Verify proof method + Refactor confidential pkg (#143)

* Move functions for converting amounts from confidential to new pkg

* Refactor of confidential pkg
* rename unblinding method
* add unblinding method for issuance
* add method to verify surjection proof
* use internal functions to increase readability
* add tests for new methods

* Add mthod to verify blinding and use it in e2e tests

internal/elementsutil/amount.go

@@ -0,0 +1,33 @@
+package elementsutil
+
+import (
+	"bytes"
+	"encoding/binary"
+	"errors"
+
+	"github.com/vulpemventures/go-elements/internal/bufferutil"
+)
+
+// SatoshiToElementsValue method converts Satoshi value to Elements value
+func SatoshiToElementsValue(val uint64) ([]byte, error) {
+	unconfPrefix := byte(1)
+	b := bytes.NewBuffer([]byte{})
+	if err := bufferutil.BinarySerializer.PutUint64(b, binary.LittleEndian, val); err != nil {
+		return nil, err
+	}
+	res := append([]byte{unconfPrefix}, bufferutil.ReverseBytes(b.Bytes())...)
+	return res, nil
+}
+
+// ElementsToSatoshiValue method converts Elements value to Satoshi value
+func ElementsToSatoshiValue(val []byte) (uint64, error) {
+	if len(val) != 9 {
+		return 0, errors.New("invalid elements value lenght")
+	}
+	if val[0] != byte(1) {
+		return 0, errors.New("invalid prefix")
+	}
+	reverseValueBuffer := bufferutil.ReverseBytes(val[1:])
+	d := bufferutil.NewDeserializer(bytes.NewBuffer(reverseValueBuffer))
+	return d.ReadUint64()
+}

internal/elementsutil/amount_test.go

@@ -0,0 +1,28 @@
+package elementsutil
+
+import (
+	"crypto/rand"
+	"math/big"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSatoshiToElementsValueRoundTrip(t *testing.T) {
+	bigInt, err := rand.Int(rand.Reader, big.NewInt(1000000000))
+	if err != nil {
+		panic(err)
+	}
+	satoshi := bigInt.Uint64()
+	elementsValue, err := SatoshiToElementsValue(satoshi)
+	if !assert.NoError(t, err) {
+		t.FailNow()
+	}
+
+	satoshiValue, err := ElementsToSatoshiValue(elementsValue)
+	if !assert.NoError(t, err) {
+		t.FailNow()
+	}
+
+	assert.Equal(t, satoshi, satoshiValue)
+}