This bash script can be used to encrypt secrets for a Travis CI configuration (`.travis.yml`) with the project specific public key.

README.md

Usage

• Download script travis-encrypt.sh
• Make it executable chmod +x travis-encrypt.sh
• Run the script with ./travis-encrypt.sh -r username/repositoryname -e example
  • It will return something like 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
• Use the encrypted secret in your .travis.yml according to https://docs.travis-ci.com/user/encryption-keys/#Usage

travis-encrypt.sh

#!/bin/bash

usage() { echo -e "Travis Encrypt Script\nUsage:\t$0 \n -r\t<username/repository> \n -e\t<string which should be encrypted>" 1>&2; exit 1; }

while getopts ":r:e:" param; do
  case "${param}" in
    r)
      r=${OPTARG}
      ;;
    e)
      e=${OPTARG}
      ;;
    *)
      usage
      ;;
  esac
done
shift $((OPTIND -1))

if [ -z "${r}" ] || [[ !(${r} =~ [[:alnum:]]/[[:alnum:]]) ]] || [ -z "${e}" ]; then
  usage
fi

key_match="\"key\":\"([^\"]+)\""
key_url="https://api.travis-ci.org/repos/${r}/key"
request_result=$(curl --silent $key_url)

if [[ !($request_result =~ $key_match) ]]; then
  echo "Couldn't retrieve key from ${key_url}. "
  usage
fi

echo -n "${e}" | openssl rsautl -encrypt -pubin -inkey <(echo -e "${BASH_REMATCH[1]}") | openssl base64 -A
echo

I've just tested it and it still works. I'm on 'OpenSSL 1.1.0h 27 Mar 2018'.
Remember it only works on public repos.