diff --git a/js/src/main/scala/Main.scala b/js/src/main/scala/Main.scala
index a35d2e7..a9caac9 100644
--- a/js/src/main/scala/Main.scala
+++ b/js/src/main/scala/Main.scala
@@ -1,3 +1,20 @@
+/*
+ * Copyright (C) 2017  Fluence Labs Limited
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
 object Main {
   def main(args: Array[String]): Unit = {
     println("Hello world!")
diff --git a/jvm/src/main/scala/fluence/crypto/algorithm/Ecdsa.scala b/jvm/src/main/scala/fluence/crypto/algorithm/Ecdsa.scala
index 4c3c414..f892a7a 100644
--- a/jvm/src/main/scala/fluence/crypto/algorithm/Ecdsa.scala
+++ b/jvm/src/main/scala/fluence/crypto/algorithm/Ecdsa.scala
@@ -21,7 +21,8 @@ import java.math.BigInteger
 import java.security._
 import java.security.interfaces.ECPrivateKey
 
-import cats.MonadError
+import cats.data.EitherT
+import cats.{ Applicative, Monad, MonadError }
 import cats.syntax.flatMap._
 import cats.syntax.functor._
 import fluence.crypto.keypair.KeyPair
@@ -42,47 +43,62 @@ import scala.util.control.NonFatal
 class Ecdsa(curveType: String, scheme: String) extends JavaAlgorithm
   with SignatureFunctions with KeyGenerator {
   import Ecdsa._
-  private def nonFatalHandling[F[_], A](a: ⇒ A)(errorText: String)(implicit F: MonadError[F, Throwable]): F[A] = {
-    try F.pure(a)
+
+  private def nonFatalHandling[F[_] : Applicative, A](a: ⇒ A)(errorText: String): EitherT[F, CryptoErr, A] = {
+    try EitherT.pure(a)
     catch {
-      case NonFatal(e) ⇒ F.raiseError(CryptoErr(errorText + " " + e.getLocalizedMessage))
+      case NonFatal(e) ⇒ EitherT.leftT(CryptoErr(errorText + " " + e.getLocalizedMessage))
     }
   }
 
-  override def generateKeyPair[F[_]](random: SecureRandom)(implicit F: MonadError[F, Throwable]): F[KeyPair] = {
+  override def generateKeyPair[F[_] : Monad](random: SecureRandom): EitherT[F, CryptoErr, KeyPair] =
     for {
-      ecSpecOp ← F.pure(Option(ECNamedCurveTable.getParameterSpec(curveType)))
-      ecSpec ← ecSpecOp match {
-        case Some(ecs) ⇒ F.pure(ecs)
-        case None      ⇒ F.raiseError[ECNamedCurveParameterSpec](CryptoErr("Parameter spec for the curve is not available."))
-      }
+      ecSpec ← EitherT.fromOption[F](
+        Option(ECNamedCurveTable.getParameterSpec(curveType)),
+        CryptoErr("Parameter spec for the curve is not available."))
       g ← getKeyPairGenerator
       _ ← nonFatalHandling(g.initialize(ecSpec, random))("Could not initialize KeyPairGenerator.")
-      keyPair ← Option(g.generateKeyPair()) match {
-        case Some(p) ⇒
-          //store S number for private key and compressed Q point on curve for public key
-          val pk = p.getPublic.asInstanceOf[ECPublicKey].getQ.getEncoded(true)
-          val sk = p.getPrivate.asInstanceOf[ECPrivateKey].getS.toByteArray
-          F.pure(KeyPair.fromBytes(pk, sk))
-        case None ⇒ F.raiseError[KeyPair](CryptoErr("Could not generate KeyPair. Unexpected."))
-      }
+      p ← EitherT.fromOption(
+        Option(g.generateKeyPair()),
+        CryptoErr("Could not generate KeyPair. Unexpected."))
+
+      keyPair ← nonFatalHandling {
+        //store S number for private key and compressed Q point on curve for public key
+        val pk = p.getPublic.asInstanceOf[ECPublicKey].getQ.getEncoded(true)
+        val sk = p.getPrivate.asInstanceOf[ECPrivateKey].getS.toByteArray
+        KeyPair.fromBytes(pk, sk)
+      }("Can't get public/private key from generated keypair")
     } yield keyPair
-  }
 
