RSA_CHECK_KEY(3ossl)

💖 RSA_CHECK_KEY 💖 — the openssl manual, bestie edition

okay so we're doing a deprecated one today and honestly? it's giving vintage. it's giving "we don't do it like this anymore but the lore is important." let's get into it 💅

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✨ NAME ✨

RSA_check_key_ex, RSA_check_key — validate private RSA keys 🔑

like, you have a key. is it a real key? is it lying to you? these functions will tell you. (they're also retired, so, y'know. read on.) 🙈

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✨ SYNOPSIS ✨

#include <openssl/rsa.h>

so here's the tea: the following functions have been deprecated since OpenSSL 3.0, and you can hide them entirely by defining OPENSSL_API_COMPAT with a suitable version value, see openssl_user_macros(7):

int RSA_check_key_ex(const RSA *rsa, BN_GENCB *cb);

int RSA_check_key(const RSA *rsa);

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✨ DESCRIPTION ✨

🚨 bestie alert 🚨 both of the functions on this page are deprecated. like, both of them. applications should instead use EVP_PKEY_public_check(3), EVP_PKEY_private_check(3) and EVP_PKEY_pairwise_check(3). that's the move now. that's the vibe. go there 🫶

RSA_check_key_ex() validates RSA keys. it checks that p and q are in fact prime, and that n = p*q. yes, it's really out here double-checking your math 😭 iconic behavior honestly.

it does not work on RSA public keys that have only the modulus and public exponent elements populated. not it. can't relate. 🙅♀️

it also checks that d*e = 1 mod (p-1*q-1), and that dmp1, dmq1 and iqmp are set correctly or are NULL. and it performs integrity checks on all the RSA key material, so the RSA key structure must contain all the private key data too. which means — and this is important, no cap — it cannot be used with any arbitrary RSA key object, even if it is otherwise fit for regular RSA operation. it needs the whole picture. it needs the full story. it needs you to be honest with it 💖

the cb parameter is a callback that will be invoked in the same manner as BN_is_prime_ex(3). cute.

RSA_check_key() is equivalent to RSA_check_key_ex() with a NULL cb. same energy, less to say. lowkey the simpler sibling ✨

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✨ RETURN VALUES ✨

• RSA_check_key_ex() and RSA_check_key() return 1 if rsa is a valid RSA key, and 0 otherwise. valid = 1 slay, invalid = 0 💀
• they return -1 if an error occurs while checking the key. so -1 is the "something went wrong, not the key's fault, just vibes off" answer.

if the key is invalid or an error occurred, the reason code can be obtained using ERR_get_error(3). go get the receipts 🧾

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✨ NOTES ✨

unlike most other RSA functions, this function does not work transparently with any underlying ENGINE implementation, because it uses the key data in the RSA structure directly. 😬

an ENGINE implementation can override the way key data is stored and handled, and can even provide support for HSM keys — in which case the RSA structure may contain no key data at all! like, zero. empty. ghosted. 👻

if the ENGINE in question is only being used for acceleration or analysis purposes, then in all likelihood the RSA key data is complete and untouched — but this can't be assumed in the general case. so like, don't count on it, bestie 🫠

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✨ BUGS ✨

a method of verifying the RSA key using opaque RSA API functions might need to be considered. 👀

right now RSA_check_key() simply uses the RSA structure elements directly, bypassing the RSA_METHOD table altogether (and completely violating encapsulation and object-orientation in the process 💀💀💀 it's giving chaos, it's giving "we don't talk about that").

the best fix will probably be to introduce a "check_key()" handler to the RSA_METHOD function table so that alternative implementations can also provide their own verifiers. like, let everyone bring their own vibe check. that's the dream 🌈

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✨ SEE ALSO ✨

• BN_is_prime_ex(3)
• ERR_get_error(3)

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✨ HISTORY ✨

• all of these functions were deprecated in OpenSSL 3.0. periodt 💅
• RSA_check_key_ex() appeared after OpenSSL 1.0.2. so it's the younger one. the baby. the little sister 🎀

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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!! 🥳 this one came straight outta the OpenSSL manual, version 3.5.9, dated 2026-09-29, filed under RSA_CHECK_KEY(3ossl). deprecated but never forgotten 💜 mwah, go use the EVP_PKEY_* ones, ily, bye!! 🌸🦋✨