diff --git a/lib/bcoin/block.js b/lib/bcoin/block.js
index 2a488bb3..e11b4965 100644
--- a/lib/bcoin/block.js
+++ b/lib/bcoin/block.js
@@ -431,10 +431,26 @@ Block.prototype.getCoinbaseHeight = function getCoinbaseHeight() {
 
 Block.prototype.getReward = function getReward(network) {
   var reward = Block.reward(this.height, network);
-  var i;
+  var i, fee;
 
-  for (i = 1; i < this.txs.length; i++)
-    reward += this.txs[i].getFee();
+  for (i = 1; i < this.txs.length; i++) {
+    fee = this.txs[i].getFee();
+
+    if (fee < 0 || fee > constants.MAX_MONEY)
+      return -1;
+
+    reward += fee;
+
+    // We don't want to go above 53 bits.
+    // This is to make the getClaimed check
+    // fail if the miner mined an evil block.
+    // Note that this check ONLY works because
+    // MAX_MONEY is 51 bits. The result of
+    // (51 bits + 51 bits) is _never_ greater
+    // than 52 bits.
+    if (reward < 0 || reward > constants.MAX_MONEY)
+      return -1;
+  }
 
   return reward;
 };
@@ -459,6 +475,8 @@ Block.prototype.getClaimed = function getClaimed() {
 Block.reward = function _reward(height, network) {
   var halvings, reward;
 
+  assert(height !== -1, 'Height is negative.');
+
   network = bcoin.network.get(network);
   halvings = height / network.halvingInterval | 0;
 
diff --git a/lib/bcoin/utils.js b/lib/bcoin/utils.js
index aabebad8..bab004c9 100644
--- a/lib/bcoin/utils.js
+++ b/lib/bcoin/utils.js
@@ -633,23 +633,16 @@ utils.assert.fatal = function fatal(value, message) {
 };
 
 /**
- * One bitcoin in satoshis.
- * @const {BN}
- * @default
- */
-
-utils.COIN = 100000000;
-
-/**
- * Convert satoshis to a BTC string. Note that
- * BTC strings _always_ have a decimal point.
- * @param {BN|Number} satoshi
+ * Convert satoshis to a BTC string.
+ * This function explicitly avoids
+ * any floating point arithmetic.
+ * @param {Number} satoshi
  * @returns {String} BTC string.
  */
 
 utils.btc = function btc(satoshi) {
-  var neg = false;
-  var btc, hi, lo;
+  var negative = false;
+  var btc;
 
   if (utils.isBTC(satoshi))
     return satoshi;
@@ -658,24 +651,22 @@ utils.btc = function btc(satoshi) {
 
   if (satoshi < 0) {
     satoshi = -satoshi;
-    neg = true;
+    negative = true;
   }
 
-  hi = Math.floor(satoshi / utils.COIN).toString(10);
-  lo = (satoshi % utils.COIN).toString(10);
+  btc = satoshi.toString(10);
 
-  while (lo.length < 8)
-    lo = '0' + lo;
+  while (btc.length < 9)
+    btc = '0' + btc;
 
-  lo = '.' + lo;
+  btc = btc.slice(0, -8) + '.' + btc.slice(-8);
 
-  lo = lo.replace(/0+$/, '');
-  if (lo === '.')
-    lo += '0';
+  btc = btc.replace(/0+$/, '');
 
-  btc = hi + lo;
+  if (btc[btc.length - 1] === '.')
+    btc = btc.slice(0, -1);
 
-  if (neg)
+  if (negative)
     btc = '-' + btc;
 
   return btc;
@@ -683,12 +674,15 @@ utils.btc = function btc(satoshi) {
 
 /**
  * Convert BTC string to satoshis.
- * @param {String|Number} btc
- * @returns {BN} Satoshis.
+ * This function explicitly avoids
+ * any floating point arithmetic.
+ * @param {String} btc
+ * @returns {Number} Satoshis.
+ * @throws on parse error
  */
 
 utils.satoshi = function satoshi(btc) {
-  var neg = false;
+  var negative = false;
   var satoshi, parts, hi, lo;
 
   if (utils.isSatoshi(btc))
@@ -697,24 +691,31 @@ utils.satoshi = function satoshi(btc) {
   assert(utils.isBTC(btc), 'Non-BTC value for conversion.');
 
   if (btc[0] === '-') {
-    neg = true;
+    negative = true;
     btc = btc.substring(1);
   }
 
   parts = btc.split('.');
+
+  assert(parts.length <= 2, 'Bad decimal point.');
+
   hi = parts[0] || '0';
   lo = parts[1] || '0';
 
