| SKESK Packet | *Symmetric-Key Encrypted Session Key* packet; contains or provides a passphrase-encrypted session key |
| PKESK Packet | *Public-Key Encrypted Session Key* packet; contains a session key encrypted using an asymmetric public key |
| Session Key | Symmetric encryption key, which is either used directly as - or to derive - the message key |
| Message Key | Symmetric encryption key used to encrypt the contents of the SEIPD packet |
## Encryption mechanism versions
OpenPGP's encryption mechanisms have evolved over time. The RFC shows an [overview of encryption mechanisms](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#section-10.3.2.1), and how they may be combined.
Two generations of encryption mechanisms are currently relevant in OpenPGP, and will co-exist for the foreseeable future.
The main difference between these lies in the symmetric part of the encryption mechanism, represented by versions 1 and 2 of the *Symmetrically Encrypted and Integrity Protected Data* packets (abbreviated as "SEIPD"). The two versions use different mechanisms to provide non-malleability. More on these below.
Older, legacy encryption mechanisms exist in OpenPGP. However, those must not be used for encryption anymore. Messages encrypted using these legacy mechanisms may still be decrypted, although with caution. For more information, see the [decryption](/decryption) chapter.
SEIPD packets are used in combination with two mechanisms that store *session keys*:
- [Public-Key Encrypted Session Key](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-public-key-encrypted-sessio) (PKESK) packets and
The typical combination of mechanisms for encryption in OpenPGP is a [hybrid cryptosystem](hybrid-cryptosystems), consisting of one or more [Public-Key Encrypted Session Key](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-public-key-encrypted-sessio) packets (PKESK), followed by a [Symmetrically Encrypted Integrity Protected Data](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-symmetrically-encrypted-int) (SEIPD) packet.
In this combination, an asymmetric cryptographic mechanism is used to protect a *session key* inside PKESK packets. The *session key*, in turn, is used to protect the plaintext using symmetric-key encryption in a SEIPD packet.
## Encrypted session keys: PKESK, SKESK
Encrypted session key (ESK) packets are a family of two mechanisms for securing symmetric key material:
- [PKESK](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-public-key-encrypted-sessio): Uses asymmetric OpenPGP key material to protect a session key, and
- [SKESK](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-symmetric-key-encrypted-ses): Uses passphrases to protect the symmetric key material, instead of OpenPGP asymmetric key material (this is less commonly used).
An arbitrary number of PKESKs and SKESKs can be used in the same message. It is also possible to mix the two, resulting in a message which can be decrypted using either one of the designated OpenPGP keys or any of the passphrases used to encrypt the message. This is useful to make a message available to a number of known recipients, with the option to provide the passphrase to future recipients.
### PKESK: Session key encrypted to an asymmetric OpenPGP key
To encrypt an OpenPGP message for a recipient, the session key is encrypted to the recipient's public key. The resulting encrypted session key is packed into a PKESK packet, which holds essential metadata, like an identifier of the recipients encryption (sub)-key.
This procedure is repeated for each recipient of the message, and all resulting PKESK packets are prepended to the SEIPD packet (see below) containing the actual message.
Typically, the sender would also include themselves as a recipient, to be able to decrypt the message with their own key at a later point in time.
### SKESK: Session key encrypted to a passphrase
As an alternative (or augmentation) to PKESK packets, a message can also be encrypted to a symmetric passphrase. This is done using a SKESK packet, which uses an S2K mechanism to derive a symmetric key from a passphrase. This key is either used directly as the session key, or more commonly, used as a key-encapsulation-key (KEK) to encrypt the session key.
Also see https://flowcrypt.com/docs/guide/send-and-receive/send-password-protected-emails.html
As for protection of secret key material, it is important to choose appropriate S2K parameters when generating an SKESK packet.
The specification currently recommends to use either *Iterated and Salted S2K* or *Argon2*.
## Symmetric encryption of data, SEIPD
*Symmetrically Encrypted Integrity Protected Data* (SEIPD) packets represent the symmetric aspect of OpenPGP's encryption mechanism. The function of these packets is entirely independent of (asymmetric) OpenPGP keys.
A SEIPD packet contains the actual payload: the ciphertext of the encrypted message. For a large encrypted message, the SEIPD packet will also be large.
```{note}
SEIPD packets are the successor to the [Symmetrically Encrypted Data](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-symmetrically-encrypted-dat) packet, which is obsolete.
```
Two versions of the SEIPD packet (differentiated by the version number) have been specified. Version 1, introduced in RFC4880, is used in OpenPGP v4 while SEIPD version 2 was introduced with OpenPGP v6. Both versions can be used with either OpenPGP v4 or v6 keys, although OpenPGP v4 keys need to announce support for SEIPD version 2 via the *Feature* signature subpacket.
When decrypted, the data contained in a SEIPD packet forms an [OpenPGP message](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-openpgp-messages). That is, the decrypted data consists of a series of OpenPGP packets.
In both versions of SEIPD, the decryptor must have obtained a *session key* in a previous step, before processing the SEIPD packet. Using this session key, the decryptor can decrypt the SEIPD packet and process the plaintext data that it contains.
