diff --git a/src/libSchnorr/src/MultiSig.cpp b/src/libSchnorr/src/MultiSig.cpp
index 55e279e..0dbe481 100644
--- a/src/libSchnorr/src/MultiSig.cpp
+++ b/src/libSchnorr/src/MultiSig.cpp
@@ -367,7 +367,7 @@ bool MultiSig::MultiSigVerify(const bytes& message, unsigned int offset,
         return false;
       }
       err2 = (BN_nnmod(challenge_built.get(), challenge_built.get(),
-                       Schnorr::GetCurveOrder(), NULL) == 0);
+                       Schnorr::GetCurveOrder(), ctx.get()) == 0);
       err = err || err2;
       if (err2) {
         // Challenge rebuild mod failed
diff --git a/src/libSchnorr/src/MultiSig_Challenge.cpp b/src/libSchnorr/src/MultiSig_Challenge.cpp
index 6a79bbd..b8c1db0 100644
--- a/src/libSchnorr/src/MultiSig_Challenge.cpp
+++ b/src/libSchnorr/src/MultiSig_Challenge.cpp
@@ -129,6 +129,11 @@ void Challenge::Set(const CommitPoint& aggregatedCommit,
 
   bytes buf(Schnorr::PUBKEY_COMPRESSED_SIZE_BYTES);
 
+  unique_ptr<BN_CTX, void (*)(BN_CTX*)> ctx(BN_CTX_new(), BN_CTX_free);
+  if (!ctx) {
+    throw std::bad_alloc();
+  }
+
   // Convert the committment to octets first
   if (EC_POINT_point2oct(Schnorr::GetCurveGroup(), aggregatedCommit.m_p.get(),
                          POINT_CONVERSION_COMPRESSED, buf.data(),
@@ -166,7 +171,8 @@ void Challenge::Set(const CommitPoint& aggregatedCommit,
     return;
   }
 
-  if (BN_nnmod(m_c.get(), m_c.get(), Schnorr::GetCurveOrder(), NULL) == 0) {
+  if (BN_nnmod(m_c.get(), m_c.get(), Schnorr::GetCurveOrder(), ctx.get()) ==
+      0) {
     // Could not reduce challenge modulo group order
     return;
   }
diff --git a/src/libSchnorr/src/MultiSig_CommitPointHash.cpp b/src/libSchnorr/src/MultiSig_CommitPointHash.cpp
index e53ddff..ee08e58 100644
--- a/src/libSchnorr/src/MultiSig_CommitPointHash.cpp
+++ b/src/libSchnorr/src/MultiSig_CommitPointHash.cpp
@@ -104,6 +104,10 @@ void CommitPointHash::Set(const CommitPoint& point) {
   // byte to 0x01.
   sha2.Update({SECOND_DOMAIN_SEPARATED_HASH_FUNCTION_BYTE});
 
+  unique_ptr<BN_CTX, void (*)(BN_CTX*)> ctx(BN_CTX_new(), BN_CTX_free);
+  if (!ctx) {
+    throw std::bad_alloc();
+  }
   // Convert the commitment to octets first
   if (EC_POINT_point2oct(Schnorr::GetCurveGroup(), point.m_p.get(),
                          POINT_CONVERSION_COMPRESSED, buf.data(),
@@ -123,7 +127,8 @@ void CommitPointHash::Set(const CommitPoint& point) {
     return;
   }
 
-  if (BN_nnmod(m_h.get(), m_h.get(), Schnorr::GetCurveOrder(), NULL) == 0) {
+  if (BN_nnmod(m_h.get(), m_h.get(), Schnorr::GetCurveOrder(), ctx.get()) ==
+      0) {
     // Could not reduce hashpoint value modulo group order
     return;
   }