+  hi = hi.replace(/^0+/, '');
+  lo = lo.replace(/0+$/, '');
+
+  assert(hi.length <= 8, 'Number exceeds MAX_MONEY.');
+  assert(+hi < 21000000, 'Number exceeds MAX_MONEY.');
+
+  assert(lo.length <= 8, 'Too many decimal places.');
+
   while (lo.length < 8)
     lo += '0';
 
-  assert(lo.length === 8);
+  satoshi = parseInt(hi + lo, 10);
 
-  satoshi = (hi + lo).replace(/^0+/, '');
-
-  satoshi = parseInt(satoshi, 10);
-
-  if (neg)
+  if (negative)
     satoshi = -satoshi;
 
   return satoshi;
@@ -747,7 +748,10 @@ utils.isSatoshi = function isSatoshi(val) {
  */
 
 utils.isBTC = function isBTC(val) {
-  return typeof val === 'string' && /^-?\d+\.\d+$/.test(val);
+  return typeof val === 'string'
+    && /^-?(\d+)?(?:\.\d*)?$/.test(val)
+    && val.length !== 0
+    && val !== '-';
 };
 
 /**
@@ -1083,7 +1087,7 @@ utils.uniq = function uniq(obj) {
 };
 
 /**
- * Convert a mantissa/compact number to a big number.
+ * Convert a compact number to a big number.
  * Used for `block.bits` -> `target` conversion.
  * @param {Number} compact
  * @returns {BN}
@@ -1115,7 +1119,7 @@ utils.fromCompact = function fromCompact(compact) {
 };
 
 /**
- * Convert a big number to a mantissa/compact number.
+ * Convert a big number to a compact number.
  * Used for `target` -> `block.bits` conversion.
  * @param {BN} num
  * @returns {Number}
@@ -1148,8 +1152,7 @@ utils.toCompact = function toCompact(num) {
   if (num.isNeg())
     compact |= 0x800000;
 
-  if (compact < 0)
-    compact += 0x100000000;
+  compact >>>= 0;
 
   return compact;
 };
@@ -1589,7 +1592,7 @@ utils.MAX_SAFE_INTEGER = 0x1fffffffffffff;
  */
 
 utils.write64N = function write64N(dst, num, off, be) {
-  var neg, hi, lo;
+  var negative, hi, lo;
 
   assert(typeof num === 'number');
 
@@ -1597,18 +1600,17 @@ utils.write64N = function write64N(dst, num, off, be) {
 
   assert(num <= utils.MAX_SAFE_INTEGER, 'Number exceeds 2^53-1');
 
-  if (num < 0)
-    neg = true;
+  negative = num < 0;
 
-  num = num < 0 ? -num : num;
-
-  if (neg)
-    num--;
+  if (negative) {
+    num = -num;
+    num -= 1;
+  }
 
   hi = num / 0x100000000 | 0;
   lo = num % 0x100000000;
 
