[SOLVED] Can I trust self-signed SSL certificate?

x-yuri · 2018-03-27 13:49:17

Hi,

I'm trying to create a certificate for localhost for local development.

I've created a certificate:

$ openssl req -x509 -out localhost.crt -keyout localhost.key \
  -newkey rsa:2048 -nodes -sha256 \
  -subj '/CN=localhost' -extensions EXT -config <( \
   printf "[dn]\nCN=localhost\n[req]\ndistinguished_name = dn\n[EXT]\nsubjectAltName=DNS:localhost\nkeyUsage=digitalSignature\nextendedKeyUsage=serverAuth")

And want to make it trusted. I tried:

# trust anchor localhost.crt
# trust extract-compat

trust puts it in category other-entry (not authority). And it makes no difference. Browsers still complain about connection not being secure.

I also tried to add it to Chromium. When adding to Your Certificates it says, "The Private Key for this Client Certificate is missing or invalid." To Authorities, "localhost: Not a Certification Authority." What do I do?

Last edited by x-yuri (2018-03-28 23:43:30)

x-yuri · 2018-03-28 23:43:07

tl;dr Generate a certificate issued by own CA (see the script below)

Here's what I've found. Correct me where I'm wrong.

There are CA's (certificate authorities). They issue certificates (sign CSR's) for other CA's (intermediate CA's), or servers (end entity certificates). Some of them are root authorities. They have self-signed certificates, issued by themselves. That is, usually there's a chain of trust that goes from server certificate to root certificate. And there's noone to vouch for a root certicate. As such, OS'es have a root certificate store (or trust policy store), a systemwide list of trusted root certificates. Browsers have their own lists of trusted certificates, which consist of systemwide list plus certificates trusted by the user.

In Chromium you manage certificates at chrome://settings/certificates. In Firefox, Preferences > Privacy & Security > Certificates > View Certificates. Both have Authorities tab, which is a list of trusted root certificates. And Servers tab, a list of trusted server certificates.

To obtain a certificate you create CSR (certificate signing request), send it to CA. CA signs the CSR, turning it into trusted certificate in the process.

Certificates and CSR's are a bunch of fields with information plus public key. Some of the fields are called extensions. CA certificate is a certificate with "basicConstraints = CA:true".

You can inspect certificate errors in Chromium in Developer Tools > Security.

Trusting certificates systemwide

When you change OS' root certificate store, you've got to restart a browser. You change it with:

# trust anchor path/to/cert.crt
# trust anchor --remove path/to/cert.crt

`trust` puts CA certificates under "authority" category (`trust list`), or "other-entry" category otherwise. CA certificates appear in Authorities tab in browsers, or else in Servers tab.

Trusting certificates in a browser

In Chromium, and Firefox you can add (import) certificates to Authorities tab. If you try to import a non-CA certificate, you get "Not a Certificate Authority" message. After choosing a file, a dialog appears where you can specify trust settings (when to trust the certificate). The relevant setting for making a site work is "Trust this certificate for identifying websites."

In Chromium, you can add (import) certificates on Servers tab. But they end up either on Authorities tab (CA certificates, and you're not presented with trust settings dialog after choosing a file), or on Others tab (if non-CA certificate).

In Firefox, you can't exactly add a certificate to Servers tab. You add exceptions. And you can trust a certificate with no extensions at all there.

Self-signed certificate extensions

My system comes with the following default settings (extensions to be added) for certificates:

basicConstraints = critical,CA:true
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always,issuer

Taken from /etc/ssl/openssl.cnf, section v3_ca. More on it here.

Additionally, Chromium considers a certificate invalid, when it doesn't have:

subjectAltName = DNS:$domain

Non-self-signed certificate extensions

From section [ usr_cert ] of /etc/ssl/openssl.cnf:

basicConstraints = CA:FALSE
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid,issuer

When browsers trust a self-signed certificate

For Chromium to trust to a self-signed certificate it's got to have "basicConstraints = CA:true", and "subjectAltName = DNS:$domain". For Firefox not even this is enough:

basicConstraints = critical,CA:true
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always,issuer
subjectAltName = DNS:$domain

When browsers trust a certificate issued by own CA

Firefox needs no extensions, but Chromium requires subjectAltName.

