Support singing via Certificate PrivateKey (#257)

This PR introduces two key changes to improve flexibility when working
with signatures in swift-certificates. Previously, it was only possible
to verify signatures using `Certificate.PrivateKey`, but not to generate
them.

- The internal `sign(bytes:signatureAlgorithm:)` method on
`Certificate.PrivateKey` is now public, allowing users to generate
signatures directly.
- `Certificate.Signature` gains a new `rawRepresentation` property that
exposes the raw byte representation of a signature.

Author: bauer-andreas

Sources/X509/CertificatePrivateKey.swift

@@ -91,8 +91,14 @@ extension Certificate {
         }
         #endif
 
+        /// Use the private key to sign the provided bytes with a given signature algorithm.
+        ///
+        /// - Parameters:
+        ///   - bytes: The data to create the signature for.
+        ///   - signatureAlgorithm: The signature algorithm to use.
+        /// - Returns: The signature.
         @inlinable
-        internal func sign<Bytes: DataProtocol>(
+        public func sign<Bytes: DataProtocol>(
             bytes: Bytes,
             signatureAlgorithm: SignatureAlgorithm
         ) throws -> Signature {

Sources/X509/Signature.swift

@@ -129,22 +129,31 @@ extension Certificate.Signature {
 }
 
 @available(macOS 10.15, iOS 13, watchOS 6, tvOS 13, macCatalyst 13, visionOS 1.0, *)
-extension ASN1BitString {
+extension Certificate.Signature {
+    /// The raw byte representation of the signature.
     @inlinable
-    init(_ signature: Certificate.Signature) {
-        switch signature.backing {
+    public var rawRepresentation: [UInt8] {
+        switch self.backing {
         case .ecdsa(let sig):
             var serializer = DER.Serializer()
             try! serializer.serialize(sig)
-            self = ASN1BitString(bytes: serializer.serializedBytes[...])
-        case .rsa(let sig):
-            self = ASN1BitString(bytes: ArraySlice(sig.rawRepresentation))
-        case .ed25519(let sig):
-            self = ASN1BitString(bytes: ArraySlice(sig))
+            return serializer.serializedBytes
+        case let .ed25519(data):
+            return .init(data)
+        case let .rsa(signature):
+            return .init(signature.rawRepresentation)
         }
     }
 }
 
+@available(macOS 10.15, iOS 13, watchOS 6, tvOS 13, macCatalyst 13, visionOS 1.0, *)
+extension ASN1BitString {
+    @inlinable
+    init(_ signature: Certificate.Signature) {
+        self.init(bytes: signature.rawRepresentation[...])
+    }
+}
+
 @available(macOS 10.15, iOS 13, watchOS 6, tvOS 13, macCatalyst 13, visionOS 1.0, *)
 extension ASN1OctetString {
     @inlinable

Tests/X509Tests/SignatureTests.swift

@@ -50,6 +50,40 @@ final class SignatureTests: XCTestCase {
     )
     #endif
 
+    func testRSASignatureBytes() throws {
+        let input = Array("Hello World".utf8)
+        let privateKey = Certificate.PrivateKey(Self.rsaKey)
+
+        let expected = try Self.rsaKey.signature(for: SHA256.hash(data: input), padding: .insecurePKCS1v1_5)
+        let found = try privateKey.sign(bytes: input, signatureAlgorithm: .sha256WithRSAEncryption)
+
+        XCTAssertEqual(.init(expected.rawRepresentation), found.rawRepresentation)
+    }
+
+    func testEd25519SignatureBytes() throws {
+        let input = Array("Hello World".utf8)
+
+        let expected = try Self.ed25519Key.signature(for: input)
+        let signature = try Certificate.Signature(
+            signatureAlgorithm: .ed25519,
+            signatureBytes: .init(bytes: Array(expected)[...])
+        )
+
+        XCTAssertEqual(.init(expected), signature.rawRepresentation)
+    }
+
+    func testECDSASignatureBytes() throws {
+        let input = Array("Hello World".utf8)
+
+        let expected = try Self.p384Key.signature(for: SHA256.hash(data: input))
+        let signature = try Certificate.Signature(
+            signatureAlgorithm: .ecdsaWithSHA256,
+            signatureBytes: .init(bytes: Array(expected.derRepresentation)[...])
+        )
+
+        XCTAssertEqual(.init(expected.derRepresentation), signature.rawRepresentation)
+    }
+
     func testP384Signature() throws {
         // This is the P384 signature over LetsEncrypt Intermediate E1.
         let signatureBytes: [UInt8] = [
@@ -75,6 +109,8 @@ final class SignatureTests: XCTestCase {
             return
         }
 
+        XCTAssertEqual(signature.rawRepresentation, .init(signatureBytes))
+
         // Validate that the signature is valid over the TBS certificate bytes.
         let issuingPublicKeyBytes: [UInt8] = [
             0x04, 0xCD, 0x9B, 0xD5, 0x9F, 0x80, 0x83, 0x0A, 0xEC, 0x09, 0x4A, 0xF3,