Add isSignatureValid method and tests for it

package.json

@@ -46,5 +46,8 @@
     "typedoc": "^0.7.1",
     "typescript": "^2.3.3",
     "webpack": "^2.6.0"
+  },
+  "dependencies": {
+    "ethereumjs-util": "^5.1.1"
   }
 }

src/ts/0x.js.ts

@@ -1,6 +1,32 @@
+import * as ethUtil from 'ethereumjs-util';
+
+/**
+ * Elliptic Curve signature
+ */
+export interface ECSignature {
+    v: number;
+    r: string;
+    s: string;
+}
+
+export type ETHAddress = string;
+
 export class ZeroEx {
-    /** Verifies the signature */
-    public verifySignature() {
-        // TODO
+    /**
+     * Checks if the signature is valid
+     */
+    public static isValidSignature(data: string, signature: ECSignature, signer: ETHAddress): boolean {
+        const dataBuff = ethUtil.toBuffer(data);
+        const msgHashBuff = ethUtil.hashPersonalMessage(dataBuff);
+        try {
+            const pubKey = ethUtil.ecrecover(msgHashBuff,
+                signature.v,
+                ethUtil.toBuffer(signature.r),
+                ethUtil.toBuffer(signature.s));
+            const retrievedAddress = ethUtil.bufferToHex(ethUtil.pubToAddress(pubKey));
+            return retrievedAddress === signer;
+        } catch (err) {
+            return false;
+        }
     }
 }

src/ts/globals.d.ts

@@ -0,0 +1 @@
+declare module 'ethereumjs-util';

test/0x.js.ts

@@ -3,10 +3,52 @@ import {expect} from 'chai';
 import 'mocha';
 
 describe('ZeroEx library', () => {
-    describe('#verifySignature', () => {
-        it('should return undefined', () => {
-            const zeroEx = new ZeroEx();
-            expect(zeroEx.verifySignature()).to.be.undefined;
+    describe('#isValidSignature', () => {
+        const data = '0xdeadbeaf';
+        const signature = {
+            v: 27,
+            r: '0xa3f20717a250c2b0b729b7e5becbff67fdaef7e0699da4de7ca5895b02a170a1',
+            s: '0x2d887fd3b17bfdce3481f10bea41f45ba9f709d39ce8325427b57afcfc994cee',
+        };
+        const address = '0x9b2055d370f73ec7d8a03e965129118dc8f5bf83';
+        describe('should return false for malformed signature', () => {
+            it('malformed v', () => {
+                const malformedSignature = {
+                    v: 34,
+                    r: signature.r,
+                    s: signature.s,
+                };
+                const isValid = ZeroEx.isValidSignature(data, malformedSignature, address);
+                expect(isValid).to.be.false;
+            });
+            it('malformed r', () => {
+                const malformedSignature = {
+                    v: signature.v,
+                    r: signature.r.replace('0x', ''),
+                    s: signature.s,
+                };
+                const isValid = ZeroEx.isValidSignature(data, malformedSignature, address);
+                expect(isValid).to.be.false;
+            });
+        });
+        describe('should return false for invalid signature', () => {
+            it('wrong data', () => {
+                const isValid = ZeroEx.isValidSignature('wrong data', signature, address);
+                expect(isValid).to.be.false;
+            });
+            it('wrong signer', () => {
+                const isValid = ZeroEx.isValidSignature(data, signature, '0xIamWrong');
+                expect(isValid).to.be.false;
+            });
+            it('wrong signature', () => {
+                const wrongSignature = Object.assign({}, signature, {v: 28});
+                const isValid = ZeroEx.isValidSignature(data, wrongSignature, address);
+                expect(isValid).to.be.false;
+            });
+        });
+        it('should return true for valid signature', () => {
+            const isValid = ZeroEx.isValidSignature(data, signature, address);
+            expect(isValid).to.be.true;
         });
     });
 });