-  override def generateKeyPair[F[_]]()(implicit F: MonadError[F, Throwable]): F[KeyPair] = {
+  override def generateKeyPair[F[_] : Monad](): EitherT[F, CryptoErr, KeyPair] =
     generateKeyPair(new SecureRandom())
-  }
 
-  override def sign[F[_]](keyPair: KeyPair, message: ByteVector)(implicit F: MonadError[F, Throwable]): F[fluence.crypto.signature.Signature] = {
+  override def sign[F[_] : Monad](keyPair: KeyPair, message: ByteVector): EitherT[F, CryptoErr, fluence.crypto.signature.Signature] =
     signMessage(keyPair.secretKey.value.toArray, message.toArray)
       .map(bb ⇒ fluence.crypto.signature.Signature(keyPair.publicKey, ByteVector(bb)))
+
+  override def verify[F[_] : Monad](signature: fluence.crypto.signature.Signature, message: ByteVector): EitherT[F, CryptoErr, Unit] = {
+    val publicKey = signature.publicKey.value.toArray
+    val messageBytes = message.toArray
+    val signatureBytes = signature.sign.toArray
+
+    for {
+      ec ← curveSpec
+      keySpec ← nonFatalHandling(new ECPublicKeySpec(ec.getCurve.decodePoint(publicKey), ec))("Cannot read public key.")
+      keyFactory ← getKeyFactory
+      signProvider ← getSignatureProvider
+      verify ← {
+        nonFatalHandling {
+          signProvider.initVerify(keyFactory.generatePublic(keySpec))
+          signProvider.update(messageBytes)
+          signProvider.verify(signatureBytes)
+        }("Cannot verify message.")
+      }
+      _ ← EitherT.cond[F](verify, (), CryptoErr("Signature is not verified"))
+    } yield ()
   }
 
-  override def verify[F[_]](signature: fluence.crypto.signature.Signature, message: ByteVector)(implicit F: MonadError[F, Throwable]): F[Boolean] = {
-    verifySign(signature.publicKey.value.toArray, message.toArray, signature.sign.toArray)
-  }
-
-  private def signMessage[F[_]](privateKey: Array[Byte], message: Array[Byte])(implicit F: MonadError[F, Throwable]): F[Array[Byte]] = {
+  private def signMessage[F[_] : Monad](privateKey: Array[Byte], message: Array[Byte]): EitherT[F, CryptoErr, Array[Byte]] = {
     for {
       ec ← curveSpec
       keySpec ← nonFatalHandling(new ECPrivateKeySpec(new BigInteger(privateKey), ec))("Cannot read private key.")
@@ -98,32 +114,16 @@ class Ecdsa(curveType: String, scheme: String) extends JavaAlgorithm
     } yield sign
   }
 
-  private def verifySign[F[_]](publicKey: Array[Byte], message: Array[Byte], signature: Array[Byte])(implicit F: MonadError[F, Throwable]): F[Boolean] = {
-    for {
-      ec ← curveSpec
-      keySpec ← nonFatalHandling(new ECPublicKeySpec(ec.getCurve.decodePoint(publicKey), ec))("Cannot read public key.")
-      keyFactory ← getKeyFactory
-      signProvider ← getSignatureProvider
-      verify ← {
-        nonFatalHandling {
-          signProvider.initVerify(keyFactory.generatePublic(keySpec))
-          signProvider.update(message)
-          signProvider.verify(signature)
-        }("Cannot verify message.")
-      }
-    } yield verify
-  }
-
-  private def curveSpec[F[_]](implicit F: MonadError[F, Throwable]) =
+  private def curveSpec[F[_] : Monad] =
     nonFatalHandling(ECNamedCurveTable.getParameterSpec(curveType).asInstanceOf[ECParameterSpec])("Cannot get curve parameters.")
 
-  private def getKeyPairGenerator[F[_]](implicit F: MonadError[F, Throwable]) =
+  private def getKeyPairGenerator[F[_] : Monad] =
     nonFatalHandling(KeyPairGenerator.getInstance(ECDSA, BouncyCastleProvider.PROVIDER_NAME))("Cannot get key pair generator.")
 
