Encoding/decoding negative integers and bigger integers

lib/asn1/decoders/der.js

@@ -254,6 +254,11 @@ DERNode.prototype._decodeInt = function decodeInt(buffer, values) {
   const raw = buffer.raw();
   let res = new bignum(raw);
 
+  // check if number is negative
+  if ((raw[0] & 0x80) > 0) {
+    res = res.fromTwos(raw.length * 8);
+  }
+
   if (values)
     res = values[res.toString(10)] || res;
 

lib/asn1/encoders/der.js

@@ -5,6 +5,7 @@ const Buffer = require('buffer').Buffer;
 
 const asn1 = require('../../asn1');
 const base = asn1.base;
+const bignum = asn1.bignum;
 
 // Import DER constants
 const der = asn1.constants.der;
@@ -197,10 +198,25 @@ DERNode.prototype._encodeInt = function encodeInt(num, values) {
 
   // Bignum, assume big endian
   if (typeof num !== 'number' && !Buffer.isBuffer(num)) {
-    const numArray = num.toArray();
-    if (!num.sign && numArray[0] & 0x80) {
-      numArray.unshift(0);
+    let numArray;
+    if (num.isNeg()) {
+      const bitLength = num.bitLength();
+      let width = Math.ceil(bitLength / 8) * 8;
+      // check if sign bit is occupied
+      if ((bitLength % 8) === 0) {
+        // when number is of form  10..0 no correction is needed
+        if (num.zeroBits() + 1 !== num.bitLength()) {
+          width += 8;
+        }
+      }
+      numArray = num.toTwos(width).toArray();
+    } else {
+      numArray = num.toArray();
+      if (numArray[0] & 0x80) {
+        numArray.unshift(0);
+      }
     }
+
     num = new Buffer(numArray);
   }
 
@@ -216,6 +232,12 @@ DERNode.prototype._encodeInt = function encodeInt(num, values) {
     return this._createEncoderBuffer(out);
   }
 
+  if (num < 0)
+    return this._encodeInt(new bignum(num));
+
+  if (num > 0x7FFFFFFF)
+    return this._encodeInt(new bignum(num));
+
   if (num < 0x80)
     return this._createEncoderBuffer(num);
 

test/der-decode-test.js

@@ -3,6 +3,7 @@
 
 const assert = require('assert');
 const asn1 = require('..');
+const bignum = asn1.bignum;
 
 const Buffer = require('buffer').Buffer;
 
@@ -54,6 +55,22 @@ describe('asn1.js DER decoder', function() {
     });
   }
 
+  it('should decode integers', function() {
+    const Int = asn1.define('Int', function () { this.int(); });
+    const values = [
+      ['020101', 1],
+      ['0203008011', 0x8011],
+      ['02020080', 128],
+      ['0201FF', -1],
+      ['020180', -128],
+      ['0202FF7F', -129]
+    ];
+    values.forEach( function (v) {
+      const decoded = Int.decode(new Buffer(v[0], 'hex'), 'der');
+      assert(decoded.eq(new bignum(v[1])), v[1]);
+    });
+  });
+
   test('should decode choice', function() {
     this.choice({
       apple: this.bool(),

test/der-encode-test.js

@@ -69,6 +69,32 @@ describe('asn1.js DER encoder', function() {
     this.int();
   }, 0x8011, '0203008011');
 
+  test('should encode int -1 properly', function() {
+    this.int();
+  }, -1, '0201FF');
+
+  test('should encode int -128 properly', function() {
+    this.int();
+  }, -128, '020180');
+
+  test('should encode int -129 properly', function() {
+    this.int();
+  }, -129, '0202FF7F');
+
+  test('should encode int 2147483647 properly', function() {
+    this.int();
+  }, 2147483647, '02047FFFFFFF');
+
+  test('should encode int 2147483648 properly', function() {
+    this.int();
+  }, 2147483648, '02050080000000');
+
+  test('should encode int 5000000000 properly', function() {
+    this.int();
+  }, 5000000000, '0205012A05F200');
+
+
+
   test('should omit default value in DER', function() {
     this.seq().obj(
       this.key('required').def(false).bool(),