LibTLS: Try to disambiguate errors in case of failure

Not particularly helpful, but better than nothing.

Libraries/LibTLS/Record.cpp

@@ -282,8 +282,9 @@ ssize_t TLSv12::handle_message(const ByteBuffer& buffer)
             auto level = plain[0];
             auto code = plain[1];
             if (level == (u8)AlertLevel::Critical) {
-                dbg() << "We were alerted of a critical error: " << code;
+                dbg() << "We were alerted of a critical error: " << code << " (" << alert_name((AlertDescription)code) << ")";
                 m_context.critical_error = code;
+                try_disambiguate_error();
                 res = (i8)Error::UnknownError;
             } else {
                 dbg() << "Alert: " << code;

Libraries/LibTLS/TLSv12.cpp

@@ -519,4 +519,28 @@ void TLSv12::ensure_hmac(size_t digest_size, bool local)
         m_hmac_remote = move(hmac);
 }
 
+void TLSv12::try_disambiguate_error() const
+{
+    dbg() << "Possible failure cause: ";
+    switch ((AlertDescription)m_context.critical_error) {
+    case AlertDescription::HandshakeFailure:
+        if (!m_context.cipher_spec_set) {
+            dbg() << "- No cipher suite in common with " << m_context.SNI;
+        } else {
+            dbg() << "- Unknown internal issue";
+        }
+        break;
+    case AlertDescription::InsufficientSecurity:
+        dbg() << "- No cipher suite in common with " << m_context.SNI << " (the server is oh so secure)";
+        break;
+    case AlertDescription::DecryptionFailed:
+        dbg() << "- Bad MAC record from our side";
+        dbg() << "- Bad block cipher padding";
+        break;
+    default:
+        dbg() << "- No one knows";
+        break;
+    }
+}
+
 }

Libraries/LibTLS/TLSv12.h

@@ -73,36 +73,55 @@ enum class CipherSuite {
     RSA_WITH_AES_256_GCM_SHA384 = 0x009D,
 };
 
+#define ENUMERATE_ALERT_DESCRIPTIONS                        \
+    ENUMERATE_ALERT_DESCRIPTION(CloseNotify, 0)             \
+    ENUMERATE_ALERT_DESCRIPTION(UnexpectedMessage, 10)      \
+    ENUMERATE_ALERT_DESCRIPTION(BadRecordMAC, 20)           \
+    ENUMERATE_ALERT_DESCRIPTION(DecryptionFailed, 21)       \
+    ENUMERATE_ALERT_DESCRIPTION(RecordOverflow, 22)         \
+    ENUMERATE_ALERT_DESCRIPTION(DecompressionFailure, 30)   \
+    ENUMERATE_ALERT_DESCRIPTION(HandshakeFailure, 40)       \
+    ENUMERATE_ALERT_DESCRIPTION(NoCertificate, 41)          \
+    ENUMERATE_ALERT_DESCRIPTION(BadCertificate, 42)         \
+    ENUMERATE_ALERT_DESCRIPTION(UnsupportedCertificate, 43) \
+    ENUMERATE_ALERT_DESCRIPTION(CertificateRevoked, 44)     \
+    ENUMERATE_ALERT_DESCRIPTION(CertificateExpired, 45)     \
+    ENUMERATE_ALERT_DESCRIPTION(CertificateUnknown, 46)     \
+    ENUMERATE_ALERT_DESCRIPTION(IllegalParameter, 47)       \
+    ENUMERATE_ALERT_DESCRIPTION(UnknownCA, 48)              \
+    ENUMERATE_ALERT_DESCRIPTION(AccessDenied, 49)           \
+    ENUMERATE_ALERT_DESCRIPTION(DecodeError, 50)            \
+    ENUMERATE_ALERT_DESCRIPTION(DecryptError, 51)           \
+    ENUMERATE_ALERT_DESCRIPTION(ExportRestriction, 60)      \
+    ENUMERATE_ALERT_DESCRIPTION(ProtocolVersion, 70)        \
+    ENUMERATE_ALERT_DESCRIPTION(InsufficientSecurity, 71)   \
+    ENUMERATE_ALERT_DESCRIPTION(InternalError, 80)          \
+    ENUMERATE_ALERT_DESCRIPTION(InappropriateFallback, 86)  \
+    ENUMERATE_ALERT_DESCRIPTION(UserCanceled, 90)           \
+    ENUMERATE_ALERT_DESCRIPTION(NoRenegotiation, 100)       \
+    ENUMERATE_ALERT_DESCRIPTION(UnsupportedExtension, 110)  \
+    ENUMERATE_ALERT_DESCRIPTION(NoError, 255)
+
 enum class AlertDescription : u8 {
-    CloseNotify = 0,
-    UnexpectedMessage = 10,
-    BadRecordMAC = 20,
-    DecryptionFailed = 21,
-    RecordOverflow = 22,
-    DecompressionFailure = 30,
-    HandshakeFailure = 40,
-    NoCertificate = 41,
-    BadCertificate = 42,
-    UnsupportedCertificate = 43,
-    CertificateRevoked = 44,
-    CertificateExpired = 45,
-    CertificateUnknown = 46,
-    IllegalParameter = 47,
-    UnknownCA = 48,
-    AccessDenied = 49,
-    DecodeError = 50,
-    DecryptError = 51,
-    ExportRestriction = 60,
-    ProtocolVersion = 70,
-    InsufficientSecurity = 71,
-    InternalError = 80,
-    InappropriateFallback = 86,
-    UserCanceled = 90,
-    NoRenegotiation = 100,
-    UnsupportedExtension = 110,
-    NoError = 255
+#define ENUMERATE_ALERT_DESCRIPTION(name, value) name = value,
+    ENUMERATE_ALERT_DESCRIPTIONS
+#undef ENUMERATE_ALERT_DESCRIPTION
 };
 
+constexpr static const char* alert_name(AlertDescription descriptor)
+{
+#define ENUMERATE_ALERT_DESCRIPTION(name, value) \
+    case AlertDescription::name:                 \
+        return #name;
+
+    switch (descriptor) {
+        ENUMERATE_ALERT_DESCRIPTIONS
+    }
+
+    return "Unknown";
+#undef ENUMERATE_ALERT_DESCRIPTION
+}
+
 enum class Error : i8 {
     NoError = 0,
     UnknownError = -1,
@@ -437,6 +456,8 @@ private:
 
     bool compute_master_secret(size_t length);
 
+    void try_disambiguate_error() const;
+
     Context m_context;
 
     OwnPtr<Crypto::Authentication::HMAC<Crypto::Hash::Manager>> m_hmac_local;