diff --git a/src/transaction_builder.js b/src/transaction_builder.js
index 259fa6b..0c02ce0 100644
--- a/src/transaction_builder.js
+++ b/src/transaction_builder.js
@@ -12,154 +12,106 @@ var ECPair = require('./ecpair')
 var ECSignature = require('./ecsignature')
 var Transaction = require('./transaction')
 
-// re-orders signatures to match pubKeys, fills undefined otherwise
-function fixMSSignatures (transaction, vin, pubKeys, signatures, prevOutScript, hashType, skipPubKey) {
-  // maintain a local copy of unmatched signatures
-  var unmatched = signatures.slice()
-  var signatureHash
-
-  return pubKeys.map(function (pubKey) {
-    // skip optionally provided pubKey
-    if (skipPubKey && bufferEquals(skipPubKey, pubKey)) return undefined
-
-    var matched
-    var keyPair2 = ECPair.fromPublicKeyBuffer(pubKey)
-
-    // check for a matching signature
-    unmatched.some(function (signature, i) {
-      // skip if undefined || OP_0
-      if (!signature) return false
-
-      if (!signatureHash) {
-        signatureHash = transaction.hashForSignature(vin, prevOutScript, hashType)
-      }
-      if (!keyPair2.verify(signatureHash, signature)) return false
-
-      // remove matched signature from unmatched
-      unmatched[i] = undefined
-      matched = signature
+// inspects a scriptSig w/ optional provided redeemScript and derives
+// all necessary input information as required by TransactionBuilder
+function expandInput (scriptSig, redeemScript) {
+  var scriptSigChunks = bscript.decompile(scriptSig)
+  var scriptSigType = bscript.classifyInput(scriptSigChunks, true)
 
-      return true
-    })
+  var hashType, pubKeys, signatures, prevOutScript
 
-    return matched || undefined
-  })
-}
+  switch (scriptSigType) {
+    case 'scripthash':
+      // FIXME: maybe depth limit instead, how possible is this anyway?
+      if (redeemScript) throw new Error('Recursive P2SH script')
 
-function extractInput (transaction, txIn, vin) {
-  if (txIn.script.length === 0) return {}
+      var redeemScriptSig = scriptSigChunks.slice(0, -1)
+      redeemScript = scriptSigChunks[scriptSigChunks.length - 1]
 
-  var scriptSigChunks = bscript.decompile(txIn.script)
-  var prevOutType = bscript.classifyInput(scriptSigChunks, true)
+      var result = expandInput(redeemScriptSig, redeemScript)
+      result.redeemScript = redeemScript
+      result.redeemScriptType = result.prevOutType
+      result.prevOutScript = bscript.scriptHashOutput(bcrypto.hash160(redeemScript))
+      result.prevOutType = 'scripthash'
+      return result
 
-  function processScript (scriptType, scriptSigChunks, redeemScriptChunks) {
-    // ensure chunks are decompiled
-    scriptSigChunks = bscript.decompile(scriptSigChunks)
-    redeemScriptChunks = redeemScriptChunks ? bscript.decompile(redeemScriptChunks) : undefined
-
-    var hashType, pubKeys, signatures, prevOutScript, redeemScript, redeemScriptType, result, parsed
-
-    switch (scriptType) {
-      case 'scripthash':
-        redeemScript = scriptSigChunks.slice(-1)[0]
-        scriptSigChunks = bscript.compile(scriptSigChunks.slice(0, -1))
-
-        redeemScriptType = bscript.classifyInput(scriptSigChunks, true)
-        prevOutScript = bscript.scriptHashOutput(bcrypto.hash160(redeemScript))
-
-        result = processScript(redeemScriptType, scriptSigChunks, bscript.decompile(redeemScript))
-
-        result.prevOutScript = prevOutScript
-        result.redeemScript = redeemScript
-        result.redeemScriptType = redeemScriptType
-
-        return result
-
-      case 'pubkeyhash':
-        parsed = ECSignature.parseScriptSignature(scriptSigChunks[0])
-        hashType = parsed.hashType
-        pubKeys = scriptSigChunks.slice(1)
-        signatures = [parsed.signature]
-        prevOutScript = bscript.pubKeyHashOutput(bcrypto.hash160(pubKeys[0]))
-
-        break
+    case 'pubkeyhash':
+      // if (redeemScript) throw new Error('Nonstandard... P2SH(P2PKH)')
+      var s = ECSignature.parseScriptSignature(scriptSigChunks[0])
+      hashType = s.hashType
+      pubKeys = scriptSigChunks.slice(1)
+      signatures = [s.signature]
 