-  if (neg) {
+  if (negative) {
     hi = ~hi >>> 0;
     lo = ~lo >>> 0;
   }
diff --git a/test/block-test.js b/test/block-test.js
index 69291caf..9e4e62da 100644
--- a/test/block-test.js
+++ b/test/block-test.js
@@ -52,8 +52,24 @@ describe('Block', function() {
   it('should be jsonified and unjsonified and still verify', function() {
     var json = block.toRaw();
     var b = bcoin.merkleblock.fromRaw(json);
-    // FIXME
-    //assert.equal(b.render(), json);
+    assert.deepEqual(b.render(), json);
     assert(b.verify());
   });
+
+  it('should calculate reward properly', function() {
+    var height = 0;
+    var total = 0;
+    var reward;
+
+    for (;;) {
+      reward = bcoin.block.reward(height);
+      total += reward;
+      if (reward === 0)
+        break;
+      height++;
+    }
+
+    assert.equal(height, 6930000);
+    assert.equal(total, 2099999997690000);
+  });
 });
diff --git a/test/tx-test.js b/test/tx-test.js
index 3a4c2b5f..4cdb7c10 100644
--- a/test/tx-test.js
+++ b/test/tx-test.js
@@ -269,4 +269,178 @@ describe('TX', function() {
       });
     });
   });
+
+  function createInput(value) {
+    var hash = bcoin.ec.random(32).toString('hex');
+    return {
+      prevout: {
+        hash: hash,
+        index: 0
+      },
+      coin: {
+        version: 1,
+        height: 0,
+        value: value,
+        script: [],
+        coinbase: false,
+        hash: hash,
+        index: 0
+      },
+      script: [],
+      witness: [],
+      sequence: 0xffffffff
+    };
+  }
+
+  it('should fail on >51 bit coin values', function () {
+    var tx = bcoin.tx({
+      version: 1,
+      inputs: [createInput(constants.MAX_MONEY + 1)],
+      outputs: [{
+        script: [],
+        value: constants.MAX_MONEY
+      }],
+      locktime: 0
+    });
+    assert.ok(tx.isSane());
+    assert.ok(!tx.checkInputs(0));
+  });
+
+  it('should handle 51 bit coin values', function () {
+    var tx = bcoin.tx({
+      version: 1,
+      inputs: [createInput(constants.MAX_MONEY)],
+      outputs: [{
+        script: [],
+        value: constants.MAX_MONEY
+      }],
+      locktime: 0
+    });
+    assert.ok(tx.isSane());
+    assert.ok(tx.checkInputs(0));
+  });
+
+  it('should fail on >51 bit output values', function () {
+    var tx = bcoin.tx({
+      version: 1,
+      inputs: [createInput(constants.MAX_MONEY)],
+      outputs: [{
+        script: [],
+        value: constants.MAX_MONEY + 1
+      }],
+      locktime: 0
+    });
+    assert.ok(!tx.isSane());
+    assert.ok(!tx.checkInputs(0));
+  });
+
+  it('should handle 51 bit output values', function () {
+    var tx = bcoin.tx({
+      version: 1,
+      inputs: [createInput(constants.MAX_MONEY)],
+      outputs: [{
+        script: [],
+        value: constants.MAX_MONEY
+      }],
+      locktime: 0
+    });
+    assert.ok(tx.isSane());
+    assert.ok(tx.checkInputs(0));
+  });
+
+  it('should fail on >51 bit fees', function () {
+    var tx = bcoin.tx({
+      version: 1,
+      inputs: [createInput(constants.MAX_MONEY + 1)],
+      outputs: [{
+        script: [],
+        value: 0
+      }],
+      locktime: 0
+    });
+    assert.ok(tx.isSane());
+    assert.ok(!tx.checkInputs(0));
+  });
+
+  it('should fail on >51 bit values from multiple', function () {
+    var tx = bcoin.tx({
+      version: 1,
+      inputs: [
+        createInput(Math.floor(constants.MAX_MONEY / 2)),
+        createInput(Math.floor(constants.MAX_MONEY / 2)),
+        createInput(Math.floor(constants.MAX_MONEY / 2))
+      ],
+      outputs: [{
+        script: [],
+        value: constants.MAX_MONEY
+      }],
+      locktime: 0
+    });
+    assert.ok(tx.isSane());
+    assert.ok(!tx.checkInputs(0));
+  });
+
+  it('should fail on >51 bit fees from multiple', function () {
+    var tx = bcoin.tx({
+      version: 1,
+      inputs: [
+        createInput(Math.floor(constants.MAX_MONEY / 2)),
+        createInput(Math.floor(constants.MAX_MONEY / 2)),
+        createInput(Math.floor(constants.MAX_MONEY / 2))
+      ],
+      outputs: [{
+        script: [],
+        value: 0
+      }],
+      locktime: 0
+    });
+    assert.ok(tx.isSane());
+    assert.ok(!tx.checkInputs(0));
+  });
+
+  it('should fail on >51 bit fees from multiple txs', function () {
+    var data = utils.merge(bcoin.network.get().genesis, { height: 0 });
+    var block = new bcoin.block(data);
+    for (var i = 0; i < 3; i++) {
+      var tx = bcoin.tx({
+        version: 1,
+        inputs: [
+          createInput(Math.floor(constants.MAX_MONEY / 2))
+        ],
+        outputs: [{
+          script: [],
+          value: 0
+        }],
+        locktime: 0
+      });
+      block.txs.push(tx);
+    }
+    assert.equal(block.getReward(), -1);
+  });
+
+  it('should fail to parse >53 bit values', function () {
+    var tx = bcoin.tx({
+      version: 1,
+      inputs: [
+        createInput(Math.floor(constants.MAX_MONEY / 2))
+      ],
+      outputs: [{
+        script: [],
+        value: 0
+      }],
+      locktime: 0
+    });
+    tx.outputs[0].value = new bn('00ffffffffffffff', 'hex');
+    assert(tx.outputs[0].value.bitLength() === 56);
+    var raw = tx.toRaw()
+    assert.throws(function() {
+      tx.fromRaw(raw);
+    });
+    tx.outputs[0].value = new bn('00ffffffffffffff', 'hex').ineg();
+    assert(tx.outputs[0].value.bitLength() === 56);
+    var raw = tx.toRaw()
+    assert.throws(function() {
+      tx.fromRaw(raw);
+    });
+  });
 });
diff --git a/test/utils-test.js b/test/utils-test.js
index 5a22c06a..e2db19cf 100644
--- a/test/utils-test.js
+++ b/test/utils-test.js
@@ -27,7 +27,7 @@ describe('Utils', function() {
     btc = utils.btc(54678 * 1000000);
     assert.equal(btc, '546.78');
     btc = utils.btc(5460 * 10000000);
-    assert.equal(btc, '546.0');
+    assert.equal(btc, '546');
   });
 
