This fixes a potential crash due to a null-pointer dereference if rsadp()
returns NULL (e.g. with an all-zero ciphertext).
And it also implements the PKCS#1 v1.5 decryption padding check in
constant time.
The timing leak caused by the previous implementation was measured at
~17.5 μs at 3 GHz, which could allow a Bleichenbacher-like attack in
LAN environments. However, because of how RSA encryption is used in
strongSwan, this is not that much of an issue in practice. The mechanism
is only used for two use cases. One is SCEP/EST via PKCS#7 enveloped
data. Fortunately, this can not be triggered in significant numbers by
an attacker. The other use case is TLS as used by EAP methods (EAP-TLS,
EAP-PEAP/TTLS) during the authentication. While the cipher suites that
use RSA encryption are still enabled by default, the TLS messages are
wrapped in EAP and encrypted by IKE, making any kind of attack difficult.
Note that the gmp plugin isn't enabled anymore by default. And even
before that, most setups had the openssl plugin enabled, which has
priority over the gmp plugin. So it's unlikely the plugin was used in
practice.
Fixes: d615ffdcf3 ("implement gmp_rsa_private_key.decrypt()")
Fixes: CVE-2026-35334
Tobias Brunner
e2fa67e8fc