-      case 'pubkey':
-        parsed = ECSignature.parseScriptSignature(scriptSigChunks[0])
-        hashType = parsed.hashType
-        signatures = [parsed.signature]
+      if (redeemScript) break
 
-        if (redeemScriptChunks) {
-          pubKeys = redeemScriptChunks.slice(0, 1)
-        }
+      prevOutScript = bscript.pubKeyHashOutput(bcrypto.hash160(pubKeys[0]))
+      break
 
-        break
+    case 'pubkey':
+      if (redeemScript) {
+        pubKeys = bscript.decompile(redeemScript).slice(0, 1)
+      }
 
-      case 'multisig':
-        signatures = scriptSigChunks.slice(1).map(function (chunk) {
-          if (chunk === ops.OP_0) return undefined
+      var ss = ECSignature.parseScriptSignature(scriptSigChunks[0])
+      hashType = ss.hashType
+      signatures = [ss.signature]
+      break
 
-          parsed = ECSignature.parseScriptSignature(chunk)
-          hashType = parsed.hashType
+    case 'multisig':
+      if (redeemScript) {
+        pubKeys = bscript.decompile(redeemScript).slice(1, -2)
+      }
 
-          return parsed.signature
-        })
+      signatures = scriptSigChunks.slice(1).map(function (chunk) {
+        if (chunk === ops.OP_0) return undefined
 
-        if (redeemScriptChunks) {
-          pubKeys = redeemScriptChunks.slice(1, -2)
+        var sss = ECSignature.parseScriptSignature(chunk)
 
-          if (pubKeys.length !== signatures.length) {
-            signatures = fixMSSignatures(transaction, vin, pubKeys, signatures, bscript.compile(redeemScriptChunks), hashType, redeemScript)
-          }
+        if (hashType !== undefined) {
+          if (sss.hashType !== hashType) throw new Error('Inconsistent hashType')
+        } else {
+          hashType = sss.hashType
         }
 
-        break
-    }
+        return sss.signature
+      })
 
-    return {
-      hashType: hashType,
-      pubKeys: pubKeys,
-      signatures: signatures,
-      prevOutScript: prevOutScript,
-      redeemScript: redeemScript,
-      redeemScriptType: redeemScriptType
-    }
+      break
   }
 
-  // Extract hashType, pubKeys, signatures and prevOutScript
-  var result = processScript(prevOutType, scriptSigChunks)
-
   return {
-    hashType: result.hashType,
-    prevOutScript: result.prevOutScript,
-    prevOutType: prevOutType,
-    pubKeys: result.pubKeys,
-    redeemScript: result.redeemScript,
-    redeemScriptType: result.redeemScriptType,
-    signatures: result.signatures
+    hashType: hashType,
+    pubKeys: pubKeys,
+    signatures: signatures,
+    prevOutScript: prevOutScript,
+    prevOutType: scriptSigType
   }
 }
 
-function extractFromOutputScript (outputScript, kpPubKey) {
-  var scriptType = bscript.classifyOutput(outputScript)
-  var outputScriptChunks = bscript.decompile(outputScript)
+function expandOutput (script, ourPubKey) {
+  typeforce(types.Buffer, script)
+
+  var scriptChunks = bscript.decompile(script)
+  var scriptType = bscript.classifyOutput(script)
+
+  var pubKeys = []
 
-  var pubKeys
   switch (scriptType) {
+    // does our hash160(pubKey) match the output scripts?
     case 'pubkeyhash':
-      var pkh1 = outputScriptChunks[2]
-      var pkh2 = bcrypto.hash160(kpPubKey)
+      if (!ourPubKey) break
 
