Add ImageType.jpeg read capability and magic-byte detection

.changeset/imagetype-jpeg-read.md

@@ -0,0 +1,5 @@
+---
+"@germ-network/autonomous-comm-protocol": minor
+---
+
+Add read capability for a `jpeg = 2` case on `ImageType`, plus `ImageType.detect(from:)` magic-byte classification (JXL codestream, JXL container, JPEG). No writer emits `.jpeg` yet: senders without a JPEG XL encoder (the App Clip) keep labeling JPEG bytes `.jpegXL` so that deployed clients — which fail the whole signed `CoreIdentity` parse on an unknown discriminant — are unaffected. Consumers should classify by bytes via `detect(from:)`, not the wire label.

Sources/CommProtocol/CoreIdentity.swift

@@ -67,6 +67,27 @@ extension CoreIdentity: LinearEncodedQuintuple {
 
 public enum ImageType: UInt8, Sendable {
 	case jpegXL = 1
+	case jpeg = 2
+
+	///Classify image bytes by magic number, independent of the declared wire label.
+	///Senders without a JPEG XL encoder (the App Clip) label their JPEG images
+	///`.jpegXL` for backward compatibility, so consumers should trust the bytes,
+	///not the label.
+	public static func detect(from data: Data) -> ImageType? {
+		//JXL raw codestream: FF 0A
+		if data.starts(with: [0xFF, 0x0A]) {
+			return .jpegXL
+		}
+		//JXL ISOBMFF container: 00 00 00 0C 4A 58 4C 20 0D 0A 87 0A
+		if data.starts(with: [0, 0, 0, 0x0C, 0x4A, 0x58, 0x4C, 0x20, 0x0D, 0x0A, 0x87, 0x0A]) {
+			return .jpegXL
+		}
+		//JPEG: FF D8 FF
+		if data.starts(with: [0xFF, 0xD8, 0xFF]) {
+			return .jpeg
+		}
+		return nil
+	}
 }
 
 public struct DescribedImage: Equatable, Sendable {

Tests/CommProtocolTests/DescribedImageTests.swift

@@ -0,0 +1,79 @@
+//
+//  DescribedImageTests.swift
+//
+//
+//  Created by Mark @ Germ on 7/22/26.
+//
+
+import CryptoKit
+import Foundation
+import Testing
+
+@testable import CommProtocol
+
+struct DescribedImageTests {
+	@Test func testImageTypeRoundTrip() throws {
+		for imageType in [ImageType.jpegXL, .jpeg] {
+			let described = DescribedImage(
+				imageType: imageType,
+				imageData: SymmetricKey(size: .bits256).rawRepresentation,
+				altText: "description"
+			)
+			let received = try DescribedImage.finalParse(described.wireFormat)
+			#expect(received == described)
+			#expect(received.imageType == imageType)
+		}
+	}
+
+	@Test func testUnknownImageTypeRejected() throws {
+		let described = DescribedImage(
+			imageType: .jpeg,
+			imageData: SymmetricKey(size: .bits256).rawRepresentation,
+			altText: nil
+		)
+		var encoded = try described.wireFormat
+		#expect(encoded.first == ImageType.jpeg.rawValue)
+		encoded[encoded.startIndex] = 3
+
+		#expect(throws: LinearEncodingError.unexpectedData) {
+			_ = try DescribedImage.finalParse(encoded)
+		}
+	}
+
+	@Test func testDetectJXLCodestream() {
+		let data = Data([0xFF, 0x0A]) + Data(repeating: 0, count: 16)
+		#expect(ImageType.detect(from: data) == .jpegXL)
+	}
+
+	@Test func testDetectJXLContainer() {
+		let box: [UInt8] = [0, 0, 0, 0x0C, 0x4A, 0x58, 0x4C, 0x20, 0x0D, 0x0A, 0x87, 0x0A]
+		let data = Data(box) + Data(repeating: 0, count: 16)
+		#expect(ImageType.detect(from: data) == .jpegXL)
+	}
+
+	@Test func testDetectJPEG() {
+		let data = Data([0xFF, 0xD8, 0xFF, 0xE0]) + Data(repeating: 0, count: 16)
+		#expect(ImageType.detect(from: data) == .jpeg)
+	}
+
+	@Test func testDetectRejectsOther() {
+		//PNG magic
+		#expect(ImageType.detect(from: Data([0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A])) == nil)
+		#expect(ImageType.detect(from: Data()) == nil)
+		#expect(ImageType.detect(from: Data([0xFF])) == nil)
+	}
+
+	///The compatibility posture: a sender without a JPEG XL encoder labels its
+	///JPEG bytes `.jpegXL`; detection recovers the true format from the bytes.
+	@Test func testFalseLabelRecoveredByDetection() {
+		let jpegBytes = Data([0xFF, 0xD8, 0xFF, 0xE0]) + Data(repeating: 0, count: 16)
+		let described = DescribedImage(
+			imageData: jpegBytes,
+			altText: nil
+		)
+		//wire label says jpegXL (the default)
+		#expect(described.imageType == .jpegXL)
+		//bytes say jpeg
+		#expect(ImageType.detect(from: jpegBytes) == .jpeg)
+	}
+}