Cert embedding a X25519 pubkey, signed by an Ed25519 key (not self signed)

Hey there,

I have a fairly unorthodox use case (that is nonetheless conforming to all specs). I want to embed an x25519 public key into a certificate as the subject PKI. That requires a public key algorithm of X25519 == OID 1.3.101.110; but not being a digital signature algorithm means it cannot self sign itself. That is accepted according to the specs.

Furthermore, I want to be able to sign this cert using an ed25519 private key directly, NOT a CA. That is required for use cases where the root key is not some CA, rather a pinned key (i.e. hard coded well known key). This is again something supported by the spec.

You can generate such a certificate with openssl:

openssl genpkey -algorithm x25519 -out my-x25519-key
openssl pkey -in my-x25519-key -pubout -out my-x25519-key.pub

openssl genpkey -algorithm ed25519 -out my-ed25519-key

openssl x509 -new -force_pubkey my-x25519-key.pub -signkey my-ed25519-key -subj="/CN=test" -out my.cert

You can inspect it via:

openssl x509 -in my.cert -text -noout

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            3a:cb:5f:69:46:b1:5d:6a:91:a2:48:0e:cd:d5:24:7c:44:8b:9c:d1
        Signature Algorithm: ED25519
        Issuer: CN=test
        Validity
            Not Before: May 23 14:22:45 2025 GMT
            Not After : Jun 22 14:22:45 2025 GMT
        Subject: CN=test
        Subject Public Key Info:
            Public Key Algorithm: X25519
                X25519 Public-Key:
                pub:
                    17:c3:5b:94:28:bc:e4:75:b6:3b:fc:84:df:eb:3e:
                    9c:a0:91:3b:c4:f6:33:c6:4a:d5:4d:8e:c8:76:4d:
                    89:41
        X509v3 extensions:
            X509v3 Subject Key Identifier: 
                B9:49:DA:F2:64:6D:26:99:FB:CF:F7:A0:C1:37:C7:1F:51:C3:BF:D8
            X509v3 Authority Key Identifier: 
                88:97:CF:C8:82:5F:6D:11:4B:76:D5:59:34:28:06:A8:12:9E:5A:0B
    Signature Algorithm: ED25519
    Signature Value:
        90:e5:16:28:1a:e8:77:bd:21:78:7a:ed:ef:15:f6:18:f2:a8:
        5b:55:30:57:38:51:4b:4a:12:a4:53:63:ec:56:e2:af:7e:e9:
        65:34:c9:fb:d4:98:d3:41:38:24:94:ee:9c:cd:02:a4:92:f1:
        32:e6:34:c9:35:72:3e:ea:68:06

or

openssl asn1parse -inform PEM -in my.cert -i

    0:d=0  hl=4 l= 289 cons: SEQUENCE          
    4:d=1  hl=3 l= 212 cons:  SEQUENCE          
    7:d=2  hl=2 l=   3 cons:   cont [ 0 ]        
    9:d=3  hl=2 l=   1 prim:    INTEGER           :02
   12:d=2  hl=2 l=  20 prim:   INTEGER           :3ACB5F6946B15D6A91A2480ECDD5247C448B9CD1
   34:d=2  hl=2 l=   5 cons:   SEQUENCE          
   36:d=3  hl=2 l=   3 prim:    OBJECT            :ED25519
   41:d=2  hl=2 l=  15 cons:   SEQUENCE          
   43:d=3  hl=2 l=  13 cons:    SET               
   45:d=4  hl=2 l=  11 cons:     SEQUENCE          
   47:d=5  hl=2 l=   3 prim:      OBJECT            :commonName
   52:d=5  hl=2 l=   4 prim:      UTF8STRING        :test
   58:d=2  hl=2 l=  30 cons:   SEQUENCE          
   60:d=3  hl=2 l=  13 prim:    UTCTIME           :250523142245Z
   75:d=3  hl=2 l=  13 prim:    UTCTIME           :250622142245Z
   90:d=2  hl=2 l=  15 cons:   SEQUENCE          
   92:d=3  hl=2 l=  13 cons:    SET               
   94:d=4  hl=2 l=  11 cons:     SEQUENCE          
   96:d=5  hl=2 l=   3 prim:      OBJECT            :commonName
  101:d=5  hl=2 l=   4 prim:      UTF8STRING        :test
  107:d=2  hl=2 l=  42 cons:   SEQUENCE          
  109:d=3  hl=2 l=   5 cons:    SEQUENCE          
  111:d=4  hl=2 l=   3 prim:     OBJECT            :X25519
  116:d=3  hl=2 l=  33 prim:    BIT STRING        
  151:d=2  hl=2 l=  66 cons:   cont [ 3 ]        
  153:d=3  hl=2 l=  64 cons:    SEQUENCE          
  155:d=4  hl=2 l=  29 cons:     SEQUENCE          
  157:d=5  hl=2 l=   3 prim:      OBJECT            :X509v3 Subject Key Identifier
  162:d=5  hl=2 l=  22 prim:      OCTET STRING      [HEX DUMP]:0414B949DAF2646D2699FBCFF7A0C137C71F51C3BFD8
  186:d=4  hl=2 l=  31 cons:     SEQUENCE          
  188:d=5  hl=2 l=   3 prim:      OBJECT            :X509v3 Authority Key Identifier
  193:d=5  hl=2 l=  24 prim:      OCTET STRING      [HEX DUMP]:301680148897CFC8825F6D114B76D559342806A8129E5A0B
  219:d=1  hl=2 l=   5 cons:  SEQUENCE          
  221:d=2  hl=2 l=   3 prim:   OBJECT            :ED25519
  226:d=1  hl=2 l=  65 prim:  BIT STRING 

Would be nice to have support for generating these keys via rcgen too.

Start by reproducing the certificate and key setup with the OpenSSL commands in the issue, then inspect rcgen's certificate-generation APIs for how subject public keys and signing keys are represented. Done means rcgen can generate a certificate embedding an X25519 public key and signed directly with an Ed25519 private key, matching the shown structure.