Ed25519 certificates in Tor

1. Scope and Preliminaries

   This document describes a certificate format that Tor uses for
   its Ed25519 internal certificates.  It is not the only
   certificate format that Tor uses.  For the certificates that
   authorities use for their signing keys, see dir-spec.txt.
   Additionally, Tor uses TLS, which depends on X.509 certificates;
   see tor-spec.txt for details.

   The certificates in this document were first introduced in
   proposal 220, and were first supported by Tor in Tor version
   0.2.7.2-alpha.

1.1. Signing

   All signatures here, unless otherwise specified, are computed
   using an Ed25519 key.

   In order to future-proof the format, before signing anything, the
   signed document is prefixed with a personalization string, which
   will be different in each case.

2. Document formats

2.1. Certificates

   When generating a signing key, we also generate a certificate for it.
   Unlike the certificates for authorities' signing keys, these
   certificates need to be sent around frequently, in significant
   numbers.  So we'll choose a compact representation.

         VERSION         [1 Byte]
         CERT_TYPE       [1 Byte]
         EXPIRATION_DATE [4 Bytes]
         CERT_KEY_TYPE   [1 byte]
         CERTIFIED_KEY   [32 Bytes]
         N_EXTENSIONS    [1 byte]
         EXTENSIONS      [N_EXTENSIONS times]
         SIGNATURE       [64 Bytes]

   The "VERSION" field holds the value [01].  The "CERT_TYPE" field
   holds a value depending on the type of certificate. (See appendix
   A.1.) The CERTIFIED_KEY field is an Ed25519 public key if
   CERT_KEY_TYPE is [01], or a SHA256 hash of some other key type
   depending on the value of CERT_KEY_TYPE. The EXPIRATION_DATE is a
   date, given in HOURS since the epoch, after which this
   certificate isn't valid. (A four-byte field here will work fine
   until 10136 A.D.)

   The EXTENSIONS field contains zero or more extensions, each of
   the format:

         ExtLength [2 bytes]
         ExtType   [1 byte]
         ExtFlags  [1 byte]
         ExtData   [ExtLength bytes]

   The meaning of the ExtData field in an extension is type-dependent.

   The ExtFlags field holds flags; this flag is currently defined:

      1 -- AFFECTS_VALIDATION. If this flag is present, then the
           extension affects whether the certificate is valid; clients
           must not accept the certificate as valid unless they
           understand the extension.

   It is an error for an extension to be truncated; such a
   certificate is invalid.

   Before processing any certificate, parties SHOULD know which
   identity key it is supposed to be signed by, and then check the
   signature.  The signature is formed by signing the first N-64
   bytes of the certificate prefixed with the string "Tor node
   signing key certificate v1".

2.2. Basic extensions

2.2.1. Signed-with-ed25519-key extension [type 04]

   In several places, it's desirable to bundle the key signing a
   certificate along with the certificate.  We do so with this
   extension.

        ExtLength = 32
        ExtData =
           An ed25519 key    [32 bytes]

   When this extension is present, it MUST match the key used to
   sign the certificate.

A.1. List of certificate types

   The values marked with asterisks are not types corresponding to
   the certificate format of section 2.1.  Instead, they are
   reserved for RSA-signed certificates to avoid conflicts between
   the certificate type enumeration of the CERTS cell and the
   certificate type enumeration of in our Ed25519 certificates.


   **[00],[01],[02],[03] - Reserved to avoid conflict with types used
          in CERTS cells.

   [04] - Ed25519 signing key with an identity key
          (see prop220 section 4.2)

   [05] - TLS link certificate signed with ed25519 signing key
          (see prop220 section 4.2)

   [06] - Ed25519 authentication key signed with ed25519 signing key
          (see prop220 section 4.2)

   [07] - RSA identity cross-certification
          (see prop220 section 4.2)

   [0A] - ntor onion key cross-certifying ntor identity key
          (see prop228 section 2.3)

A.2. List of extension types

   [01] - signed-with-ed25519-key (section 2.2.1)

A.3. List of signature prefixes

   We describe various documents as being signed with a prefix. Here
   are those prefixes:

   "Tor node signing key certificate v1" (section 2.1)
   "Tor router descriptor signature v1" (see dir-spec.txt)

A.4. List of certified key types

   [01] ed25519 key
   [02] SHA256 hash of an RSA key
   [03] SHA256 hash of an X.509 certificate