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Essential support of TLS for fs2.

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This library adds sport of TLS for fs2 programs. Library provides two levels of TLS:

  • TLSEngine A Low level abstraction of java's SSLEngine allowing asynchronous functional usage of SSLEngine
  • TLSSocket A wrapper of standard that allows to receive/send data over TLS/SSL


Add this to your sbt build file :

libraryDependencies += "com.spinoco" %% "fs2-crypto" % "0.4.0"


version scala fs2
0.5.0-M1 2.11, 2.12, 2.13 1.1.0-M1
0.4.0 2.11, 2.12 1.0.0
0.2.0 2.11, 2.12 0.10.0
0.1.2 2.11, 2.12 0.9.7


To create encrypted TLS socket, library allows very simple usage by just using current User just need to create tcp socket and then pass it to TLSSocket that creates encrypted version of the socket.

Following is the example how this can be done in scala :



import spinoco.fs2.crypto._

import java.nio.channels.AsynchronousChannelGroup
import java.nio.channels.spi.AsynchronousChannelProvider
import java.util.concurrent.{Executors, ThreadFactory}
import cats.effect.IO._
import scala.concurrent.ExecutionContext
import scala.{Stream => _ }
import fs2.Stream

implicit val executionContext: ExecutionContext =

val sslCtx = SSLContext.getInstance("TLS")
sslCtx.init(null, null, null)

implicit val tcpACG: AsynchronousChannelGroup = AsynchronousChannelProvider
  .openAsynchronousChannelGroup(Executors.newCachedThreadPool(), 8)

val ctx = SSLContext.getInstance("TLS")
ctx.init(null, null, null)

val engine = sslCtx.createSSLEngine()

val address = new InetSocketAddress("", 6060)

client(address) flatMap { socket => TLSSocket(socket, engine) } flatMap { tlsSocket =>

  // perform any operations with tlsSocket as you would with normal Socket.


Note that client initiates initial TLS handshake. As such, client needs to send always some data to server to trigger SSL handshake. It is engough to send empty chunk of bytes to trigger initial TLS handshake from client side.


Java's SSLEngine is not most pleasant api to work with, and therefore there is TLSEngine. TLSEngine wraps SSLEngine and provides nonblocking asynchronous interface to SSLEngine that can be used to create TLS (with java 9 even DTLS) encrypted connections.

TLSEngine is used to create TLSSocket, which can be used as real example on how TLSEngine can construct the higher level abstractions. TLSEgine provides two simple methods:

  • encrypt to encrypt data from application and send resulting data over transport layer
  • decrypt to decrypt data received from network transport.

TLSEngine encryption

When encrypting data with TLSEngine, you use the encrypt method that may return any of three results:

  • Encrypted(data) indicating that data was successfully encrypted supplying the resulting frame to be sent over network
  • Closed indicating that engine is closed and no more application data may be encrypted
  • Handshake(data, next) providing data to be sent to network party and next operation to perfom immediately after the data will be send to remote party, but before any other encrypt invocation.

TLSEngine decryption

When decrypting data with TLSEngine, you use the decrypt method, that may return any of three possible results:

  • Decrypted(data) indicating data that were decrypted. Note that data may be empty, for example in case the received data was not enough to form full TLS Record. If the data supplied to decrypt forms multiple TLS records, then this will return content of both TLS Recors, concatenated.
  • Closed idicating that engine is closed and no more data can be decrypted.
  • Handshake(data, signalSentEventually) providing data to be sent to remote party during handshake. Eventually signalSentEventually may be provided to perform operation immediately when the data will be sent to network, before any other decrypt.