods only. When encoding strings you need to be mindful of the different ways that strings are represented and encoded. When a glyph has an accent or another adornment it may be represented as a single complex code point (`é`) or as a simple code point (`e`) followed by its modifiers (`´`). When the entire glyph is a single code point that’s called [NFC][nfc]; when it’s multiple it’s [NFD][nfd]. Though we use UTF-8 whenever we read or write strings in I/O, when they are in memory Java Strings use an obsolete character encoding called UTF-16. It is a bad encoding because it uses a 16-bit `char` for most characters, but some don’t fit. In particular, most emoji use two Java chars. This is problematic because `String.length()` returns a surprising result: the number of UTF-16 chars and not the natural number of glyphs. | | Café 🍩 | Café 🍩 | | --------------------: | :---------------------------| :------------------------------| | Form | [NFC][nfc] | [NFD][nfd] | | Code Points | `c  a  f  é    ␣   🍩     ` | `c  a  f  e  ´    ␣   🍩     ` | | UTF-8 bytes | `43 61 66 c3a9 20 f09f8da9` | `43 61 66 65 cc81 20 f09f8da9` | | String.codePointCount | 6 | 7 | | String.length | 7 | 8 | | Utf8.size | 10 | 11 | For the most part Okio lets you ignore these problems and focus on your data. But when you need them, there are convenient APIs for dealing with low-level UTF-8 strings. Use `Utf8.size()` to count the number of bytes required to encode a string as UTF-8 without actually encoding it. This is handy in length-prefixed encodings like protocol buffers. Use `BufferedSource.readUtf8CodePoint()` to read a single variable-length code point, and `BufferedSink.writeUtf8CodePoint()` to write one. === "Java" ```java public void dumpStringData(String s) throws IOException { System.out.println(" " + s); System.out.println(" String.length: " + s.length()); System.out.println("String.codePointCount: " + s.codePointCount(0, s.length())); System.out.println(" Utf8.size: " + Utf8.size(s)); System.out.println(" UTF-8 bytes: " + ByteString.encodeUtf8(s).hex()); System.out.println(); } ``` === "Kotlin" ```kotlin fun dumpStringData(s: String) { println(" " + s) println(" String.length: " + s.length) println("String.codePointCount: " + s.codePointCount(0, s.length)) println(" Utf8.size: " + s.utf8Size()) println(" UTF-8 bytes: " + s.encodeUtf8().hex()) println() } ``` Golden Values ([Java][GoldenValue]/[Kotlin][GoldenValueKt]) ----------------------------------------------------------- Okio likes testing. The library itself is heavily tested, and it has features that are often helpful when testing application code. One pattern we’ve found to be quite useful is “golden value” testing. The goal of such tests is to confirm that data encoded with earlier versions of a program can safely be decoded by the current program. We’ll illustrate this by encoding a value using Java Serialization. Though we must disclaim that Java Serialization is an awful encoding system and most programs should prefer other formats like JSON or protobuf! In any case, here’s a method that takes an object, serializes it, and returns the result as a `ByteString`: === "Java" ```Java private ByteString serialize(Object o) throws IOException { Buffer buffer = new Buffer(); try (ObjectOutputStream objectOut = new ObjectOutputStream(buffer.outputStream())) { objectOut.writeObject(o); } return buffer.readByteString(); } ``` === "Kotlin" ```Kotlin @Throws(IOException::class) private fun serialize(o: Any?): ByteString { val buffer = Buffer() ObjectOutputStream(buffer.outputStream()).use { objectOut -> objectOut.writeObject(o) } return buffer.readByteString() } ``` There’s a lot going on here. 1. We create a buffer as a holding space for our serialized data. It’s a convenient replacement for `ByteArrayOutputStream`. 