OpenSSL Cookbook Notes and Summaries#

One of the most important open source projects. It is widely used and a large portion of internet infrastrcuture relies on it.

Project contents:

  • Key cryptographic algorithms
  • Complete TLS and PKI Stack
  • Command line toolkit

If you are in IT security, web development and system administration it is an unavoidable tool.

OpenSSL, which is not very well documented; what you can find on the Internet is often wrong and outdated

1. OpenSSL#

Consists of:

  • Cryptographic library
  • TLS Toolkit

Licensing of OpenSSL is a mess and GnutLS is favoured.

Getting Started#

OpenSSL Versions#

On macOS:

$ openssl version
LibreSSL 2.2.7

On ubuntu:

$ openssl version
OpenSSL 1.1.1f  31 Mar 2020

On debian:

$ openssl version
OpenSSL 1.1.1g  21 Apr 2020

You can get more information with:

openssl version -a

OpenSSL 1.1.1g  21 Apr 2020
built on: Tue Apr 21 14:33:04 2020 UTC
platform: debian-amd64
options:  bn(64,64) rc4(16x,int) des(int) blowfish(ptr) 
compiler: gcc -fPIC -pthread -m64 -Wa,--noexecstack -Wall -Wa,--noexecstack -g -O2 -fdebug-prefix-map=/build/openssl-RvYbbo/openssl-1.1.1g=. -specs=/usr/share/dpkg/no-pie-compile.specs -fstack-protector-strong -Wformat -Werror=format-security -DOPENSSL_USE_NODELETE -DL_ENDIAN -DOPENSSL_PIC -DOPENSSL_CPUID_OBJ -DOPENSSL_IA32_SSE2 -DOPENSSL_BN_ASM_MONT -DOPENSSL_BN_ASM_MONT5 -DOPENSSL_BN_ASM_GF2m -DSHA1_ASM -DSHA256_ASM -DSHA512_ASM -DKECCAK1600_ASM -DRC4_ASM -DMD5_ASM -DAESNI_ASM -DVPAES_ASM -DGHASH_ASM -DECP_NISTZ256_ASM -DX25519_ASM -DPOLY1305_ASM -DNDEBUG -Wdate-time -D_FORTIFY_SOURCE=2
OPENSSLDIR: "/usr/lib/ssl"
ENGINESDIR: "/usr/lib/x86_64-linux-gnu/engines-1.1"
Seeding source: os-specific

The OPENSSLDIR tells you where OpenSSL will look for configuration.

/usr/lib/ssl is usually an alias for /etc/ssl

lrwxrwxrwx  1 root root   14 Apr 24  2019 certs -> /etc/ssl/certs
drwxr-xr-x  2 root root 4096 May  8 19:21 misc
lrwxrwxrwx  1 root root   20 Apr 21 16:33 openssl.cnf -> /etc/ssl/openssl.cnf
lrwxrwxrwx  1 root root   16 Apr 24  2019 private -> /etc/ssl/private

The misc folder is for scripts allowing for the implementation of a private CA (certificate authority)

Building from Source#

Sometimes to get a recent version you need to install from source

Releases can be pulled from:

cd /opt

sha256sum openssl-1.1.1g.tar.gz
# output should equal contents of openssl-1.1.1g.tar.gz.sha256

You can install it in a different location:

sudo tar xf openssl-1.1.1g.tar.gz
cd openssl-1.1.1g

sudo ./config --prefix=/opt/openssl --openssldir=/opt/openssl enable-ec_nistp_64_gcc_128

make depend
sudo make install

The contents of /opt/openssl:

drwxr-xr-x 9 root root  4096 Jun 11 09:06 .
drwxr-xr-x 5 root root  4096 Jun 11 09:06 ..
drwxr-xr-x 2 root root  4096 Jun 11 09:06 bin
drwxr-xr-x 2 root root  4096 Jun 11 09:06 certs
-rw-r--r-- 1 root root   412 Jun 11 09:06 ct_log_list.cnf
-rw-r--r-- 1 root root   412 Jun 11 09:06 ct_log_list.cnf.dist
drwxr-xr-x 3 root root  4096 Jun 11 09:06 include
drwxr-xr-x 4 root root  4096 Jun 11 09:06 lib
drwxr-xr-x 2 root root  4096 Jun 11 09:06 misc
-rw-r--r-- 1 root root 10909 Jun 11 09:06 openssl.cnf
-rw-r--r-- 1 root root 10909 Jun 11 09:06 openssl.cnf.dist
drwxr-xr-x 2 root root  4096 Jun 11 09:06 private
drwxr-xr-x 4 root root  4096 Jun 11 09:07 share

The /private folder is empty - as you do not have any private keys.

