Introduce modexp precompile (EIP101)

Author: axic

lib/index.js

@@ -8,6 +8,7 @@ const num01 = require('./precompiled/01-ecrecover.js')
 const num02 = require('./precompiled/02-sha256.js')
 const num03 = require('./precompiled/03-ripemd160.js')
 const num04 = require('./precompiled/04-identity.js')
+const num09 = require('./precompiled/09-modexp.js')
 
 module.exports = VM
 
@@ -43,6 +44,7 @@ function VM (opts = {}) {
   this._precompiled['0000000000000000000000000000000000000002'] = num02
   this._precompiled['0000000000000000000000000000000000000003'] = num03
   this._precompiled['0000000000000000000000000000000000000004'] = num04
+  this._precompiled['0000000000000000000000000000000000000009'] = num09
 
   if (this.opts.activatePrecompiles) {
     for (var i = 1; i <= 4; i++) {

lib/precompiled/09-modexp.js

@@ -0,0 +1,54 @@
+/* 
+At the address 0x0000....09, add a precompile that expects input in the following format:
+
+<length of B, as a 32-byte-padded integer> <bytes of B> <length of E, as a 32-byte-padded integer> <bytes of E> <bytes of M>
+
+This should return B**E % M, and the data should be returned in the same format as above. If M = 0, it returns zero. Gas cost GMODEXPBASE + GARITHWORD * ceil(<total length of input data> / 32) + <length of M> * <length of M> * <length of E> / GQUADDIVISOR.
+ *
+ */
+
+const utils = require('ethereumjs-util')
+const BN = utils.BN
+const error = require('../constants.js').ERROR
+
+const Gmodexpbase = 45
+const Garithword = 6
+const Gquaddivisor = 32
+
+module.exports = function (opts) {
+  var results = {}
+
+  var i = 0
+  var Blen = new BN(data.slice(i, 32)).toNumber()
+  i += 32
+  var B = new BN(data.slice(i, i + Blen))
+  i += Blen
+  var Elen = new BN(data.slice(i, i + 32)).toNumber()
+  i += 32
+  var E = new BN(data.slice(i, Elen))
+  i += Elen
+  var Mlen = data.length - i
+  var M = new BN(data.slice(i, Mlen))
+  
+  console.log('MODEXP input', Blen, B, Elen, E, Mlen, M)
+
+  result.gasUsed = new BN(Gmodexpbase).addi(Math.ceil(data.length / 32) * Garithword).addi(Blen * Mlen * Elen / Gquaddivisor)
+
+  if (opts.gasLimit.cmp(results.gasUsed) === -1) {
+    results.gasUsed = opts.gasLimit
+    results.exception = 0 // 0 means VM fail (in this case because of OOG)
+    results.exceptionError = error.OUT_OF_GAS
+    return results
+  }
+
+  // FIXME: use reduction contexts
+  var R = B.pow(E).mod(M).toArrayLike(Buffer, 'be')
+  var Rlen = new BN(R).toArrayLike(Buffer, 'be', 32)
+
+  results.return = Buffer.concat(Rlen, R)
+  results.exception = 1
+
+  console.log('MODEXP output', Rlen, R)
+
+  return results
+}