Add crypto extern to behavioral-model

targets/simple_switch/sample_externs/macsec/Makefile

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+crypto_SOURCES = crypto.cpp
+crypto_LDADD = -lcrypto
+
+CPPFLAGS += -I../../ -fPIC -shared
+
+libexterncrypto.so : $(crypto_SOURCES)
+	g++ $(CPPFLAGS) $(crypto_SOURCES) -o libexterncrypto.so $(crypto_LDADD)
+
+clean:
+	rm -rf libexterncrypto.so
+

targets/simple_switch/sample_externs/macsec/README.md

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+# Add macsec extern to simple_switch
+# macsec is layer-2 security.
+
+For reference, the extern is implemented in crypto.cpp in this directory.
+Steps to use the macsec extern are described below.
+
+TODO
+
+The code is crypto.cpp is also available in the repo URL included below
+so that testing can be done using MACsec (MAC security) code.  A
+README.md is available at the repo below with steps for how to test
+the code.
+
+https://github.com/uni-tue-kn/p4-macsec
+
+The extern definition is included below.
+
+extern ExternCrypt {
+    ExternCrypt();
+    void protect(in bit<128> SAK,
+                in bit<64> SCI,
+                in bit<32> PN,
+                in bit<48> src_addr,
+                in bit<48> dst_addr,
+                in bit<128> sectag,
+                in bit<16> ethertype,
+                in bit<8> prepend_ipv4_hdr,
+                in bit<160> ipv4_hdr);
+    void validate(in bit<128> SAK,
+                in bit<64> SCI,
+                in bit<32> PN,
+                in bit<48> src_addr,
+                in bit<48> dst_addr,
+                in bit<128> sectag,
+                out bit<8> valid,
+                out bit<16> ethertype);
+}
+
+Additionally the extern is used in P4 code as crypt.validate() which is
+decryption and crypt.protect() which is encryption.
+
+https://github.com/uni-tue-kn/p4-macsec/p4/p4/basic.p4
+
+basic.p4 is used to test the cryto extern code with simple_switch as follows:
+
+In p4-macsec/p4/p4 directory, run `make run`. This will compile the P4 program
+and start the switches and mininet.
+
+basic.p4 is compiled using p4c with args shown in
+
+https://github.com/uni-tue-kn/p4-macsec/blob/master/p4/p4/Makefile