diff --git a/banjax-config.yaml b/banjax-config.yaml
index 76f8d69..47afcfe 100644
--- a/banjax-config.yaml
+++ b/banjax-config.yaml
@@ -111,3 +111,5 @@ sites_to_disable_baskerville:
   localhost: true
 use_user_agent_in_cookie:
   localhost: true
+# difficulty of sha-inv page, setting above 10 might cause solving to fail
+sha_inv_expected_zero_bits: 10
diff --git a/internal/config.go b/internal/config.go
index 12f5678..11f981b 100644
--- a/internal/config.go
+++ b/internal/config.go
@@ -51,6 +51,7 @@ type Config struct {
 	TooManyFailedChallengesThreshold       int                 `yaml:"too_many_failed_challenges_threshold"`
 	PasswordCookieTtlSeconds               int                 `yaml:"password_cookie_ttl_seconds"`
 	ShaInvCookieTtlSeconds                 int                 `yaml:"sha_inv_cookie_ttl_seconds"`
+	ShaInvExpectedZeroBits                 uint32              `yaml:"sha_inv_expected_zero_bits"`
 	RestartTime                            int
 	ReloadTime                             int
 	Hostname                               string
diff --git a/internal/http_server.go b/internal/http_server.go
index b016845..d3eec99 100644
--- a/internal/http_server.go
+++ b/internal/http_server.go
@@ -21,6 +21,11 @@ import (
 	"github.com/gin-gonic/gin"
 )
 
+const (
+	PasswordCookieName  = "deflect_password3"
+	ChallengeCookieName = "deflect_challenge3"
+)
+
 func RunHttpServer(
 	config *Config,
 	decisionListsMutex *sync.Mutex,
@@ -252,16 +257,16 @@ func getUserAgentOrIp(c *gin.Context, config *Config) string {
 
 func passwordChallenge(c *gin.Context, config *Config, roaming bool) {
 	cookieTtl := getPerSiteCookieTtlOrDefault(config, c.Request.Header.Get("X-Requested-Host"), config.PasswordCookieTtlSeconds)
-	challenge(c, config, "deflect_password3", cookieTtl, config.HmacSecret, roaming)
+	challenge(c, config, PasswordCookieName, cookieTtl, config.HmacSecret, roaming)
 	sessionCookieEndPoint(c, config)
 	c.Data(401, "text/html", applyArgsToPasswordPage(config, config.PasswordPageBytes, roaming, cookieTtl))
 	c.Abort()
 }
 
 func shaInvChallenge(c *gin.Context, config *Config) {
-	challenge(c, config, "deflect_challenge3", config.ShaInvCookieTtlSeconds, config.HmacSecret, false)
+	challenge(c, config, ChallengeCookieName, config.ShaInvCookieTtlSeconds, config.HmacSecret, false)
 	sessionCookieEndPoint(c, config)
-	c.Data(429, "text/html", applyCookieMaxAge(config.ChallengerBytes, "deflect_challenge3", config.ShaInvCookieTtlSeconds))
+	c.Data(429, "text/html", applyArgsToShaInvPage(config))
 	c.Abort()
 }
 
@@ -303,14 +308,28 @@ func applyCookieDomain(pageBytes []byte, cookieName string) (modifiedPageBytes [
 
 func applyArgsToPasswordPage(config *Config, pageBytes []byte, roaming bool, cookieTtl int) (modifiedPageBytes []byte) {
 	// apply default or site specific expire time
-	modifiedPageBytes = applyCookieMaxAge(pageBytes, "deflect_password3", cookieTtl)
+	modifiedPageBytes = applyCookieMaxAge(pageBytes, PasswordCookieName, cookieTtl)
 
 	if !roaming {
 		return
 	}
 
 	// apply domain scope if allow banjax roaming
-	modifiedPageBytes = applyCookieDomain(modifiedPageBytes, "deflect_password3")
+	modifiedPageBytes = applyCookieDomain(modifiedPageBytes, PasswordCookieName)
+	return
+}
+
+func applyArgsToShaInvPage(config *Config) (modifiedPageBytes []byte) {
+	modifiedPageBytes = applyCookieMaxAge(
+		config.ChallengerBytes,
+		ChallengeCookieName,
+		config.ShaInvCookieTtlSeconds,
+	)
+	modifiedPageBytes = modifyHTMLContent(
+		modifiedPageBytes,
+		"new_solver(10)",
+		fmt.Sprintf("new_solver(%d)", config.ShaInvExpectedZeroBits),
+	)
 	return
 }
 
@@ -468,10 +487,10 @@ func sendOrValidateShaChallenge(
 	requestedHost := c.Request.Header.Get("X-Requested-Host")
 	requestedPath := c.Request.Header.Get("X-Requested-Path")
 	clientUserAgent := c.Request.Header.Get("X-Client-User-Agent")
-	challengeCookie, err := c.Cookie("deflect_challenge3")
+	challengeCookie, err := c.Cookie(ChallengeCookieName)
 	requestedMethod := c.Request.Method
 	if err == nil {
-		err := ValidateShaInvCookie(config.HmacSecret, challengeCookie, time.Now(), getUserAgentOrIp(c, config), 10) // XXX config
+		err := ValidateShaInvCookie(config.HmacSecret, challengeCookie, time.Now(), getUserAgentOrIp(c, config), config.ShaInvExpectedZeroBits)
 		if err != nil {
 			// log.Println("Sha-inverse challenge failed")
 			// log.Println(err)
@@ -570,7 +589,7 @@ func sendOrValidatePassword(
 	requestedHost := c.Request.Header.Get("X-Requested-Host")
 	requestedPath := c.Request.Header.Get("X-Requested-Path")
 	clientUserAgent := c.Request.Header.Get("X-Client-User-Agent")
-	passwordCookie, err := c.Cookie("deflect_password3")
+	passwordCookie, err := c.Cookie(PasswordCookieName)
 	requestedMethod := c.Request.Method
 	// log.Println("passwordCookie: ", passwordCookie)
 	if err == nil {
@@ -798,7 +817,7 @@ func decisionForNginx2(
 	decisionForNginxResult.DecisionListResult = NotSet
 
 	// check if user has a valid password cookie, if so, allow them through
-	passwordCookie, passwordCookieErr := c.Cookie("deflect_password3")
+	passwordCookie, passwordCookieErr := c.Cookie(PasswordCookieName)
 	if passwordCookieErr == nil {
 		var grantPriorityPass bool = false
 		expectedHashedPassword, hasPasswordHash := passwordProtectedPaths.SiteToPasswordHash[requestedHost]
diff --git a/internal/sha-inverse-challenge.html b/internal/sha-inverse-challenge.html
index 18d8d79..965cbf9 100644
--- a/internal/sha-inverse-challenge.html
+++ b/internal/sha-inverse-challenge.html
@@ -279,6 +279,7 @@
     </style>
 </head>
 
+<!-- XXX: http_server.go applyArgsToShaInvPage() will apply sha_inv_expected_zero_bits in the following line -->
 <body onload="new_solver(10)">
     <div class="header">
       <div class="header-wrapper">