diff --git a/card-functionality/src/cards.rs b/card-functionality/src/cards.rs
index 7f2c563..2a8a5c1 100644
--- a/card-functionality/src/cards.rs
+++ b/card-functionality/src/cards.rs
@@ -41,7 +41,7 @@ pub struct TestCardData {
 }
 
 impl TestCardData {
-    pub(crate) fn get_card(&self) -> Result<Box<dyn CardBackend>> {
+    pub(crate) fn get_card(&self) -> Result<Box<dyn CardBackend + Send + Sync>> {
         self.tc.open()
     }
 
@@ -92,7 +92,7 @@ pub enum TestCard {
 }
 
 impl TestCard {
-    pub fn open(&self) -> Result<Box<dyn CardBackend>> {
+    pub fn open(&self) -> Result<Box<dyn CardBackend + Send + Sync>> {
         match self {
             Self::Pcsc(ident) => {
                 // Attempt to shutdown SCD, if it is running.
@@ -103,7 +103,7 @@ impl TestCard {
                 // Make three attempts to open the card before failing
                 // (this can be useful in ShareMode::Exclusive)
                 let mut i = 1;
-                let card: Result<Box<dyn CardBackend>, Error> = loop {
+                let card: Result<Box<dyn CardBackend + Send + Sync>, Error> = loop {
                     let res = PcscBackend::open_by_ident(ident, SHARE_MODE);
 
                     if i == 3 {
diff --git a/card-functionality/src/tests.rs b/card-functionality/src/tests.rs
index f79cc68..fa57d07 100644
--- a/card-functionality/src/tests.rs
+++ b/card-functionality/src/tests.rs
@@ -49,7 +49,10 @@ pub enum TestError {
 }
 
 /// Run after each "upload keys", if key *was* uploaded (?)
-pub fn test_decrypt(card: &mut dyn CardBackend, param: &[&str]) -> Result<TestOutput, TestError> {
+pub fn test_decrypt(
+    card: &mut (dyn CardBackend + Send + Sync),
+    param: &[&str],
+) -> Result<TestOutput, TestError> {
     let mut pgp = OpenPgp::new(card);
     let mut pgpt = pgp.transaction()?;
 
@@ -75,7 +78,10 @@ pub fn test_decrypt(card: &mut dyn CardBackend, param: &[&str]) -> Result<TestOu
 }
 
 /// Run after each "upload keys", if key *was* uploaded (?)
-pub fn test_sign(card: &mut dyn CardBackend, param: &[&str]) -> Result<TestOutput, TestError> {
+pub fn test_sign(
+    card: &mut (dyn CardBackend + Send + Sync),
+    param: &[&str],
+) -> Result<TestOutput, TestError> {
     let mut pgp = OpenPgp::new(card);
     let mut pgpt = pgp.transaction()?;
 
@@ -139,7 +145,7 @@ fn check_key_upload_algo_attrs() -> Result<()> {
 }
 
 pub fn test_print_caps(
-    card: &mut dyn CardBackend,
+    card: &mut (dyn CardBackend + Send + Sync),
     _param: &[&str],
 ) -> Result<TestOutput, TestError> {
     let mut pgp = OpenPgp::new(card);
@@ -163,7 +169,7 @@ pub fn test_print_caps(
 }
 
 pub fn test_print_algo_info(
-    card: &mut dyn CardBackend,
+    card: &mut (dyn CardBackend + Send + Sync),
     _param: &[&str],
 ) -> Result<TestOutput, TestError> {
     let mut pgp = OpenPgp::new(card);
@@ -185,7 +191,7 @@ pub fn test_print_algo_info(
 }
 
 pub fn test_upload_keys(
-    card: &mut dyn CardBackend,
+    card: &mut (dyn CardBackend + Send + Sync),
     param: &[&str],
 ) -> Result<TestOutput, TestError> {
     let mut pgp = OpenPgp::new(card);
@@ -215,7 +221,10 @@ pub fn test_upload_keys(
 }
 
 /// Generate keys for each of the three KeyTypes
-pub fn test_keygen(card: &mut dyn CardBackend, param: &[&str]) -> Result<TestOutput, TestError> {
+pub fn test_keygen(
+    card: &mut (dyn CardBackend + Send + Sync),
+    param: &[&str],
+) -> Result<TestOutput, TestError> {
     let mut pgp = OpenPgp::new(card);
     let mut pgpt = pgp.transaction()?;
 
@@ -257,7 +266,10 @@ pub fn test_keygen(card: &mut dyn CardBackend, param: &[&str]) -> Result<TestOut
 }
 
 /// Construct public key based on data from the card
-pub fn test_get_pub(card: &mut dyn CardBackend, _param: &[&str]) -> Result<TestOutput, TestError> {
+pub fn test_get_pub(
+    card: &mut (dyn CardBackend + Send + Sync),
+    _param: &[&str],
+) -> Result<TestOutput, TestError> {
     let mut pgp = OpenPgp::new(card);
     let mut pgpt = pgp.transaction()?;
 
@@ -296,7 +308,10 @@ pub fn test_get_pub(card: &mut dyn CardBackend, _param: &[&str]) -> Result<TestO
     Ok(vec![])
 }
 
