Cherry pick v1.7 into master

vendor/github.com/transip/gotransip/sign.go

@@ -0,0 +1,49 @@
+package gotransip
+
+import (
+	"crypto"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/sha512"
+	"crypto/x509"
+	"encoding/base64"
+	"encoding/pem"
+	"errors"
+	"fmt"
+	"net/url"
+)
+
+var (
+	asn1Header = []byte{
+		0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03,
+		0x04, 0x02, 0x03, 0x05, 0x00, 0x04, 0x40,
+	}
+)
+
+func signWithKey(params *soapParams, key []byte) (string, error) {
+	// create SHA512 hash of given parameters
+	h := sha512.New()
+	h.Write([]byte(params.Encode()))
+
+	// prefix ASN1 header to SHA512 hash
+	digest := append(asn1Header, h.Sum(nil)...)
+
+	// prepare key struct
+	block, _ := pem.Decode(key)
+	if block == nil {
+		return "", errors.New("could not decode private key")
+	}
+	parsed, err := x509.ParsePKCS8PrivateKey(block.Bytes)
+	if err != nil {
+		return "", fmt.Errorf("could not parse private key: %s", err.Error())
+	}
+
+	pkey := parsed.(*rsa.PrivateKey)
+
+	enc, err := rsa.SignPKCS1v15(rand.Reader, pkey, crypto.Hash(0), digest)
+	if err != nil {
+		return "", fmt.Errorf("could not sign data: %s", err.Error())
+	}
+
+	return url.QueryEscape(base64.StdEncoding.EncodeToString(enc)), nil
+}