create intermediate CA certificates + bug fixes

**Note**: This change introduces breaking changes to the `easypki` API:

* The `GenerateCertificate` function had a typo in its name. It used to
  be `GenerateCertifcate` (missing an `i`).
* The `GenerateCertificate` function now takes a struct as a parameter,
  making it easier to use.

The main reason behind this change was to provide the ability to
generate intermediate CA certificates. This will allow people and
organizations to use `easypki` to create a multi-layered tree of trust.
In addition to that, the ability to set the maximum path length on CA
certificates was added to make the keys safer (less prone for abuse).

You can now generate intermediate certificates using the
`--intermediate` flag. This flag effectively creates a new CA
certificate, within the CA, but doesn't overwrite the `ca.crt` or
`ca.key` file. Instead, it uses the same logic as regular certificates
and saves the cert and key within the `issued/` and `private/`
directories respectively. It's suggested that the `--max-path-len` flag
be used when generating CA certificates.

You can now set the maximum path depth for a CA certificate by using the
`--max-path-len` flag. If you want to generate an offline root CA and
ensure that your intermediates cannot generate valid intermediate CA
certificates themselves, you would set `--max-path-len 1` when
generating the root CA. It's recommended to always use this flag when
generating CA certificates, otherwise that certificate will be valid for
an "infinite" number of intermediate certificates.

With the features above added, a few bugs were discovered in the
certificates being generated by `easypki`. Specifically we needed to fix
some issues with the KeyUsage and ExtKeyUsage settings of the certs.

While troubleshooting an issue with Consul, trying to do
verification of a TLS chain generated by `easypki`, I ran in to a
situation where the certificates were failing to validate. It turns out
there were a few issues that caused this to happen.

I found an issue on Hashicorp's Vault project referencing a similar
issue with CA certificates generated by Vault itself. This guided me to
the first bug that needed patching:

* https://github.com/hashicorp/vault/pull/852

>Assign ExtKeyUsageAny to CA certs to help with validation with the
>Windows Crypto API and Go's validation logic

The solution: when generating CAs, we now set the `ExtKeyUsage` to
`ExtKeyUsageAny`. This will mark the CA certificate as being valid for
any usage. Some X.509 validation systems require that all certificates
in the chain contain the requested usage, including in Go.

The second was that the server certificates weren't being assigned
`ExtKeyUsageClientAuth` causing issues with applications trying to use
the certificates as both client and server certificates. The fix is to
also give server certificates `ExtKeyUsageClientAuth`. Upon
investigation of certificates deployed for public Internet services, it
seems that `ExtKeyUsageClientAuth` is pretty common in server
certificates.

fixes #2
fixes #3
fixes #4

1 files changed, 65 insertions, 24 deletions

diff --git a/pkg/easypki/easyca.go b/pkg/easypki/easyca.go
index 0c908d2..c38a295 100644
--- a/pkg/easypki/easyca.go
+++ b/pkg/easypki/easyca.go
@@ -66,22 +66,36 @@ func GeneratePrivateKey(path string) (*rsa.PrivateKey, error) {
 	return key, nil
 }
 
-func GenerateCertifcate(pkiroot, name string, template *x509.Certificate) error {
+// GenerationRequest is a struct for providing configuration to
+// GenerateCertifcate when actioning a certification generation request.
+type GenerationRequest struct {
+	PKIRoot             string
+	Name                string
+	Template            *x509.Certificate
+	MaxPathLen          int
+	IsIntermediateCA    bool
+	IsClientCertificate bool
+}
+
+// GenerateCertificate is a function for helping to generate new x509
+// certificates and keys from the GenerationRequest. This function renders the
+// content out to the filesystem.
+func GenerateCertificate(genReq *GenerationRequest) error {
 	// TODO(jclerc): check that pki has been init
 
 	var crtPath string
-	privateKeyPath := filepath.Join(pkiroot, "private", name+".key")
-	if name == "ca" {
-		crtPath = filepath.Join(pkiroot, name+".crt")
+	privateKeyPath := filepath.Join(genReq.PKIRoot, "private", genReq.Name+".key")
+	if genReq.Name == "ca" {
+		crtPath = filepath.Join(genReq.PKIRoot, genReq.Name+".crt")
 	} else {
-		crtPath = filepath.Join(pkiroot, "issued", name+".crt")
+		crtPath = filepath.Join(genReq.PKIRoot, "issued", genReq.Name+".crt")
 	}
 
 	var caCrt *x509.Certificate
 	var caKey *rsa.PrivateKey
 
 	if _, err := os.Stat(privateKeyPath); err == nil {
-		return fmt.Errorf("a key pair for %v already exists", name)
+		return fmt.Errorf("a key pair for %v already exists", genReq.Name)
 	}
 
 	privateKey, err := GeneratePrivateKey(privateKeyPath)
@@ -94,39 +108,66 @@ func GenerateCertifcate(pkiroot, name string, template *x509.Certificate) error
 		return fmt.Errorf("marshal public key: %v", err)
 	}
 	subjectKeyId := sha1.Sum(publicKeyBytes)
-	template.SubjectKeyId = subjectKeyId[:]
+	genReq.Template.SubjectKeyId = subjectKeyId[:]
 