The /certs folder is also empty - OpenSSL does not include any root certificates - mainting trust stores is outside the scope of the project.

Luckily your operating system usually comes with a trust store. I think they are in cd /etc/ssl/certs/. You can also build your own trust store.

OpenSSL Available Commands#

It is the swiss army knife of cryptography.

Typing openssl help you get some info

Standard commands
asn1parse         ca                ciphers           cms               
crl               crl2pkcs7         dgst              dh                
dhparam           dsa               dsaparam          ec                
ecparam           enc               engine            errstr            
gendh             gendsa            genpkey           genrsa            
nseq              ocsp              passwd            pkcs12            
pkcs7             pkcs8             pkey              pkeyparam         
pkeyutl           prime             rand              req               
rsa               rsautl            s_client          s_server          
s_time            sess_id           smime             speed             
spkac             srp               ts                verify            
version           x509

To get info on the above just write: man openssl <command>

Also output is the message digest commands:

Message Digest commands (see the `dgst' command for more details)
md4               md5               rmd160            sha               

and cipher commands:

Cipher commands (see the `enc' command for more details)
aes-128-cbc       aes-128-ecb       aes-192-cbc       aes-192-ecb       
aes-256-cbc       aes-256-ecb       base64            bf                
bf-cbc            bf-cfb            bf-ecb            bf-ofb            
camellia-128-cbc  camellia-128-ecb  camellia-192-cbc  camellia-192-ecb  
camellia-256-cbc  camellia-256-ecb  cast              cast-cbc          
cast5-cbc         cast5-cfb         cast5-ecb         cast5-ofb         
des               des-cbc           des-cfb           des-ecb           
des-ede           des-ede-cbc       des-ede-cfb       des-ede-ofb       
des-ede3          des-ede3-cbc      des-ede3-cfb      des-ede3-ofb      
des-ofb           des3              desx              rc2               
rc2-40-cbc        rc2-64-cbc        rc2-cbc           rc2-cfb           
rc2-ecb           rc2-ofb           rc4               rc4-40            
seed              seed-cbc          seed-cfb          seed-ecb          

Building a Trust Store#

OpenSSL does not come with a trust store, trusted root certificates.

You can rely on the outdated trust store of your operating system or something like mozilla. Check this link

mozzila certs, unfortunately it is in a kak proprietary format.

You can get it in a PEM (Privacy Enhanced Mail) format at:

Key and Certificate Management#

Most users have a webserver they want to use to support SSL


  1. Generate a strong private key
  2. Create a certificate signing request (CSR)
  3. Install the CA provided certificate in your webserver

Key Generation#

Ask yourself?

  • Key algorithm - RSA, DSA and ECDSA
  • Key size - 256bit for ECDSA and 2048 and above for RSA
  • Passphrase - optional - convenience vs security

To generate a key:

openssl genrsa -aes128 -out fd.key 2048

The private keys are stored in a PEM format:

Proc-Type: 4,ENCRYPTED
DEK-Info: AES-256-CBC,EA701D65440153BF0C560E351D781E77


View the key’s structure:

openssl rsa -text -in fd.key

To get the public key seperately:

openssl rsa -in fd.key -pubout -out fd-public.key

Contents of the public key:

-----END PUBLIC KEY-----

Important to use the correct command, if you didn’t specify -pubout the output would be the private key

Generate a ECDSA key:

openssl ecparam -genkey -name secp256r1 | openssl ec -out ec.key -aes128

You can also use the genpkey command:

openssl genpkey

Creating Certificate Signing Requests#

Once you have a private key, you can create a CSR.

A formal request asking a CA to sign a certificate, contains a public key from the entity requesting the certificate and some information about the entity.

openssl req -new -key fd.key -out fd.csr

It will ask some info about the company

You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
Country Name (2 letter code) [AU]:ZA
State or Province Name (full name) [Some-State]:Gauteng
Locality Name (eg, city) []:Johannesburg
Organization Name (eg, company) [Internet Widgits Pty Ltd]
Organizational Unit Name (eg, section) []:
Common Name (e.g. server FQDN or YOUR name) []
Email Address []

challenge password is used for certificate revocation

After a CSR is generated, use it to sign your own certificate or send it to a public CA asking them to sign your certificate.

