How should I generate a self signed SSL for an intranet server, following these specific requisites?

My knowledge about SSL is very basic and I need to generate self-signed SSL certificates for a local server [server.local] so I can use it in a mobile app for SSL pinning.

How do I create a self-signed certificate, following the rules established by Apple below, via openssl x509?

• All TLS server certificates must comply with these new security requirements in iOS 13 and macOS 10.15; connections to TLS servers violating these new requirements will fail and may cause network failures, apps to fail, and websites to not load in Safari:
  • Certificates and issuing CAs using RSA keys must use key sizes ≥2048 bits; RSA key sizes <2048 bits are no longer trusted
  • Certificates, and issuing CAs, must use a hash algorithm from the SHA2 family in the signature algorithm; SHA1 signed certificates are no longer trusted for TLS
  • Certificates must present the DNS name of the server in the SubjectAltName extension of the certificate; DNS names in the CommonName of a certificate are no longer trusted
  • Certificates issued after July 1, 2019, as indicated in the NotBefore field of the certificate, must follow these guidelines:
    • Certificates must contain an extendedKeyUsage (EKU) extension containing the id-kp-serverAuth OID
    • Certificates must have a validity period of ≤825 days, as expressed in the NotBefore and NotAfter fields of the certificate.

I created the certificate via:

openssl genrsa -des3 -out myCA.key 2048
openssl req -x509 -new -nodes -key myCA.key -sha256 -days 825 -out myCA.pem

After installing it on the server and device, when I try to use this inside iOS, I get error:

SSL hostname does not match name(s) in certificate,
Extended key usage does not match certificate usage, Root is not trusted`

1 Answer

Self-signed certificates should never be used, as they have no Chain of Trust and are therefore insecure and pointless; the recommended way is to create a self-signed CA, using that CA to sign the certificate via an openssl.cnf, such as this custom one.

• The commonName [CN] cannot be an IP or DNS name, since doing so is insecure; I always recommend using the CN to name the cert (e.g. Apache Web Server X, VPN Server 1, etc.)
• SAN [SubjectAltName] profiles can contain:
  • email: email address
  • URI: Uniform Resource Indicator
  • DNS: DNS domain name [local or FQDN]
  • ID: Registered ID: Object Identifier [OID]
  • IP: IP address
  • dirName: Distinguished Name
  • otherName: OID;content [arbitrary data associated with an OID]
    ^{(Content value is in standard ASN1_generate_nconf format)}

Execute all commands from within the directory that will contain openssl.cnf:

1. Download the custom openssl.cnf: (Linux/Windows)

   # Linux: wget
   # Windows: wget 

   
   
2. Edit the SAN profiles of the certs to be created: (lines 151 - 244)
   1. CA: Edit line 170 [Router.1] to name your CA

      [ alt_ca_main ]
      DNS.1 = Router.1
      IP.1 = 127.0.0.1

      • [ alt_ca_main ]
alt_: specifies a SAN profile
        ca_main: custom name for the SAN profile
        ^{(If renaming: update CA V3 profile line 264 [@alt_ca_main])}
      • DNS.1:
        Names the CA to make it easier when managing multiple CAs/ICAs
      • IP.1:
        Some router OSes require the loopback IP to be specified in the master CA
   2. Client/Server Cert: Edit lines 189 - 193

      [ alt_sophos ]
      IP.1 = 192.168.2.1
      IP.2 = 127.0.0.1
      DNS.1 = UTM.WRT
      DNS.2 = your.ddns.com

      • [ alt_sophos ]:
        SAN profile name; change sophos only if wishing to rename
        ^{(If renaming: update V3 profile lines 331 [v3_sophos] and 337 [@alt_sophos])}
      • IP.1:
        Server IP (if accessible via SSH, loopback IP must be specified [IP.2])
        ^{(list additional IPs on new lines with chronological listings; e.g. IP.3)}
      • DNS.1:
        LocalHostname.LocalDomain
      • DNS.2:
        DDNS/FQDN
        ^{(list additional names on new lines with chronological listings; e.g. DNS.3)}
3. Create prerequisite files and directories: (lines 436 - 455)

