Add option for custom OID formatting.

certinfo.go

@@ -1,6 +1,7 @@
 package certinfo
 
 import (
+	"bufio"
 	"bytes"
 	"crypto/dsa"
 	"crypto/ecdsa"
@@ -12,6 +13,7 @@ import (
 	"fmt"
 	"math/big"
 	"net"
+	"strings"
 	"time"
 )
 
@@ -252,7 +254,14 @@ func printSignature(sigAlgo x509.SignatureAlgorithm, sig []byte, buf *bytes.Buff
 // CertificateText returns a human-readable string representation
 // of the certificate cert. The format is similar (but not identical)
 // to the OpenSSL way of printing certificates.
-func CertificateText(cert *x509.Certificate) (string, error) {
+func CertificateText(cert *x509.Certificate, opts ...Option) (string, error) {
+	o := &options{
+		formatters: make(map[string]Formatter),
+	}
+	for _, fn := range opts {
+		fn(o)
+	}
+
 	var buf bytes.Buffer
 	buf.Grow(4096) // 4KiB should be enough
 
@@ -519,6 +528,14 @@ func CertificateText(cert *x509.Certificate) (string, error) {
 				} else {
 					buf.WriteString(fmt.Sprintf("%12sNetscape Comment:\n%16s%s\n", "", "", comment))
 				}
+			} else if format, ok := o.formatters[ext.Id.String()]; ok {
+				// If configured, use custom formatter.
+				scanner := bufio.NewScanner(strings.NewReader(format(ext)))
+				for scanner.Scan() {
+					// Prepend padding so that formatted string appears in-line
+					// with other content.
+					buf.WriteString(fmt.Sprintf("%12s%s\n", "", scanner.Text()))
+				}
 			} else {
 				buf.WriteString(fmt.Sprintf("%12sUnknown extension %s\n", "", ext.Id.String()))
 			}

certinfo_test.go

@@ -1,11 +1,12 @@
 package certinfo
 
 import (
-	"bytes"
 	"crypto/x509"
 	"encoding/pem"
 	"io/ioutil"
 	"testing"
+
+	"github.com/google/go-cmp/cmp"
 )
 
 type InputType int
@@ -54,9 +55,9 @@ func testPair(t *testing.T, certFile, refFile string, inputType InputType) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if !bytes.Equal(resultData, refData) {
+	if diff := cmp.Diff(resultData, refData); diff != "" {
 		t.Logf("'%s' did not match reference '%s'\n", certFile, refFile)
-		t.Errorf("Dump follows:\n%s\n", result)
+		t.Errorf("Dump follows:\n%s\n", diff)
 	}
 }
 

formatter_test.go

@@ -0,0 +1,53 @@
+package certinfo
+
+import (
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"encoding/asn1"
+	"encoding/pem"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestFormatter(t *testing.T) {
+	pemData, err := ioutil.ReadFile("test_certs/root1.cert.pem")
+	if err != nil {
+		t.Fatal(err)
+	}
+	block, rest := pem.Decode([]byte(pemData))
+	if block == nil || len(rest) > 0 {
+		t.Fatal("Certificate decoding error")
+	}
+	cert, err := x509.ParseCertificate(block.Bytes)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	oid := asn1.ObjectIdentifier{1, 2, 3, 4}
+
+	cert.Extensions = append(cert.Extensions, pkix.Extension{
+		Id:    oid,
+		Value: []byte("foo"),
+	})
+
+	got, err := CertificateText(cert, WithFormatter(oid, func(ext pkix.Extension) string {
+		return fmt.Sprintf("Custom:\n%4s%s", "", string(ext.Value))
+	}))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	want, err := os.ReadFile("test_certs/root1.cert.customfield.text")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if diff := cmp.Diff(string(want), got); diff != "" {
+		t.Log(got)
+		t.Error(diff)
+	}
+}

go.mod

@@ -0,0 +1,5 @@
+module github.com/grantae/certinfo
+
+go 1.18
+
+require github.com/google/go-cmp v0.5.8 // indirect

