RFC 9804: Simple Public Key Infrastructure (SPKI) S-Expressions

2 eadmund 4 7/3/2025, 10:41:45 AM datatracker.ietf.org ↗

Comments (4)

eadmund · 15h ago

After 29 years, Rivest’s S-expression draft is an RFC.

They are a straightforward, easy-to-parse S-expression format whose canonical representation is useful for cryptography. They are suitable as a general replacement for JSON, XML, HTML, ASN.1 and more.

eadmund · 14h ago

This XML (from https://www.w3schools.com/xml/note.xml):

    <note>
      <to>Tove</to>
      <from>Jani</from>
      <heading>Reminder</heading>
      <body>Don't forget me this weekend!</body>
    </note>

could be this S-expression:

    (note
     (to Tove)
     (from Jani)
     (heading Reminder)
     (body "Don't forget me this weekend"))

But if every note must have a body, this might make even more sense:

    (note
     (to Tove)
     (from Jani)
     (heading Reminder)
     "Don't forget me this weekend")

eadmund · 14h ago

This JSON (taken from https://www.w3schools.com/js/js_json_intro.asp):

    {"name":"John", "age":30, "car":null}

could be this S-expression:

    ((name John)
     (age 30)
     (car ()))

The canonical representation (suitable for cryptographic hashing) would be ((4:name4:John)(3:age2:30)(3:car())).

eadmund · 14h ago

The DER-encoded ASN.1 byte sequence Base64-encoded to MBMCAQUWDkFueWJvZHkgdGhlcmU/ could be represented as:

    ((tracking-number 5)
     (question "Anybody there?"))

While we are all familiar with opaque X.509 certificates such as (from https://www.fm4dd.com/openssl/source/PEM/certs/512b-rsa-exam...):

an SPKI certificate might be:

    (sequence
        (public-key
         (rsa-pkcs1-md5
          (e #11#)
          (n
           |ALNdAXftavTBG2zHV7BEV59gntNlxtJYqfWIi2kTcFIgIPSjKlHleyi9s
           5dDcQbVNMzjRjF+z8TrICEn9Msy0vXB00WYRtw/7aH2WAZx+x8erOWR+yn
           1CTRLS/68IWB6Wc1x8hiPycMbiICAbSYjHC/ghq2mwCZO7VQXJENzYr45|)))
        (do hash md5)
        (cert
         (issuer (hash md5 |+gbUgUltGysNgewRwu/3hQ==|))
         (subject
          (keyholder (hash md5 |+gbUgUltGysNgewRwu/3hQ==|)))
         (tag
          (* set
           (name "Carl M. Ellison")
           (street "207 Grindall St.")
           (city "Baltimore MD")
           (zip "21230-4103")))
         (not-after "1998-04-15_00:00:00"))
        (signature
         (hash md5 |54LeOBILOUpskE5xRTSmmA==|)
         (hash md5 |+gbUgUltGysNgewRwu/3hQ==|)
         |HU6ptoaEd7v4rTKBiRrpJBqDKWX9fBfLY/MeHyJRryS8iA34+nixf+8Yh/
         buBin9xgcu1lIZ3Gu9UPLnu5bSbiJGDXwKlOuhTRG+lolZWHaAd5YnqmV9h
         Khws7UM4KoenAhfouKshc8Wgb3RmMepi6t80Arcc6vIuAF4PCP+zxc=|)))

Note that this is not a translation of the X.509 certificate above, though — I pulled it from <https://datatracker.ietf.org/doc/html/draft-ietf-spki-cert-e...>. Note that this is a very 90s example: MD5 and a bespoke data format instead of SHA-2 and ISO 8601.

I think it’s clear that an SPKI certificate is much, much more readable.