Set both key generation and ECDSA nonce to use mixed entropy

* Move random entropy functions to new package randentropy
* Add function to get n bytes entropy where up to first 32
  bytes are mixed with OS entropy sources

crypto/randentropy/rand_entropy.go

@@ -0,0 +1,82 @@
+package randentropy
+
+import (
+	crand "crypto/rand"
+	"encoding/binary"
+	"github.com/ethereum/go-ethereum/crypto/sha3"
+	"io"
+	"os"
+	"strings"
+	"time"
+)
+
+type RandEntropy struct {
+}
+
+func (*RandEntropy) Read(bytes []byte) (n int, err error) {
+	readBytes := GetEntropyMixed(len(bytes))
+	copy(bytes, readBytes)
+	return len(bytes), nil
+}
+
+// TODO: copied from crypto.go , move to sha3 package?
+func Sha3(data []byte) []byte {
+	d := sha3.NewKeccak256()
+	d.Write(data)
+
+	return d.Sum(nil)
+}
+
+// TODO: verify. this needs to be audited
+// we start with crypt/rand, then XOR in additional entropy from OS
+func GetEntropyMixed(n int) []byte {
+	startTime := time.Now().UnixNano()
+	// for each source, we take SHA3 of the source and use it as seed to math/rand
+	// then read bytes from it and XOR them onto the bytes read from crypto/rand
+	mainBuff := GetEntropyCSPRNG(n)
+	// 1. OS entropy sources
+	startTimeBytes := make([]byte, 32)
+	binary.PutVarint(startTimeBytes, startTime)
+	startTimeHash := Sha3(startTimeBytes)
+	mixBytes(mainBuff, startTimeHash)
+
+	pid := os.Getpid()
+	pidBytes := make([]byte, 32)
+	binary.PutUvarint(pidBytes, uint64(pid))
+	pidHash := Sha3(pidBytes)
+	mixBytes(mainBuff, pidHash)
+
+	osEnv := os.Environ()
+	osEnvBytes := []byte(strings.Join(osEnv, ""))
+	osEnvHash := Sha3(osEnvBytes)
+	mixBytes(mainBuff, osEnvHash)
+
+	// not all OS have hostname in env variables
+	osHostName, err := os.Hostname()
+	if err != nil {
+		osHostNameBytes := []byte(osHostName)
+		osHostNameHash := Sha3(osHostNameBytes)
+		mixBytes(mainBuff, osHostNameHash)
+	}
+	return mainBuff
+}
+
+func GetEntropyCSPRNG(n int) []byte {
+	mainBuff := make([]byte, n)
+	_, err := io.ReadFull(crand.Reader, mainBuff)
+	if err != nil {
+		panic("reading from crypto/rand failed: " + err.Error())
+	}
+	return mainBuff
+}
+
+func mixBytes(buff []byte, mixBuff []byte) []byte {
+	bytesToMix := len(buff)
+	if bytesToMix > 32 {
+		bytesToMix = 32
+	}
+	for i := 0; i < bytesToMix; i++ {
+		buff[i] ^= mixBuff[i]
+	}
+	return buff
+}