Add Support for Manta Object Storage as a Gateway (#5025)

Manta is an Object Storage by [Joyent](https://www.joyent.com/)

This PR adds initial support for Manta. It is intended as non-production 
ready so that feedback can be obtained.

vendor/github.com/joyent/triton-go/authentication/private_key_signer.go

@@ -0,0 +1,106 @@
+package authentication
+
+import (
+	"crypto"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/base64"
+	"encoding/pem"
+	"errors"
+	"fmt"
+	"strings"
+
+	"github.com/hashicorp/errwrap"
+	"golang.org/x/crypto/ssh"
+)
+
+type PrivateKeySigner struct {
+	formattedKeyFingerprint string
+	keyFingerprint          string
+	algorithm               string
+	accountName             string
+	hashFunc                crypto.Hash
+
+	privateKey *rsa.PrivateKey
+}
+
+func NewPrivateKeySigner(keyFingerprint string, privateKeyMaterial []byte, accountName string) (*PrivateKeySigner, error) {
+	keyFingerprintMD5 := strings.Replace(keyFingerprint, ":", "", -1)
+
+	block, _ := pem.Decode(privateKeyMaterial)
+	if block == nil {
+		return nil, errors.New("Error PEM-decoding private key material: nil block received")
+	}
+
+	rsakey, err := x509.ParsePKCS1PrivateKey(block.Bytes)
+	if err != nil {
+		return nil, errwrap.Wrapf("Error parsing private key: {{err}}", err)
+	}
+
+	sshPublicKey, err := ssh.NewPublicKey(rsakey.Public())
+	if err != nil {
+		return nil, errwrap.Wrapf("Error parsing SSH key from private key: {{err}}", err)
+	}
+
+	matchKeyFingerprint := formatPublicKeyFingerprint(sshPublicKey, false)
+	displayKeyFingerprint := formatPublicKeyFingerprint(sshPublicKey, true)
+	if matchKeyFingerprint != keyFingerprintMD5 {
+		return nil, errors.New("Private key file does not match public key fingerprint")
+	}
+
+	signer := &PrivateKeySigner{
+		formattedKeyFingerprint: displayKeyFingerprint,
+		keyFingerprint:          keyFingerprint,
+		accountName:             accountName,
+
+		hashFunc:   crypto.SHA1,
+		privateKey: rsakey,
+	}
+
+	_, algorithm, err := signer.SignRaw("HelloWorld")
+	if err != nil {
+		return nil, fmt.Errorf("Cannot sign using ssh agent: %s", err)
+	}
+	signer.algorithm = algorithm
+
+	return signer, nil
+}
+
+func (s *PrivateKeySigner) Sign(dateHeader string) (string, error) {
+	const headerName = "date"
+
+	hash := s.hashFunc.New()
+	hash.Write([]byte(fmt.Sprintf("%s: %s", headerName, dateHeader)))
+	digest := hash.Sum(nil)
+
+	signed, err := rsa.SignPKCS1v15(rand.Reader, s.privateKey, s.hashFunc, digest)
+	if err != nil {
+		return "", errwrap.Wrapf("Error signing date header: {{err}}", err)
+	}
+	signedBase64 := base64.StdEncoding.EncodeToString(signed)
+
+	keyID := fmt.Sprintf("/%s/keys/%s", s.accountName, s.formattedKeyFingerprint)
+	return fmt.Sprintf(authorizationHeaderFormat, keyID, "rsa-sha1", headerName, signedBase64), nil
+}
+
+func (s *PrivateKeySigner) SignRaw(toSign string) (string, string, error) {
+	hash := s.hashFunc.New()
+	hash.Write([]byte(toSign))
+	digest := hash.Sum(nil)
+
+	signed, err := rsa.SignPKCS1v15(rand.Reader, s.privateKey, s.hashFunc, digest)
+	if err != nil {
+		return "", "", errwrap.Wrapf("Error signing date header: {{err}}", err)
+	}
+	signedBase64 := base64.StdEncoding.EncodeToString(signed)
+	return signedBase64, "rsa-sha1", nil
+}
+
+func (s *PrivateKeySigner) KeyFingerprint() string {
+	return s.formattedKeyFingerprint
+}
+
+func (s *PrivateKeySigner) DefaultAlgorithm() string {
+	return s.algorithm
+}