### v1 SEIPD, based on MDC
The [version 1 SEIPD](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#version-one-seipd) mechanism is supported by all modern OpenPGP version 4 implementations. It was introduced in [RFC 4880](https://www.rfc-editor.org/rfc/rfc4880.html#section-5.13) as a replacement for the *SED* (Symmetrically Encrypted Data) packet. SEIPDv1 provides integrity protection of the ciphertext using a SHA-1 checksum of the plaintext as modification detection code.
Version 1 SEIPD can only be combined with [version 3 PKESK](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#v3-pkesk) and/or [version 4 SKESK](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#v4-skesk) packets.
In this version of the SEIPD packet, the session key is used directly as message key, meaning the payload is encrypted symmetrically using the session key.
When communicating with a mix of recipients, some of whose OpenPGP software only supports OpenPGP version 4, then this mechanism must be used.
```{figure} ../plain_svg/SEIPDv1-PKESK.svg
:name: fig-encryption-seipdv1-pkesk
:alt: Depicts a dotted hexagon labeled "Plaintext", from which a curved arrow passes another dotted hexagon "Session Key" and finally points to a "SEIPDv1" packet. Two more curved arrows originate from the session key and pass Alice' and Bob's encryption key, ending in two PKESK packets.
With SEIPDv1, the session key is directly used as message key to encrypt the payload
```
(quick-check-and-mdc)=
#### Preparing the plaintext with quick check and modification detection code
Before encrypting the plaintext, the data is modified by adding both a prepended "quick check", as well as an appended modification detection code.
The quick check comprises of 16 randomly chosen bytes plus 2 bytes which are the last two of the 16 random bytes repeated.
This mechanism is useful to quickly check, whether the correct session key was used when decrypting the message.
These quick-check bytes are prepended to the plaintext.
The modification detection code on the other hand is added to allow detection of unwanted modification of the ciphertext.
First, the two marker bytes `0xD3` and `0x14` are appended to the plaintext. Then, the SHA1 checksum of the entire plaintext including quick check and marker bytes is calculated and appended to the plaintext.
```{figure} ../plain_svg/mdc.svg
:name: fig-encryption-mdc
:alt: Depicts, how the prior to encryption, the plaintext bytes are prepended with 18 quick check bytes and appended with 22 bytes of modification detection code. The quick check comprises of 16 random bytes plus 2 repeated bytes. The modification detection code starts with the marker bytes 0xD314, followed by the SHA1 checksum of the entire plaintext including quick check and marker bytes.
The plaintext inside of a SEIPDv1 packet contains quick check bytes, the actual plaintext and modification detection code
```
Lastly, the whole prepared plaintext is encrypted symmetrically.
(seipd-v2)=
### v2 SEIPD, based on AEAD
The [version 2 SEIPD](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#version-two-seipd) mechanism was introduced in OpenPGP version 6. Consequently, it can only be used for encryption when all recipients explicitly announce support for it using a *Feature* signature subpacket.
It provides integrity protection of the ciphertext using *AEAD* (authenticated encryption with additional data).
v2 SEIPD can only be combined with either [version 6 PKESK](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#v6-pkesk) and/or [version 6 SKESK](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#v6-skesk) packets.
In version 2 SEIPD, the *session key* is transformed into a *message key*, based on a per-message salt value stored separately in the v2 SEIPD packet. The message key is then used in an AEAD scheme to encrypt the message payload.
```{note}
The session key can use a different symmetric algorithm than the message key.
```
```{figure} ../plain_svg/SEIPDv2-PKESK.svg
:name: fig-encryption-seipdv2-pkesk
:alt: TODO
With SEIPDv2, the message key is derived from the session key in an extra step.
```
This additional step introduces key-separation into the protocol, which protects against certain attacks, such as an [OpenPGP SEIP downgrade attack](https://www.metzdowd.com/pipermail/cryptography/2015-October/026685.html).
## Encrypt to multiple/single subkey per certificate?
A recipient's certificate can contain more than one usable encryption subkey.
This raises the question, should the message be encrypted for all of them?
There is the argument that a powerful attacker might have managed to add an attacker-controlled encryption subkey to the victim's certificate.
In this case, only encrypting to the "newest" encryption key would help uncover such an attack. However, a powerful attacker could just MitM any sent messages and just add a PKESK for the victim-controlled encryption keys to hide the fact that the sender used a different key.
On the other hand, a user might have multiple encryption subkeys on purpose.
Picture, for example, a scenario where the same certificate is used on multiple devices, but each device has dedicated encryption subkeys to allow for smoother revocation in case of a lost device.
In this scenario, it is important that the sender encrypts the message to all available encryption subkeys.
## "Negotiating" algorithms based on recipients preference subpackets
### Prevent "downgrade" -> Policy
Each implementation should define a "minimum" level of security when it comes to algorithms and key lengths.
If the lowest common denominator of symmetric encryption algorithms preferred by a set of recipients provides too little security, the implementation should either use a configured fallback algorithm instead, or fail to produce a message at all.