   mkdir crl; echo 01 > crl\crlnumber; echo > index; echo > rand; echo 00 > serial

   • crlnumber:
     serial for the next CRL [Certificate Revocation List] signed (lines 443 - 446)
   • index:
     list of all certs created (lines 448 - 451 and 642 - 686)
     • openssl ca must be used to maintain the index automatically, which I don't cover, as it overcomplicates the process (manually maintain: lines 642 - 686)
   • rand:
     random characters used for certificate/key creation (lines 453 - 455)
   • serial:
     serial [hex] of last cert signed, can be any number (lines 438 - 441 and 671 - 681)
4. Create CA:

   # CA key should have a secure passphrase of at least 20 characters, containing:
   # 2 uppercase, 2 lowercase, 2 numbers, and 2 symbols # Request: openssl req -x509 -new -sha512 -days 3650 -newkey rsa:4096 -keyout 'CA.key.pem' -out 'CA.crt.pem' -config '.\openssl.cnf' -extensions v3_ca # Generate CA CRL Cert: openssl ca -gencrl -keyfile 'CA.key.pem' -cert 'CA.crt.pem' -out 'CA.crl.pem' -config '.\openssl.cnf' # Convert CA CRL Cert to DER CRL: openssl crl -inform PEM -in '.\CA.crl.pem' -outform DER -out '.\CA.crl'

   ^{Most can ignore, as vast majority are fine using RSA:
   Type of key chosen determines the Key Exchange Algorithms that can be used [below]}
   • It's imperative the CA V3 profile's KUs are not altered (lines 260 - 265)
5. (Optional) Create ICA [Intermediate CA]:
   ^{Useful if needing multiple CAs to lower risk of any one being compromised}

   # ICA key should have a secure passphrase of at least 20 characters, containing:
   # 2 uppercase, 2 lowercase, 2 numbers, and 2 symbols # Request: openssl req -out '.\ICA.csr' -new -days 3650 -sha512 -newkey rsa:4096 -keyout 'ICA.key.pem' -config '.\openssl.cnf' -extensions v3_ica_router2 # Sign ICA with CA: openssl x509 -req -sha512 -days 3650 -in '.\ICA.csr' -CA 'CA.crt.pem' -CAkey 'CA.key.pem' -CAserial '.\serial' -out 'ICA.crt.pem' -extfile '.\openssl.cnf' -extensions v3_ica_router2 # Generate ICA CRL Cert: openssl ca -config '.\openssl.cnf' -gencrl -keyfile 'ICA.key.pem' -cert 'ICA.crt.pem' -out '.\ICA.crl.pem' # Convert ICA CRL Cert to DER CRL: openssl crl -inform PEM -in '.\ICA.crl.pem' -outform DER -out '.\ICA.crl' # Concatenate ICA and CA Cert: # Linux: cat './ICA.crt.pem' './CA.crt.pem' > './CA-ICA-Chain.crt.pem' # Windows: cmd /c type '.\ICA.crt.pem' '.\CA.crt.pem' > '.\CA-ICA-Chain.crt.pem'

   ^{Most can ignore, as vast majority are fine using RSA:
   Type of key chosen determines the Key Exchange Algorithms that can be used [below]}
   • It's imperative the ICA V3 profiles' KUs are not altered (lines 267 - 279)
   • ICA V3 profiles contian pathlen:0 (they can sign certs, but not CAs/ICAs); if pathlen is not specified/number set, it can sign an infinite/specified number of CAs/ICAs
6. Create certificates: (if V3 profile was renamed in 2.2, update here)

   # Server certs: add -nodes to the end of the Request command; else server requires
   # manually entering encryption passphrase when starting (impractical) # Request: openssl req -out '.\server.csr' -new -days 825 -sha512 -newkey rsa:2048 -keyout '.\server.key.pem' -config '.\openssl.cnf' -extensions v3_sophos -nodes # Sign: # CA only: openssl x509 -req -sha512 -days 825 -in '.\server.csr' -CA '.\CA.crt.pem' -CAkey '.\CA.key.pem' -CAserial '.\serial' -out '.\server.crt.pem' -extfile '.\openssl.cnf' -extensions v3_sophos # ICA: (change to) -CA '.\CA-ICA-Chain.crt.pem' -CAkey '.\ICA.key.pem' # (Optional) Export: # CA only: openssl pkcs12 -export -out '.\server.p12' -inkey '.\server.key.pem' -in '.\server.crt.pem' -certfile 'CA.crt.pem' # ICA: (change to) -certfile 'CA-ICA-Chain.crt.pem' # CA - ICA chain cert must be exported to maintain the Chain of Trust of: Cert → ICA → CA