-      if (!bufferEquals(pkh1, pkh2)) throw new Error('privateKey cannot sign for this input')
-      pubKeys = [kpPubKey]
+      var pkh1 = scriptChunks[2]
+      var pkh2 = bcrypto.hash160(ourPubKey)
+      if (bufferEquals(pkh1, pkh2)) pubKeys = [ourPubKey]
       break
 
     case 'pubkey':
-      pubKeys = outputScriptChunks.slice(0, 1)
+      pubKeys = scriptChunks.slice(0, 1)
       break
 
     case 'multisig':
-      pubKeys = outputScriptChunks.slice(1, -2)
+      pubKeys = scriptChunks.slice(1, -2)
       break
 
     default: return
@@ -167,7 +119,8 @@ function extractFromOutputScript (outputScript, kpPubKey) {
 
   return {
     pubKeys: pubKeys,
-    scriptType: scriptType
+    scriptType: scriptType,
+    signatures: pubKeys.map(function () { return undefined })
   }
 }
 
@@ -204,34 +157,47 @@ TransactionBuilder.prototype.setVersion = function (version) {
 TransactionBuilder.fromTransaction = function (transaction, network) {
   var txb = new TransactionBuilder(network)
 
-  // Copy other transaction fields
-  txb.tx.version = transaction.version
-  txb.tx.locktime = transaction.locktime
-
-  // Extract/add inputs
-  transaction.ins.forEach(function (txIn) {
-    txb.addInput(txIn.hash, txIn.index, txIn.sequence)
-  })
+  // Copy transaction fields
+  txb.setVersion(transaction.version)
+  txb.setLockTime(transaction.locktime)
 
-  // Extract/add outputs
+  // Copy outputs (done first to avoid signature invalidation)
   transaction.outs.forEach(function (txOut) {
     txb.addOutput(txOut.script, txOut.value)
   })
 
-  // Extract/add signatures
-  txb.inputs = transaction.ins.map(function (txIn, vin) {
-    // TODO: verify whether extractInput is sane with coinbase scripts
-    if (Transaction.isCoinbaseHash(txIn.hash)) {
-      throw new Error('coinbase inputs not supported')
-    }
+  // Copy inputs
+  transaction.ins.forEach(function (txIn) {
+    txb.__addInputUnsafe(txIn.hash, txIn.index, txIn.sequence, txIn.script)
+  })
+
+  // fix some things not possible through the public API
+  txb.inputs.forEach(function (input, i) {
+    // attempt to fix any multisig inputs if they exist
+    if ((input.redeemScriptType || input.prevOutType) === 'multisig') {
+      // pubKeys will only exist for 'multisig' if a redeemScript was found
+      if (!input.pubKeys || !input.signatures) return
+      if (input.pubKeys.length === input.signatures.length) return
 
-    return extractInput(transaction, txIn, vin)
+      txb.__fixMultisigOrder(transaction, i)
+    }
   })
 
   return txb
 }
 
 TransactionBuilder.prototype.addInput = function (txHash, vout, sequence, prevOutScript) {
+  // if signatures exist, adding inputs is only acceptable if SIGHASH_ANYONECANPAY is used
+  // throw if any signatures *didn't* use SIGHASH_ANYONECANPAY
+  if (!this.inputs.every(function (otherInput) {
+    // no signature
+    if (otherInput.hashType === undefined) return true
+
+    return otherInput.hashType & Transaction.SIGHASH_ANYONECANPAY
+  })) {
+    throw new Error('No, this would invalidate signatures')
+  }
+
   // is it a hex string?
   if (typeof txHash === 'string') {
     // transaction hashs's are displayed in reverse order, un-reverse it
@@ -243,49 +209,42 @@ TransactionBuilder.prototype.addInput = function (txHash, vout, sequence, prevOu
     txHash = txHash.getHash()
   }
 
-  var input = {}
-  if (prevOutScript) {
-    var prevOutScriptChunks = bscript.decompile(prevOutScript)
-    var prevOutType = bscript.classifyOutput(prevOutScriptChunks)
+  return this.__addInputUnsafe(txHash, vout, sequence, null, prevOutScript)
+}
 
-    // if we can, extract pubKey information
-    switch (prevOutType) {
-      case 'multisig':
-        input.pubKeys = prevOutScriptChunks.slice(1, -2)
-        input.signatures = input.pubKeys.map(function () { return undefined })
+TransactionBuilder.prototype.__addInputUnsafe = function (txHash, vout, sequence, scriptSig, prevOutScript) {
+  if (Transaction.isCoinbaseHash(txHash)) {
+    throw new Error('coinbase inputs not supported')
+  }
 
