Panic when using inline SSL certificate or key (elastic#23858)

* Panic when using inline SSL certificate or key

When the key or certificate was smaller than 256bytes the system was
throwing a panic, the problem was generate by a debug message. Instead
of logging part of the keys or certificate in the log we are just
writing "inline".

Fixes: elastic#23820

* changelog

CHANGELOG.next.asciidoc

@@ -31,6 +31,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - API address is a required setting in `add_cloudfoundry_metadata`. {pull}21759[21759]
 - Update to ECS 1.7.0. {pull}22571[22571]
 - Add support for SCRAM-SHA-512 and SCRAM-SHA-256 in Kafka output. {pull}12867[12867]
+- Fix panic with inline SSL when the certificate or key were small than 256 bytes. {pull}23820[23820]
 
 *Auditbeat*
 

libbeat/common/transport/tlscommon/tls.go

@@ -214,9 +214,7 @@ type PEMReader struct {
 // NewPEMReader returns a new PEMReader.
 func NewPEMReader(certificate string) (*PEMReader, error) {
 	if IsPEMString(certificate) {
-		// Take a substring of the certificate so we do not leak the whole certificate or private key in the log.
-		debugStr := certificate[0:256] + "..."
-		return &PEMReader{reader: ioutil.NopCloser(strings.NewReader(certificate)), debugStr: debugStr}, nil
+		return &PEMReader{reader: ioutil.NopCloser(strings.NewReader(certificate)), debugStr: "inline"}, nil
 	}
 
 	r, err := os.Open(certificate)

libbeat/common/transport/tlscommon/tls_test.go

@@ -546,6 +546,7 @@ NHy+PwkYsHhbrPl4dgStTNXLenJLIJ+Ke0Pcld4ZPfYdSyu/Tv4rNswZBNpNsW9K
 nygO9KTJuUiBrLr0AHEnqko=
 -----END PRIVATE KEY-----
 `
+
 		t.Run("embed", func(t *testing.T) {
 			// Create a dummy configuration and append the CA after.
 			cfg, err := load(`
@@ -568,6 +569,48 @@ supported_protocols: null
 			assert.NotNil(t, tlsC)
 		})
 
+		t.Run("embed small key", func(t *testing.T) {
+			// Create a dummy configuration and append the CA after.
+			cfg, err := load(`
+enabled: true
+verification_mode: null
+certificate: null
+key: null
+key_passphrase: null
+certificate_authorities:
+cipher_suites: null
+curve_types: null
+supported_protocols: null
+  `)
+			certificate := `
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
+`
+			key := `
+-----BEGIN PRIVATE KEY-----
+MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgFDQJ1CPLXrUbUFqj
+ED8dqsGuVQdcPK7CHpsCeTtAgQqhRANCAAQFztQS8F37lIZnpBB+AFo+H9UCV/ui
++lr/B9KZsd8GF5751O90A7Wn3SVqecKRmZzs59YCVDV5OAlBbrrwInAT
+-----END PRIVATE KEY-----
+`
+			cfg.Certificate.Certificate = certificate
+			cfg.Certificate.Key = key
+
+			tlsC, err := LoadTLSConfig(cfg)
+			assert.NoError(t, err)
+
+			assert.NotNil(t, tlsC)
+		})
+
 		t.Run("From disk", func(t *testing.T) {
 			k, err := ioutil.TempFile("", "certificate.key")
 			k.WriteString(key)