   ^{Most can ignore, as vast majority are fine using RSA:
   Type of key chosen determines the Key Exchange Algorithms that can be used [below]}

References:

Certificate Verification:

• Certificate:

  openssl x509 -text -noout -in 'cert.crt.pem'

• Certificate Signing Request [CSR]:

  openssl req -text -noout -verify -in 'cert.csr'

• Key:

  openssl rsa -check -in 'cert.key.pem'

• PKCS12 [.pfx/.p12]:

  openssl pkcs12 -info -in 'cert.p12'

  
  

Hex ⟷ Dec Conversion:

• Linux:

  # hex → dec: (returns 10) printf '%d\n' 0x0a
  # dec → hex: (returns 0a) printf '%x\n' 10

• Windows:
  Calculator has programmer mode

index:

• # Manually maintain the index file by inputting one cert entry per line in the following format:
  # 1 2-----------> 3-> 4> 5-----> 6----------------------------------------------------------------------------> V 261231235959Z 0a unknown /C=US/ST=State/L=Locality/O=Org/OU=Unit/CN=Common Name/emailaddress= # 1 Status of Certificate: V # Valid R # Revoked E # Expired # 2 Expiration Date: YYMMDDHHMMSSZ # Date format followed by 'Z' (2026.12.31 @ 23:59:59) # 3 Revocation Date: (empty if not revoked) YYMMDDHHMMSSZ,reason # Valid reasons are: ## keyCompromise ## CACompromise ## affiliationChanged ## superseded ## cessationOfOperation ## certificateHold ## privilegeWithdrawn ## AACompromise # Certain distros error out without a whitespace for 3 # 4 Serial number in hex format: 0a # hex for 10 # 5 Certificate Filename or Literal String unknown # Certificate filename or literal string 'unknown' # 6 Distinguished Name

Key Exchange Algorithms:

• RSA:
  Encrypts a random value, chosen by the client, via the server public key
  • Required:
    Server public key must be an RSA key
    Server certificate must have KU keyAgreement
• DH_RSA:
  Key exchange occurs via a static Diffie-Hellman key
  • Required:
    CA must be using an RSA signing key
    Server public key must be a Diffie-Hellman key
    Diffie-Hellman key must have been issued by a CA
• DH_DSA:
  Like DH_RSA, except the CA used a DSA key in lieu of RSA
• DHE_RSA:
  Key exchange occurs via an Ephemeral Diffie-Hellman
  • Server dynamically generates & signs a DH public key, sending it to the client
  • Required:
    Server public key must be an RSA key
    Server certificate must have KU digitalSignature
• DHE_DSA:
  Like DHE_RSA, except the CA used a DSA key in lieu of RSA

Elliptic-Curve Key Exchange Algorithms:

• ECDH_RSA:
  Like DH_RSA, but with elliptic-curves
  • Required:
    Server public key must be an ECDH key
    Server certificate must be issued by a CA using an RSA public key
• ECDH_ECDSA:
  Like ECDH_RSA, except the CA used an ECDSA key
• ECDHE_RSA: Server sends dynamically generated EC Diffie-Hellman key, signing it via it's RSA key
  • Server public key signs the Ephemeral EC Diffie-Hellman key
• ECDHE_ECDSA:
  Like ECDHE_RSA, except server public key is an ECDSA key
  • Equivalent to DHE_DSS, but with elliptic curves for both the Diffie-Hellman & signature

KUs & EKUs:

• See this answer

Man Pages:

• openssl
  • Config
    V3 Profile
  • ca
  • crl
  • pkcs12
  • req
  • rsa
  • verify
  • x509
• printf