Fix ElGamal param range and PKCS1 decoding (#1169)

* Fix ElGamal sampling range

* Stricter PKCS1 decoding

src/crypto/crypto.js

@@ -133,8 +133,10 @@ export default {
         const c2 = data_params[1].toBN();
         const p = key_params[0].toBN();
         const x = key_params[3].toBN();
-        const result = new type_mpi(await publicKey.elgamal.decrypt(c1, c2, p, x));
+        const result = new type_mpi(await publicKey.elgamal.decrypt(c1, c2, p, x)); // MPI and BN.js discard any leading zeros
-        return pkcs1.eme.decode(result.toString());
+        return pkcs1.eme.decode(
+          util.Uint8Array_to_str(result.toUint8Array('be', p.byteLength())) // re-introduce leading zeros
+        );
       }
       case enums.publicKey.ecdh: {
         const oid = key_params[0];

src/crypto/pkcs1.js

@@ -106,10 +106,6 @@ eme.encode = async function(M, k) {
  * @returns {String} message, an octet string
  */
 eme.decode = function(EM) {
-  // leading zeros truncated by bn.js
-  if (EM.charCodeAt(0) !== 0) {
-    EM = String.fromCharCode(0) + EM;
-  }
   const firstOct = EM.charCodeAt(0);
   const secondOct = EM.charCodeAt(1);
   let i = 2;

src/crypto/public_key/elgamal.js

@@ -25,8 +25,6 @@
 import BN from 'bn.js';
 import random from '../random';
 
-const zero = new BN(0);
-
 export default {
   /**
    * ElGamal Encryption function
@@ -42,8 +40,9 @@ export default {
     const mred = m.toRed(redp);
     const gred = g.toRed(redp);
     const yred = y.toRed(redp);
-    // See Section 11.5 here: https://crypto.stanford.edu/~dabo/cryptobook/BonehShoup_0_4.pdf
+    // OpenPGP uses a "special" version of ElGamal where g is generator of the full group Z/pZ*
-    const k = await random.getRandomBN(zero, p); // returns in [0, p-1]
+    // hence g has order p-1, and to avoid that k = 0 mod p-1, we need to pick k in [1, p-2]
+    const k = await random.getRandomBN(new BN(1), p.subn(1));
     return {
       c1: gred.redPow(k).fromRed(),
       c2: yred.redPow(k).redMul(mred).fromRed()

src/crypto/public_key/rsa.js

@@ -501,7 +501,11 @@ export default {
       });
       key = { key: pem, padding: nodeCrypto.constants.RSA_PKCS1_PADDING };
     }
-    return util.Uint8Array_to_str(nodeCrypto.privateDecrypt(key, data));
+    try {
+      return util.Uint8Array_to_str(nodeCrypto.privateDecrypt(key, data));
+    } catch (err) {
+      throw new Error('Decryption error');
+    }
   },
 
   bnDecrypt: async function(data, n, e, d, p, q, u) {
@@ -540,7 +544,8 @@ export default {
       result = result.redMul(unblinder);
     }
 
-    return pkcs1.eme.decode((new type_mpi(result)).toString());
+    result = new type_mpi(result).toUint8Array('be', n.byteLength()); // preserve leading zeros
+    return pkcs1.eme.decode(util.Uint8Array_to_str(result));
   },
 
   prime: prime

test/general/openpgp.js

@@ -374,6 +374,69 @@ EQr2Mx42THr260IFYp5E/rIA
 =oA0b
 -----END PGP PRIVATE KEY BLOCK-----`;
 
