LibWeb: Implement skeleton of RSA-OAEP decrypt for SubtleCrypto

The actual Crypto algorithm part isn't implemented yet, so we just copy
the ciphertext and claim that's the plaintext :^)

Author: ADKaster

Userland/Libraries/LibWeb/Crypto/CryptoAlgorithms.cpp

@@ -373,6 +373,34 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<JS::ArrayBuffer>> RSAOAEP::encrypt(Algorith
     return JS::ArrayBuffer::create(realm, move(ciphertext));
 }
 
+// https://w3c.github.io/webcrypto/#rsa-oaep-operations
+WebIDL::ExceptionOr<JS::NonnullGCPtr<JS::ArrayBuffer>> RSAOAEP::decrypt(AlgorithmParams const& params, JS::NonnullGCPtr<CryptoKey> key, AK::ByteBuffer const& ciphertext)
+{
+    auto& realm = m_realm;
+    auto& vm = realm.vm();
+    auto const& normalized_algorithm = static_cast<RsaOaepParams const&>(params);
+
+    // 1. If the [[type]] internal slot of key is not "private", then throw an InvalidAccessError.
+    if (key->type() != Bindings::KeyType::Private)
+        return WebIDL::InvalidAccessError::create(realm, "Key is not a private key"_fly_string);
+
+    // 2. Let label be the contents of the label member of normalizedAlgorithm or the empty octet string if the label member of normalizedAlgorithm is not present.
+    [[maybe_unused]] auto const& label = normalized_algorithm.label;
+
+    // 3. Perform the decryption operation defined in Section 7.1 of [RFC3447] with the key represented by key as the recipient's RSA private key,
+    //    the contents of ciphertext as the ciphertext to be decrypted, C, and label as the label, L, and with the hash function specified by the hash attribute
+    //    of the [[algorithm]] internal slot of key as the Hash option and MGF1 (defined in Section B.2.1 of [RFC3447]) as the MGF option.
+
+    // 4. If performing the operation results in an error, then throw an OperationError.
+
+    // 5. Let plaintext the value M that results from performing the operation.
+    // FIXME: Actually decrypt the data
+    auto plaintext = TRY_OR_THROW_OOM(vm, ByteBuffer::copy(ciphertext));
+
+    // 6. Return the result of creating an ArrayBuffer containing plaintext.
+    return JS::ArrayBuffer::create(realm, move(plaintext));
+}
+
 // https://w3c.github.io/webcrypto/#rsa-oaep-operations
 WebIDL::ExceptionOr<Variant<JS::NonnullGCPtr<CryptoKey>, JS::NonnullGCPtr<CryptoKeyPair>>> RSAOAEP::generate_key(AlgorithmParams const& params, bool extractable, Vector<Bindings::KeyUsage> const& key_usages)
 {

Userland/Libraries/LibWeb/Crypto/CryptoAlgorithms.h

@@ -165,6 +165,7 @@ class AlgorithmMethods {
 class RSAOAEP : public AlgorithmMethods {
 public:
     virtual WebIDL::ExceptionOr<JS::NonnullGCPtr<JS::ArrayBuffer>> encrypt(AlgorithmParams const&, JS::NonnullGCPtr<CryptoKey>, ByteBuffer const&) override;
+    virtual WebIDL::ExceptionOr<JS::NonnullGCPtr<JS::ArrayBuffer>> decrypt(AlgorithmParams const&, JS::NonnullGCPtr<CryptoKey>, ByteBuffer const&) override;
 
     virtual WebIDL::ExceptionOr<Variant<JS::NonnullGCPtr<CryptoKey>, JS::NonnullGCPtr<CryptoKeyPair>>> generate_key(AlgorithmParams const&, bool, Vector<Bindings::KeyUsage> const&) override;
 

Userland/Libraries/LibWeb/Crypto/SubtleCrypto.cpp

@@ -516,7 +516,7 @@ SupportedAlgorithmsMap supported_algorithms()
     define_an_algorithm<RSAOAEP>("exportKey"_string, "RSA-OAEP"_string);
     define_an_algorithm<RSAOAEP, RsaHashedImportParams>("importKey"_string, "RSA-OAEP"_string);
     define_an_algorithm<RSAOAEP, RsaOaepParams>("encrypt"_string, "RSA-OAEP"_string);
-    // FIXME: decrypt
+    define_an_algorithm<RSAOAEP, RsaOaepParams>("decrypt"_string, "RSA-OAEP"_string);
 
     return internal_object;
 }