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# Encryption

[Encryption](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#section-2.1) is one of the core facilities of OpenPGP. It provides confidentiality.

## High-Level overview of the message encryption process

Core concept:

- The plaintext is encrypted with a symmetric "session key."
- The "session key" itself is stored in encrypted form, possibly multiple times:
  - The session key is encrypted to the encryption keys of each intended recipient of the message.
  - Alternatively, or additionally, the session key may be encrypted using a passphrase (this mode of operation doesn't require any OpenPGP certificates.)

## Generations of encryption

### SEIPD w/ AEAD (v2)

### SEIPD (v1)

## Advanced topics

### Encrypt for multiple/single subkey per certificate?

### "Negotiating" algorithms based on recipients preference subpackets

#### Prevent "downgrade" -> Policy

### Implications of how a recipient cert is "addressed" (fingerprint/key-ID vs. user-ID) (preferences, expiration, revocation)

## Zooming in: Packet structure

### Encryption yields a 'wrapped' openpgp packet stream

### SKESK

Also see https://flowcrypt.com/docs/guide/send-and-receive/send-password-protected.html