CVE-2019-1551: rsaz_512_sqr overflow bug on x86_64

There is an overflow bug in the x64_64 Montgomery squaring procedure used in exponentiation with 512-bit moduli. No EC algorithms are affected. Analysis suggests that attacks against 2-prime RSA1024, 3-prime RSA1536, and DSA1024 as a result of this defect would be very difficult to perform and are not believed likely. Attacks against DH512 are considered just feasible. However, for an attack the target would have to re-use the DH512 private key, which is not recommended anyway. Also applications directly using the low level API BN_mod_exp may be affected if they use BN_FLG_CONSTTIME. OpenSSL versions 1.1.1 and 1.0.2 are affected by this issue. However due to the low severity of this issue we are not creating new releases at this time. The 1.1.1 mitigation for this issue can be found in commit 419102400. The 1.0.2 mitigation for this issue can be found in commit f1c5eea8a. This issue was found by OSS-Fuzz and Guido Vranken and reported to OpenSSL on 12th September 2019. The fix was developed by Andy Polyakov with additional analysis by Bernd Edlinger. ## Impact Breaking DH512, according to the advisory.