crypto: allow deriving public from private keys

This change allows passing private key objects to
crypto.createPublicKey, resulting in a key object that represents a
valid public key for the given private key. The returned public key
object can be used and exported safely without revealing information
about the private key.

Backport-PR-URL: #26688
PR-URL: #26278
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>

doc/api/crypto.md

@@ -1817,28 +1817,35 @@ must be an object with the properties described above.
 <!-- YAML
 added: v11.6.0
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/26278
+    description: The `key` argument can now be a `KeyObject` with type
+                 `private`.
   - version: v11.7.0
     pr-url: https://github.com/nodejs/node/pull/25217
     description: The `key` argument can now be a private key.
 -->
-* `key` {Object | string | Buffer}
+* `key` {Object | string | Buffer | KeyObject}
   - `key`: {string | Buffer}
   - `format`: {string} Must be `'pem'` or `'der'`. **Default:** `'pem'`.
   - `type`: {string} Must be `'pkcs1'` or `'spki'`. This option is required
     only if the `format` is `'der'`.
 * Returns: {KeyObject}
 
 Creates and returns a new key object containing a public key. If `key` is a
-string or `Buffer`, `format` is assumed to be `'pem'`; otherwise, `key`
-must be an object with the properties described above.
+string or `Buffer`, `format` is assumed to be `'pem'`; if `key` is a `KeyObject`
+with type `'private'`, the public key is derived from the given private key;
+otherwise, `key` must be an object with the properties described above.
 
 If the format is `'pem'`, the `'key'` may also be an X.509 certificate.
 
 Because public keys can be derived from private keys, a private key may be
 passed instead of a public key. In that case, this function behaves as if
 [`crypto.createPrivateKey()`][] had been called, except that the type of the
-returned `KeyObject` will be `public` and that the private key cannot be
-extracted from the returned `KeyObject`.
+returned `KeyObject` will be `'public'` and that the private key cannot be
+extracted from the returned `KeyObject`. Similarly, if a `KeyObject` with type
+`'private'` is given, a new `KeyObject` with type `'public'` will be returned
+and it will be impossible to extract the private key from the returned object.
 
 ### crypto.createSecretKey(key)
 <!-- YAML

lib/internal/crypto/keys.js

@@ -26,6 +26,12 @@ const { isArrayBufferView } = require('internal/util/types');
 
 const kKeyType = Symbol('kKeyType');
 
+// Key input contexts.
+const kConsumePublic = 0;
+const kConsumePrivate = 1;
+const kCreatePublic = 2;
+const kCreatePrivate = 3;
+
 const encodingNames = [];
 for (const m of [[kKeyEncodingPKCS1, 'pkcs1'], [kKeyEncodingPKCS8, 'pkcs8'],
                  [kKeyEncodingSPKI, 'spki'], [kKeyEncodingSEC1, 'sec1']])
@@ -203,7 +209,7 @@ function parseKeyEncoding(enc, keyType, isPublic, objName) {
 // when this is used to parse an input encoding and must be a valid key type if
 // used to parse an output encoding.
 function parsePublicKeyEncoding(enc, keyType, objName) {
-  return parseKeyFormatAndType(enc, keyType, true, objName);
+  return parseKeyEncoding(enc, keyType, keyType ? true : undefined, objName);
 }
 
 // Parses the private key encoding based on an object. keyType must be undefined
@@ -213,26 +219,31 @@ function parsePrivateKeyEncoding(enc, keyType, objName) {
   return parseKeyEncoding(enc, keyType, false, objName);
 }
 
-function getKeyObjectHandle(key, isPublic, allowKeyObject) {
-  if (!allowKeyObject) {
+function getKeyObjectHandle(key, ctx) {
+  if (ctx === kCreatePrivate) {
     throw new ERR_INVALID_ARG_TYPE(
       'key',
       ['string', 'Buffer', 'TypedArray', 'DataView'],
       key
     );
   }
-  if (isPublic != null) {
-    const expectedType = isPublic ? 'public' : 'private';
-    if (key.type !== expectedType)
-      throw new ERR_CRYPTO_INVALID_KEY_OBJECT_TYPE(key.type, expectedType);
+
+  if (key.type !== 'private') {
+    if (ctx === kConsumePrivate || ctx === kCreatePublic)
+      throw new ERR_CRYPTO_INVALID_KEY_OBJECT_TYPE(key.type, 'private');
+    if (key.type !== 'public') {
+      throw new ERR_CRYPTO_INVALID_KEY_OBJECT_TYPE(key.type,
+                                                   'private or public');
+    }
   }
+
   return key[kHandle];
 }
 
