OpenSSH sshbuf_put_bignum2_bytes() writes uint32(len) + mpint_data to the buffer (confirmed from sshbuf-getput-basic.c line 569). Both kex_gen_hash() via sshbuf_putb() and kex_derive_keys() via ssh_digest_update_buffer() consume the full buffer including the uint32 prefix. Fixes 'incorrect signature' error on OpenSSH 10.2.