-  private def getKeyFactory[F[_]](implicit F: MonadError[F, Throwable]) =
+  private def getKeyFactory[F[_] : Monad] =
     nonFatalHandling(KeyFactory.getInstance(ECDSA, BouncyCastleProvider.PROVIDER_NAME))("Cannot get key factory instance.")
 
-  private def getSignatureProvider[F[_]](implicit F: MonadError[F, Throwable]) =
+  private def getSignatureProvider[F[_] : Monad] =
     nonFatalHandling(Signature.getInstance(scheme, BouncyCastleProvider.PROVIDER_NAME))("Cannot get signature instance.")
 }
 
diff --git a/jvm/src/test/scala/fluence/crypto/SignatureSpec.scala b/jvm/src/test/scala/fluence/crypto/SignatureSpec.scala
index 661e1a1..7248996 100644
--- a/jvm/src/test/scala/fluence/crypto/SignatureSpec.scala
+++ b/jvm/src/test/scala/fluence/crypto/SignatureSpec.scala
@@ -18,68 +18,74 @@
 package fluence.crypto
 
 import java.io.File
-import java.security.SecureRandom
 
+import cats.data.EitherT
 import cats.instances.try_._
 import fluence.crypto.algorithm.{ CryptoErr, Ecdsa }
-import fluence.crypto.keypair.KeyPair
 import org.scalatest.{ BeforeAndAfterAll, Matchers, WordSpec }
 import scodec.bits.ByteVector
 
-import scala.util.Random
+import scala.util.{ Random, Try }
 
 class SignatureSpec extends WordSpec with Matchers with BeforeAndAfterAll {
 
   def rndBytes(size: Int) = Random.nextString(10).getBytes
+
   def rndByteVector(size: Int) = ByteVector(rndBytes(size))
 
+  private implicit class TryEitherTExtractor[A, B](et: EitherT[Try, A, B]) {
+    def extract: B = et.value.get.right.get
+
+    def isOk: Boolean = et.value.fold(_ ⇒ false, _.isRight)
+  }
+
   "ecdsa algorithm" should {
     "correct sign and verify data" in {
       val algorithm = Ecdsa.ecdsa_secp256k1_sha256
 
-      val keys = algorithm.generateKeyPair().get
+      val keys = algorithm.generateKeyPair[Try]().extract
       val data = rndByteVector(10)
-      val sign = algorithm.sign(keys, data).get
+      val sign = algorithm.sign[Try](keys, data).extract
 
-      algorithm.verify(sign, data).get shouldBe true
+      algorithm.verify[Try](sign, data).isOk shouldBe true
 
       val randomData = rndByteVector(10)
-      val randomSign = algorithm.sign(keys, randomData).get
+      val randomSign = algorithm.sign(keys, randomData).extract
 
-      algorithm.verify(sign.copy(sign = randomSign.sign), data).get shouldBe false
+      algorithm.verify(sign.copy(sign = randomSign.sign), data).isOk shouldBe false
 
-      algorithm.verify(sign, randomData).get shouldBe false
+      algorithm.verify(sign, randomData).isOk shouldBe false
     }
 
     "correctly work with signer and checker" in {
       val algo = new SignAlgo(Ecdsa.ecdsa_secp256k1_sha256)
-      val keys = algo.generateKeyPair().get
+      val keys = algo.generateKeyPair().extract
       val signer = algo.signer(keys)
 
       val data = rndByteVector(10)
-      val sign = signer.sign(data).get
+      val sign = signer.sign(data).extract
 
-      algo.checker.check(sign, data).get shouldBe true
+      algo.checker.check(sign, data).isOk shouldBe true
 
-      val randomSign = signer.sign(rndByteVector(10)).get
-      algo.checker.check(randomSign, data).get shouldBe false
+      val randomSign = signer.sign(rndByteVector(10)).extract
+      algo.checker.check(randomSign, data).isOk shouldBe false
     }
 