-        break
+  var prevTxOut = txHash.toString('hex') + ':' + vout
+  if (this.prevTxMap[prevTxOut]) throw new Error('Duplicate TxOut: ' + prevTxOut)
 
-      case 'pubkey':
-        input.pubKeys = prevOutScriptChunks.slice(0, 1)
-        input.signatures = [undefined]
+  var input = {}
 
-        break
-    }
+  // derive what we can from the scriptSig
+  if (scriptSig) {
+    input = expandInput(scriptSig)
+  }
 
-    if (prevOutType !== 'scripthash') {
-      input.scriptType = prevOutType
+  // derive what we can from the previous transactions output script
+  if (!input.prevOutScript && prevOutScript) {
+    var prevOutScriptChunks = bscript.decompile(prevOutScript)
+    var prevOutType = bscript.classifyOutput(prevOutScriptChunks)
+
+    if (!input.pubKeys && !input.signatures) {
+      var expanded = expandOutput(prevOutScript)
+      if (expanded) {
+        input.pubKeys = expanded.pubKeys
+        input.signatures = expanded.signatures
+      }
     }
 
     input.prevOutScript = prevOutScript
     input.prevOutType = prevOutType
   }
 
-  // if signatures exist, adding inputs is only acceptable if SIGHASH_ANYONECANPAY is used
-  // throw if any signatures *didn't* use SIGHASH_ANYONECANPAY
-  if (!this.inputs.every(function (otherInput) {
-    // no signature
-    if (otherInput.hashType === undefined) return true
-
-    return otherInput.hashType & Transaction.SIGHASH_ANYONECANPAY
-  })) {
-    throw new Error('No, this would invalidate signatures')
-  }
-
-  var prevTxOut = txHash.toString('hex') + ':' + vout
-  if (this.prevTxMap[prevTxOut]) throw new Error('Duplicate TxOut: ' + prevTxOut)
-
-  var vin = this.tx.addInput(txHash, vout, sequence)
+  var vin = this.tx.addInput(txHash, vout, sequence, scriptSig)
   this.inputs[vin] = input
   this.prevTxMap[prevTxOut] = vin
 
@@ -334,43 +293,56 @@ var canBuildTypes = {
   'pubkeyhash': true
 }
 
-function buildFromInputData (input, scriptType, parentType, redeemScript, allowIncomplete) {
+function buildFromInputData (input, scriptType, allowIncomplete) {
+  var signatures = input.signatures
   var scriptSig
 
   switch (scriptType) {
     case 'pubkeyhash':
-      var pkhSignature = input.signatures[0].toScriptSignature(input.hashType)
+      // remove blank signatures
+      signatures = signatures.filter(function (x) { return x })
+
+      if (signatures.length < 1) throw new Error('Not enough signatures provided')
+      if (signatures.length > 1) throw new Error('Too many signatures provided')
+
+      var pkhSignature = signatures[0].toScriptSignature(input.hashType)
       scriptSig = bscript.pubKeyHashInput(pkhSignature, input.pubKeys[0])
       break
 
     case 'pubkey':
-      var pkSignature = input.signatures[0].toScriptSignature(input.hashType)
+      // remove blank signatures
+      signatures = signatures.filter(function (x) { return x })
+
+      if (signatures.length < 1) throw new Error('Not enough signatures provided')
+      if (signatures.length > 1) throw new Error('Too many signatures provided')
+
+      var pkSignature = signatures[0].toScriptSignature(input.hashType)
       scriptSig = bscript.pubKeyInput(pkSignature)
       break
 
+    // ref https://github.com/bitcoin/bitcoin/blob/d612837814020ae832499d18e6ee5eb919a87907/src/script/sign.cpp#L232
     case 'multisig':
-      var msSignatures = input.signatures.map(function (signature) {
+      signatures = signatures.map(function (signature) {
         return signature && signature.toScriptSignature(input.hashType)
       })
 