   it('should convert btc to satoshi', function() {
@@ -35,7 +35,76 @@ describe('Utils', function() {
     assert(btc === 5460);
     btc = utils.satoshi('546.78');
     assert(btc === 54678 * 1000000);
-    btc = utils.satoshi('546.0');
+    btc = utils.satoshi('546');
     assert(btc === 5460 * 10000000);
   });
+
+  var unsigned = [
+    new bn('ffeeffee'),
+    new bn('001fffeeffeeffee'),
+    new bn('eeffeeff'),
+    new bn('001feeffeeffeeff'),
+    new bn(0),
+    new bn(1)
+  ];
+
+  var signed = [
+    new bn('ffeeffee'),
+    new bn('001fffeeffeeffee'),
+    new bn('eeffeeff'),
+    new bn('001feeffeeffeeff'),
+    new bn(0),
+    new bn(1),
+    new bn('ffeeffee').ineg(),
+    new bn('001fffeeffeeffee').ineg(),
+    new bn('eeffeeff').ineg(),
+    new bn('001feeffeeffeeff').ineg(),
+    new bn(0).ineg(),
+    new bn(1).ineg()
+  ];
+
+  unsigned.forEach(function(num) {
+    var buf1 = new Buffer(8);
+    var buf2 = new Buffer(8);
+    var msg = 'should write+read a ' + num.bitLength() + ' bit unsigned int';
+    it(msg, function() {
+      utils.writeU64(buf1, num, 0);
+      utils.writeU64N(buf2, num.toNumber(), 0);
+      assert.deepEqual(buf1, buf2);
+      var n1 = utils.readU64(buf1, 0);
+      var n2 = utils.readU64N(buf2, 0);
+      assert.equal(n1.toNumber(), n2);
+    });
+  });
+
+  signed.forEach(function(num) {
+    var buf1 = new Buffer(8);
+    var buf2 = new Buffer(8);
+    var msg = 'should write+read a ' + num.bitLength()
+      + ' bit ' + (num.isNeg() ? 'negative' : 'positive') + ' int';
+    it(msg, function() {
+      utils.write64(buf1, num, 0);
+      utils.write64N(buf2, num.toNumber(), 0);
+      assert.deepEqual(buf1, buf2);
+      var n1 = utils.read64(buf1, 0);
+      var n2 = utils.read64N(buf2, 0);
+      assert.equal(n1.toNumber(), n2);
+    });
+    var msg = 'should write+read a ' + num.bitLength()
+      + ' bit ' + (num.isNeg() ? 'negative' : 'positive') + ' int as unsigned';
+    it(msg, function() {
+      utils.writeU64(buf1, num, 0);
+      utils.writeU64N(buf2, num.toNumber(), 0);
+      assert.deepEqual(buf1, buf2);
+      var n1 = utils.readU64(buf1, 0);
+      if (num.isNeg()) {
+        assert.throws(function() {
+          utils.readU64N(buf2, 0);
+        });
+      } else {
+        var n2 = utils.readU64N(buf2, 0);
+        assert.equal(n1.toNumber(), n2);
+      }
+    });
+  });
 });