Merge pull request #10 from apple/dn-implicitly-tagged-optional-field

Add support for implicitly tagged optional fields

Author: Lukasa

Sources/SwiftASN1/ASN1.swift

@@ -223,6 +223,41 @@ extension DER {
         // We're good: pass the node on.
         return try T(derEncoded: &nodes, withIdentifier: tag)
     }
+    
+    /// Parses an optional implicitly tagged element.
+    ///
+    /// - parameters:
+    ///     - nodes: The ``ASN1NodeCollection/Iterator`` to parse this element out of.
+    ///     - tagNumber: The number of the explicit tag.
+    ///     - tagClass: The class of the explicit tag.
+    ///     - builder: A closure that will be called with the node for the element, if the element is present.
+    ///
+    /// - returns: The result of `builder` if the element was present, or `nil` if it was not.
+    @inlinable
+    public static func optionalImplicitlyTagged<Result>(
+        _ nodes: inout ASN1NodeCollection.Iterator,
+        tagNumber: UInt,
+        tagClass: ASN1Identifier.TagClass,
+        _ builder: (ASN1Node) throws -> Result
+    ) rethrows -> Result? {
+        var localNodesCopy = nodes
+        guard let node = localNodesCopy.next() else {
+            // Node not present, return nil.
+            return nil
+        }
+        
+        let expectedNodeID = ASN1Identifier(tagWithNumber: tagNumber, tagClass: tagClass)
+        guard node.identifier == expectedNodeID else {
+            // Node is a mismatch, with the wrong tag. Our optional isn't present.
+            return nil
+        }
+        
+        // We have the right optional, so let's consume it.
+        nodes = localNodesCopy
+        
+        // We're good: pass the node on.
+        return try builder(node)
+    }
 }
 
 // MARK: - DEFAULT

Tests/SwiftASN1Tests/ASN1Tests.swift

@@ -783,6 +783,24 @@ O9zxi7HTvuXyQr7QKSBtdC%mHym+WoPsbA==
         }
         XCTAssertEqual(int, 1)
     }
+    
+    func testOptionalImplicitlyTaggedWithBuilder() throws {
+        var serializer = DER.Serializer()
+        try serializer.appendConstructedNode(identifier: .sequence) { serializer in
+            try serializer.serializeOptionalImplicitlyTagged(1, withIdentifier: ASN1Identifier(tagWithNumber: 1, tagClass: .contextSpecific))
+        }
+        let bytes = serializer.serializedBytes
+
+        XCTAssertEqual(bytes, [0x30, 0x03, 0x81, 0x1, 0x1])
+
+        let parseResult = try DER.parse(bytes)
+        let int = try DER.sequence(parseResult, identifier: .sequence) { nodes in
+            try DER.optionalImplicitlyTagged(&nodes, tagNumber: 1, tagClass: .contextSpecific) { node in
+                try Int(derEncoded: node, withIdentifier: .init(tagWithNumber: 1, tagClass: .contextSpecific))
+            }
+        }
+        XCTAssertEqual(int, 1)
+    }
 
     func testPrintingOIDs() {
         let oid: ASN1ObjectIdentifier = [1, 2, 865, 11241, 3]