Verify CA from on-disk file (#237)

* Verify CA from on-disk file

Revert part of #213 again, because `SSL_CTX_load_verify_locations`
is just more reliable at setting up the trust store.

It looks like it's able to reference the .pem multiple times in
those cases where the root issuer of the CA is also embedded in
the file (which is the case with e.g. Let's Encrypt).

This is better than the current implementation, and I couldn't
easily find a way to do the same in-memory. I'd rather use the
standard API here.

See 7a85d3cac7

Sources/CTunnelKitOpenVPNProtocol/TLSBox.m

@@ -62,7 +62,7 @@ const NSInteger TLSBoxDefaultSecurityLevel = 0;
 
 @interface TLSBox ()
 
-@property (nonatomic, strong) NSString *caPEM;
+@property (nonatomic, strong) NSString *caPath;
 @property (nonatomic, strong) NSString *clientCertificatePEM;
 @property (nonatomic, strong) NSString *clientKeyPEM;
 @property (nonatomic, assign) BOOL checksEKU;
@@ -113,33 +113,6 @@ static BIO *create_BIO_from_PEM(NSString *pem) {
     return hex;
 }
 
-+ (NSString *)md5ForCertificatePEM:(NSString *)pem error:(NSError * _Nullable __autoreleasing * _Nullable)error
-{
-    const EVP_MD *alg = EVP_get_digestbyname("MD5");
-    uint8_t md[16];
-    unsigned int len;
-
-    BIO *bio = create_BIO_from_PEM(pem);
-    if (!bio) {
-        return NULL;
-    }
-    X509 *cert = PEM_read_bio_X509(bio, NULL, NULL, NULL);
-    if (!cert) {
-        BIO_free(bio);
-        return NULL;
-    }
-    X509_digest(cert, alg, md, &len);
-    X509_free(cert);
-    BIO_free(bio);
-    NSCAssert2(len == sizeof(md), @"Unexpected MD5 size (%d != %lu)", len, sizeof(md));
-
-    NSMutableString *hex = [[NSMutableString alloc] initWithCapacity:2 * sizeof(md)];
-    for (int i = 0; i < sizeof(md); ++i) {
-        [hex appendFormat:@"%02x", md[i]];
-    }
-    return hex;
-}
-
 + (NSString *)decryptedPrivateKeyFromPath:(NSString *)path passphrase:(NSString *)passphrase error:(NSError * _Nullable __autoreleasing *)error
 {
     BIO *bio;
@@ -200,15 +173,15 @@ static BIO *create_BIO_from_PEM(NSString *pem) {
     return nil;
 }
 
-- (instancetype)initWithCA:(nonnull NSString *)caPEM
+- (instancetype)initWithCAPath:(nonnull NSString *)caPath
-         clientCertificate:(nullable NSString *)clientCertificatePEM
+             clientCertificate:(nullable NSString *)clientCertificatePEM
-                 clientKey:(nullable NSString *)clientKeyPEM
+                     clientKey:(nullable NSString *)clientKeyPEM
-                 checksEKU:(BOOL)checksEKU
+                     checksEKU:(BOOL)checksEKU
-             checksSANHost:(BOOL)checksSANHost
+                 checksSANHost:(BOOL)checksSANHost
-                  hostname:(nullable NSString *)hostname
+                      hostname:(nullable NSString *)hostname
 {
     if ((self = [super init])) {
-        self.caPEM = caPEM;
+        self.caPath = caPath;
         self.clientCertificatePEM = clientCertificatePEM;
         self.clientKeyPEM = clientKeyPEM;
         self.checksEKU = checksEKU;
@@ -245,34 +218,14 @@ static BIO *create_BIO_from_PEM(NSString *pem) {
         SSL_CTX_set_security_level(self.ctx, (int)self.securityLevel);
     }
 
