Locate the Public Key

Public keys may be distributed in many ways, but for email only four methods are available:

  1. Public keys may be given to a recipient out-of-band, as, for example, by delivery of a floppy disk containing the public keys. Using an out-of-band method, the public keys may be stored long before the email is sent.

  2. Public keys may be embedded in a message. Typically, they are located in the header part of a message as part of a special header. For example:

    X-Public-Key: c3NoLWRzcyBBQUFBQjNOemFDMWtjM01BQUFDQkFKOTh2MXloQVp
    VWjBYM3ZMUVhiemVwY1hienkvdnh6T1NEN3E0a25Ed3loSWFoYm
    dLclMzK2RIV3hzOUErSTRrV2YrODlBLzMzU3VGOCtBOFdwUTc2W
    ld2K2JHMUZiUzg0WW5XeWtNUlY3Z3NzY2VlQUs4OXM2ZDcvSlR3
    VDhiZi9OVTFlT2hvWUdjamJONFBHVHhHajB1bW9nWlBaRiswdEZ
    SMm45b3hVcFpBQUFBRlFDdGpUUVBCS016cXM1Y0QxZVAydXJEZX
    NXSERRQUFBSUFwMEg5dG9YZ21yekJJdjN0aUtVWWcrN0JvSndsW
    HdWTnNiR1lPcVlzdWpxUlZKUWQ0SXRlcVo1WVo2VG5Rbk5DZUho
    V2tjVFNPa3NFcVhsemlIemtudS9pRUp4MTloQnlYaXFzYmlQQ2V
    ZRU1pZUp2Z2crWWZVQTlXb0QwWk00bEs2VHhKUTB2U09PV3E0Yn
    ZFYzNCMzI3ZGh6dS9QaGNqenNNLzMzQ05pVHdBQUFJQXNMWVduU
    HFMNnVkNFR0RTRFYXIyVXBaQ282WEg1ZDk2cVRHNHhUdlpLMnpl
    NTVyRi9Rc1pXNVdod2ZvYkhRWmM5WlRRZzdMeFRtSFhDZmVHT1U
    3eGhrTGpPUTJqMVB0ZXlYd2FTUVpiek1ITU8zaW10ejNwdVB4Vn
    J5a0owTVc0NHdPd1VzbWRvSElqOE5Za094QmNzU1FLUzN6NTdXb
    0VOSnZKbFZuSjBjdz09IGJjeEBsYWR5Cg==

  3. Public keys may be downloaded. DKIM, for example, specifies that public keys be downloaded using DNS:

    % dig txt mypub._domainkey.example.com

  4. A special header may specify a web URL:

    X-Public-Key-Location: https://www.example.com/keys/email/A459b.pub ...

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