-      // fill in blanks with OP_0
       if (allowIncomplete) {
-        for (var i = 0; i < msSignatures.length; ++i) {
-          msSignatures[i] = msSignatures[i] || ops.OP_0
+        // fill in blanks with OP_0
+        for (var i = 0; i < signatures.length; ++i) {
+          signatures[i] = signatures[i] || ops.OP_0
         }
-
-      // remove blank signatures
       } else {
-        msSignatures = msSignatures.filter(function (x) { return x })
+        // remove blank signatures
+        signatures = signatures.filter(function (x) { return x })
       }
 
-      scriptSig = bscript.multisigInput(msSignatures, allowIncomplete ? undefined : redeemScript)
+      scriptSig = bscript.multisigInput(signatures, allowIncomplete ? undefined : input.redeemScript)
       break
   }
 
   // wrap as scriptHash if necessary
-  if (parentType === 'scripthash') {
-    scriptSig = bscript.scriptHashInput(scriptSig, redeemScript)
+  if (input.prevOutType === 'scripthash') {
+    scriptSig = bscript.scriptHashInput(scriptSig, input.redeemScript)
   }
 
   return scriptSig
@@ -387,24 +359,22 @@ TransactionBuilder.prototype.__build = function (allowIncomplete) {
   // Create script signatures from inputs
   this.inputs.forEach(function (input, index) {
     var scriptType = input.redeemScriptType || input.prevOutType
-    var scriptSig
 
     if (!allowIncomplete) {
       if (!scriptType) throw new Error('Transaction is not complete')
       if (!canBuildTypes[scriptType]) throw new Error(scriptType + ' not supported')
 
-      // XXX: only relevant to types that need signatures
+      // FIXME: only relevant to types that need signatures
       if (!input.signatures) throw new Error('Transaction is missing signatures')
     }
 
-    if (input.signatures) {
-      scriptSig = buildFromInputData(input, scriptType, input.prevOutType, input.redeemScript, allowIncomplete)
-    }
+    // FIXME: only relevant to types that need signatures
+    // skip if no scriptSig exists
+    if (!input.signatures) return
 
-    // did we build a scriptSig? Buffer('') is allowed
-    if (scriptSig) {
-      tx.setInputScript(index, scriptSig)
-    }
+    // build a scriptSig
+    var scriptSig = buildFromInputData(input, scriptType, allowIncomplete)
+    tx.setInputScript(index, scriptSig)
   })
 
   return tx
@@ -416,12 +386,10 @@ TransactionBuilder.prototype.sign = function (index, keyPair, redeemScript, hash
   hashType = hashType || Transaction.SIGHASH_ALL
 
   var input = this.inputs[index]
-  var canSign = input.hashType &&
-    input.prevOutScript &&
-    input.prevOutType &&
-    input.pubKeys &&
-    input.redeemScriptType &&
-    input.signatures &&
+  var canSign = input.hashType !== undefined &&
+    input.prevOutScript !== undefined &&
+    input.pubKeys !== undefined &&
+    input.signatures !== undefined &&
     input.signatures.length === input.pubKeys.length
 
   var kpPubKey = keyPair.getPublicKeyBuffer()
@@ -437,44 +405,46 @@ TransactionBuilder.prototype.sign = function (index, keyPair, redeemScript, hash
 
   // no? prepare
   } else {
-    // must be pay-to-scriptHash?
     if (redeemScript) {
-      // if we have a prevOutScript, enforce scriptHash equality to the redeemScript
+      var redeemScriptHash = bcrypto.hash160(redeemScript)
+
+      // if redeemScript exists, it is pay-to-scriptHash
+      // if we have a prevOutScript, enforce hash160(redeemScriptequality)  to the redeemScript
       if (input.prevOutScript) {
         if (input.prevOutType !== 'scripthash') throw new Error('PrevOutScript must be P2SH')
 
-        var scriptHash = bscript.decompile(input.prevOutScript)[1]
-        if (!bufferEquals(scriptHash, bcrypto.hash160(redeemScript))) throw new Error('RedeemScript does not match ' + scriptHash.toString('hex'))
-      }
-
-      var extracted = extractFromOutputScript(redeemScript, kpPubKey)
-      if (!extracted) throw new Error('RedeemScript not supported "' + bscript.toASM(redeemScript) + '"')
+        var prevOutScriptScriptHash = bscript.decompile(input.prevOutScript)[1]
+        if (!bufferEquals(prevOutScriptScriptHash, redeemScriptHash)) throw new Error('Inconsistent hash160(RedeemScript)')
 
