diff --git a/server/common/config.go b/server/common/config.go
index b2542a20..b9a296d3 100644
--- a/server/common/config.go
+++ b/server/common/config.go
@@ -15,7 +15,6 @@ import (
 var (
 	Config Configuration
 	configPath string = filepath.Join(GetCurrentDir(), CONFIG_PATH + "config.json")
-	SECRET_KEY string
 )
 
 type Configuration struct {
@@ -321,7 +320,7 @@ func (this *Configuration) Initialise() {
 		}
 		this.Save()
 	}
-	SECRET_KEY = this.Get("general.secret_key").String()
+	InitSecretDerivate(this.Get("general.secret_key").String())
 }
 
 func (this Configuration) Save() Configuration {
diff --git a/server/common/constants.go b/server/common/constants.go
index bd3517cc..5e464471 100644
--- a/server/common/constants.go
+++ b/server/common/constants.go
@@ -16,4 +16,22 @@ const (
 	URL_SETUP = "/admin/setup"
 )
 
-var BUILD_NUMBER string
+var (
+	BUILD_NUMBER string
+	SECRET_KEY   string
+	SECRET_KEY_DERIVATE_FOR_PROOF string
+	SECRET_KEY_DERIVATE_FOR_ADMIN string
+	SECRET_KEY_DERIVATE_FOR_USER  string
+)
+
+
+/*
+ * Improve security by calculating derivative of the secret key to restrict the attack surface
+ * in the worst case scenario with one compromise secret key
+ */
+func InitSecretDerivate(secret string) {
+	SECRET_KEY = secret
+	SECRET_KEY_DERIVATE_FOR_PROOF = Hash("PROOF_" + SECRET_KEY, len(SECRET_KEY))
+	SECRET_KEY_DERIVATE_FOR_ADMIN = Hash("ADMIN_" + SECRET_KEY, len(SECRET_KEY))
+	SECRET_KEY_DERIVATE_FOR_USER = Hash("USER_" + SECRET_KEY, len(SECRET_KEY))
+}
diff --git a/server/common/crypto.go b/server/common/crypto.go
index 2d0eb0f6..485b6db7 100644
--- a/server/common/crypto.go
+++ b/server/common/crypto.go
@@ -6,8 +6,7 @@ import (
 	"crypto/aes"
 	"crypto/cipher"
 	"crypto/rand"
-	"crypto/sha1"
-	"encoding/base32"
+	"crypto/sha256"
 	"encoding/base64"
 	"io"
 	"io/ioutil"
@@ -45,10 +44,19 @@ func DecryptString(secret string, data string) (string, error){
 	return string(d), nil
 }
 
-func Hash(str string) string {
-	hasher := sha1.New()
+func Hash(str string, n int) string {
+	hasher := sha256.New()
 	hasher.Write([]byte(str))
-	return "sha1::" + base32.HexEncoding.EncodeToString(hasher.Sum(nil))
+	d := hasher.Sum(nil)
+	size := len(Letters)
+	h := ""
+	for i:=0; i<len(d); i++ {
+		if n > 0 && i >= n {
+			break
+		}
+		h += string(Letters[int(d[i]) % size])
+	}
+	return h
 }
 
 func RandomString(n int) string {
@@ -140,8 +148,8 @@ func verify(something []byte) ([]byte, error) {
 
 // Create a unique ID that can be use to identify different session
 func GenerateID(ctx *App) string {
+	p := ""
 	params := ctx.Session
-	p := "salt => " + SECRET_KEY
 	if params["type"] != "" {
 		p += "type =>" + params["type"]
 	}
@@ -172,8 +180,10 @@ func GenerateID(ctx *App) string {
 	if params["token"] != "" {
 		p += "token =>" + params["token"]
 	}
-	if p == "salt => " + SECRET_KEY {
-		return ""
+
+	if p == "" {
+		return Hash("N/A", 20)
 	}
-	return Hash(p)
+	p += "salt => " + SECRET_KEY
+	return Hash(p, 20)
 }
diff --git a/server/ctrl/admin.go b/server/ctrl/admin.go
index 4ccb0163..e4619eff 100644
--- a/server/ctrl/admin.go
+++ b/server/ctrl/admin.go
@@ -22,7 +22,7 @@ func AdminSessionGet(ctx App, res http.ResponseWriter, req *http.Request) {
 		return c.Value
 	}()
 
-	str, err := DecryptString(SECRET_KEY, obfuscate);
+	str, err := DecryptString(SECRET_KEY_DERIVATE_FOR_ADMIN, obfuscate);
 	if err != nil {
 		SendSuccessResult(res, false)
 		return
@@ -61,7 +61,7 @@ func AdminSessionAuthenticate(ctx App, res http.ResponseWriter, req *http.Reques
 
 	// Step 3: Send response to the client
 	body, _ := json.Marshal(NewAdminToken())
-	obfuscate, err := EncryptString(SECRET_KEY, string(body))
+	obfuscate, err := EncryptString(SECRET_KEY_DERIVATE_FOR_ADMIN, string(body))
 	if err != nil {
 		SendErrorResult(res, err)
 		return
@@ -85,7 +85,7 @@ func AdminBackend(ctx App, res http.ResponseWriter, req *http.Request) {
 	}
 
 	if c, err := json.Marshal(backends); err == nil {
-		hash := Hash(string(c))
+		hash := Hash(string(c), 20)
 		if req.Header.Get("If-None-Match") == hash {
 			res.WriteHeader(http.StatusNotModified)
 			return
diff --git a/server/ctrl/config.go b/server/ctrl/config.go
index 8d1bdf62..15c450d7 100644
--- a/server/ctrl/config.go
+++ b/server/ctrl/config.go
@@ -92,7 +92,7 @@ func PublicConfigHandler(ctx App, res http.ResponseWriter, req *http.Request) {
 	cfg := Config.Export()
 