+const rsaPrivateKeyPKCS1 = `-----BEGIN PGP PRIVATE KEY BLOCK-----
+
+xcLYBF7yFJcBCACv2ad3tpfA8agLV+7ZO+7vWAS8f4CgCLsW2fvyIG0X3to9
+O9c+iKFk4QgfOhwb58JKSJpZtbZRyxFODCK8XqZEeONdlyXjXOKTCwb9G0qz
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+q0CQr/3PZ6elz0w6WyY2Rn8N7hC+GOYyKmiVoMLiM2+fodSiQ2YH79Nn8QrG
+YmdrQm9VEmPk8+ypDgulsoVAcP3nAshXuBVcT1QKCw8FKcoNlE1pbJR0DBjQ
+tKdNLmJdGCAtQunn8zqciCsilqH9JJ+gA0ZVLPMlodoKCxdN3PlM30ZJABEB
+AAEAB/kBdF+NL5Ktko2+S6gm64QqsRRZxxZKFN+URVQFMKuunsMv3J56Li9a
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+v8LqdOR/dKuuyt2oRQL0c3y5FkTR2OCp2UGqnzMbEdGS1c6hTL8IV3+xo6Cj
+/77XeeO2KiLKTzog6FORunPbqdh5USIQ92pO2iSTx20v+82dOQeHwaJJHrwF
+5nd3llJn/thisTvYDwwg5YoK0n93hvgebUwWuUTsCuAA1K0lqwW3NS0agLf2
+IMq6OV/eCedB8bF4bqoU+zGdGh+XwJkoYVVF6DtG+gIcceHUjC0eXHw=
+=dSNv
+-----END PGP PRIVATE KEY BLOCK-----
+`;
+
+
 function withCompression(tests) {
   const compressionTypes = Object.keys(openpgp.enums.compression).map(k => openpgp.enums.compression[k]);
 
@@ -2464,6 +2527,52 @@ describe('OpenPGP.js public api tests', function() {
         await expect(openpgp.decrypt(decOpt)).to.be.rejectedWith(/Session key decryption failed/);
       });
 
+      it('RSA decryption with PKCS1 padding of wrong length should fail', async function() {
+        const key = (await openpgp.key.readArmored(rsaPrivateKeyPKCS1)).keys[0];
+        // the paddings of these messages are prefixed by 0x02 and 0x000002 instead of 0x0002
+        // the code should discriminate between these cases by checking the length of the padded plaintext
+        const padding02 = `-----BEGIN PGP MESSAGE-----
+Version: OpenPGP.js VERSION
+Comment: https://openpgpjs.org
+
+wcBMAxbpoSTRSSl3AQf/fepDhqeam4Ecy8GUFChc47U3hbkdgINobI9TORAf
+eGFZVcyTQKVIt7fB8bwQwjxRmU98xCjF7VkLhPQJkzKlkT9cIDBKswU+d3fw
+lHAVYo77yUkFkVLXrQTZj/OjsA12V7lfRagO375XB3EpJUHVPvYQFFr3aSlo
+FbsCrpZoS6FXxRYVjGpIeMjam3a7qDavQpKhjOQ+Sfm0tk2JZkQwpFom6x7c
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+J9I8AcH94nE77JUtCm7s1kOlo0EIshZsAqJwGveDGdAuabfViVwVxG4I24M6
+8sqJYJd9FpNjSbYlrLT0R9zy
+=+n/4
+-----END PGP MESSAGE-----`;
+        const padding000002 = `-----BEGIN PGP MESSAGE-----
+Version: OpenPGP.js VERSION
+Comment: https://openpgpjs.org
+
+wcBMAxbpoSTRSSl3AQf/fepDhqeam4Ecy8GUFChc47U3hbkdgINobI9TORAf
+eGFZVcyTQKVIt7fB8bwQwjxRmU98xCjF7VkLhPQJkzKlkT9cIDBKswU+d3fw
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+8sqJYJd9FpNjSbYlrLT0R9zy
+=+n/4
+-----END PGP MESSAGE-----`;
+
+        const decOpt02 = {
+          message: await openpgp.message.readArmored(padding02),
+          privateKeys: key
+        };
+        await expect(openpgp.decrypt(decOpt02)).to.be.rejectedWith(/Decryption error/);
+
+        const decOpt000002 = {
+          message: await openpgp.message.readArmored(padding000002),
+          privateKeys: key
+        };
+        await expect(openpgp.decrypt(decOpt000002)).to.be.rejectedWith(/Decryption error/);
+      });
+
       it('should decrypt with two passwords message which GPG fails on', async function() {
         const decOpt = {
           message: await openpgp.message.readArmored(twoPasswordGPGFail),