Using openssl

`openssl genpkey -algorithm RSA -out "$domain".key` - generate private key

`openssl req -x509 -key "$domain".key -out "$domain".crt` - generate self-signed certificate

Without `-subj` it will ask questions regarding distinguished name (DN), like common name (CN), organization (O), locality (L). You can answer them "in advance": `-subj "/CN=$domain/O=$org"`.

To add subjectAltName extension, you've got to either have a config where it all is specified, or add a section to config and tell openssl its name with `-extensions` switch:

    -config <(cat /etc/ssl/openssl.cnf - <<END
[ x509_ext ]
basicConstraints = critical,CA:true
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always,issuer
subjectAltName = DNS:$domain
END
    ) -extensions x509_ext

`openssl req -new -key "$domain".key -out "$domain".csr` - generate CSR, it can take `-subj` option

`openssl x509 -req -in "$domain".csr -days 365 -out "$domain".crt \
-CA ca.crt -CAkey ca.key -CAcreateserial` - sign CSR

Doesn't work without `-CAcreateserial`. It creates a ca.srl file, where it keeps serial number of the last generated certificate. To add subjectAltName, you're gonna need `-extfile` switch:

    -extfile <(cat <<END
basicConstraints = CA:FALSE
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid,issuer
subjectAltName = DNS:$domain
END
    )

`openssl req -in $domain.csr -text -noout` - view CSR

`openssl x509 -in $domain.crt -text -noout` - view certificate

Generate self-signed certificate

(you're gonna need an exception in Firefox for it to work)

#!/usr/bin/env bash
set -eu
org=localhost
domain=localhost

sudo trust anchor --remove "$domain".crt || true

openssl genpkey -algorithm RSA -out "$domain".key
openssl req -x509 -key "$domain".key -out "$domain".crt \
    -subj "/CN=$domain/O=$org" \
    -config <(cat /etc/ssl/openssl.cnf - <<END
[ x509_ext ]
basicConstraints = critical,CA:true
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always,issuer
subjectAltName = DNS:$domain
END
    ) -extensions x509_ext

sudo trust anchor "$domain".crt

Generate a certificate issued by own CA

#!/usr/bin/env bash
set -eu
org=localhost-ca
domain=localhost

sudo trust anchor --remove ca.crt || true

openssl genpkey -algorithm RSA -out ca.key
openssl req -x509 -key ca.key -out ca.crt \
    -subj "/CN=$org/O=$org"

openssl genpkey -algorithm RSA -out "$domain".key
openssl req -new -key "$domain".key -out "$domain".csr \
    -subj "/CN=$domain/O=$org"

openssl x509 -req -in "$domain".csr -days 365 -out "$domain".crt \
    -CA ca.crt -CAkey ca.key -CAcreateserial \
    -extfile <(cat <<END
basicConstraints = CA:FALSE
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid,issuer
subjectAltName = DNS:$domain
END
    )

sudo trust anchor ca.crt

Webserver configuration

Nginx:

server {
    listen  443  ssl;
    ssl_certificate  ssl/localhost.crt;
    ssl_certificate_key  ssl/localhost.key;
    ...

Morbo:

carton exec morbo --listen='https://*:3000?cert=localhost.crt&key=localhost.key' site.pl

P.S. I'm running Chromium 65.0.3325.162, Firefox 59.0, and openssl-1.1.0.g.

Additional reading

OpenSSL
genpkey
req
x509
OpenSSL Certificate Authority
Certificates for localhost
iamaCA - Become your own certificate authority and dispense certifications
Firefox and Self-Signed Certs
Bypassing certificate error page in Chrome

Last edited by x-yuri (2018-03-29 09:37:28)

Lone_Wolf · 2018-03-29 12:12:36

Nice overview, maybe you can put it in arch wiki ?

Arch Linux

#1 2018-03-27 13:49:17