diff --git a/lib/period.js b/lib/period.js
index f4c99b7..d9c25c2 100644
--- a/lib/period.js
+++ b/lib/period.js
@@ -64,6 +64,10 @@ export default class Period {
     }
   }
 
+  usePrevPeriod() {
+    this.usePrev = true;
+  }
+
   // helpful: https://docs.google.com/drawings/d/13oFjua-v_E_yaFYUbralluI-EysgtTaZb_sSgGbxG_A
   periodData() {
     if (typeof this.slotId === 'undefined' || !this.ownerAddr) {
@@ -85,7 +89,7 @@ export default class Period {
       
     // make metaRoot
     let metaRoot = Buffer.alloc(32, 0);
-    if (this.prevHash) {
+    if (this.usePrev) {
       rootBuf = Buffer.alloc(64, 0);
       toBuffer(this.prevHash).copy(rootBuf, 32);
       metaRoot = keccak256(rootBuf);
@@ -115,6 +119,44 @@ export default class Period {
     return periodRoot;
   }
 
+  prevPeriodProof() {
+    if (!this.usePrev) {
+      throw Error('not set to use prev period in proofs');
+    }
+
+    const { casRoot,  metaRoot } = this.periodData();
+
+    const proof = [];
+
+    // fees hash
+    proof.push(bufferToHex(Buffer.alloc(32, 0)));
+    proof.push(this.prevHash);
+    proof.push(this.merkleRoot());
+    proof.push(bufferToHex(casRoot));
+
+    return proof;
+  }
+
+  // returns current period
+  static verifyPrevPeriodProof(proof) {
+    let result;
+    result = Buffer.alloc(64, 0);
+    // fees hash
+    toBuffer(proof[0]).copy(result);
+    // prevPeriod
+    toBuffer(proof[1]).copy(result, 32);
+    const metaRoot = keccak256(result);
+    result = Buffer.alloc(64, 0);
+    //blocks root
+    toBuffer(proof[2]).copy(result);
+    metaRoot.copy(result, 32);
+    const consensusRoot = keccak256(result);
+    result = Buffer.alloc(64, 0);
+    toBuffer(consensusRoot).copy(result);
+    toBuffer(proof[3]).copy(result, 32);
+    return bufferToHex(keccak256(result));  
+  }
+
   proof(tx) {
     let periodPos = -1;
     for (let i = 0; i < this.blockList.length; i++) {
diff --git a/lib/period.spec.js b/lib/period.spec.js
index 826e89b..b6c26e0 100644
--- a/lib/period.spec.js
+++ b/lib/period.spec.js
@@ -255,38 +255,21 @@ describe('periods', () => {
 
     const blocks = [];
     let block;
-    for (let i = 0; i < 31; i++) {
-      if (i % 2) {
-        block = new Block(i).addTx(Tx.deposit(i, value, ADDR, color));
-        blocks.push(block);
-      } else {
-        blocks.push(new Block(i));
-      }
+    for (let i = 0; i < 30; i++) {
+      block = new Block(i).addTx(Tx.deposit(i, value, ADDR, color));
+      blocks.push(block);
     }
     block = new Block(31).addTx(transfer);
     blocks.push(block);
 
-    const period = new Period("'0x8b04de057fe524a3118eb7c8e14a2e55323c67fd7b6080583d1047b700b2d674'", blocks);
+    const period = new Period("0x8b04de057fe524a3118eb7c8e14a2e55323c67fd7b6080583d1047b700b2d674", blocks);
     period.setValidatorData(slotId, ADDR);
-    const proof = period.proof(transfer);
-    expect(proof).to.eql([
-      '0xec7034f5cf812842edc14044fa453c60958c90063a31b623cf44f50402c14938',
-      '0x300800d0000000000000003e0000000003127777777777777777777777777777',
-      '0x77777777777777777777777777777777777700c2b5d5b0953f40dd9bbc5e534f',
-      '0x2db3d4847d897b6615103a157bd69c640490c07945dc613cd500f12a009aad63',
-      '0x1c530264507c1f5a8591ddc053bd5dc87bcf291b000000000000000000000000',
-      '0x0000000000000000000000000000000002f34f60053982e8c6cf42c8d1ff9594',
-      '0xb17a3f50e94a12cc860f00000000000000000000000000000000000000000000',
-      '0x00000000000002f34f60053982e8c6cf42c8d1ff9594b17a3f50e94a12cc860f',
-      '0x0000000000000000000000000000000000000000000000000000000000000000',
-      '0x0000000000000000000000000000000000000000000000000000000000000000',
-      '0x7d63e2113d90d4316fd6fd3f25837061858b8401ac1331295e922a21fe5e9579',
-      '0x04fc4e57a69f3fc7b76e12212f843a49837b36dd7280539e52b12f303d1355c4',
-      '0x3d5087be87669128a31314d9a3cf9d52af2e6dc96132eef574f84ed4ba260b2c',
-      '0x7e85398f3d5b052c378cc3d2a15209a7e8fbaf2a2b65349f80f5faf033ddf172',
-      '0x4f23fef08ddaf966bed1b1a57541da6c5a6779fbb51c2d70e4674397ae33712f',
-      '0x03bfe7f7bc5ba5d5119009b30c5eb918b6ab5050af363b4c3844ec5565e4a604'
-    ]);
+    const periodAfter = new Period(period.periodRoot(), blocks);
+    periodAfter.setValidatorData(slotId, ADDR);
+    periodAfter.usePrevPeriod();
+    
+    const proof = periodAfter.prevPeriodProof();
+    expect(Period.verifyPrevPeriodProof(proof)).to.eql(periodAfter.periodRoot());
     done();
   });