2020 03 17 secp256k1jni md (#1248)

* First draft secp256k1.md

Fix imports/names

Fix nits

* Address nadav's code review

docs/secp256k1/secp256k1.md

@@ -0,0 +1,87 @@
+---
+id: secp256k1
+title: Secp256k1
+---
+
+[Libsecp256k1](https://github.com/bitcoin-core/secp256k1) is used to preform cryptographic operations on the secp256k1 curve.
+This is the curve that bitcoin uses. There is a _signficant_ speedup when using this library compared to java crypto libraries
+like bouncy castle.
+
+In bitcoin-s, we support native binaries for libsecp256k1
+
+1. [linux 32 bit](../../secp256k1jni/natives/linux_32)
+2. [linux 64 bit](../../secp256k1jni/natives/linux_64)
+3. [mac osx 64 bit](../../secp256k1jni/natives/osx_64)
+4. [windows 64 bit](../../secp256k1jni/natives/windows_64)
+
+Bitcoin-s uses a zero dependency library called [`native-lib-loader`](https://github.com/scijava/native-lib-loader). 
+That does the appropriate loading of the library onto your classpath to be accessed.
+
+#### Using libsecp256k1
+
+To tell if you have access to libsecp256k1 you can do the following
+
+```scala mdoc:invisible
+import org.bitcoin._
+import org.bitcoins.core.crypto._
+
+```
+
+```scala mdoc:compile-only
+val isEnabled = org.bitcoin.Secp256k1Context.isEnabled()
+
+println(s"Secp256k1Context.isEnabled=${isEnabled}")
+```
+
+If libsecp256k1 is enabled, you can use [NativeSecp256k1](../../secp256k1jni/src/main/java/org/bitcoin/NativeSecp256k1.java)
+with static method defined in the class.
+
+
+```scala mdoc:compile-only
+val privKey = ECPrivateKey.freshPrivateKey
+val pubKey = privKey.publicKey
+val dataToSign = DoubleSha256Digest.empty
+
+val signature = NativeSecp256k1.sign(dataToSign.bytes.toArray, privKey.bytes.toArray)
+    
+val verify = NativeSecp256k1.verify(dataToSign.bytes.toArray, signature, pubKey.bytes.toArray)
+
+println(s"Verified with NativeSecp256k1 signature=${verify}")
+
+//you can also just directly sign with the ECKey interface:
+val signature2 = privKey.sign(dataToSign)
+
+val verified2 = pubKey.verify(dataToSign, signature2)
+
+println(s"Verified with NativeSecp256k1 again=${verified2}")
+
+```
+
+### When libsecp256k1 isn't available, or you want to turn it off
+
+There are two reasons you wouldn't want to use libsecp256k1 
+
+1. You don't trust the pre-compiled binaries we are using
+2. Your OS/arch is not supported
+
+There are two ways you can circumvent libsecp256k1
+
+1. Set `DISABLE_SECP256K1=true` in your environment variables. This will force `Secp256k1Context.isEnabled()` to return false
+2. Call Bouncy castle methods in `ECKey`. 
+
+Here is an example of calling bouncy castle methods in `ECKey`
+
+```scala mdoc:to-string
+
+val privKey = ECPrivateKey.freshPrivateKey
+val publicKey = privKey.publicKeyWithBouncyCastle
+val dataToSign = DoubleSha256Digest.empty
+
+val signature = privKey.signWithBouncyCastle(dataToSign.bytes)
+
+val verified = publicKey.verifyWithBouncyCastle(dataToSign.bytes, signature)
+
+println(s"Verified with bouncy castle=${verified}")
+
+```
+

website/sidebars.json

@@ -41,6 +41,9 @@
       "rpc/rpc-eclair",
       "rpc/rpc-bitcoind"
     ],
+    "Secp256k1": [
+      "secp256k1/secp256k1"
+    ],
     "Testkit": [
       "testkit/testkit"
     ],