2. We ask the buffer for its output stream. Writes to a buffer or its output stream always append data to the end of the buffer. 3. We create an `ObjectOutputStream` (the encoding API for Java serialization) and write our object. The try block takes care of closing the stream for us. Note that closing a buffer has no effect. 4. Finally we read a byte string from the buffer. The `readByteString()` method allows us to specify how many bytes to read; here we don’t specify a count in order to read the entire thing. Reads from a buffer always consume data from the front of the buffer. With our `serialize()` method handy we are ready to compute and print a golden value. === "Java" ```Java Point point = new Point(8.0, 15.0); ByteString pointBytes = serialize(point); System.out.println(pointBytes.base64()); ``` === "Kotlin" ```Kotlin val point = Point(8.0, 15.0) val pointBytes = serialize(point) println(pointBytes.base64()) ``` We print the `ByteString` as [base64][base64] because it’s a compact format that’s suitable for embedding in a test case. The program prints this: ``` rO0ABXNyAB5va2lvLnNhbXBsZXMuR29sZGVuVmFsdWUkUG9pbnTdUW8rMji1IwIAAkQAAXhEAAF5eHBAIAAAAAAAAEAuAAAAAAAA ``` That’s our golden value! We can embed it in our test case using base64 again to convert it back into a `ByteString`: === "Java" ```Java ByteString goldenBytes = ByteString.decodeBase64("rO0ABXNyAB5va2lvLnNhbXBsZ" + "XMuR29sZGVuVmFsdWUkUG9pbnTdUW8rMji1IwIAAkQAAXhEAAF5eHBAIAAAAAAAAEAuA" + "AAAAAAA"); ``` === "Kotlin" ```Kotlin val goldenBytes = ("rO0ABXNyACRva2lvLnNhbXBsZXMuS290bGluR29sZGVuVmFsdWUkUG9pbnRF9yaY7cJ9EwIAA" + "kQAAXhEAAF5eHBAIAAAAAAAAEAuAAAAAAAA").decodeBase64() ``` The next step is to deserialize the `ByteString` back into our value class. This method reverses the `serialize()` method above: we append a byte string to a buffer then consume it using an `ObjectInputStream`: === "Java" ```Java private Object deserialize(ByteString byteString) throws IOException, ClassNotFoundException { Buffer buffer = new Buffer(); buffer.write(byteString); try (ObjectInputStream objectIn = new ObjectInputStream(buffer.inputStream())) { return objectIn.readObject(); } } ``` === "Kotlin" ```Kotlin @Throws(IOException::class, ClassNotFoundException::class) private fun deserialize(byteString: ByteString): Any? { val buffer = Buffer() buffer.write(byteString) ObjectInputStream(buffer.inputStream()).use { objectIn -> return objectIn.readObject() } } ``` Now we can test the decoder against the golden value: === "Java" ```Java ByteString goldenBytes = ByteString.decodeBase64("rO0ABXNyAB5va2lvLnNhbXBsZ" + "XMuR29sZGVuVmFsdWUkUG9pbnTdUW8rMji1IwIAAkQAAXhEAAF5eHBAIAAAAAAAAEAuA" + "AAAAAAA"); Point decoded = (Point) deserialize(goldenBytes); assertEquals(new Point(8.0, 15.0), decoded); ``` === "Kotlin" ```Kotlin val goldenBytes = ("rO0ABXNyACRva2lvLnNhbXBsZXMuS290bGluR29sZGVuVmFsdWUkUG9pbnRF9yaY7cJ9EwIAA" + "kQAAXhEAAF5eHBAIAAAAAAAAEAuAAAAAAAA").decodeBase64()!! val decoded = deserialize(goldenBytes) as Point assertEquals(point, decoded) ``` With this test we can change the serialization of the `Point` class without breaking compatibility. Write a binary file ([Java][BitmapEncoder]/[Kotlin][BitmapEncoderKt]) --------------------------------------------------------------------- Encoding a binary file is not unlike encoding a text file. Okio uses the same `BufferedSink` and `BufferedSource` bytes for both. This is handy for binary formats that include both byte and character data. Writing binary data is more hazardous than text because if you make a mistake it is often quite difficult to diagnose. Avoid such mistakes by being careful around these traps: * **The width of each field.** This is the number of bytes used. Okio doesn't include a mechanism to emit partial bytes. If you need that, you’ll need to do your own bit shifting and masking before writing. * **The endianness of each field.