 	if c, err := json.Marshal(cfg); err == nil {
-		hash := Hash(string(c))
+		hash := Hash(string(c), 20)
 		if req.Header.Get("If-None-Match") == hash {
 			res.WriteHeader(http.StatusNotModified)
 			return
diff --git a/server/ctrl/files.go b/server/ctrl/files.go
index 38d76fbf..30988d7d 100644
--- a/server/ctrl/files.go
+++ b/server/ctrl/files.go
@@ -85,7 +85,7 @@ func FileLs(ctx App, res http.ResponseWriter, req *http.Request) {
 			Tmp1 interface{}
 		}{ files, perms }
 		if j, err := json.Marshal(tmp); err == nil {
-			return Hash(string(j))
+			return Hash(string(j), 20)
 		}
 		return ""
 	}()
diff --git a/server/ctrl/session.go b/server/ctrl/session.go
index d40a8189..0ccf183d 100644
--- a/server/ctrl/session.go
+++ b/server/ctrl/session.go
@@ -72,7 +72,7 @@ func SessionAuthenticate(ctx App, res http.ResponseWriter, req *http.Request) {
 		SendErrorResult(res, NewError(err.Error(), 500))
 		return
 	}
-	obfuscate, err := EncryptString(SECRET_KEY, string(s))
+	obfuscate, err := EncryptString(SECRET_KEY_DERIVATE_FOR_USER, string(s))
 	if err != nil {
 		SendErrorResult(res, NewError(err.Error(), 500))
 		return
diff --git a/server/ctrl/share.go b/server/ctrl/share.go
index b6ea0027..3d16cb2a 100644
--- a/server/ctrl/share.go
+++ b/server/ctrl/share.go
@@ -44,7 +44,7 @@ func ShareUpsert(ctx App, res http.ResponseWriter, req *http.Request) {
 				a, err := req.Cookie(COOKIE_NAME_AUTH)
 				if err != nil {
 					return ""
-				}
+				}				
 				return a.Value
 			}
 			return ctx.Share.Auth
@@ -146,7 +146,7 @@ func ShareVerifyProof(ctx App, res http.ResponseWriter, req *http.Request) {
 	}
 
 	if submittedProof.Key != "" {
-		submittedProof.Id = Hash(submittedProof.Key + "::" + submittedProof.Value)
+		submittedProof.Id = Hash(submittedProof.Key + "::" + submittedProof.Value, 20)
 		verifiedProof = append(verifiedProof, submittedProof)
 	}
 
@@ -158,7 +158,7 @@ func ShareVerifyProof(ctx App, res http.ResponseWriter, req *http.Request) {
 		Name: COOKIE_NAME_PROOF,
 		Value: func(p []model.Proof) string {
 			j, _ := json.Marshal(p)
-			str, _ := EncryptString(SECRET_KEY, string(j))
+			str, _ := EncryptString(SECRET_KEY_DERIVATE_FOR_PROOF, string(j))
 			return str
 		}(verifiedProof),
 		Path: COOKIE_PATH,
diff --git a/server/middleware/session.go b/server/middleware/session.go
index 94fe8c77..90bca82f 100644
--- a/server/middleware/session.go
+++ b/server/middleware/session.go
@@ -30,7 +30,7 @@ func AdminOnly(fn func(App, http.ResponseWriter, *http.Request)) func(ctx App, r
 				return
 			}
 
-			str, err := DecryptString(SECRET_KEY, c.Value);
+			str, err := DecryptString(SECRET_KEY_DERIVATE_FOR_ADMIN, c.Value);
 			if err != nil {
 				SendErrorResult(res, ErrPermissionDenied)
 				return
@@ -195,7 +195,7 @@ func _extractSession(req *http.Request, ctx *App) (map[string]string, error) {
 	var session map[string]string = make(map[string]string)
 
 	if ctx.Share.Id != "" {
-		str, err = DecryptString(SECRET_KEY, ctx.Share.Auth)
+		str, err = DecryptString(SECRET_KEY_DERIVATE_FOR_USER, ctx.Share.Auth)
 		if err != nil {
 			// This typically happen when changing the secret key
 			return session, nil
@@ -223,7 +223,7 @@ func _extractSession(req *http.Request, ctx *App) (map[string]string, error) {
 			return session, nil
 		}
 		str = cookie.Value
-		str, err = DecryptString(SECRET_KEY, str)
+		str, err = DecryptString(SECRET_KEY_DERIVATE_FOR_USER, str)
 		if err != nil {
 			// This typically happen when changing the secret key
 			return session, nil
diff --git a/server/model/share.go b/server/model/share.go
index f173741f..3fa930ac 100644
--- a/server/model/share.go
+++ b/server/model/share.go
@@ -261,7 +261,7 @@ func ShareProofGetAlreadyVerified(req *http.Request) []Proof {
 	if len(cookieValue) > 500 {
 		return p
 	}
-	j, err := DecryptString(SECRET_KEY, cookieValue)
+	j, err := DecryptString(SECRET_KEY_DERIVATE_FOR_PROOF, cookieValue)
 	if err != nil {
 		return p
 	}
@@ -306,7 +306,7 @@ func shareProofAreEquivalent(ref Proof,  p Proof) bool {
 	}
 	for _, chunk := range strings.Split(ref.Value, ",") {
 		chunk = strings.Trim(chunk, " ")
-		if p.Id == Hash(ref.Key + "::" + chunk) {
+		if p.Id == Hash(ref.Key + "::" + chunk, 20) {
 			return true
 		}
 	}