ed25519 signing does not work

[Sami Farin]

Using Ubuntu 20.04 LTS.

# apt-cache policy opendkim
opendkim:
  Installed: 2.11.0~beta2-1
  Candidate: 2.11.0~beta2-1
  Version table:
 *** 2.11.0~beta2-1 500
        500 http://archive.ubuntu.com/ubuntu focal/universe amd64 Packages
        100 /var/lib/dpkg/status

# opendkim -V
opendkim: OpenDKIM Filter v2.11.0
 Compiled with OpenSSL 1.1.1f 31 Mar 2020
 SMFI_VERSION 0x1000001
 libmilter version 1.0.1
 Supported signing algorithms:
  rsa-sha1
  rsa-sha256
  ed25519-sha256
 Supported canonicalization algorithms:
  relaxed
  simple
 Active code options:
  QUERY_CACHE
  USE_DB
  USE_LDAP
  USE_LUA
  USE_ODBX
  USE_UNBOUND
  _FFR_ATPS
  _FFR_RBL
  _FFR_REPLACE_RULES
  _FFR_SENDER_MACRO
  _FFR_STATS
  _FFR_VBR
 libopendkim 2.11.0: atps query_cache

However, when configured to use "MultipleSignatures yes" and two Selectors:

opendkim[113250]: 6307460C4C: SSL error:0607907F:digital envelope routines:EVP_PKEY_get0_RSA:expecting an rsa key
opendkim[113250]: 6307460C4C: dkim_eom(): resource unavailable: EVP_PKEY_get1_RSA() failed

[David Bürgin (glts)]

Unfortunately this is not supported by OpenDKIM today, see:
https://github.com/trusteddomainproject/OpenDKIM/issues/6

OpenDKIM in this version only supports a single signature (parameter
SignatureAlgorithm), eg *either* ‘rsa-sha256’ *or* ‘ed25519-sha256’.