LibCrypto: Add methods to convert OpenSSL BN <-> UnsignedBigInteger

These methods allow to convert between OpenSSL big numbers and ours.

Libraries/LibCrypto/CMakeLists.txt

@@ -1,6 +1,7 @@
 add_cxx_compile_options(-Wvla)
 
 set(SOURCES
+    OpenSSL.cpp
     AEAD/ChaCha20Poly1305.cpp
     ASN1/ASN1.cpp
     ASN1/DER.cpp

Libraries/LibCrypto/OpenSSL.cpp

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2025, Altomani Gianluca <[email protected]>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/ByteBuffer.h>
+#include <LibCrypto/OpenSSL.h>
+#include <openssl/evp.h>
+
+namespace Crypto {
+
+ErrorOr<OpenSSL_BN> unsigned_big_integer_to_openssl_bignum(UnsignedBigInteger const& integer)
+{
+    auto bn = TRY(OpenSSL_BN::create());
+    auto buf = TRY(ByteBuffer::create_uninitialized(integer.byte_length()));
+    auto integer_size = integer.export_data(buf.bytes());
+    OPENSSL_TRY_PTR(BN_bin2bn(buf.bytes().data(), integer_size, bn.ptr()));
+    return bn;
+}
+
+ErrorOr<UnsignedBigInteger> openssl_bignum_to_unsigned_big_integer(OpenSSL_BN const& bn)
+{
+    auto size = BN_num_bytes(bn.ptr());
+    auto buf = TRY(ByteBuffer::create_uninitialized(size));
+    BN_bn2bin(bn.ptr(), buf.bytes().data());
+    return UnsignedBigInteger::import_data(buf.bytes().data(), size);
+}
+
+}

Libraries/LibCrypto/OpenSSL.h

@@ -8,7 +8,9 @@
 
 #include <AK/Error.h>
 #include <AK/Format.h>
+#include <LibCrypto/BigInt/UnsignedBigInteger.h>
 
+#include <openssl/bn.h>
 #include <openssl/err.h>
 #include <openssl/evp.h>
 
@@ -82,6 +84,16 @@ private:                                                                \
                                                                         \
     openssl_type* m_ptr { nullptr };
 
+class OpenSSL_BN {
+    OPENSSL_WRAPPER_CLASS(OpenSSL_BN, BIGNUM, BN);
+
+public:
+    static ErrorOr<OpenSSL_BN> create()
+    {
+        return OpenSSL_BN(OPENSSL_TRY_PTR(BN_new()));
+    }
+};
+
 class OpenSSL_PKEY {
     OPENSSL_WRAPPER_CLASS(OpenSSL_PKEY, EVP_PKEY, EVP_PKEY);
 
@@ -104,4 +116,7 @@ class OpenSSL_MD_CTX {
 
 #undef OPENSSL_WRAPPER_CLASS
 
+ErrorOr<OpenSSL_BN> unsigned_big_integer_to_openssl_bignum(UnsignedBigInteger const& integer);
+ErrorOr<UnsignedBigInteger> openssl_bignum_to_unsigned_big_integer(OpenSSL_BN const& bn);
+
 }