diff --git a/passwd.go b/passwd.go
index 4ce9245..3353061 100644
--- a/passwd.go
+++ b/passwd.go
@@ -62,6 +62,12 @@ func (p *Passwd) Passwd(pass, hash string) (string, error) {
 func (p *Passwd) IsPreferred(hash string) bool {
 	_, algo := p.getAlgo(hash)
 	if algo != nil && algo == p.d {
+
+		// if the algorithem defines its own check for preference.
+		if ck, ok := algo.(interface{ IsPreferred(string) bool }); ok {
+			return ck.IsPreferred(hash)
+		}
+
 		return true
 	}
 	return false
diff --git a/pkg/argon2/argon2.go b/pkg/argon2/argon2.go
index 628f5b0..1fed682 100644
--- a/pkg/argon2/argon2.go
+++ b/pkg/argon2/argon2.go
@@ -101,6 +101,36 @@ func (p *argon) Passwd(pass string, check string) (string, error) {
 func (p *argon) ApplyPasswd(passwd *passwd.Passwd) {
 	passwd.Register(p.name, p)
 }
+func (s *argon) IsPreferred(hash string) bool {
+	args, err := s.parseArgs(hash)
+	if err != nil {
+		return false
+	}
+
+	if args.version < s.version {
+		return false
+	}
+	if args.time < s.time {
+		return false
+	}
+	if args.memory < s.memory {
+		return false
+	}
+	if args.threads < s.threads {
+		return false
+	}
+	if args.keyLen < s.keyLen {
+		return false
+	}
+	if len(args.salt) < int(s.saltLen) {
+		return false
+	}
+	if len(args.hash) < int(s.keyLen) {
+		return false
+	}
+
+	return true
+}
 func (p *argon) defaultArgs() *pwArgs {
 	return &pwArgs{
 		name:    p.name,
diff --git a/pkg/scrypt/scrypt.go b/pkg/scrypt/scrypt.go
index d07401f..31d85c1 100644
--- a/pkg/scrypt/scrypt.go
+++ b/pkg/scrypt/scrypt.go
@@ -93,6 +93,30 @@ func (s *scryptpw) ApplyPasswd(p *passwd.Passwd) {
 		p.SetFallthrough(s)
 	}
 }
+func (s *scryptpw) IsPreferred(hash string) bool {
+	args, err := s.parseArgs(hash)
+	if err != nil {
+		return false
+	}
+
+	if args.N < s.N {
+		return false
+	}
+	if args.R < s.R {
+		return false
+	}
+	if args.P < s.P {
+		return false
+	}
+	if args.SaltLen < s.SaltLen {
+		return false
+	}
+	if args.DKLen < s.DKLen {
+		return false
+	}
+
+	return true
+}
 func (s *scryptpw) defaultArgs() *scryptArgs {
 	return &scryptArgs{
 		name:    s.name,
@@ -145,11 +169,13 @@ func (s *scryptpw) parseArgs(hash string) (*scryptArgs, error) {
 	if err != nil {
 		return nil, fmt.Errorf("%w: corrupt salt part", passwd.ErrBadHash)
 	}
+	args.SaltLen = len(args.salt)
 
 	args.hash, err = s.encoder.DecodeString(hash)
 	if err != nil {
 		return nil, fmt.Errorf("%w: corrupt hash part", passwd.ErrBadHash)
 	}
+	args.DKLen = len(args.hash)
 
 	return args, nil
 }