Bug 2451094 (CVE-2026-31790)

Issue summary: Applications using RSASVE key encapsulation to establish a secret
encryption key can send contents of an uninitialized memory buffer to a malicious peer.

Impact summary: The uninitialized buffer might contain sensitive data from the previous
execution of the application process which leads to sensitive data leakage to an attacker.

RSA_public_encrypt() returns the number of bytes written on success and -1
on error. The affected code tests only whether the return value is non-zero.
As a result, if RSA encryption fails, encapsulation can still return success to
the caller, set the output lengths, and leave the caller to use the contents of
the ciphertext buffer as if a valid KEM ciphertext had been produced.

If applications use EVP_PKEY_encapsulate() with RSA/RSASVE on an
attacker-supplied invalid RSA public key without first validating that key,
then this may cause stale or uninitialized contents of the caller-provided ciphertext
buffer to be disclosed to the attacker in place of the KEM ciphertext.

As a workaround calling EVP_PKEY_public_check() or EVP_PKEY_public_check_quick()
before EVP_PKEY_encapsulate() will mitigate the issue.