** All fields that have more than one byte have _endianness_: whether the bytes are ordered most-significant to least (big endian) or least-significant to most (little endian). Okio uses the `Le` suffix for little-endian methods; methods without a suffix are big-endian. * **Signed vs. Unsigned.** Java doesn’t have unsigned primitive types (except for `char`!) so coping with this is often something that happens at the application layer. To make this a little easier Okio accepts `int` types for `writeByte()` and `writeShort()`. You can pass an “unsigned” byte like 255 and Okio will do the right thing. | Method | Width | Endianness | Value | Encoded Value | | :----------- | ----: | :--------- | --------------: | :------------------------ | | writeByte | 1 | | 3 | `03` | | writeShort | 2 | big | 3 | `00 03` | | writeInt | 4 | big | 3 | `00 00 00 03` | | writeLong | 8 | big | 3 | `00 00 00 00 00 00 00 03` | | writeShortLe | 2 | little | 3 | `03 00` | | writeIntLe | 4 | little | 3 | `03 00 00 00` | | writeLongLe | 8 | little | 3 | `03 00 00 00 00 00 00 00` | | writeByte | 1 | | Byte.MAX_VALUE | `7f` | | writeShort | 2 | big | Short.MAX_VALUE | `7f ff` | | writeInt | 4 | big | Int.MAX_VALUE | `7f ff ff ff` | | writeLong | 8 | big | Long.MAX_VALUE | `7f ff ff ff ff ff ff ff` | | writeShortLe | 2 | little | Short.MAX_VALUE | `ff 7f` | | writeIntLe | 4 | little | Int.MAX_VALUE | `ff ff ff 7f` | | writeLongLe | 8 | little | Long.MAX_VALUE | `ff ff ff ff ff ff ff 7f` | This code encodes a bitmap following the [BMP file format][bmp]. === "Java" ```Java void encode(Bitmap bitmap, BufferedSink sink) throws IOException { int height = bitmap.height(); int width = bitmap.width(); int bytesPerPixel = 3; int rowByteCountWithoutPadding = (bytesPerPixel * width); int rowByteCount = ((rowByteCountWithoutPadding + 3) / 4) * 4; int pixelDataSize = rowByteCount * height; int bmpHeaderSize = 14; int dibHeaderSize = 40; // BMP Header sink.writeUtf8("BM"); // ID. sink.writeIntLe(bmpHeaderSize + dibHeaderSize + pixelDataSize); // File size. sink.writeShortLe(0); // Unused. sink.writeShortLe(0); // Unused. sink.writeIntLe(bmpHeaderSize + dibHeaderSize); // Offset of pixel data. // DIB Header sink.writeIntLe(dibHeaderSize); sink.writeIntLe(width); sink.writeIntLe(height); sink.writeShortLe(1); // Color plane count. sink.writeShortLe(bytesPerPixel * Byte.SIZE); sink.writeIntLe(0); // No compression. sink.writeIntLe(16); // Size of bitmap data including padding. sink.writeIntLe(2835); // Horizontal print resolution in pixels/meter. (72 dpi). sink.writeIntLe(2835); // Vertical print resolution in pixels/meter. (72 dpi). sink.writeIntLe(0); // Palette color count. sink.writeIntLe(0); // 0 important colors. // Pixel data. for (int y = height - 1; y >= 0; y--) { for (int x = 0; x < width; x++) { sink.writeByte(bitmap.blue(x, y)); sink.writeByte(bitmap.green(x, y)); sink.writeByte(bitmap.red(x, y)); } // Padding for 4-byte alignment. for (int p = rowByteCountWithoutPadding; p < rowByteCount; p++) { sink.writeByte(0); } } } ``` === "Kotlin" ```Kotlin @Throws(IOException::class) fun encode(bitmap: Bitmap, sink: BufferedSink) { val height = bitmap.height val width = bitmap.width val bytesPerPixel = 3 val rowByteCountWithoutPadding = bytesPerPixel * width val rowByteCount = (rowByteCountWithoutPadding + 3) / 4 * 4 val pixelDataSize = rowByteCount * height val bmpHeaderSize = 14 val dibHeaderSize = 40 // BMP Header sink.writeUtf8("BM") // ID. sink.writeIntLe(bmpHeaderSize + dibHeaderSize + pixelDataSize) // File size. sink.writeShortLe(0) // Unused. sink.writeShortLe(0) // Unused. sink.writeIntLe(bmpHeaderSize + dibHeaderSize) // Offset of pixel data. // DIB Header sink.writeIntLe(dibHeaderSize) sink.writeIntLe(width) sink.writeIntLe(height) sink.writeShortLe(1) // Color plane count. sink.writeShortLe(bytesPerPixel * Byte.SIZE_BITS) sink.writeIntLe(0) // No