Email encryption

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Email encryption is used to send sensitive information via email from the sender to the recipient. Encryption between the end devices of the sender and recipient is possible as end-to-end encryption .

E-mail encryption often goes hand in hand with the digital signature and is actually combined with it in many standards such as X.509 or PGP . The aim of a digitally signed e-mail is to send information from the sender to the recipient in such a way that the sender can be clearly identified and nobody can manipulate the e-mail unnoticed on its way from the sender to the recipient. The email signature thus satisfies the need for authenticity and integrity , but does not guarantee confidentiality ; this requires encryption.

Independent of the end-to-end encryption, the transmission between e-mail servers and end devices (clients) is also encrypted by default. Due to the protocol stack used, TLS or STARTTLS are used here. The integrity and confidentiality of the e-mail can also be guaranteed by signing the e-mail or encrypting it end-to-end.

Application forms in comparison

There are various forms of application for e-mail encryption and e-mail signature.

Client-based email encryption and signature

The classic e-mail encryption and signature takes place from client to client (end-to-end encryption).

Example: Alice sends an encrypted and signed message to Bob by email.

  1. Alice's e-mail client handles the encryption and signature of the message. Bob's public key is used for encryption. The signature is made with Alice's private key.
  2. Bob's email client takes care of the decryption and signature verification of the message. The decryption is done with Bob's private key. The signature is checked using Alice's public key.

Client-based solutions have the disadvantage that they are too complex for many organizations (companies, associations, ...). Because the corresponding IT infrastructures do not exist, there is a great temptation to forego e-mail encryption and signature entirely in the organization.

Server-based email encryption and signature

To avoid the disadvantages of client-based encryption, server-based solutions are the method of choice. The work of encryption and signature is not done by clients, but by servers.

Example 1: Alice works in company A and sends an encrypted and signed message to Bob by email.

  1. Alice's message is encrypted and signed by an email server (a so-called encryption gateway) located in company A.
  2. Bob's email client takes care of the decryption and signature verification of the message.

Example 2: Alice works in company A and sends an encrypted and signed message to Bob by email. Bob works in a company B.

  1. Alice's message is encrypted and signed by an e-mail server located in company A.
  2. An e-mail server located in company B takes care of the decryption and signature verification of the message.

The advantages of a server-based solution are as follows:

  • The members of the organization (e.g. employees in the company) do not have to deal with the subject of encryption and signature. The work is done by the administrator who maintains the central server.
  • Nevertheless, all e-mail traffic can be encrypted and signed, provided that the internal users want it and the external communication partners participate.

The disadvantage of this solution is that the administrator or third parties can eavesdrop on the path between the sending e-mail client and the internal mail server (encryption gateway) and thus read and change e-mails.

Server-based solutions can offer the administrator the following services:

  • Automatically generate, manage and, if necessary, publish secret and public keys for internal users (e.g. for public LDAP directories)
  • Automatically query, validate and possibly save the certificates of external communication partners for later use
  • Issue certificates fully automatically

PKI-based email encryption and signature

The most common method of achieving confidentiality and authenticity in email is through PKI-based email encryption and signature. PKI stands for Public Key Infrastructure . One of the following two standards is almost always used for PKI-based email encryption and signature:

  1. S / MIME : Secure / Multipurpose Internet Mail Extensions
  2. OpenPGP : Open Pretty Good Privacy

PKI-based email encryption and signature is used in both client-based solutions and server-based solutions.

Password-based email encryption

Password-based email encryption is an option that server-based solutions can offer. It solves the following problem:

  • If server-based solutions work PKI-based, then they can relieve the internal communication partners of the operating organization from complicated PKI, but not the external communication partners. The external communication partners must either operate a server-based solution in their organization themselves or, if this is not possible, operate their PKI client-based. If you cannot do both, then e-mail encryption, at least PKI-based, is not possible.

In order to avoid that there is no encryption at all, server-based solutions can offer password-based e-mail encryption in addition to PKI-based e-mail encryption. For external communication partners who have a PKI, PKI-based encryption is then used. For communication partners who do not have a PKI, password-based encryption can be used.

Working principle

There are various options for implementing password-based email encryption.

Example of one of many possibilities:

  • Alice works in a company with a server-based solution. Bob doesn't have any PKI.
  • Alice emails a message to Bob.
  • The server-based solution does not find any certificates for Bob and automatically decides on a password-based delivery of the message to Bob.
  • Alice's message is put on hold.
  • Bob receives an email notification that a message is waiting for him.
  • Bob sets up an account on a web server and assigns a password for himself.
  • The message in the queue is then automatically converted into a PDF file, the content of the PDF file is encrypted with the password specified by Bob and the protected PDF is sent to Bob by email (as an attachment).
  • Bob opens the PDF, enters his password into the PDF reader and can read Alice's message.
  • Every further message from the company Alice works for is now automatically sent to Bob in a password-encrypted PDF format.

Advantages for the external communication partner

  • No certificates are required on the recipient side.
  • The automated password management replaces the complex certificate issuing process at trust centers for the external communication partner. The only requirement is that he has standard software (e.g. web browser or PDF reader).

S / MIME-based email encryption and signature in detail

As with pure hybrid encryption, each communication partner must generate a key pair before he can sign e-mails or receive encrypted e-mails. Without your own key pair, you can only verify third-party signatures and encrypt messages.

In the S / MIME world, it is common for new communication partners to have their public key signed by a certification authority . To do this, the public key is sent to the certification authority. Depending on the security class, the certification authority checks more or less strictly whether the public key actually belongs to the person who claims it. After passing the test, the certification authority creates a certificate for the key by signing it with its secret signature key. The certificate consists of the public key itself, the signature and administrative data. In addition to the signature key used for signing, there is a public verification key with which the signature can be verified. There is also a certificate for this verification key of the certification authority, the CA certificate, which in turn was signed by a certification authority. This creates a chain of CA certificates. The last link in such a chain is called the root CA certificate. The root CA certificate was signed with itself, so that in practice further methods are used to ensure that the root CA certificate is genuine.

Messages can be signed as well as encrypted. A signature ensures that a message has not been changed and provides information about the identity of the author. The encryption guarantees the confidentiality of the message, whereby it is usually ensured that the sender and all recipients of a message can decrypt it.

application areas

Email encryption and signing are used in the following situations, among others:

  • Respect for privacy
  • Ensuring the integrity of the email content
  • Compliance with legal data protection regulations in authorities and institutes

literature

  • Bruce Schneier: Applied Cryptography . Protocols, Algorithms, and Source Code in C, Second Edition. 1996, ISBN 0-471-11709-9 .
  • Niels Ferguson and Bruce Schneier: Practical Cryptography . 2003, ISBN 978-0-471-22357-3 .
  • Bruce Schneier: E-mail Security . How to Keep Your Electronic Messages Private. 1995, ISBN 978-0-471-05318-7 .

Web links

Individual evidence

  1. ↑ BSI for Citizens - Information - Private remains private - encrypted communication with e-mails. Retrieved March 6, 2020 .