[slow sign in] Reduce number of kdf iterations for PDF database key (#1343)

`kdf_iter` parameter is reduced to 3200. This change is done because of
performance reasons, currently key derivation is too slow on some mobile
devices. The number of iterations before this commit is 64000, default
value in `sqlcipher` from version `3.0.0`.
fda4c68bb4/CHANGELOG.md (300---2013-11-05)

Implementation:
`sqlcipher_export` is used for migration, check out the link below
for details
https://www.zetetic.net/sqlcipher/sqlcipher-api/#sqlcipher_export

services/shhext/chat/sql_lite_persistence.go

@@ -21,6 +21,17 @@ import (
 // A safe max number of rows
 const maxNumberOfRows = 100000000
 
+// The default number of kdf iterations in sqlcipher (from version 3.0.0)
+// https://github.com/sqlcipher/sqlcipher/blob/fda4c68bb474da7e955be07a2b807bda1bb19bd2/CHANGELOG.md#300---2013-11-05
+// https://www.zetetic.net/sqlcipher/sqlcipher-api/#kdf_iter
+const defaultKdfIterationsNumber = 64000
+
+// The reduced number of kdf iterations (for performance reasons) which is
+// currently used for derivation of the database key
+// https://github.com/status-im/status-go/pull/1343
+// https://notes.status.im/i8Y_l7ccTiOYq09HVgoFwA
+const kdfIterationsNumber = 3200
+
 // SQLLitePersistence represents a persistence service tied to an SQLite database
 type SQLLitePersistence struct {
 	db             *sql.DB
@@ -70,7 +81,7 @@ func MigrateDBFile(oldPath string, newPath string, oldKey string, newKey string)
 		return err
 	}
 
-	db, err := openDB(newPath, oldKey)
+	db, err := openDB(newPath, oldKey, defaultKdfIterationsNumber)
 	if err != nil {
 		return err
 	}
@@ -85,7 +96,66 @@ func MigrateDBFile(oldPath string, newPath string, oldKey string, newKey string)
 
 }
 
-func openDB(path string, key string) (*sql.DB, error) {
+// MigrateDBKeyKdfIterations changes the number of kdf iterations executed
+// during the database key derivation. This change is necessary because
+// of performance reasons.
+// https://github.com/status-im/status-go/pull/1343
+// `sqlcipher_export` is used for migration, check out this link for details:
+// https://www.zetetic.net/sqlcipher/sqlcipher-api/#sqlcipher_export
+func MigrateDBKeyKdfIterations(oldPath string, newPath string, key string) error {
+	_, err := os.Stat(oldPath)
+
+	// No files, nothing to do
+	if os.IsNotExist(err) {
+		return nil
+	}
+
+	// Any other error, throws
+	if err != nil {
+		return err
+	}
+
+	db, err := openDB(oldPath, key, defaultKdfIterationsNumber)
+	if err != nil {
+		return err
+	}
+
+	attach := fmt.Sprintf(
+		"ATTACH DATABASE '%s' AS newdb KEY '%s'",
+		newPath,
+		key)
+
+	if _, err = db.Exec(attach); err != nil {
+		return err
+	}
+
+	changeKdfIter := fmt.Sprintf(
+		"PRAGMA newdb.kdf_iter = %d",
+		kdfIterationsNumber)
+
+	if _, err = db.Exec(changeKdfIter); err != nil {
+		return err
+	}
+
+	exportDB := "SELECT sqlcipher_export('newdb')"
+
+	if _, err = db.Exec(exportDB); err != nil {
+		return err
+	}
+
+	if err = db.Close(); err != nil {
+		return err
+	}
+
+	if err = os.Remove(oldPath); err != nil {
+		return err
+	}
+
+	return nil
+
+}
+
+func openDB(path string, key string, kdfIter int) (*sql.DB, error) {
 	db, err := sql.Open("sqlite3", path)
 	if err != nil {
 		return nil, err
@@ -104,7 +174,9 @@ func openDB(path string, key string) (*sql.DB, error) {
 		return nil, err
 	}
 
-	if _, err = db.Exec("PRAGMA cypher_page_size=4096"); err != nil {
+	kdfString := fmt.Sprintf("PRAGMA kdf_iter = '%d'", kdfIter)
+
+	if _, err = db.Exec(kdfString); err != nil {
 		return nil, err
 	}
 	return db, nil
@@ -136,7 +208,7 @@ func (s *SQLLitePersistence) GetSessionStorage() dr.SessionStorage {
 
 // Open opens a file at the specified path
 func (s *SQLLitePersistence) Open(path string, key string) error {
-	db, err := openDB(path, key)
+	db, err := openDB(path, key, kdfIterationsNumber)
 	if err != nil {
 		return err
 	}

services/shhext/service.go

@@ -129,6 +129,7 @@ func (s *Service) InitProtocol(address string, password string) error {
 	v0Path := filepath.Join(s.dataDir, fmt.Sprintf("%x.db", address))
 	v1Path := filepath.Join(s.dataDir, fmt.Sprintf("%s.db", s.installationID))
 	v2Path := filepath.Join(s.dataDir, fmt.Sprintf("%s.v2.db", s.installationID))
+	v3Path := filepath.Join(s.dataDir, fmt.Sprintf("%s.v3.db", s.installationID))
 
 	if err := chat.MigrateDBFile(v0Path, v1Path, "ON", password); err != nil {
 		return err
@@ -141,7 +142,12 @@ func (s *Service) InitProtocol(address string, password string) error {
 		os.Remove(v2Path)
 	}
 
-	persistence, err := chat.NewSQLLitePersistence(v2Path, hashedPassword)
+	if err := chat.MigrateDBKeyKdfIterations(v2Path, v3Path, hashedPassword); err != nil {
+		os.Remove(v2Path)
+		os.Remove(v3Path)
+	}
+
+	persistence, err := chat.NewSQLLitePersistence(v3Path, hashedPassword)
 	if err != nil {
 		return err
 	}