Nostr mail large MIME support

Overview#

This specification defines how to handle large emails (MIME > 65KB) by storing encrypted MIME messages on Blossom servers. Small emails remain stored inline in the kind 1301 event content.

Motivation#

NIP-44 encryption has a hard limit of 65535 bytes for plaintext messages.

Traditional emails can easily exceed this limit. This spec provides a hybrid approach:

• Small emails (< 60KB): MIME stored inline in kind 1301 content field
• Large emails (≥ 60KB): MIME encrypted with AES-GCM and stored on Blossom, kind 1301 contains reference + decryption key

This allows Nostr Mail to support emails of any size.

Event Kind 1301: Email#

Inline MIME (small emails < 65KB)#

{
  "kind": 1301,
  "pubkey": "<sender_npub>",
  "tags": [
    ["p", "<recipient_npub>"],
    ["subject", "Hello"],
    ["from", "alice@nostr.mail"],
    ["to", "bob@example.com"],
    ["date", "1735401600"]
  ],
  "content": "From: alice@nostr.mail\nTo: bob@example.com\nSubject: Hello\nDate: Sat, 28 Dec 2024 12:00:00 +0000\n\nHey Bob!"
}

Blossom MIME (large emails ≥ 65KB)#

{
  "kind": 1301,
  "pubkey": "<sender_npub>",
  "tags": [
    ["p", "<recipient_npub>"],
    ["encryption-algorithm", "aes-gcm"],
    ["decryption-key", "<base64_key>"],
    ["decryption-nonce", "<base64_nonce>"],
    ["x", "<sha256_hash>"],
    ["subject", "Hello"],
    ["from", "alice@nostr.mail"],
    ["to", "bob@example.com"],
    ["date", "1735401600"]
  ],
  "content": ""
}

Tags#

Tag	Required	Description
p	Yes	Recipient pubkey (or bridge pubkey)
encryption-algorithm	Conditional	Encryption algorithm aes-gcm (only for Blossom MIME)
decryption-key	Conditional	The decryption key (only for Blossom MIME)
decryption-nonce	Conditional	The decryption nonce (only for Blossom MIME)
x	Conditional	SHA-256 hex of encrypted MIME (only for Blossom MIME)
subject	No	Email subject
from	No	From address
to	No	To address
date	No	Email timestamp (Unix epoch)

Detection#

Clients detect the storage mode:

Condition	Mode	Action
content non-empty	Inline MIME	Parse content directly
content empty, x tag exists	Blossom MIME	Download from Blossom, then decrypt

Encryption Scheme#

Inline MIME (small emails)#

MIME is stored in plaintext in the content field.

Blossom MIME (large emails)#

The MIME content is encrypted using AES-GCM.

RFC 2822 MIME message
    ↓
Generate random 256-bit key and 96-bit nonce
    ↓
AES-GCM Encrypt (key, nonce, mime_bytes)
    ↓
Encrypted MIME blob
    ↓
Upload to recipient Blossom servers
    ↓
Create kind 1301 with x tag + decryption key + nonce