typedef struct { ... } SilcAttributeObjPk;

    Data type for public key, certificate or digital signatures.  The
    caller must free the data inside the structure.  The 'type' is one
    of following: "silc-rsa", "silc-dss, "ssh-rsa", "ssh-dss",
    "pgp-sign-rsa", "pgp-sign-dss", "x509v3-sign-rsa", "x509v3-sign-dss".
    The 'type' is NULL when this structure includes a digital signature.

    In SILC, at least the "silc-rsa" must be supported.  In this case
    the key is normal SILC Public key.  To verify a signature with the
    SILC Public key, construct the signature data with the
    silc_attribute_get_verify_data and verify the signature with
    for example silc_pkcs_verify_with_hash function.  The public key
    to the verification is the `data' and `data_len', and can be decoded
    with silc_pkcs_public_key_decode function.

    typedef struct SilcAttributeObjPkStruct {
      char *type;                   /* public key/certificate type, NULL
                                       when contains digital signature. */
      unsigned char *data;          /* public key/cert/signature data. The
                                       encoding depends of the `type'. */
      SilcUInt32 data_len;          /* data length */
    } SilcAttributeObjPk;