Apache License
As the OpenSSL License is Apache License 1.0, but not Apache License 2.0, it requires the phrase "this product includes software developed by the OpenSSL Project for use in the OpenSSL Toolkit" to appear in advertising material and any redistributions (Sections 3 and 6 of the OpenSSL License). Due to this restriction, the OpenSSL License and the Apache License 1.0 are incompatible with the GNU GPL.