     "throw an errors on invalid data" in {
       val algo = new SignAlgo(Ecdsa.ecdsa_secp256k1_sha256)
-      val keys = algo.generateKeyPair().get
+      val keys = algo.generateKeyPair().extract
       val signer = algo.signer(keys)
       val data = rndByteVector(10)
 
-      val sign = signer.sign(data).get
+      val sign = signer.sign(data).extract
 
-      the[CryptoErr] thrownBy algo.checker.check(sign.copy(sign = rndByteVector(10)), data).get
-      the[CryptoErr] thrownBy algo.checker.check(sign.copy(publicKey = sign.publicKey.copy(value = rndByteVector(10))), data).get
+      the[CryptoErr] thrownBy algo.checker.check(sign.copy(sign = rndByteVector(10)), data).value.flatMap(_.toTry).get
+      the[CryptoErr] thrownBy algo.checker.check(sign.copy(publicKey = sign.publicKey.copy(value = rndByteVector(10))), data).value.flatMap(_.toTry).get
     }
 
     "store and read key from file" in {
       val algo = new SignAlgo(Ecdsa.ecdsa_secp256k1_sha256)
-      val keys = algo.generateKeyPair().get
+      val keys = algo.generateKeyPair().extract
 
       val keyFile = File.createTempFile("test", "")
       if (keyFile.exists()) keyFile.delete()
@@ -92,10 +98,10 @@ class SignatureSpec extends WordSpec with Matchers with BeforeAndAfterAll {
 
       val signer = algo.signer(keys)
       val data = rndByteVector(10)
-      val sign = signer.sign(data).get
+      val sign = signer.sign(data).extract
 
-      algo.checker.check(sign.copy(publicKey = keysRead.publicKey), data).get shouldBe true
-      algo.checker.check(sign, data).get shouldBe true
+      algo.checker.check(sign.copy(publicKey = keysRead.publicKey), data).isOk shouldBe true
+      algo.checker.check(sign, data).isOk shouldBe true
 
       //try to store key into previously created file
       storage.storeSecretKey(keys).isFailure shouldBe true
diff --git a/src/main/scala/fluence/crypto/SignAlgo.scala b/src/main/scala/fluence/crypto/SignAlgo.scala
index 32fd9f9..7c3b55b 100644
--- a/src/main/scala/fluence/crypto/SignAlgo.scala
+++ b/src/main/scala/fluence/crypto/SignAlgo.scala
@@ -19,8 +19,9 @@ package fluence.crypto
 
 import java.security.SecureRandom
 
-import cats.MonadError
-import fluence.crypto.algorithm.{ DumbSign, KeyGenerator, SignatureFunctions }
+import cats.data.EitherT
+import cats.{ Monad, MonadError }
+import fluence.crypto.algorithm.{ CryptoErr, DumbSign, KeyGenerator, SignatureFunctions }
 import fluence.crypto.keypair.KeyPair
 import fluence.crypto.signature.{ Signature, SignatureChecker, Signer }
 import scodec.bits.ByteVector
@@ -33,15 +34,17 @@ import scala.language.higherKinds
  */
 class SignAlgo(algo: KeyGenerator with SignatureFunctions) {
 
-  def generateKeyPair[F[_]]()(implicit F: MonadError[F, Throwable]): F[KeyPair] = algo.generateKeyPair()
-  def generateKeyPair[F[_]](seed: ByteVector)(implicit F: MonadError[F, Throwable]): F[KeyPair] = algo.generateKeyPair(new SecureRandom(seed.toArray))
+  def generateKeyPair[F[_] : Monad](): EitherT[F, CryptoErr, KeyPair] = algo.generateKeyPair()
+  def generateKeyPair[F[_] : Monad](seed: ByteVector): EitherT[F, CryptoErr, KeyPair] = algo.generateKeyPair(new SecureRandom(seed.toArray))
 
-  def signer[F[_]](kp: KeyPair)(implicit F: MonadError[F, Throwable]): Signer[F] = new Signer[F] {
-    override def sign(plain: ByteVector): F[Signature] = algo.sign(kp, plain)
+  def signer(kp: KeyPair): Signer = new Signer {
     override def publicKey: KeyPair.Public = kp.publicKey
+    override def sign[F[_] : Monad](plain: ByteVector): EitherT[F, CryptoErr, Signature] = algo.sign(kp, plain)
   }
 