-    if (self.caPEM) {
+    if (self.caPath) {
-        BIO *bio = create_BIO_from_PEM(self.caPEM);
+        if (!SSL_CTX_load_verify_locations(self.ctx, [self.caPath cStringUsingEncoding:NSASCIIStringEncoding], NULL)) {
-        if (!bio) {
-            if (error) {
-                *error = OpenVPNErrorWithCode(OpenVPNErrorCodeTLSCARead);
-            }
-            return NO;
-        }
-        X509 *ca = PEM_read_bio_X509_AUX(bio, NULL, NULL, NULL);
-        if (!ca) {
-            if (error) {
-                *error = OpenVPNErrorWithCode(OpenVPNErrorCodeTLSCARead);
-            }
-            BIO_free(bio);
-            return NO;
-        }
-        BIO_free(bio);
-        X509_STORE *trustedStore = SSL_CTX_get_cert_store(self.ctx);
-        X509_STORE_set_flags(trustedStore, X509_V_FLAG_PARTIAL_CHAIN);
-        if (!X509_STORE_add_cert(trustedStore, ca)) {
             ERR_print_errors_fp(stdout);
             if (error) {
                 *error = OpenVPNErrorWithCode(OpenVPNErrorCodeTLSCAUse);
             }
-            X509_free(ca);
             return NO;
         }
-        X509_free(ca);
     }
 
     if (self.clientCertificatePEM) {

Sources/CTunnelKitOpenVPNProtocol/include/TLSBox.h

@@ -55,16 +55,15 @@ extern const NSInteger TLSBoxDefaultSecurityLevel;
 @property (nonatomic, assign) NSInteger securityLevel; // TLSBoxDefaultSecurityLevel for default
 
 + (nullable NSString *)md5ForCertificatePath:(NSString *)path error:(NSError **)error;
-+ (nullable NSString *)md5ForCertificatePEM:(NSString *)pem error:(NSError **)error;
 + (nullable NSString *)decryptedPrivateKeyFromPath:(NSString *)path passphrase:(NSString *)passphrase error:(NSError **)error;
 + (nullable NSString *)decryptedPrivateKeyFromPEM:(NSString *)pem passphrase:(NSString *)passphrase error:(NSError **)error;
 
-- (instancetype)initWithCA:(nonnull NSString *)caPEM
+- (instancetype)initWithCAPath:(nonnull NSString *)caPath
-         clientCertificate:(nullable NSString *)clientCertificatePEM
+             clientCertificate:(nullable NSString *)clientCertificatePEM
-                 clientKey:(nullable NSString *)clientKeyPEM
+                     clientKey:(nullable NSString *)clientKeyPEM
-                 checksEKU:(BOOL)checksEKU
+                     checksEKU:(BOOL)checksEKU
-             checksSANHost:(BOOL)checksSANHost
+                 checksSANHost:(BOOL)checksSANHost
-                  hostname:(nullable NSString *)hostname;
+                      hostname:(nullable NSString *)hostname;
 
 - (BOOL)startWithError:(NSError **)error;
 

Sources/TunnelKitOpenVPNAppExtension/OpenVPNTunnelProvider.swift

@@ -241,7 +241,7 @@ open class OpenVPNTunnelProvider: NEPacketTunnelProvider {
 
         let session: OpenVPNSession
         do {
-            session = try OpenVPNSession(queue: tunnelQueue, configuration: cfg.sessionConfiguration)
+            session = try OpenVPNSession(queue: tunnelQueue, configuration: cfg.sessionConfiguration, cachesURL: cachesURL)
             refreshDataCount()
         } catch let e {
             completionHandler(e)

Sources/TunnelKitOpenVPNProtocol/OpenVPNSession.swift

@@ -72,6 +72,10 @@ public class OpenVPNSession: Session {
         case reconnect
     }
     
+    private struct Caches {
+        static let ca = "ca.pem"
+    }
+
     // MARK: Configuration
     
     /// The session base configuration.
@@ -166,6 +170,14 @@ public class OpenVPNSession: Session {
     
     private var authenticator: OpenVPN.Authenticator?
     