compression. sink.writeIntLe(16) // Size of bitmap data including padding. sink.writeIntLe(2835) // Horizontal print resolution in pixels/meter. (72 dpi). sink.writeIntLe(2835) // Vertical print resolution in pixels/meter. (72 dpi). sink.writeIntLe(0) // Palette color count. sink.writeIntLe(0) // 0 important colors. // Pixel data. for (y in height - 1 downTo 0) { for (x in 0 until width) { sink.writeByte(bitmap.blue(x, y)) sink.writeByte(bitmap.green(x, y)) sink.writeByte(bitmap.red(x, y)) } // Padding for 4-byte alignment. for (p in rowByteCountWithoutPadding until rowByteCount) { sink.writeByte(0) } } } ``` The trickiest part of this program is the format’s required padding. The BMP format expects each row to begin on a 4-byte boundary so it is necessary to add zeros to maintain the alignment. Encoding other binary formats is usually quite similar. Some tips: * Write tests with golden values! Confirming that your program emits the expected result can make debugging easier. * Use `Utf8.size()` to compute the number of bytes of an encoded string. This is essential for length-prefixed formats. * Use `Float.floatToIntBits()` and `Double.doubleToLongBits()` to encode floating point values. Communicate on a Socket ([Java][SocksProxyServer]/[Kotlin][SocksProxyServerKt]) ------------------------------------------------------------------------------- Note that Okio doesn't yet support sockets on Kotlin/Native or Kotlin/JS. Sending and receiving data over the network is a bit like writing and reading files. We use `BufferedSink` to encode output and `BufferedSource` to decode input. Like files, network protocols can be text, binary, or a mix of both. But there are also some substantial differences between the network and the file system. With a file you’re either reading or writing but with the network you can do both! Some protocols handle this by taking turns: write a request, read a response, repeat. You can implement this kind of protocol with a single thread. In other protocols you may read and write simultaneously. Typically you’ll want one dedicated thread for reading. For writing you can use either a dedicated thread or use `synchronized` so that multiple threads can share a sink. Okio’s streams are not safe for concurrent use. Sinks buffer outbound data to minimize I/O operations. This is efficient but it means you must manually call `flush()` to transmit data. Typically message-oriented protocols flush after each message. Note that Okio will automatically flush when the buffered data exceeds some threshold. This is intended to save memory and you shouldn’t rely on it for interactive protocols. Okio builds on `java.io.Socket` for connectivity. Create your socket as a server or as a client, then use `Okio.source(Socket)` to read and `Okio.sink(Socket)` to write. These APIs also work with `SSLSocket`. You should use SSL unless you have a very good reason not to! Cancel a socket from any thread by calling `Socket.close()`; this will cause its sources and sinks to immediately fail with an `IOException`. You can also configure timeouts for all socket operations. You don’t need a reference to the socket to adjust timeouts: `Source` and `Sink` expose timeouts directly. This API works even if the streams are decorated. As a complete example of networking with Okio we wrote a [basic SOCKS proxy][SocksProxyServer] server. Some highlights: === "Java" ```Java Socket fromSocket = ... BufferedSource fromSource = Okio.buffer(Okio.source(fromSocket)); BufferedSink fromSink = Okio.buffer(Okio.sink(fromSocket)); ``` === "Kotlin" ```Kotlin val fromSocket: Socket = ... val fromSource = fromSocket.source().buffer() val fromSink = fromSocket.sink().buffer() ``` Creating sources and sinks for sockets is the same as creating them for files. Once you create a `Source` or `Sink` for a socket you