-  def checker[F[_]](implicit F: MonadError[F, Throwable]): SignatureChecker[F] = (signature: Signature, plain: ByteVector) ⇒ algo.verify(signature, plain)
+  def checker: SignatureChecker = new SignatureChecker {
+    override def check[F[_] : Monad](signature: Signature, plain: ByteVector): EitherT[F, CryptoErr, Unit] = algo.verify(signature, plain)
+  }
 }
 
 object SignAlgo {
diff --git a/src/main/scala/fluence/crypto/algorithm/DumbSign.scala b/src/main/scala/fluence/crypto/algorithm/DumbSign.scala
index ed52bda..22bcd81 100644
--- a/src/main/scala/fluence/crypto/algorithm/DumbSign.scala
+++ b/src/main/scala/fluence/crypto/algorithm/DumbSign.scala
@@ -1,8 +1,26 @@
+/*
+ * Copyright (C) 2017  Fluence Labs Limited
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
 package fluence.crypto.algorithm
 
 import java.security.SecureRandom
 
-import cats.MonadError
+import cats.Monad
+import cats.data.EitherT
 import fluence.crypto.keypair.KeyPair
 import fluence.crypto.signature.Signature
 import scodec.bits.ByteVector
@@ -10,15 +28,15 @@ import scodec.bits.ByteVector
 import scala.language.higherKinds
 
 class DumbSign extends KeyGenerator with SignatureFunctions {
-  override def generateKeyPair[F[_]](random: SecureRandom)(implicit F: MonadError[F, Throwable]): F[KeyPair] =
-    F.pure(KeyPair.fromBytes(random.generateSeed(10), random.generateSeed(10)))
+  override def generateKeyPair[F[_] : Monad](random: SecureRandom): EitherT[F, CryptoErr, KeyPair] =
+    EitherT.pure(KeyPair.fromBytes(random.generateSeed(10), random.generateSeed(10)))
 
-  override def generateKeyPair[F[_]]()(implicit F: MonadError[F, Throwable]): F[KeyPair] =
+  override def generateKeyPair[F[_] : Monad](): EitherT[F, CryptoErr, KeyPair] =
     generateKeyPair(new SecureRandom())
 
-  override def sign[F[_]](keyPair: KeyPair, message: ByteVector)(implicit F: MonadError[F, Throwable]): F[Signature] =
-    F.pure(Signature(keyPair.publicKey, message.reverse))
+  override def sign[F[_] : Monad](keyPair: KeyPair, message: ByteVector): EitherT[F, CryptoErr, Signature] =
+    EitherT.pure(Signature(keyPair.publicKey, message.reverse))
 
-  override def verify[F[_]](signature: Signature, message: ByteVector)(implicit F: MonadError[F, Throwable]): F[Boolean] =
-    F.pure(signature.sign == message.reverse)
+  override def verify[F[_] : Monad](signature: Signature, message: ByteVector): EitherT[F, CryptoErr, Unit] =
+    EitherT.cond[F](signature.sign == message.reverse, (), CryptoErr("Invalid Signature"))
 }
diff --git a/src/main/scala/fluence/crypto/algorithm/KeyGenerator.scala b/src/main/scala/fluence/crypto/algorithm/KeyGenerator.scala
index 7a33bea..e30574b 100644
--- a/src/main/scala/fluence/crypto/algorithm/KeyGenerator.scala
+++ b/src/main/scala/fluence/crypto/algorithm/KeyGenerator.scala
@@ -19,12 +19,14 @@ package fluence.crypto.algorithm
 
 import java.security.SecureRandom
 
-import cats.MonadError
+import cats.Monad
+import cats.data.EitherT
 import fluence.crypto.keypair.KeyPair
 
