Fixed BMPString encoding

Sources/SwiftASN1/Basic ASN1 Types/ASN1Strings.swift

@@ -284,7 +284,19 @@ public struct ASN1BMPString: DERImplicitlyTaggable, BERImplicitlyTaggable, Hasha
 
     @inlinable
     public init(stringLiteral value: StringLiteralType) {
-        self.bytes = ArraySlice(value.utf8)
+        guard
+            value.utf16.allSatisfy({ codeUnit in
+                !(0xD800...0xDFFF).contains(codeUnit)
+            })
+        else {
+            fatalError("BMPString cannot contain characters outside the Basic Multilingual Plane: '\(value)'")
+        }
+
+        self.bytes = ArraySlice(
+            value.utf16.flatMap { codeUnit in
+                [UInt8(truncatingIfNeeded: codeUnit >> 8), UInt8(truncatingIfNeeded: codeUnit)]
+            }
+        )
     }
 
     @inlinable

Tests/SwiftASN1Tests/ASN1StringTests.swift

@@ -103,6 +103,37 @@ final class ASN1StringTests: XCTestCase {
         string.withUnsafeBytes { XCTAssertTrue($0.elementsEqual([1, 2, 3, 4])) }
     }
 
+    func testBMPStringStringLiteral() throws {
+        typealias TestCase = (literal: String, utf16: [UInt8], asn1: [UInt8])
+
+        let testCases: [TestCase] = [
+            TestCase(
+                "Test",
+                [0, 84, 0, 101, 0, 115, 0, 116],
+                [30, 8, 0, 84, 0, 101, 0, 115, 0, 116]
+            ),
+            TestCase(
+                "Tests",
+                [0, 84, 0, 101, 0, 115, 0, 116, 0, 115],
+                [30, 10, 0, 84, 0, 101, 0, 115, 0, 116, 0, 115]
+            ),
+            TestCase(
+                "中文",
+                [78, 45, 101, 135],
+                [30, 4, 78, 45, 101, 135]
+            ),
+        ]
+
+        try testCases.forEach { testCase in
+            let string = ASN1BMPString(stringLiteral: testCase.literal)
+            XCTAssertEqual(Array(string.bytes), testCase.utf16)
+
+            var serializer = DER.Serializer()
+            try serializer.serialize(string)
+            XCTAssertEqual(serializer.serializedBytes, testCase.asn1)
+        }
+    }
+
     func testUTF8StringCanCreateAString() throws {
         let string = "hello, world!"
         let utf8String = ASN1UTF8String(string)