tmp/ccRPqfPB.o:function.cpp:(.text+0x1f2): undefined reference to `EVP_CIPHER_CTX_new' tmp/ccRPqfPB.o:function.cpp:(.text+0x1d8): undefined reference to `EVP_CIPHER_CTX_free' tmp/ccRPqfPB.o:function.cpp:(.text+0x1b8): undefined reference to `EVP_EncryptFinal_ex' tmp/ccRPqfPB.o:function.cpp:(.text+0x183): undefined reference to `EVP_EncryptUpdate' tmp/ccRPqfPB.o:function.cpp:(.text+0x14d): undefined reference to `EVP_EncryptInit_ex' tmp/ccRPqfPB.o:function.cpp:(.text+0x128): undefined reference to `EVP_aes_256_cbc' tmp/ccRPqfPB.o:function.cpp:(.text+0x110): undefined reference to `EVP_CIPHER_CTX_new' tmp/ccRPqfPB.o:function.cpp:(.text+0xf4): undefined reference to `ERR_free_strings' tmp/ccRPqfPB.o:function.cpp:(.text+0xef): undefined reference to `EVP_cleanup' tmp/ccRPqfPB.o:function.cpp:(.text+0x92): undefined reference to `BIO_dump_fp' tmp/ccRPqfPB.o:function.cpp:(.text+0x30): undefined reference to `OPENSSL_config' tmp/ccRPqfPB.o:function.cpp:(.text+0x24): undefined reference to `OPENSSL_add_all_algorithms_noconf' tmp/ccRPqfPB.o:function.cpp:(.text+0x1f): undefined reference to `ERR_load_crypto_strings' G++ -m32 -o openssl openssl.c-lssl -lcrypt -L c:/cygwin/usr/local/ssl/lib/ -c -g-include /cygdrive/c/cygwin/usr/local/ssl/include/ -MMD -MP-MF "build/Debug/Cygwin_4.x-Windows/function.o.d" -o build/Debug/Cygwin_4.x-Windows/function.o function.cpp Openssl was trying to put through the installer, and to collect from source. This file I found in the source, tried to connect using the-I option of the compiler, using #include directly in the project - no effect. G++: error: openssl.c: No such file or directory Did not get to collect a simple application to encrypt the data.