five.yml

Icambridge\Entity\User:
    id: encoder.super_duper_silly

four.xml

<service id="encoder.super_duper_silly" class="Icambridge\Encoder\SuperDuperSillyEncoder" />

one.yml

bcrypt:
  algorithm: bcrypt

seven.yml

Icambridge\Entity\InsecureUser:
  algorithm: plaintext
bcrypt:
  algorithm: bcrypt

six.php

<?php

namespace Icambridge\Encoder;

class User implements EncoderAwareInterface
{
  public function getEncoderName()
  {
    return "bcrypt";
  }
}

three.php

<?php

namespace Icambridge\Encoder;

use Symfony\Component\Security\Core\Exception\BadCredentialsException;
use Symfony\Component\Security\Core\Encoder\BasePasswordEncoder

/**
 * A super duper silly password encoder, no one should ever use!
 */
class SuperDuperSillyEncoder extends BasePasswordEncoder
{  
    private $ignorePasswordCase;

    public function encodePassword($raw, $salt)
    {
        if ($this->isPasswordTooLong($raw)) {
            throw new BadCredentialsException('Invalid password.');
        }

        $rawMd5 = md5($raw);
        $saltSha = sha1($salt);

        $encodableString = $this->mergePasswordAndSalt($rawmd5, $saltSha);
        $encodedString = $encodableString . $encodableString[0] . $encodableString[1];

        return $encodedString;
    }

    public function isPasswordValid($encoded, $raw, $salt)
    {
        if ($this->isPasswordTooLong($raw)) {
            return false;
        }

        $pass2 = $this->encodePassword($raw, $salt);

        if (!$this->ignorePasswordCase) {
            return $this->comparePasswords($encoded, $pass2);
        }

        return $this->comparePasswords(strtolower($encoded), strtolower($pass2));
    }
}

two.php

$encoder = $encoderFactory->getEncoder('bcrypt');