diff --git a/Tests/LibCrypto/TestBigInteger.cpp b/Tests/LibCrypto/TestBigInteger.cpp
index 7af6eb4a1d4..94beca95539 100644
--- a/Tests/LibCrypto/TestBigInteger.cpp
+++ b/Tests/LibCrypto/TestBigInteger.cpp
@@ -642,3 +642,19 @@ TEST_CASE(test_signed_multiplication_with_two_big_numbers)
     EXPECT_EQ(result.unsigned_value().words(), expected_results);
     EXPECT(result.is_negative());
 }
+
+TEST_CASE(test_negative_zero_is_not_allowed)
+{
+    Crypto::SignedBigInteger zero(Crypto::UnsignedBigInteger(0), true);
+    EXPECT(!zero.is_negative());
+
+    zero.negate();
+    EXPECT(!zero.is_negative());
+
+    Crypto::SignedBigInteger positive_five(Crypto::UnsignedBigInteger(5), false);
+    Crypto::SignedBigInteger negative_five(Crypto::UnsignedBigInteger(5), true);
+    zero = positive_five.plus(negative_five);
+
+    EXPECT(zero.unsigned_value().is_zero());
+    EXPECT(!zero.is_negative());
+}
diff --git a/Userland/Libraries/LibCrypto/BigInt/SignedBigInteger.h b/Userland/Libraries/LibCrypto/BigInt/SignedBigInteger.h
index c8850fd5abf..5b5468a728a 100644
--- a/Userland/Libraries/LibCrypto/BigInt/SignedBigInteger.h
+++ b/Userland/Libraries/LibCrypto/BigInt/SignedBigInteger.h
@@ -25,6 +25,7 @@ public:
         : m_sign(sign)
         , m_unsigned_data(move(unsigned_data))
     {
+        ensure_sign_is_valid();
     }
 
     explicit SignedBigInteger(UnsignedBigInteger unsigned_data)
@@ -72,9 +73,18 @@ public:
     const Vector<u32, STARTING_WORD_SIZE> words() const { return m_unsigned_data.words(); }
     bool is_negative() const { return m_sign; }
 
-    void negate() { m_sign = !m_sign; }
+    void negate()
+    {
+        if (!m_unsigned_data.is_zero())
+            m_sign = !m_sign;
+    }
+
+    void set_to_0()
+    {
+        m_unsigned_data.set_to_0();
+        m_sign = false;
+    }
 
-    void set_to_0() { m_unsigned_data.set_to_0(); }
     void set_to(i32 other)
     {
         m_unsigned_data.set_to((u32)other);
@@ -129,6 +139,12 @@ public:
     bool operator>(const UnsignedBigInteger& other) const;
 
 private:
+    void ensure_sign_is_valid()
+    {
+        if (m_sign && m_unsigned_data.is_zero())
+            m_sign = false;
+    }
+
     bool m_sign { false };
     UnsignedBigInteger m_unsigned_data;
 };
diff --git a/Userland/Libraries/LibCrypto/BigInt/UnsignedBigInteger.cpp b/Userland/Libraries/LibCrypto/BigInt/UnsignedBigInteger.cpp
index ef560153957..a8082721d1d 100644
--- a/Userland/Libraries/LibCrypto/BigInt/UnsignedBigInteger.cpp
+++ b/Userland/Libraries/LibCrypto/BigInt/UnsignedBigInteger.cpp
@@ -145,6 +145,16 @@ void UnsignedBigInteger::set_to(const UnsignedBigInteger& other)
     m_cached_hash = 0;
 }
 
+bool UnsignedBigInteger::is_zero() const
+{
+    for (size_t i = 0; i < length(); ++i) {
+        if (m_words[i] != 0)
+            return false;
+    }
+
+    return true;
+}
+
 size_t UnsignedBigInteger::trimmed_length() const
 {
     if (!m_cached_trimmed_length.has_value()) {
diff --git a/Userland/Libraries/LibCrypto/BigInt/UnsignedBigInteger.h b/Userland/Libraries/LibCrypto/BigInt/UnsignedBigInteger.h
index 14c9a90b4c6..3f91aa8d514 100644
--- a/Userland/Libraries/LibCrypto/BigInt/UnsignedBigInteger.h
+++ b/Userland/Libraries/LibCrypto/BigInt/UnsignedBigInteger.h
@@ -72,6 +72,7 @@ public:
         m_cached_hash = 0;
     }
 
+    bool is_zero() const;
     bool is_odd() const { return m_words.size() && (m_words[0] & 1); }
     bool is_invalid() const { return m_is_invalid; }