error: field 'ctx' has incomplete type EVP_CIPHER_CTX

wget https://github.com/cisco/libsrtp/archive/v2.1.0.tar.gz
tar xfv v2.1.0.tar.gz
cd libsrtp-2.1.0
./configure --prefix=/usr --enable-openssl
make shared_library && sudo make install

Get the latest version of libsrtp.

It appears that there is a dependency on OpenSSL, but the version of OpenSSL you are using is incompatible. You are using OpenSSL 1.1.0 but you need to use OpenSSL 1.0.2

cause newer OpenSSL don't expose strcut EVP_CIPHER_CTX ,

try this

EVP_CIPHER_CTX *ctx = EVP_CIPHER_CTX_new();
EVP_CIPHER_CTX_init(ctx);
//do sth here
//...
EVP_CIPHER_CTX_free(ctx);