prober: speed up TestCRL ~450x by baking in some test keys

Fixes #16290
Updates tailscale/corp#28679

Change-Id: Ic90129b686779d0ed1cb40acf187cfcbdd39eb83
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

prober/tls_test.go

@@ -6,6 +6,7 @@ package prober
 import (
 	"bytes"
 	"context"
+	"crypto/ecdsa"
 	"crypto/rand"
 	"crypto/rsa"
 	"crypto/tls"
@@ -140,16 +141,60 @@ func (s *CRLServer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	w.Write(s.crlBytes)
 }
 
-func TestCRL(t *testing.T) {
-	// Generate CA key and self-signed CA cert
-	caKey, err := rsa.GenerateKey(rand.Reader, 4096)
+// someECDSAKey{1,2,3} are different EC private keys in PEM format
+// as generated by:
+//
+//	openssl ecparam -name prime256v1 -genkey -noout -out -
+//
+// They're used in tests to avoid burning CPU at test time to just
+// to make some arbitrary test keys.
+const (
+	someECDSAKey1 = `
+-----BEGIN EC PRIVATE KEY-----
+MHcCAQEEIDKggO47Si0/JgqF0q9m0HfQ92lbERWsBaKS5YihtuheoAoGCCqGSM49
+AwEHoUQDQgAE/JtNZkfFmAGQJHW5Xgz0Eoyi9MKVxl77sXjIFDMX233QDIWPEM/B
+vmNMvdFkuYBjwbq6H+SNf1NXRNladEGU/Q==
+-----END EC PRIVATE KEY-----
+`
+	someECDSAKey2 = `
+-----BEGIN EC PRIVATE KEY-----
+MHcCAQEEIPIJhRf4MpzLil1ZKcRqMx+jPeJXw96KtYYzV2AcgBzgoAoGCCqGSM49
+AwEHoUQDQgAEhA9CSWFmUvdvXMzyt+as+6f+0luydHU1x/gEksVByYIgYxahaGts
+xbSKj6F2WgAN/ok1gFLqhH3UWMNVthM1wA==
+-----END EC PRIVATE KEY-----
+`
+	someECDSAKey3 = `
+-----BEGIN EC PRIVATE KEY-----
+MHcCAQEEIKgZ1OJjK2St9O0i52N1K+IgSiu2/NSMk9Yt2+kDMHd7oAoGCCqGSM49
+AwEHoUQDQgAExFp80etkjy/AEUtSgJjXRA39jTU7eiEmCGRREewFQhwcEscBEfrg
+6NN31r9YlEs+hZ8gXE1L3Deu6jn5jW3pig==
+-----END EC PRIVATE KEY-----
+`
+)
+
+// parseECKey parses an EC private key from a PEM-encoded string.
+func parseECKey(t *testing.T, pemPriv string) *ecdsa.PrivateKey {
+	t.Helper()
+	block, _ := pem.Decode([]byte(pemPriv))
+	if block == nil {
+		t.Fatal("failed to decode PEM")
+	}
+	key, err := x509.ParseECPrivateKey(block.Bytes)
 	if err != nil {
-		t.Fatal(err)
+		t.Fatalf("failed to parse EC key: %v", err)
 	}
+	return key
+}
+
+func TestCRL(t *testing.T) {
+	// Generate CA key and self-signed CA cert
+	caKey := parseECKey(t, someECDSAKey1)
+
 	caTpl := issuerCertTpl
 	caTpl.BasicConstraintsValid = true
 	caTpl.IsCA = true
 	caTpl.KeyUsage = x509.KeyUsageCertSign | x509.KeyUsageCRLSign | x509.KeyUsageDigitalSignature
+	caTpl.SignatureAlgorithm = x509.ECDSAWithSHA256
 	caBytes, err := x509.CreateCertificate(rand.Reader, &caTpl, &caTpl, &caKey.PublicKey, caKey)
 	if err != nil {
 		t.Fatal(err)
@@ -162,11 +207,9 @@ func TestCRL(t *testing.T) {
 	// Issue a leaf cert signed by the CA
 	leaf := leafCert
 	leaf.SerialNumber = big.NewInt(20001)
+	leaf.SignatureAlgorithm = x509.ECDSAWithSHA256
 	leaf.Issuer = caCert.Subject
-	leafKey, err := rsa.GenerateKey(rand.Reader, 4096)
-	if err != nil {
-		t.Fatal(err)
-	}
+	leafKey := parseECKey(t, someECDSAKey2)
 	leafBytes, err := x509.CreateCertificate(rand.Reader, &leaf, caCert, &leafKey.PublicKey, caKey)
 	if err != nil {
 		t.Fatal(err)
@@ -182,10 +225,8 @@ func TestCRL(t *testing.T) {
 	noCRLCert.CRLDistributionPoints = []string{}
 	noCRLCert.NotBefore = time.Unix(letsEncryptStartedStaplingCRL, 0).Add(-48 * time.Hour)
 	noCRLCert.Issuer = caCert.Subject
-	noCRLCertKey, err := rsa.GenerateKey(rand.Reader, 4096)
-	if err != nil {
-		t.Fatal(err)
-	}
+	noCRLCert.SignatureAlgorithm = x509.ECDSAWithSHA256
+	noCRLCertKey := parseECKey(t, someECDSAKey3)
 	noCRLStapledBytes, err := x509.CreateCertificate(rand.Reader, &noCRLCert, caCert, &noCRLCertKey.PublicKey, caKey)
 	if err != nil {
 		t.Fatal(err)