+    // MARK: Caching
+
+    private let cachesURL: URL
+
+    private var caURL: URL {
+        return cachesURL.appendingPathComponent(Caches.ca)
+    }
+
     // MARK: Init
 
     /**
@@ -174,13 +186,14 @@ public class OpenVPNSession: Session {
      - Parameter queue: The `DispatchQueue` where to run the session loop.
      - Parameter configuration: The `Configuration` to use for this session.
      */
-    public init(queue: DispatchQueue, configuration: OpenVPN.Configuration) throws {
+    public init(queue: DispatchQueue, configuration: OpenVPN.Configuration, cachesURL: URL) throws {
-        guard let _ = configuration.ca else {
+        guard let ca = configuration.ca else {
             throw ConfigurationError.missingConfiguration(option: "ca")
         }
         
         self.queue = queue
         self.configuration = configuration
+        self.cachesURL = cachesURL
 
         withLocalOptions = true
         keys = [:]
@@ -201,10 +214,14 @@ public class OpenVPNSession: Session {
         } else {
             controlChannel = OpenVPN.ControlChannel()
         }
+
+        // cache CA locally (mandatory for OpenSSL)
+        try ca.pem.write(to: caURL, atomically: true, encoding: .ascii)
     }
     
     deinit {
         cleanup()
+        cleanupCache()
     }
     
     // MARK: Session
@@ -315,6 +332,13 @@ public class OpenVPNSession: Session {
         stopError = nil
     }
 
+    func cleanupCache() {
+        let fm = FileManager.default
+        for url in [caURL] {
+            try? fm.removeItem(at: url)
+        }
+    }
+
     // MARK: Loop
 
     // Ruby: start
@@ -576,13 +600,13 @@ public class OpenVPNSession: Session {
     
     private func hardResetPayload() -> Data? {
         guard !(configuration.usesPIAPatches ?? false) else {
-            guard let ca = configuration.ca else {
+            guard let _ = configuration.ca else {
                 log.error("Configuration doesn't have a CA")
                 return nil
             }
             let caMD5: String
             do {
-                caMD5 = try TLSBox.md5(forCertificatePEM: ca.pem)
+                caMD5 = try TLSBox.md5(forCertificatePath: caURL.path)
             } catch {
                 log.error("CA MD5 could not be computed, skipping custom HARD_RESET")
                 return nil
@@ -723,7 +747,7 @@ public class OpenVPNSession: Session {
             return
         }
         
-        guard let ca = configuration.ca else {
+        guard let _ = configuration.ca else {
             log.error("Configuration doesn't have a CA")
             return
         }
@@ -751,7 +775,7 @@ public class OpenVPNSession: Session {
             log.debug("Start TLS handshake")
 
             let tls = TLSBox(
-                ca: ca.pem,
+                caPath: caURL.path,
                 clientCertificate: configuration.clientCertificate?.pem,
                 clientKey: configuration.clientKey?.pem,
                 checksEKU: configuration.checksEKU ?? false,
@@ -1215,6 +1239,7 @@ public class OpenVPNSession: Session {
             switch method {
             case .shutdown:
                 self?.doShutdown(error: error)
+                self?.cleanupCache()
                 
             case .reconnect:
                 self?.doReconnect(error: error)

Tests/TunnelKitOpenVPNTests/EncryptionTests.swift

@@ -118,11 +118,6 @@ class EncryptionTests: XCTestCase {
         let exp = "e2fccccaba712ccc68449b1c56427ac1"
         print(md5)
         XCTAssertEqual(md5, exp)
-        
-        let pem = try! String(contentsOfFile: path, encoding: .ascii)
-        let md5FromPEM = try! TLSBox.md5(forCertificatePEM: pem)
-        print(md5FromPEM)
-        XCTAssertEqual(md5FromPEM, exp)
     }
     
     func testPrivateKeyDecryption() {