/etc/ipsec.secrets is in strongswan-starter 5.3.5-1ubuntu3.8.
This file is owned by root:root, with mode 0o600.
The actual contents of the file can be viewed below.
1 2 3 4 | # This file holds shared secrets or RSA private keys for authentication.
# RSA private key for this host, authenticating it to any other host
# which knows the public part.
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