Add a way to extract the public key from a CSR to the Matter framework.

src/darwin/Framework/CHIP/MTRCSRInfo.h

@@ -19,8 +19,6 @@
 
 #import <Matter/MTRDefines.h>
 
-typedef NSData * MTRCSRDERBytes;
-
 NS_ASSUME_NONNULL_BEGIN
 
 /**

src/darwin/Framework/CHIP/MTRCertificates.h

@@ -20,8 +20,7 @@
 
 #import <Foundation/Foundation.h>
 
-typedef NSData * MTRCertificateDERBytes;
-typedef NSData * MTRCertificateTLVBytes;
+#import <Matter/MTRDefines.h>
 
 NS_ASSUME_NONNULL_BEGIN
 
@@ -128,8 +127,8 @@ NS_ASSUME_NONNULL_BEGIN
  * On failure returns nil and if "error" is not null sets *error to the relevant
  * error.
  */
-+ (NSData * _Nullable)createCertificateSigningRequest:(id<MTRKeypair>)keypair
-                                                error:(NSError * __autoreleasing _Nullable * _Nullable)error;
++ (MTRCSRDERBytes _Nullable)createCertificateSigningRequest:(id<MTRKeypair>)keypair
+                                                      error:(NSError * __autoreleasing _Nullable * _Nullable)error;
 
 /**
  * Convert the given X.509v3 DER encoded certificate to the Matter certificate
@@ -151,6 +150,15 @@ NS_ASSUME_NONNULL_BEGIN
  */
 + (MTRCertificateDERBytes _Nullable)convertMatterCertificate:(MTRCertificateTLVBytes)matterCertificate MTR_NEWLY_AVAILABLE;
 
+/**
+ * Extract the public key from the given PKCS#10 certificate signing request.
+ * This is the public key that a certificate issued in response to the request
+ * would need to have.
+ */
++ (NSData * _Nullable)extractPublicKeyFromCertificateSigningRequest:(MTRCSRDERBytes)certificateSigningRequest
+                                                              error:(NSError * __autoreleasing _Nullable * _Nullable)error
+    MTR_NEWLY_AVAILABLE;
+
 @end
 
 @interface MTRCertificates (Deprecated)

src/darwin/Framework/CHIP/MTRCertificates.mm

@@ -221,6 +221,24 @@ + (MTRCertificateDERBytes _Nullable)convertMatterCertificate:(MTRCertificateTLVB
     return AsData(derCertBytes);
 }
 
++ (NSData * _Nullable)extractPublicKeyFromCertificateSigningRequest:(MTRCSRDERBytes)certificateSigningRequest
+                                                              error:(NSError * __autoreleasing _Nullable * _Nullable)error
+{
+    auto requestSpan = AsByteSpan(certificateSigningRequest);
+    P256PublicKey publicKey;
+    CHIP_ERROR err = VerifyCertificateSigningRequest(requestSpan.data(), requestSpan.size(), publicKey);
+    if (err != CHIP_NO_ERROR) {
+        MTR_LOG_ERROR("extractPublicKeyFromCertificateSigningRequest: %s", chip::ErrorStr(err));
+        if (error) {
+            *error = [MTRError errorForCHIPErrorCode:err];
+        }
+        return nil;
+    }
+
+    P256PublicKeySpan publicKeySpan(publicKey.ConstBytes());
+    return AsData(publicKeySpan);
+}
+
 @end
 
 @implementation MTRCertificates (Deprecated)

src/darwin/Framework/CHIP/MTRDefines.h

@@ -26,3 +26,6 @@
 #endif
 
 typedef NSData * MTRTLVBytes;
+typedef NSData * MTRCSRDERBytes;
+typedef NSData * MTRCertificateDERBytes;
+typedef NSData * MTRCertificateTLVBytes;

src/darwin/Framework/CHIPTests/MTRCertificateTests.m

@@ -284,7 +284,16 @@ - (void)testGenerateCSR
     __auto_type * csr = [MTRCertificates createCertificateSigningRequest:testKeys error:nil];
     XCTAssertNotNil(csr);
 
-    // Wish there was something we could test here about the CSR.
+    __auto_type * publicKey = [MTRCertificates extractPublicKeyFromCertificateSigningRequest:csr error:nil];
+    XCTAssertNotNil(publicKey);
+
+    SecKeyRef originalKeyRef = [testKeys publicKey];
+    XCTAssertTrue(originalKeyRef != NULL);
+
+    NSData * originalPublicKey = (__bridge_transfer NSData *) SecKeyCopyExternalRepresentation(originalKeyRef, nil);
+    XCTAssertNotNil(originalPublicKey);
+
+    XCTAssertEqualObjects(publicKey, originalPublicKey);
 }
 
 @end