-function prepareAsymmetricKey(key, isPublic, allowKeyObject = true) {
+function prepareAsymmetricKey(key, ctx) {
   if (isKeyObject(key)) {
     // Best case: A key object, as simple as that.
-    return { data: getKeyObjectHandle(key, isPublic, allowKeyObject) };
+    return { data: getKeyObjectHandle(key, ctx) };
   } else if (typeof key === 'string' || isArrayBufferView(key)) {
     // Expect PEM by default, mostly for backward compatibility.
     return { format: kKeyFormatPEM, data: key };
@@ -241,32 +252,32 @@ function prepareAsymmetricKey(key, isPublic, allowKeyObject = true) {
     // The 'key' property can be a KeyObject as well to allow specifying
     // additional options such as padding along with the key.
     if (isKeyObject(data))
-      return { data: getKeyObjectHandle(data, isPublic, allowKeyObject) };
+      return { data: getKeyObjectHandle(data, ctx) };
     // Either PEM or DER using PKCS#1 or SPKI.
     if (!isStringOrBuffer(data)) {
       throw new ERR_INVALID_ARG_TYPE(
         'key',
         ['string', 'Buffer', 'TypedArray', 'DataView',
-         ...(allowKeyObject ? ['KeyObject'] : [])],
+         ...(ctx !== kCreatePrivate ? ['KeyObject'] : [])],
         key);
     }
-    return { data, ...parseKeyEncoding(key, undefined, isPublic) };
+    return { data, ...parseKeyEncoding(key, undefined) };
   } else {
     throw new ERR_INVALID_ARG_TYPE(
       'key',
       ['string', 'Buffer', 'TypedArray', 'DataView',
-       ...(allowKeyObject ? ['KeyObject'] : [])],
+       ...(ctx !== kCreatePrivate ? ['KeyObject'] : [])],
       key
     );
   }
 }
 
-function preparePrivateKey(key, allowKeyObject) {
-  return prepareAsymmetricKey(key, false, allowKeyObject);
+function preparePrivateKey(key) {
+  return prepareAsymmetricKey(key, kConsumePrivate);
 }
 
-function preparePublicOrPrivateKey(key, allowKeyObject) {
-  return prepareAsymmetricKey(key, undefined, allowKeyObject);
+function preparePublicOrPrivateKey(key) {
+  return prepareAsymmetricKey(key, kConsumePublic);
 }
 
 function prepareSecretKey(key, bufferOnly = false) {
@@ -296,14 +307,15 @@ function createSecretKey(key) {
 }
 
 function createPublicKey(key) {
-  const { format, type, data } = preparePublicOrPrivateKey(key, false);
+  const { format, type, data } = prepareAsymmetricKey(key, kCreatePublic);
   const handle = new KeyObjectHandle(kKeyTypePublic);
   handle.init(data, format, type);
   return new PublicKeyObject(handle);
 }
 
 function createPrivateKey(key) {
-  const { format, type, data, passphrase } = preparePrivateKey(key, false);
+  const { format, type, data, passphrase } =
+    prepareAsymmetricKey(key, kCreatePrivate);
   const handle = new KeyObjectHandle(kKeyTypePrivate);
   handle.init(data, format, type, passphrase);
   return new PrivateKeyObject(handle);

test/parallel/test-crypto-key-objects.js

@@ -59,9 +59,27 @@ const privatePem = fixtures.readSync('test_rsa_privkey.pem', 'ascii');
 }
 
 {
-  // Passing an existing key object should throw.
+  // Passing an existing public key object to createPublicKey should throw.
   const publicKey = createPublicKey(publicPem);
   common.expectsError(() => createPublicKey(publicKey), {
+    type: TypeError,
+    code: 'ERR_CRYPTO_INVALID_KEY_OBJECT_TYPE',
+    message: 'Invalid key object type public, expected private.'
+  });
+
+  // Constructing a private key from a public key should be impossible, even
+  // if the public key was derived from a private key.
+  common.expectsError(() => createPrivateKey(createPublicKey(privatePem)), {
+    type: TypeError,
+    code: 'ERR_INVALID_ARG_TYPE',
+    message: 'The "key" argument must be one of type string, Buffer, ' +
+             'TypedArray, or DataView. Received type object'
+  });
+
+  // Similarly, passing an existing private key object to createPrivateKey
+  // should throw.
+  const privateKey = createPrivateKey(privatePem);
+  common.expectsError(() => createPrivateKey(privateKey), {
     type: TypeError,
     code: 'ERR_INVALID_ARG_TYPE',
     message: 'The "key" argument must be one of type string, Buffer, ' +
@@ -80,6 +98,12 @@ const privatePem = fixtures.readSync('test_rsa_privkey.pem', 'ascii');
   assert.strictEqual(privateKey.asymmetricKeyType, 'rsa');
   assert.strictEqual(privateKey.symmetricKeySize, undefined);
 
+  // It should be possible to derive a public key from a private key.
+  const derivedPublicKey = createPublicKey(privateKey);
+  assert.strictEqual(derivedPublicKey.type, 'public');
+  assert.strictEqual(derivedPublicKey.asymmetricKeyType, 'rsa');
+  assert.strictEqual(derivedPublicKey.symmetricKeySize, undefined);
+
   const publicDER = publicKey.export({
     format: 'der',
     type: 'pkcs1'
@@ -95,8 +119,18 @@ const privatePem = fixtures.readSync('test_rsa_privkey.pem', 'ascii');
 
   const plaintext = Buffer.from('Hello world', 'utf8');
   const ciphertexts = [
+    // Encrypt using the public key.
     publicEncrypt(publicKey, plaintext),
     publicEncrypt({ key: publicKey }, plaintext),
+
+    // Encrypt using the private key.
+    publicEncrypt(privateKey, plaintext),
+    publicEncrypt({ key: privateKey }, plaintext),
+
+    // Encrypt using a public key derived from the private key.
+    publicEncrypt(derivedPublicKey, plaintext),
+    publicEncrypt({ key: derivedPublicKey }, plaintext),
+
     // Test distinguishing PKCS#1 public and private keys based on the
     // DER-encoded data only.
     publicEncrypt({ format: 'der', type: 'pkcs1', key: publicDER }, plaintext),