-pub fn test_reset(card: &mut dyn CardBackend, _param: &[&str]) -> Result<TestOutput, TestError> {
+pub fn test_reset(
+    card: &mut (dyn CardBackend + Send + Sync),
+    _param: &[&str],
+) -> Result<TestOutput, TestError> {
     let mut pgp = OpenPgp::new(card);
     let mut pgpt = pgp.transaction()?;
 
@@ -310,7 +325,7 @@ pub fn test_reset(card: &mut dyn CardBackend, _param: &[&str]) -> Result<TestOut
 /// Returns an empty TestOutput, throws errors for unexpected Status codes
 /// and for unequal field values.
 pub fn test_set_user_data(
-    card: &mut dyn CardBackend,
+    card: &mut (dyn CardBackend + Send + Sync),
     _param: &[&str],
 ) -> Result<TestOutput, TestError> {
     let mut pgp = OpenPgp::new(card);
@@ -347,7 +362,7 @@ pub fn test_set_user_data(
 }
 
 pub fn test_private_data(
-    card: &mut dyn CardBackend,
+    card: &mut (dyn CardBackend + Send + Sync),
     _param: &[&str],
 ) -> Result<TestOutput, TestError> {
     let mut pgp = OpenPgp::new(card);
@@ -443,7 +458,7 @@ pub fn test_private_data(
 // }
 
 pub fn test_pw_status(
-    card: &mut dyn CardBackend,
+    card: &mut (dyn CardBackend + Send + Sync),
     _param: &[&str],
 ) -> Result<TestOutput, TestError> {
     let mut pgp = OpenPgp::new(card);
@@ -473,7 +488,10 @@ pub fn test_pw_status(
 /// Outputs:
 /// - verify pw3 (check) -> Status
 /// - verify pw1 (check) -> Status
-pub fn test_verify(card: &mut dyn CardBackend, _param: &[&str]) -> Result<TestOutput, TestError> {
+pub fn test_verify(
+    card: &mut (dyn CardBackend + Send + Sync),
+    _param: &[&str],
+) -> Result<TestOutput, TestError> {
     let mut pgp = OpenPgp::new(card);
     let mut pgpt = pgp.transaction()?;
 
@@ -540,7 +558,7 @@ pub fn test_verify(card: &mut dyn CardBackend, _param: &[&str]) -> Result<TestOu
 }
 
 pub fn test_change_pw(
-    card: &mut dyn CardBackend,
+    card: &mut (dyn CardBackend + Send + Sync),
     _param: &[&str],
 ) -> Result<TestOutput, TestError> {
     let mut pgp = OpenPgp::new(card);
@@ -601,7 +619,7 @@ pub fn test_change_pw(
 }
 
 pub fn test_reset_retry_counter(
-    card: &mut dyn CardBackend,
+    card: &mut (dyn CardBackend + Send + Sync),
     _param: &[&str],
 ) -> Result<TestOutput, TestError> {
     let mut pgp = OpenPgp::new(card);
@@ -666,7 +684,7 @@ pub fn test_reset_retry_counter(
 
 pub fn run_test(
     tc: &mut TestCardData,
-    t: fn(&mut (dyn CardBackend), &[&str]) -> Result<TestOutput, TestError>,
+    t: fn(&mut (dyn CardBackend + Send + Sync), &[&str]) -> Result<TestOutput, TestError>,
     param: &[&str],
 ) -> Result<TestOutput, TestError> {
     let mut card = tc.get_card()?;
diff --git a/openpgp-card/src/openpgp.rs b/openpgp-card/src/openpgp.rs
index 328129a..518575d 100644
--- a/openpgp-card/src/openpgp.rs
+++ b/openpgp-card/src/openpgp.rs
@@ -23,11 +23,11 @@ use crate::{
 /// Users of this crate can keep a long lived OpenPgp object. All operations must be performed on
 /// a short lived `OpenPgpTransaction`.
 pub struct OpenPgp<'a> {
-    card: &'a mut dyn CardBackend,
+    card: &'a mut (dyn CardBackend + Send + Sync),
 }
 
 impl<'a> OpenPgp<'a> {
-    pub fn new(card: &'a mut dyn CardBackend) -> Self {
+    pub fn new(card: &'a mut (dyn CardBackend + Send + Sync)) -> Self {
         Self { card }
     }
 
diff --git a/tools/src/bin/opgpcard/main.rs b/tools/src/bin/opgpcard/main.rs
index c3e5261..5e1b051 100644
--- a/tools/src/bin/opgpcard/main.rs
+++ b/tools/src/bin/opgpcard/main.rs
@@ -154,7 +154,7 @@ fn set_identity(ident: &str, id: u8) -> Result<(), Box<dyn std::error::Error>> {
 }
 
 fn print_status(ident: Option<String>, verbose: bool) -> Result<()> {
-    let mut card: Box<dyn CardBackend> = if let Some(ident) = ident {
+    let mut card: Box<dyn CardBackend + Send + Sync> = if let Some(ident) = ident {
         Box::new(util::open_card(&ident)?)
     } else {
         let mut cards = util::cards()?;