 import scala.language.higherKinds
 
 trait KeyGenerator {
-  def generateKeyPair[F[_]](random: SecureRandom)(implicit F: MonadError[F, Throwable]): F[KeyPair]
-  def generateKeyPair[F[_]]()(implicit F: MonadError[F, Throwable]): F[KeyPair]
+  def generateKeyPair[F[_] : Monad](random: SecureRandom): EitherT[F, CryptoErr, KeyPair]
+
+  def generateKeyPair[F[_] : Monad](): EitherT[F, CryptoErr, KeyPair]
 }
diff --git a/src/main/scala/fluence/crypto/algorithm/SignatureFunctions.scala b/src/main/scala/fluence/crypto/algorithm/SignatureFunctions.scala
index a76740f..7b97174 100644
--- a/src/main/scala/fluence/crypto/algorithm/SignatureFunctions.scala
+++ b/src/main/scala/fluence/crypto/algorithm/SignatureFunctions.scala
@@ -17,7 +17,8 @@
 
 package fluence.crypto.algorithm
 
-import cats.MonadError
+import cats.Monad
+import cats.data.EitherT
 import fluence.crypto.keypair.KeyPair
 import fluence.crypto.signature.Signature
 import scodec.bits.ByteVector
@@ -25,6 +26,7 @@ import scodec.bits.ByteVector
 import scala.language.higherKinds
 
 trait SignatureFunctions {
-  def sign[F[_]](keyPair: KeyPair, message: ByteVector)(implicit F: MonadError[F, Throwable]): F[Signature]
-  def verify[F[_]](signature: Signature, message: ByteVector)(implicit F: MonadError[F, Throwable]): F[Boolean]
+  def sign[F[_] : Monad](keyPair: KeyPair, message: ByteVector): EitherT[F, CryptoErr, Signature]
+
+  def verify[F[_] : Monad](signature: Signature, message: ByteVector): EitherT[F, CryptoErr, Unit]
 }
diff --git a/src/main/scala/fluence/crypto/signature/SignatureChecker.scala b/src/main/scala/fluence/crypto/signature/SignatureChecker.scala
index 2a55a76..75cf39b 100644
--- a/src/main/scala/fluence/crypto/signature/SignatureChecker.scala
+++ b/src/main/scala/fluence/crypto/signature/SignatureChecker.scala
@@ -17,10 +17,13 @@
 
 package fluence.crypto.signature
 
+import cats.Monad
+import cats.data.EitherT
+import fluence.crypto.algorithm.CryptoErr
 import scodec.bits.ByteVector
 
 import scala.language.higherKinds
 
-trait SignatureChecker[F[_]] {
-  def check(signature: Signature, plain: ByteVector): F[Boolean]
+trait SignatureChecker {
+  def check[F[_] : Monad](signature: Signature, plain: ByteVector): EitherT[F, CryptoErr, Unit]
 }
diff --git a/src/main/scala/fluence/crypto/signature/Signer.scala b/src/main/scala/fluence/crypto/signature/Signer.scala
index e70c171..3b3b435 100644
--- a/src/main/scala/fluence/crypto/signature/Signer.scala
+++ b/src/main/scala/fluence/crypto/signature/Signer.scala
@@ -17,23 +17,27 @@
 
 package fluence.crypto.signature
 
-import cats.MonadError
+import cats.Monad
+import cats.data.EitherT
+import fluence.crypto.algorithm.CryptoErr
 import fluence.crypto.keypair.KeyPair
 import scodec.bits.ByteVector
 
 import scala.language.higherKinds
 
-trait Signer[F[_]] {
+trait Signer {
   def publicKey: KeyPair.Public
 
-  def sign(plain: ByteVector): F[Signature]
+  def sign[F[_] : Monad](plain: ByteVector): EitherT[F, CryptoErr, Signature]
 }
 
 object Signer {
-  class DumbSigner[F[_]](keyPair: KeyPair)(implicit F: MonadError[F, Throwable]) extends Signer[F] {
+
+  class DumbSigner(keyPair: KeyPair) extends Signer {
     override def publicKey: KeyPair.Public = keyPair.publicKey
 
-    override def sign(plain: ByteVector): F[Signature] =
-      F.pure(Signature(keyPair.publicKey, plain.reverse))
+    override def sign[F[_] : Monad](plain: ByteVector): EitherT[F, CryptoErr, Signature] =
+      EitherT.pure(Signature(keyPair.publicKey, plain.reverse))
   }
+
 }