must not use its `InputStream` or `OutputStream`, respectively. === "Java" ```Java Buffer buffer = new Buffer(); for (long byteCount; (byteCount = source.read(buffer, 8192L)) != -1; ) { sink.write(buffer, byteCount); sink.flush(); } ``` === "Kotlin" ```Kotlin val buffer = Buffer() var byteCount: Long while (source.read(buffer, 8192L).also { byteCount = it } != -1L) { sink.write(buffer, byteCount) sink.flush() } ``` The above loop copies data from the source to the sink, flushing after each read. If we didn’t need the flushing we could replace this loop with a single call to `BufferedSink.writeAll(Source)`. The `8192` argument to `read()` is the maximum number of bytes to read before returning. We could have passed any value here, but we like 8 KiB because that’s the largest value Okio can do in a single system call. Most of the time application code doesn’t need to deal with such limits! === "Java" ```Java int addressType = fromSource.readByte() & 0xff; int port = fromSource.readShort() & 0xffff; ``` === "Kotlin" ```Kotlin val addressType = fromSource.readByte().toInt() and 0xff val port = fromSource.readShort().toInt() and 0xffff ``` Okio uses signed types like `byte` and `short`, but often protocols want unsigned values. The bitwise `&` operator is Java’s preferred idiom to convert a signed value into an unsigned value. Here’s a cheat sheet for bytes, shorts, and ints: | Type | Signed Range | Unsigned Range | Signed to Unsigned | | :---- | :---------------------------: | :--------------- | :-------------------------- | | byte | -128..127 | 0..255 | `int u = s & 0xff;` | | short | -32,768..32,767 | 0..65,535 | `int u = s & 0xffff;` | | int | -2,147,483,648..2,147,483,647 | 0..4,294,967,295 | `long u = s & 0xffffffffL;` | Java has no primitive type that can represent unsigned longs. Hashing ([Java][Hashing]/[Kotlin][HashingKt]) --------------------------------------------- We’re bombarded by hashing in our lives as Java programmers. Early on we're introduced to the `hashCode()` method, something we know we need to override otherwise unforeseen bad things happen. Later we’re shown `LinkedHashMap` and its friends. These build on that `hashCode()` method to organize data for fast retrieval. Elsewhere we have cryptographic hash functions. These get used all over the place. HTTPS certificates, Git commits, BitTorrent integrity checking, and Blockchain blocks all use cryptographic hashes. Good use of hashes can improve the performance, privacy, security, and simplicity of an application. Each cryptographic hash function accepts a variable-length stream of input bytes and produces a fixed-length byte string value called the “hash”. Hash functions have these important qualities: * Deterministic: each input always produces the same output. * Uniform: each output byte string is equally likely. It is very difficult to find or create pairs of different inputs that yield the same output. This is called a “collision”. * Non-reversible: knowing an output doesn't help you to find the input. Note that if you know some possible inputs you can hash them to see if their hashes match. * Well-known: the hash is implemented everywhere and rigorously understood. Good hash functions are very cheap to compute (dozens of microseconds) and expensive to reverse (quintillions of millenia). Steady advances in computing and mathematics have caused once-great hash functions to become inexpensive to reverse. When choosing a hash function, beware that not all are created equal! Okio supports these well-known cryptographic hash functions: * **MD5**: a 128-bit (16 byte) cryptographic hash. It is both insecure and obsolete because it is inexpensive to reverse! This hash is offered because it is popular and convenient for use in legacy systems that are not security-sensitive. * **SHA-1**: a 160-bit (20 byte) cryptographic hash. It