.\"	$OpenBSD: RSA_check_key.3,v 1.10 2023/11/19 21:06:15 tb Exp $
.\"	OpenSSL 6859cf74 Sep 25 13:33:28 2002 +0000
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.\" This file was written by Ulf Moeller <ulf@openssl.org> and
.\" Geoff Thorpe <geoff@openssl.org>.
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.Dd $Mdocdate: November 19 2023 $
.Dt RSA_CHECK_KEY 3
.Os
.Sh NAME
.Nm RSA_check_key
.Nd validate private RSA keys
.Sh SYNOPSIS
.In openssl/rsa.h
.Ft int
.Fo RSA_check_key
.Fa "RSA *rsa"
.Fc
.Sh DESCRIPTION
This function validates RSA keys.
It checks that
.Fa rsa->p
and
.Fa rsa->q
are in fact prime, and that
.Fa rsa->n
satisfies n = p*q.
.Pp
It also checks that
.Fa rsa->d
and
.Fa rsa->e
satisfy d*e = 1 mod ((p-1)*(q-1)),
and that
.Fa rsa->dmp1 ,
.Fa rsa->dmq1 ,
and
.Fa resa->iqmp
are set correctly or are
.Dv NULL .
.Pp
This function does not work on RSA public keys that have only the
modulus and public exponent elements populated.
It performs integrity checks on all the RSA key material, so the
.Vt RSA
key structure must contain all the private key data too.
Therefore, it cannot be used with any arbitrary
.Vt RSA
key object, even if it is otherwise fit for regular RSA operation.
.Sh RETURN VALUES
.Fn RSA_check_key
returns 1 if
.Fa rsa
is a valid RSA key, and 0 otherwise.
-1 is returned if an error occurs while checking the key.
.Pp
If the key is invalid or an error occurred, the reason code can be
obtained using
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr BN_is_prime_ex 3 ,
.Xr RSA_get0_key 3 ,
.Xr RSA_new 3
.Sh HISTORY
.Fn RSA_check_key
first appeared in OpenSSL 0.9.4 and has been available since
.Ox 2.6 .
.Sh BUGS
A method of verifying the RSA key using opaque RSA API functions might
need to be considered.
Right now
.Fn RSA_check_key
simply uses the
.Vt RSA
structure elements directly, bypassing the
.Vt RSA_METHOD
table altogether (and completely violating encapsulation and
object-orientation in the process).
The best fix will probably be to introduce a
.Fn check_key
handler
to the
.Vt RSA_METHOD
function table so that alternative implementations can also provide
their own verifiers.