Assistance with an iRule

Interesting challenge. Try this?

when RULE_INIT {
  set static::key "fc2ed2bf490ad801c04ccd46b9d85b0c"
  set static::iv "7e2fe021d573c0eedd93c4b2704b1b3e"
}

when HTTP_REQUEST_SEND {
  set time [clock seconds]
  set encryptedtime [CRYPTO::encrypt -alg aes-128-cbc -keyhex $static::key -ivhex $static:iv $time]
  HTTP::header remove SECRETTIME
  HTTP::header insert SECRETTIME [b64encode $encryptedtime]
}

You can test it like by applying this rule to a test VIP:

when RULE_INIT {
  set static::key "fc2ed2bf490ad801c04ccd46b9d85b0c"
  set static::iv "7e2fe021d573c0eedd93c4b2704b1b3e"
}

when HTTP_REQUEST {
  set time [clock seconds]
  set encryptedtime [CRYPTO::encrypt -alg aes-128-cbc -keyhex $static::key -ivhex $static::iv $time]
  HTTP::respond 200 SECRETTIME [b64encode $encryptedtime]
}

And then test decrypting with curl and openssl:

curl -sI <ip> | grep SECRETTIME | awk '{print $2}' | openssl enc -a -d -aes-128-cbc -K fc2ed2bf490ad801c04ccd46b9d85b0c -iv 7e2fe021d573c0eedd93c4b2704b1b3e

Edit, made key + iv static to avoid being shamed by the iRules community.
Edit2: Added an example and changed to 32 bit hex key to follow the spec. F5 accepts 16 but openssl does not accept it.