As a user, I want to be able to login with my SSH public key, so that access to my files is secure.

Acceptance Criteria:

1. Database tables are created to store usernames and SSH public keys. A user can have multiple SSH public keys.
2. Spring Data creates the tables automatically if they do not exist.
3. A user who has a valid username and SSH public key can log in using Putty.

Notes:

1. Initializing databases with Spring: https://docs.spring.io/spring-boot/docs/2.1.1.RELEASE/reference/html/howto-database-initialization.html#howto-initialize-a-database-using-spring-jdbc
2. Implement PublickeyAuthenticator (https://mina.apache.org/sshd-project/apidocs/org/apache/sshd/server/auth/pubkey/PublickeyAuthenticator.html) to facilitate logging in using database-provided usernames and public keys. Once implemented, set the SSH server to use this new authenticator.

As an application developer, I want the initial SSH server setup, so that I can start using SFTP.

Acceptance Criteria:

• The port is configurable in application.properties, and defaults to port 2200
• A login prompt is displayed when Putty connects to port 2200
• The initial connection fails due to an authentication error
• The SSH server is configured as a Spring Service
• Uses SimpleGeneratorHostKeyProvider
• Does not address configuration items not discussed above
• Fully tested with automated tests

Notes:

• See https://github.com/apache/mina-sshd/blob/master/docs/server-setup.md

Blocked by #2

As a user, I want the S3 SFTP File System Accessor implemented and configured, so I can read and write my files from S3.

Acceptance Criteria:

1. The AWS SDK is added as a dependency.
2. SftpFileSystemAccessor is implemented using S3 for file operations.
3. Operations that are not supposed by S3 throw an UnimplementedException.
4. Listing directories shows the contents of the given S3 folder.
5. Files can be uploaded through Filezilla and show up in the S3 bucket.
6. Files can be downloaded through Filezilla.
7. Appropriate exceptions are thrown for missing files or invalid folders.
8. MinIO can be used in place of the real S3 for testing.