was recently demonstrated that it is feasible to create SHA-1 collisions. Consider upgrading from SHA-1 to SHA-256. * **SHA-256**: a 256-bit (32 byte) cryptographic hash. SHA-256 is widely understood and expensive to reverse. This is the hash most systems should use. * **SHA-512**: a 512-bit (64 byte) cryptographic hash. It is expensive to reverse. Each hash creates a `ByteString` of the specified length. Use `hex()` to get the conventional human-readable form. Or leave it as a `ByteString` because that’s a convenient model type! Okio can produce cryptographic hashes from byte strings: === "Java" ```Java ByteString byteString = readByteString(Path.get("README.md")); System.out.println(" md5: " + byteString.md5().hex()); System.out.println(" sha1: " + byteString.sha1().hex()); System.out.println("sha256: " + byteString.sha256().hex()); System.out.println("sha512: " + byteString.sha512().hex()); ``` === "Kotlin" ```Kotlin val byteString = readByteString("README.md".toPath()) println(" md5: " + byteString.md5().hex()) println(" sha1: " + byteString.sha1().hex()) println(" sha256: " + byteString.sha256().hex()) println(" sha512: " + byteString.sha512().hex()) ``` From buffers: === "Java" ```Java Buffer buffer = readBuffer(Path.get("README.md")); System.out.println(" md5: " + buffer.md5().hex()); System.out.println(" sha1: " + buffer.sha1().hex()); System.out.println("sha256: " + buffer.sha256().hex()); System.out.println("sha512: " + buffer.sha512().hex()); ``` === "Kotlin" ```Kotlin val buffer = readBuffer("README.md".toPath()) println(" md5: " + buffer.md5().hex()) println(" sha1: " + buffer.sha1().hex()) println(" sha256: " + buffer.sha256().hex()) println(" sha512: " + buffer.sha512().hex()) ``` While streaming from a source: === "Java" ```Java try (HashingSink hashingSink = HashingSink.sha256(Okio.blackhole()); BufferedSource source = Okio.buffer(FileSystem.SYSTEM.source(path))) { source.readAll(hashingSink); System.out.println("sha256: " + hashingSink.hash().hex()); } ``` === "Kotlin" ```Kotlin sha256(blackholeSink()).use { hashingSink -> FileSystem.SYSTEM.source(path).buffer().use { source -> source.readAll(hashingSink) println(" sha256: " + hashingSink.hash.hex()) } } ``` While streaming to a sink: === "Java" ```Java try (HashingSink hashingSink = HashingSink.sha256(Okio.blackhole()); BufferedSink sink = Okio.buffer(hashingSink); Source source = FileSystem.SYSTEM.source(path)) { sink.writeAll(source); sink.close(); // Emit anything buffered. System.out.println("sha256: " + hashingSink.hash().hex()); } ``` === "Kotlin" ```Kotlin sha256(blackholeSink()).use { hashingSink -> hashingSink.buffer().use { sink -> FileSystem.SYSTEM.source(path).use { source -> sink.writeAll(source) sink.close() // Emit anything buffered. println(" sha256: " + hashingSink.hash.hex()) } } } ``` Okio also supports HMAC (Hash Message Authentication Code) which combines a secret and a hash. Applications use HMAC for data integrity and authentication. === "Java" ```Java ByteString secret = ByteString.decodeHex("7065616e7574627574746572"); System.out.println("hmacSha256: " + byteString.hmacSha256(secret).hex()); ``` === "Kotlin" ```Kotlin val secret = "7065616e7574627574746572".decodeHex() println("hmacSha256: " + byteString.hmacSha256(secret).hex()) ``` As with hashing, you can generate an HMAC from a `ByteString`, `Buffer`, `HashingSource`, and `HashingSink`. Note that Okio doesn’t implement HMAC for MD5. On Android and Java, Okio uses Java’s `java.security.MessageDigest` for cryptographic hashes and `javax.crypto.Mac` for HMAC. On other platforms Okio uses its own optimized implementation of these algorithms. Encryption and Decryption ------------------------- On Android and Java it's easy to encrypt streams. Callers are responsible for the initialization of the encryption or decryption cipher with the chosen