go.sum

@@ -0,0 +1,2 @@
+github.com/google/go-cmp v0.5.8 h1:e6P7q2lk1O+qJJb4BtCQXlK8vWEO8V1ZeuEdJNOqZyg=
+github.com/google/go-cmp v0.5.8/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=

option.go

@@ -0,0 +1,26 @@
+package certinfo
+
+import (
+	"crypto/x509/pkix"
+	"encoding/asn1"
+)
+
+// Option provides configurable options to output formatting.
+type Option func(*options)
+
+type options struct {
+	// formatters maps oid -> format funcs
+	formatters map[string]Formatter
+}
+
+// WithFormatter configures a custom formatting function for the given OID.
+func WithFormatter(oid asn1.ObjectIdentifier, fn Formatter) Option {
+	return func(opts *options) {
+		opts.formatters[oid.String()] = fn
+	}
+}
+
+// Formatter returns a formatted string for a given pkix.Extension.
+// Formatters should return relative strings - padding will be prepended
+// automatically when the certificate is printed.
+type Formatter func(ext pkix.Extension) string

test_certs/leaf2.csr.text

@@ -25,7 +25,7 @@ Certificate Request:
                     71:bc:10:a6:45:dd:a0:55:e1:77:02:28:84:58:09:
                     da:f9:ad:16:6e:22:3f:13:f7:91:71:44:5b:5f:98:
                     8c:92:21:13
-    Signature Algorithm: 0
+    Signature Algorithm: DSA-SHA256
          30:2c:02:14:6e:2f:4d:43:42:fe:ef:dd:d6:5d:82:ce:40:35:
          0f:df:f6:03:d0:56:02:14:2c:af:8d:a5:7e:00:cb:18:c9:eb:
          03:ee:9b:92:32:c0:15:73:6a:29

test_certs/root1.cert.customfield.text

@@ -0,0 +1,37 @@
+Certificate:
+    Data:
+        Version: 3 (0x2)
+        Serial Number: 1 (0x1)
+    Signature Algorithm: SHA256-RSA
+        Issuer: C=US,ST=California,O=World Widget Authority,OU=Identity Affairs,CN=worldwidgetauthority.com,emailAddress=nobody@worldwidgetauthority.com
+        Validity
+            Not Before: Jul 23 18:56:47 2020 UTC
+            Not After : Jun 30 07:37:21 2040 UTC
+        Subject: C=US,ST=California,O=World Widget Authority,OU=Identity Affairs,CN=worldwidgetauthority.com,emailAddress=nobody@worldwidgetauthority.com
+        Subject Public Key Info:
+            Public Key Algorithm: RSA
+                Public-Key: (512 bit)
+                Modulus:
+                    b5:d6:60:b9:f9:31:09:fe:97:34:c4:f7:6b:7b:06:
+                    01:f4:8b:fe:1a:e0:65:8f:fd:30:c0:82:30:3c:61:
+                    f7:c2:1d:98:7c:3a:ed:9f:b4:5e:8f:15:ce:90:8b:
+                    45:de:db:23:0e:aa:4d:95:e9:af:3b:79:26:a5:ce:
+                    71:8a:3a:bd
+                Exponent: 65537 (0x10001)
+        X509v3 extensions:
+            X509v3 Basic Constraints:
+                CA:TRUE
+            Netscape Comment:
+                This is a test certificate only
+            X509v3 Subject Key Identifier:
+                7C:0F:26:9D:ED:C8:7A:C0:05:1E:99:A3:5D:A5:9E:8D:A6:A6:96:5E
+            X509v3 Authority Key Identifier:
+                keyid:7C:0F:26:9D:ED:C8:7A:C0:05:1E:99:A3:5D:A5:9E:8D:A6:A6:96:5E
+            Custom:
+                foo
+
+    Signature Algorithm: SHA256-RSA
+         60:bd:b4:c4:9a:09:0d:7a:d7:b4:6b:e2:85:3b:78:0b:97:de:
+         57:47:34:19:37:2a:82:1a:79:c3:3f:0b:71:46:fe:9b:db:ce:
+         c7:41:42:2b:17:22:b4:d5:f1:fc:18:c3:31:af:c9:c4:4d:2d:
+         92:16:f7:a6:6d:4f:5d:e0:8c:83