LibCrypto: Avoid overly big allocs in intermediate ModularPower results

If we don't limit the sizes of the intermediate results, they will grow
indefinitely, causing each iteration to take longer and longer (in both
memcpy time, and algorithm runtime).
While calculating the trimmed length is fairly expensive, it's a small
cost to pay for uniform iteration times.

Userland/Libraries/LibCrypto/BigInt/UnsignedBigInteger.cpp

@@ -156,6 +156,13 @@ size_t UnsignedBigInteger::trimmed_length() const
     return m_cached_trimmed_length.value();
 }
 
+void UnsignedBigInteger::clamp_to_trimmed_length()
+{
+    auto length = trimmed_length();
+    if (m_words.size() > length)
+        m_words.resize(length);
+}
+
 FLATTEN UnsignedBigInteger UnsignedBigInteger::plus(const UnsignedBigInteger& other) const
 {
     UnsignedBigInteger result;

Userland/Libraries/LibCrypto/BigInt/UnsignedBigInteger.h

@@ -81,6 +81,8 @@ public:
     // The "trimmed length" is the number of words after trimming leading zeroed words
     size_t trimmed_length() const;
 
+    void clamp_to_trimmed_length();
+
     UnsignedBigInteger plus(const UnsignedBigInteger& other) const;
     UnsignedBigInteger minus(const UnsignedBigInteger& other) const;
     UnsignedBigInteger bitwise_or(const UnsignedBigInteger& other) const;

Userland/Libraries/LibCrypto/NumberTheory/ModularFunctions.cpp

@@ -150,6 +150,13 @@ UnsignedBigInteger ModularPower(const UnsignedBigInteger& b, const UnsignedBigIn
         UnsignedBigInteger::multiply_without_allocation(base, base, temp_1, temp_2, temp_3, temp_4, temp_multiply);
         UnsignedBigInteger::divide_without_allocation(temp_multiply, m, temp_1, temp_2, temp_3, temp_4, temp_quotient, temp_remainder);
         base.set_to(temp_remainder);
+
+        // Note that not clamping here would cause future calculations (multiply, specifically) to allocate even more unused space
+        // which would then persist through the temp bigints, and significantly slow down later loops.
+        // To avoid that, we can clamp to a specific max size, or just clamp to the min needed amount of space.
+        ep.clamp_to_trimmed_length();
+        exp.clamp_to_trimmed_length();
+        base.clamp_to_trimmed_length();
     }
     return exp;
 }