Secp256k1 for Kotlin/Multiplatform

Kotlin/Multiplatform wrapper for Bitcoin Core's secp256k1 library. Targets: JVM, Android, iOS & Linux.

Installation

secp256k1-kmp is available on maven central

Then, the actual dependency depends on your targeted platform(s):

Multiplatform

Add the secp256k1 dependency to the common sourceSet, and the JNI dependencies to JVM and Android sourcesets:

// build.gradle.kts

kotlin {
    jvm()
    android()
    linuxX64("linux")
    ios()

    sourceSets {
        val commonMain by getting {
            dependencies {
                implementation(kotlin("stdlib-common"))
                implementation("fr.acinq.secp256k1:secp256k1-kmp:$secp256k1_version")
            }
        }
        val jvmMain by getting {
            dependencies {
                implementation(kotlin("stdlib"))
                implementation("fr.acinq.secp256k1:secp256k1-kmp-jni-jvm:$secp256k1_version")
            }
        }
        val androidMain by getting {
            dependencies {
                implementation(kotlin("stdlib"))
                implementation("fr.acinq.secp256k1:secp256k1-kmp-jni-android:$secp256k1_version")
            }
        }
    }
}

Native targets (iOS, linux64)

Native targets include libsecp256k1, called through KMP's c-interop, simply add the fr.acinq.secp256k1:secp256k1 dependency.

JVM targets & Android

The JVM library uses JNI bindings for libsecp256k1, which is much faster than BouncyCastle. It will extract and load native bindings for your operating system in a temporary directory.

JNI libraries are included for:

• Linux 64 bits (x86_64 and arm64)
• Windows 64 bits (x86_64)
• Macos 64 bits (x86_64 and arm64)

Along this library, you must specify which JNI native library to use in your dependency manager:

• For desktop or server JVMs, you must add the dependency:
  • Either the fr.acinq.secp256k1:secp256k1-kmp-jni-jvm dependency which imports all supported platforms.
  • Or the platform specific dependencies (note that you can add multiple as they do not conflict):
    • fr.acinq.secp256k1:secp256k1-kmp-jni-jvm-linux for Linux
    • fr.acinq.secp256k1:secp256k1-kmp-jni-jvm-darwin for Mac OS X
    • fr.acinq.secp256k1:secp256k1-kmp-jni-jvm-mingw for Windows
• For Android, you must add the fr.acinq.secp256k1:secp256k1-kmp-jni-android dependency

If you are using the JVM on an OS for which we don't provide JNI bindings (32 bits OS for example), you can use your own library native library by adding the fr.acinq.secp256k1:secp256k1-kmp-jni-jvm dependency and specifying its path with -Dfr.acinq.secp256k1.lib.path and optionally its name with -Dfr.acinq.secp256k1.lib.name (if unspecified bitcoink use the standard name for your OS i.e. libsecp256k1.so on Linux, secp256k1.dll on Windows, ...).

To compile your own JNI bindings, have a look add the native/build.sh and jni/build.sh scripts.

You can also specify the temporary directory where the library will be extracted with -Djava.io.tmpdir or -Dfr.acinq.secp256k1.tmpdir (if you want to use a different directory from -Djava.io.tmpdir).

Usage

Please have a look at unit tests, more samples will be added soon.

Building

secp256k1-kmp is a Kotlin Multiplatform wrapper for Bitcoin Core's secp256k1 library.

To build the library you need the following:

• Window 64 bits, Linux 64 bits, or MacOs 64 Bits
• OpenJDK11 (we recommend using packages provided by https://adoptopenjdk.net/ but there are other options)
• (optional) Android SDK

It may work with other Operating Systems and JDKs, but then you're on your own (in particular we don't plan to support 32 bits Operating Systems). To build the library and publish compiled artefacts locally (so they can be used by other projects):

./gradlew :build
./gradlew :publishToMavenLocal

To run all tests on all platforms:

./gradlew allTests

To run tests on a single platform, for example the JVM:

./gradlew jvmTest

If you want to skip building Android artefacts create a local.properties file in the project's root directory and add the following line:

skip.android=true

Contributing to and extending the library

secp256k1-kmp follows 2 simples rules:

• copy as literally as possible what the original secp256k1 library does: use the same function names, parameters, options, ...
• follow JNI best practices (memory allocation, error handling, ...)

"Porting" C/C++ code that uses secp256k1 should be a no-brainer and we should not have to document secp256k1-kmp

To extend this library and support methods that have been added to specific versions of secp256k1 you have to:

• add new methods to the Secp256k1 interface src/commonMain/kotlin/fr/acinq/secp256k1/Secp256k1.kt (please follow rule #1 above and try and match secp256k1's interface as much as possible)
• implement these new methods in jni/src/main/kotlin/fr/acinq/secp256k1/NativeSecp256k1.kt (JNI implementation) and src/nativeMain/kotlin/fr/acinq/secp256k1/Secp256k1Native.kt (native linux/ios/... implementation)
• update the JNI interface src/main/java/fr/acinq/secp256k1/Secp256k1CFunctions.java (NativeSecp256k1 calls Secp256k1CFunctions)
• generate a new JNI header file jni/c/headers/java/fr_acinq_secp256k1_Secp256k1CFunctions.h with javac -h jni/c/headers/java jni/src/main/java/fr/acinq/secp256k1/Secp256k1CFunctions.java
• implement the new methods in jni/c/src/fr_acinq_secp256k1_Secp256k1CFunctions.c

You may also need to modify build files if you need to compile secp256k1 with custom options

We use secp256k1 through git submodules so you may also need to change what they point to