-      // if we don't have a prevOutScript, generate a P2SH script
-      if (!input.prevOutScript) {
-        input.prevOutScript = bscript.scriptHashOutput(bcrypto.hash160(redeemScript))
+      // or, we don't have a prevOutScript, so generate a P2SH script
+      } else {
+        input.prevOutScript = bscript.scriptHashOutput(redeemScriptHash)
         input.prevOutType = 'scripthash'
       }
 
-      input.pubKeys = extracted.pubKeys
+      var expanded = expandOutput(redeemScript, kpPubKey)
+      if (!expanded) throw new Error('RedeemScript not supported "' + bscript.toASM(redeemScript) + '"')
+
+      input.pubKeys = expanded.pubKeys
       input.redeemScript = redeemScript
-      input.redeemScriptType = extracted.scriptType
-      input.signatures = extracted.pubKeys.map(function () { return undefined })
+      input.redeemScriptType = expanded.scriptType
+      input.signatures = expanded.signatures
+
+    // no redeemScript
     } else {
       // pay-to-scriptHash is not possible without a redeemScript
       if (input.prevOutType === 'scripthash') throw new Error('PrevOutScript is P2SH, missing redeemScript')
 
-      // if we don't have a scriptType, assume pubKeyHash otherwise
-      if (!input.scriptType) {
+      // if we don't have a scriptType, assume pubKeyHash
+      if (!input.prevOutType) {
         input.prevOutScript = bscript.pubKeyHashOutput(bcrypto.hash160(kpPubKey))
         input.prevOutType = 'pubkeyhash'
 
         input.pubKeys = [kpPubKey]
-        input.scriptType = input.prevOutType
         input.signatures = [undefined]
       } else {
         // throw if we can't sign with it
-        if (!input.pubKeys || !input.signatures) throw new Error(input.scriptType + ' not supported')
+        if (!input.pubKeys || !input.signatures) throw new Error(input.prevOutType + ' not supported')
       }
     }
 
@@ -482,8 +452,8 @@ TransactionBuilder.prototype.sign = function (index, keyPair, redeemScript, hash
   }
 
   // ready to sign?
-  var signatureScript = input.redeemScript || input.prevOutScript
-  var signatureHash = this.tx.hashForSignature(index, signatureScript, hashType)
+  var hashScript = input.redeemScript || input.prevOutScript
+  var signatureHash = this.tx.hashForSignature(index, hashScript, hashType)
 
   // enforce in order signing of public keys
   var valid = input.pubKeys.some(function (pubKey, i) {
@@ -491,11 +461,45 @@ TransactionBuilder.prototype.sign = function (index, keyPair, redeemScript, hash
     if (input.signatures[i]) throw new Error('Signature already exists')
 
     input.signatures[i] = keyPair.sign(signatureHash)
-
     return true
   })
 
   if (!valid) throw new Error('Key pair cannot sign for this input')
 }
 
+TransactionBuilder.prototype.__fixMultisigOrder = function (transaction, vin) {
+  var input = this.inputs[vin]
+  var hashScriptType = input.redeemScriptType || input.prevOutType
+  if (hashScriptType !== 'multisig') throw new TypeError('Expected multisig input')
+
+  var hashType = input.hashType || Transaction.SIGHASH_ALL
+  var hashScript = input.redeemScript || input.prevOutScript
+
+  // maintain a local copy of unmatched signatures
+  var unmatched = input.signatures.concat()
+  var signatureHash = transaction.hashForSignature(vin, hashScript, hashType)
+
+  input.signatures = input.pubKeys.map(function (pubKey, y) {
+    var keyPair = ECPair.fromPublicKeyBuffer(pubKey)
+    var match
+
+    // check for a signature
+    unmatched.some(function (signature, i) {
+      // skip if undefined || OP_0
+      if (!signature) return false
+
+      // skip if signature does not match pubKey
+      if (!keyPair.verify(signatureHash, signature)) return false
+
+      // remove matched signature from unmatched
+      unmatched[i] = undefined
+      match = signature
+
+      return true
+    })
+
+    return match || undefined
+  })
+}
+
 module.exports = TransactionBuilder