diff --git a/matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/crypto/verification/SASDefaultVerificationTransaction.kt b/matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/crypto/verification/SASDefaultVerificationTransaction.kt
index 22a190c68e..c7885ce449 100644
--- a/matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/crypto/verification/SASDefaultVerificationTransaction.kt
+++ b/matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/crypto/verification/SASDefaultVerificationTransaction.kt
@@ -31,6 +31,7 @@ import org.matrix.android.sdk.internal.extensions.toUnsignedInt
 import org.matrix.olm.OlmSAS
 import org.matrix.olm.OlmUtility
 import timber.log.Timber
+import java.util.Locale
 
 /**
  * Represents an ongoing short code interactive key verification between two devices.
@@ -344,7 +345,7 @@ internal abstract class SASDefaultVerificationTransaction(
     }
 
     protected fun hashUsingAgreedHashMethod(toHash: String): String? {
-        if ("sha256".toLowerCase() == accepted?.hash?.toLowerCase()) {
+        if ("sha256" == accepted?.hash?.toLowerCase(Locale.ROOT)) {
             val olmUtil = OlmUtility()
             val hashBytes = olmUtil.sha256(toHash)
             olmUtil.releaseUtility()
@@ -354,12 +355,11 @@ internal abstract class SASDefaultVerificationTransaction(
     }
 
     private fun macUsingAgreedMethod(message: String, info: String): String? {
-        if (SAS_MAC_SHA256_LONGKDF.toLowerCase() == accepted?.messageAuthenticationCode?.toLowerCase()) {
-            return getSAS().calculateMacLongKdf(message, info)
-        } else if (SAS_MAC_SHA256.toLowerCase() == accepted?.messageAuthenticationCode?.toLowerCase()) {
-            return getSAS().calculateMac(message, info)
+        return when (accepted?.messageAuthenticationCode?.toLowerCase(Locale.ROOT)) {
+            SAS_MAC_SHA256_LONGKDF -> getSAS().calculateMacLongKdf(message, info)
+            SAS_MAC_SHA256         -> getSAS().calculateMac(message, info)
+            else                   -> null
         }
-        return null
     }
 
     override fun getDecimalCodeRepresentation(): String {
diff --git a/matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/util/Hash.kt b/matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/util/Hash.kt
index 3d80ad01d5..e19b1bcca7 100644
--- a/matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/util/Hash.kt
+++ b/matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/util/Hash.kt
@@ -17,6 +17,7 @@
 package org.matrix.android.sdk.internal.util
 
 import java.security.MessageDigest
+import java.util.Locale
 
 /**
  * Compute a Hash of a String, using md5 algorithm
@@ -26,7 +27,7 @@ fun String.md5() = try {
     digest.update(toByteArray())
     digest.digest()
             .joinToString("") { String.format("%02X", it) }
-            .toLowerCase()
+            .toLowerCase(Locale.ROOT)
 } catch (exc: Exception) {
     // Should not happen, but just in case
     hashCode().toString()
diff --git a/vector/lint.xml b/vector/lint.xml
index 51d795b669..572f937406 100644
--- a/vector/lint.xml
+++ b/vector/lint.xml
@@ -41,6 +41,7 @@
     <issue id="ObsoleteSdkInt" severity="error" />
     <issue id="Recycle" severity="error" />
     <issue id="KotlinPropertyAccess" severity="error" />
+    <issue id="DefaultLocale" severity="error" />
 
     <issue id="InvalidPackage">
         <!-- Ignore error from HtmlCompressor lib -->
diff --git a/vector/src/main/java/im/vector/app/core/intent/VectorMimeType.kt b/vector/src/main/java/im/vector/app/core/intent/VectorMimeType.kt
index c8a2bf65d5..1299f4086b 100644
--- a/vector/src/main/java/im/vector/app/core/intent/VectorMimeType.kt
+++ b/vector/src/main/java/im/vector/app/core/intent/VectorMimeType.kt
@@ -21,6 +21,7 @@ import android.net.Uri
 import android.webkit.MimeTypeMap
 import im.vector.app.core.utils.getFileExtension
 import timber.log.Timber
+import java.util.Locale
 
 /**
  * Returns the mimetype from a uri.
@@ -44,7 +45,7 @@ fun getMimeTypeFromUri(context: Context, uri: Uri): String? {
 
         if (null != mimeType) {
             // the mimetype is sometimes in uppercase.
-            mimeType = mimeType.toLowerCase()
+            mimeType = mimeType.toLowerCase(Locale.ROOT)
         }
     } catch (e: Exception) {
         Timber.e(e, "Failed to open resource input stream")
diff --git a/vector/src/main/java/im/vector/app/core/utils/FileUtils.kt b/vector/src/main/java/im/vector/app/core/utils/FileUtils.kt
index ab99ba61bd..aa36dd0959 100644
--- a/vector/src/main/java/im/vector/app/core/utils/FileUtils.kt
+++ b/vector/src/main/java/im/vector/app/core/utils/FileUtils.kt
@@ -19,6 +19,7 @@ package im.vector.app.core.utils
 import android.content.Context
 import timber.log.Timber
 import java.io.File
+import java.util.Locale
 
 // Implementation should return true in case of success
 typealias ActionOnFile = (file: File) -> Boolean
@@ -113,7 +114,7 @@ fun getFileExtension(fileUri: String): String? {
                 val ext = filename.substring(dotPos + 1)
 
                 if (ext.isNotBlank()) {
-                    return ext.toLowerCase()
+                    return ext.toLowerCase(Locale.ROOT)
                 }
             }
         }