Introduce entropy checking of string

This will hopefully reduce the number of false positives when it comes
to hard coded credentials. The zxcvbn library is used to calculate the
entropy of the string. By default the first 16 characters are considered
as doing the entropy check for strings much longer than that introduces
a fairly significant performance hit.

rules/hardcoded_credentials.go

@@ -15,15 +15,32 @@
 package rules
 
 import (
+	"fmt"
 	gas "github.com/GoASTScanner/gas/core"
 	"go/ast"
 	"go/token"
 	"regexp"
+
+	"github.com/nbutton23/zxcvbn-go"
+	"strconv"
 )
 
 type Credentials struct {
 	gas.MetaData
-	pattern *regexp.Regexp
+	pattern          *regexp.Regexp
+	entropyThreshold float64
+	perCharThreshold float64
+	truncate         int64
+	ignoreEntropy    bool
+}
+
+func (r *Credentials) isHighEntropyString(str string) bool {
+	s := fmt.Sprintf("%.*s", r.truncate, str)
+	info := zxcvbn.PasswordStrength(s, []string{})
+	entropyPerChar := info.Entropy / float64(len(s))
+	return (info.Entropy >= r.entropyThreshold ||
+		(info.Entropy >= (r.entropyThreshold/2) &&
+			entropyPerChar >= r.perCharThreshold))
 }
 
 func (r *Credentials) Match(n ast.Node, ctx *gas.Context) (*gas.Issue, error) {
@@ -41,8 +58,10 @@ func (r *Credentials) matchAssign(assign *ast.AssignStmt, ctx *gas.Context) (*ga
 		if ident, ok := i.(*ast.Ident); ok {
 			if r.pattern.MatchString(ident.Name) {
 				for _, e := range assign.Rhs {
-					if rhs, ok := e.(*ast.BasicLit); ok && rhs.Kind == token.STRING {
-						return gas.NewIssue(ctx, assign, r.What, r.Severity, r.Confidence), nil
+					if val, err := gas.GetString(e); err == nil {
+						if r.ignoreEntropy || (!r.ignoreEntropy && r.isHighEntropyString(val)) {
+							return gas.NewIssue(ctx, assign, r.What, r.Severity, r.Confidence), nil
+						}
 					}
 				}
 			}
@@ -75,11 +94,43 @@ func (r *Credentials) matchGenDecl(decl *ast.GenDecl, ctx *gas.Context) (*gas.Is
 
 func NewHardcodedCredentials(conf map[string]interface{}) (gas.Rule, []ast.Node) {
 	pattern := `(?i)passwd|pass|password|pwd|secret|token`
+	entropyThreshold := 80.0
+	perCharThreshold := 3.0
+	ignoreEntropy := false
+	var truncateString int64 = 16
 	if val, ok := conf["G101"]; ok {
-		pattern = val.(string)
+		conf := val.(map[string]string)
+		if configPattern, ok := conf["pattern"]; ok {
+			pattern = configPattern
+		}
+		if configIgnoreEntropy, ok := conf["ignore_entropy"]; ok {
+			if parsedBool, err := strconv.ParseBool(configIgnoreEntropy); err == nil {
+				ignoreEntropy = parsedBool
+			}
+		}
+		if configEntropyThreshold, ok := conf["entropy_threshold"]; ok {
+			if parsedNum, err := strconv.ParseFloat(configEntropyThreshold, 64); err == nil {
+				entropyThreshold = parsedNum
+			}
+		}
+		if configCharThreshold, ok := conf["per_char_threshold"]; ok {
+			if parsedNum, err := strconv.ParseFloat(configCharThreshold, 64); err == nil {
+				perCharThreshold = parsedNum
+			}
+		}
+		if configTruncate, ok := conf["truncate"]; ok {
+			if parsedInt, err := strconv.ParseInt(configTruncate, 10, 64); err == nil {
+				truncateString = parsedInt
+			}
+		}
 	}
+
 	return &Credentials{
-		pattern: regexp.MustCompile(pattern),
+		pattern:          regexp.MustCompile(pattern),
+		entropyThreshold: entropyThreshold,
+		perCharThreshold: perCharThreshold,
+		ignoreEntropy:    ignoreEntropy,
+		truncate:         truncateString,
 		MetaData: gas.MetaData{
 			What:       "Potential hardcoded credentials",
 			Confidence: gas.Low,

rules/hardcoded_credentials_test.go

@@ -25,6 +25,51 @@ func TestHardcoded(t *testing.T) {
 	analyzer := gas.NewAnalyzer(config, nil)
 	analyzer.AddRule(NewHardcodedCredentials(config))
 
+	issues := gasTestRunner(
+		`
+		package samples
+
+		import "fmt"
+
+		func main() {
+			username := "admin"
+			password := "f62e5bcda4fae4f82370da0c6f20697b8f8447ef"
+			fmt.Println("Doing something with: ", username, password)
+		}`, analyzer)
+
+	checkTestResults(t, issues, 1, "Potential hardcoded credentials")
+}
+
+func TestHardcodedWithEntropy(t *testing.T) {
+	config := map[string]interface{}{"ignoreNosec": false}
+	analyzer := gas.NewAnalyzer(config, nil)
+	analyzer.AddRule(NewHardcodedCredentials(config))
+
+	issues := gasTestRunner(
+		`
+		package samples
+
+		import "fmt"
+
+		func main() {
+			username := "admin"
+			password := "secret"
+			fmt.Println("Doing something with: ", username, password)
+		}`, analyzer)
+
+	checkTestResults(t, issues, 0, "Potential hardcoded credentials")
+}
+
+func TestHardcodedIgnoreEntropy(t *testing.T) {
+	config := map[string]interface{}{
+		"ignoreNosec": false,
+		"G101": map[string]string{
+			"ignore_entropy": "true",
+		},
+	}
+	analyzer := gas.NewAnalyzer(config, nil)
+	analyzer.AddRule(NewHardcodedCredentials(config))
+
 	issues := gasTestRunner(
 		`
 		package samples
@@ -50,7 +95,7 @@ func TestHardcodedGlobalVar(t *testing.T) {
 
 		import "fmt"
 
-		var password = "admin"
+		var password = "f62e5bcda4fae4f82370da0c6f20697b8f8447ef"
 
 		func main() {
 			username := "admin"
@@ -70,7 +115,7 @@ func TestHardcodedConstant(t *testing.T) {
 
 		import "fmt"
 
-		const password = "secret"
+		const password = "f62e5bcda4fae4f82370da0c6f20697b8f8447ef"
 
 		func main() {
 			username := "admin"
@@ -92,7 +137,7 @@ func TestHardcodedConstantMulti(t *testing.T) {
 
 		const (
 			username = "user"
-			password = "secret"
+			password = "f62e5bcda4fae4f82370da0c6f20697b8f8447ef"
 		)
 
 		func main() {
@@ -110,7 +155,7 @@ func TestHardecodedVarsNotAssigned(t *testing.T) {
 		package main
 		var password string
 		func init() {
-			password = "this is a secret string"
+			password = "f62e5bcda4fae4f82370da0c6f20697b8f8447ef"
 		}`, analyzer)
 	checkTestResults(t, issues, 1, "Potential hardcoded credentials")
 }
@@ -140,7 +185,7 @@ func TestHardcodedConstString(t *testing.T) {
 		package main
 
 		const (
-			ATNStateTokenStart = "foo bar"
+			ATNStateTokenStart = "f62e5bcda4fae4f82370da0c6f20697b8f8447ef"
 		)
 		func main() {
 			println(ATNStateTokenStart)