algorithm, the key, and algorithm-specific additional parameters like the initialization vector. The following example shows a typical usage with AES encryption, in which `key` and `iv` parameters should both be 16 bytes long. === "Java" Use `Okio.cipherSink(Sink, Cipher)` or `Okio.cipherSource(Source, Cipher)` to encrypt or decrypt a stream using a block cipher. ```java void encryptAes(ByteString bytes, Path path, byte[] key, byte[] iv) throws GeneralSecurityException, IOException { Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding"); cipher.init(Cipher.ENCRYPT_MODE, new SecretKeySpec(key, "AES"), new IvParameterSpec(iv)); try (BufferedSink sink = Okio.buffer( Okio.cipherSink(FileSystem.SYSTEM.sink(path), cipher))) { sink.write(bytes); } } ByteString decryptAesToByteString(Path path, byte[] key, byte[] iv) throws GeneralSecurityException, IOException { Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding"); cipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(key, "AES"), new IvParameterSpec(iv)); try (BufferedSource source = Okio.buffer( Okio.cipherSource(FileSystem.SYSTEM.source(path), cipher))) { return source.readByteString(); } } ``` === "Kotlin" Encryption and decryption functions are extensions on `Cipher`: ```kotlin fun encryptAes(bytes: ByteString, path: Path, key: ByteArray, iv: ByteArray) { val cipher = Cipher.getInstance("AES/CBC/PKCS5Padding") cipher.init(Cipher.ENCRYPT_MODE, SecretKeySpec(key, "AES"), IvParameterSpec(iv)) val cipherSink = FileSystem.SYSTEM.sink(path).cipherSink(cipher) cipherSink.buffer().use { it.write(bytes) } } fun decryptAesToByteString(path: Path, key: ByteArray, iv: ByteArray): ByteString { val cipher = Cipher.getInstance("AES/CBC/PKCS5Padding") cipher.init(Cipher.DECRYPT_MODE, SecretKeySpec(key, "AES"), IvParameterSpec(iv)) val cipherSource = FileSystem.SYSTEM.source(path).cipherSource(cipher) return cipherSource.buffer().use { it.readByteString() } } ``` [base64]: https://tools.ietf.org/html/rfc4648#section-4 [bmp]: https://en.wikipedia.org/wiki/BMP_file_format [nfd]: https://docs.oracle.com/javase/7/docs/api/java/text/Normalizer.Form.html#NFD [nfc]: https://docs.oracle.com/javase/7/docs/api/java/text/Normalizer.Form.html#NFC [BitmapEncoderKt]: https://github.com/square/okio/blob/master/samples/src/jvmMain/kotlin/okio/samples/BitmapEncoder.kt [BitmapEncoder]: https://github.com/square/okio/blob/master/samples/src/jvmMain/java/okio/samples/BitmapEncoder.java [ExploreCharsetsKt]: https://github.com/square/okio/blob/master/samples/src/jvmMain/kotlin/okio/samples/ExploreCharsets.kt [ExploreCharsets]: https://github.com/square/okio/blob/master/samples/src/jvmMain/java/okio/samples/ExploreCharsets.java [GoldenValueKt]: https://github.com/square/okio/blob/master/samples/src/jvmMain/kotlin/okio/samples/GoldenValue.kt [GoldenValue]: https://github.com/square/okio/blob/master/samples/src/jvmMain/java/okio/samples/GoldenValue.java [HashingKt]: https://github.com/square/okio/blob/master/samples/src/jvmMain/kotlin/okio/samples/Hashing.kt [Hashing]: https://github.com/square/okio/blob/master/samples/src/jvmMain/java/okio/samples/Hashing.java [ReadFileLineByLine]: https://github.com/square/okio/blob/master/samples/src/jvmMain/java/okio/samples/ReadFileLineByLine.java [ReadFileLineByLineKt]: https://github.com/square/okio/blob/master/samples/src/jvmMain/kotlin/okio/samples/ReadFileLineByLine.kt [SocksProxyServerKt]: https://github.com/square/okio/blob/master/samples/src/jvmMain/kotlin/okio/samples/SocksProxyServer.kt [SocksProxyServer]: https://github.com/square/okio/blob/master/samples/src/jvmMain/java/okio/samples/SocksProxyServer.java [WriteFile]: https://github.com/square/okio/blob/master/samples/src/jvmMain/java/okio/samples/WriteFile.java [WriteFileKt]: https://github.com/square/okio/blob/master/samples/src/jvmMain/kotlin/okio/samples/WriteFile.kt