Encryption key for ESP decode


Hello. I am running a 3DES MD5 VPN with preshared keys and wish to decode ESP packets in Wireshark. When entering the ESP protocol preferences, I have the SAs and algorithms correct, and I believe the authentication key is simply the PSK, but the hex encryption key is where I’m stuck. I’ve been through several RFCs and all documentation I can find, but still can’t figure out where to get this value. I would think it’s included in the key exchange payload but cannot determine which 192 bits to use of the 768 that make up the payload. Anyone familiar with this? Thanks.