BN_CTX_NEW(3ossl)

💖 BN_CTX_NEW(3ossl) 💖

okay bestie, gather round, because today we're talking about the OpenSSL manual page BN_CTX_NEW(3ossl) — the one about making and unmaking BN_CTX structures. it's giving ✨memory management✨ and honestly? it's iconic.

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🌸 NAME 🌸

BN_CTX_new_ex, BN_CTX_new, BN_CTX_secure_new_ex, BN_CTX_secure_new, BN_CTX_free — allocate and free BN_CTX structures.

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🎀 SYNOPSIS 🎀

#include <openssl/bn.h>

BN_CTX *BN_CTX_new_ex(OSSL_LIB_CTX *ctx);
BN_CTX *BN_CTX_new(void);

BN_CTX *BN_CTX_secure_new_ex(OSSL_LIB_CTX *ctx);
BN_CTX *BN_CTX_secure_new(void);

void BN_CTX_free(BN_CTX *c);

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🦋 DESCRIPTION 🦋

so, a BN_CTX is a structure that holds BIGNUM temporary variables which library functions use. and the reason it exists at all? because dynamic memory allocation to create BIGNUMs is rather expensive when you're doing it over and over inside repeated subroutine calls. like, imagine allocating fresh every single time — no ma'am. so we have the BN_CTX structure instead. 💅

BN_CTX_new_ex() allocates and initializes a BN_CTX structure for the given library context ctx. the <ctx> value may be NULL, in which case the default library context will be used. BN_CTX_new() is the same as BN_CTX_new_ex() except that the default library context is always used.

BN_CTX_secure_new_ex() allocates and initializes a BN_CTX structure but uses the secure heap (see CRYPTO_secure_malloc(3)) to hold the BIGNUMs, for the given library context ctx. the <ctx> value may be NULL, in which case the default library context will be used. BN_CTX_secure_new() is the same as BN_CTX_secure_new_ex() except that the default library context is always used.

BN_CTX_free() frees the components of the BN_CTX and the structure itself. and here's the tea 🫖: since BN_CTX_start() is required in order to obtain BIGNUMs from the BN_CTX, in most cases BN_CTX_end() must be called before the BN_CTX may be freed by BN_CTX_free(). if c is NULL, nothing is done. love that for us.

⚠️ okay serious face for one sec. a given BN_CTX must only be used by a single thread of execution. no locking is performed, and the internal pool allocator will not properly handle multiple threads of execution. no sharing, no passing it around the group chat. 🚫🧵

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✨ RETURN VALUES ✨

• BN_CTX_new() and BN_CTX_secure_new() return a pointer to the BN_CTX. if the allocation fails, they return NULL and set an error code that can be obtained by ERR_get_error(3). 😭
• BN_CTX_free() has no return values. it just does the thing and dips.

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💀 REMOVED FUNCTIONALITY 💀

void BN_CTX_init(BN_CTX *c);

BN_CTX_init() is no longer available as of OpenSSL 1.1.0. rip to a real one. 🙈 applications should replace use of BN_CTX_init with BN_CTX_new instead:

BN_CTX *ctx;
ctx = BN_CTX_new();
if (!ctx)
    /* error */
...
BN_CTX_free(ctx);

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👑 SEE ALSO 👑

ERR_get_error(3), BN_add(3), BN_CTX_start(3)

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🌈 HISTORY 🌈

BN_CTX_init() was removed in OpenSSL 1.1.0. periodt.

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📜 COPYRIGHT 📜

Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.

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and that's the whole page bestie, you ate it up 🍓 this was OpenSSL version 3.5.9, dated 2026-09-29 — go forth and free your BN_CTXs responsibly 💖✨🧚