-	template.NotBefore = time.Now()
-	template.SignatureAlgorithm = x509.SHA256WithRSA
-	if template.IsCA {
+	genReq.Template.NotBefore = time.Now()
+	genReq.Template.SignatureAlgorithm = x509.SHA256WithRSA
+
+	if genReq.Template.IsCA {
 		serialNumberLimit := new(big.Int).Lsh(big.NewInt(1), 128)
 		serialNumber, err := rand.Int(rand.Reader, serialNumberLimit)
 		if err != nil {
 			return fmt.Errorf("failed to generate ca serial number: %s", err)
 		}
-		template.SerialNumber = serialNumber
-		template.KeyUsage = x509.KeyUsageCertSign | x509.KeyUsageCRLSign
-		template.BasicConstraintsValid = true
-		template.Issuer = template.Subject
-		template.AuthorityKeyId = template.SubjectKeyId
+		genReq.Template.SerialNumber = serialNumber
+		genReq.Template.KeyUsage = x509.KeyUsageCertSign | x509.KeyUsageCRLSign
+		genReq.Template.BasicConstraintsValid = true
+		genReq.Template.Issuer = genReq.Template.Subject
+		genReq.Template.AuthorityKeyId = genReq.Template.SubjectKeyId
 
-		caCrt = template
+		// if the maximum path length was provided be sure to enforce it
+		if genReq.MaxPathLen >= 0 {
+			genReq.Template.MaxPathLen = genReq.MaxPathLen
+			genReq.Template.MaxPathLenZero = true // doesn't force to zero
+		}
+
+		// Go performs validation not according to spec but according to the Windows
+		// Crypto API, so we add all usages to CA certs
+		// - https://github.com/hashicorp/vault/pull/852
+		genReq.Template.ExtKeyUsage = []x509.ExtKeyUsage{x509.ExtKeyUsageAny}
+
+		caCrt = genReq.Template
 		caKey = privateKey
-	} else {
-		template.KeyUsage = x509.KeyUsageDigitalSignature | x509.KeyUsageKeyEncipherment
-		serialNumber, err := NextNumber(pkiroot, "serial")
+	}
+
+	// if this is not a CA certificate...
+	// or if this is an intermediate certificate...
+	// we want to sign it with our parent CA's key
+	if !genReq.Template.IsCA || genReq.IsIntermediateCA {
+		if !genReq.IsIntermediateCA {
+			// set the usage for non-CA certificates
+			genReq.Template.KeyUsage = x509.KeyUsageDigitalSignature | x509.KeyUsageKeyEncipherment
+			genReq.Template.ExtKeyUsage = append(genReq.Template.ExtKeyUsage, x509.ExtKeyUsageClientAuth)
+
+			// set UsageServerAuth only if this isn't a client cert
+			if !genReq.IsClientCertificate {
+				genReq.Template.ExtKeyUsage = append(genReq.Template.ExtKeyUsage, x509.ExtKeyUsageServerAuth)
+			}
+		}
+
+		serialNumber, err := NextNumber(genReq.PKIRoot, "serial")
 		if err != nil {
 			return fmt.Errorf("get next serial: %v", err)
 		}
-		template.SerialNumber = serialNumber
+		genReq.Template.SerialNumber = serialNumber
 
-		caCrt, caKey, err = GetCA(pkiroot)
+		caCrt, caKey, err = GetCA(genReq.PKIRoot)
 		if err != nil {
 			return fmt.Errorf("get ca: %v", err)
 		}
 	}
 
-	crt, err := x509.CreateCertificate(rand.Reader, template, caCrt, privateKey.Public(), caKey)
+	crt, err := x509.CreateCertificate(rand.Reader, genReq.Template, caCrt, privateKey.Public(), caKey)
 	if err != nil {
 		return fmt.Errorf("create certificate: %v", err)
 	}
@@ -146,8 +187,8 @@ func GenerateCertifcate(pkiroot, name string, template *x509.Certificate) error
 	}
 
 	// I do not think we have to write the ca.crt in the index
-	if !template.IsCA {
-		WriteIndex(pkiroot, name, template)
+	if !genReq.Template.IsCA {
+		WriteIndex(genReq.PKIRoot, genReq.Name, genReq.Template)
 		if err != nil {
 			return fmt.Errorf("write index: %v", err)
 		}