Double check that the CSR is correct:

openssl req -text -in fd.csr -noout

Certificate Request:
    Version: 0 (0x0)
    Subject: C=ZA, ST=Gauteng, L=Johannesburg,,
    Subject Public Key Info:
        Public Key Algorithm: rsaEncryption
            Public-Key: (2048 bit)

Create a new CSR from an existing certificate (Renew)

openssl x509 -x509toreq -in fd.crt -out fd.csr -signkey fd.key

You can create a config file for the csr.

Signing your own Certificates#

If you are installing a TLS server for your own use, you don’t need a go to a CA for a publicly trusted certificate. It is much easier to sign your own - by generating a self signed certificate.

If you are on firefox on your first visit you can add a certificate exception, after which the site will be secure as if it were protected by a publicy trusted certificate.

There is an illusion that self signed certificates are not secure and that only publicly trusted certs are. It is a myth. A self-signed certificate is as secure as one signed by a root CA…the only difference is it is not trusted by default via certificate chain and a root CA.

Create a cert from a CSR:

openssl x509 -req -days 365 -in fd.csr -signkey fd.key -out fd.crt

You don’t have to create a CSR in a seperate step, this command does it with the key alone:

openssl req -new -x509 -days 365 -key fd.key -out fd.crt

Creating Certs Valid for Multiple Hostnames#

By default certs have 1 common name and are valid for one hostname.

More info in the book

Examining Certificates#

use the x509 command to get info

openssl x509 -text -in fd.crt -noout

        Version: 3 (0x2)
        Serial Number: 17080102344465494832 (0xed08b696892eb330)
    Signature Algorithm: sha256WithRSAEncryption
        Issuer: C=ZA, ST=Gauteng, L=Joburg,,
            Not Before: Jun 11 10:58:51 2020 GMT
            Not After : Jun 11 10:58:51 2021 GMT
        Subject: C=ZA, ST=Gauteng, L=Joburg,,
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (2048 bit)
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Subject Key Identifier: 
            X509v3 Authority Key Identifier: 

            X509v3 Basic Constraints: 
    Signature Algorithm: sha256WithRSAEncryption

Now lets get the certificate from

openssl s_client -connect > outlook.crt
# remove the other stuff and leave the certificate

openssl x509 -text -in outlook.crt -noout

        Version: 3 (0x2)
        Serial Number:
    Signature Algorithm: sha256WithRSAEncryption
        Issuer: C=US, O=DigiCert Inc, CN=DigiCert Cloud Services CA-1
            Not Before: Feb 25 00:00:00 2020 GMT
            Not After : Feb 25 12:00:00 2022 GMT
        Subject: C=US, ST=Washington, L=Redmond, O=Microsoft Corporation,
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (2048 bit)
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Authority Key Identifier: 

            X509v3 Subject Key Identifier: 
            X509v3 Subject Alternative Name: 
                DNS:*, DNS:*, DNS:*, DNS:*, DNS:*, DNS:*, DNS:*, DNS:*, DNS:*,,,,,,,,,,,,,,
            X509v3 Key Usage: critical
                Digital Signature, Key Encipherment
            X509v3 Extended Key Usage: 
                TLS Web Server Authentication, TLS Web Client Authentication
            X509v3 CRL Distribution Points:

                Full Name:

                Full Name:

            X509v3 Certificate Policies: 
                Policy: 2.16.840.1.114412.1.1

            Authority Information Access: 
                OCSP - URI:
                CA Issuers - URI:

            X509v3 Basic Constraints: critical
            CT Precertificate SCTs: 
                Signed Certificate Timestamp:
                    Version   : v1(0)
                    Log ID    : ...
                    Timestamp : Feb 25 21:17:59.486 2020 GMT
                    Extensions: none
                    Signature : ecdsa-with-SHA256
                Signed Certificate Timestamp:
                    Version   : v1(0)
                    Log ID    : ...
                    Timestamp : Feb 25 21:17:59.458 2020 GMT
                    Extensions: none
                    Signature : ecdsa-with-SHA256
                Signed Certificate Timestamp:
                    Version   : v1(0)
                    Log ID    : ...
                    Timestamp : Feb 25 21:17:59.503 2020 GMT
                    Extensions: none
                    Signature : ecdsa-with-SHA256
    Signature Algorithm: sha256WithRSAEncryption

Basic Constraints mark a certificate as belonging to a CA - giving them the ability to sign other certificates. Non-CA certificates have this omitted or set as false.

X509v3 Basic Constraints: critical

Key Usage and Extended Key Usage restrict what a certificate can be used for.

X509v3 Key Usage: critical
    Digital Signature, Key Encipherment
X509v3 Extended Key Usage: 
    TLS Web Server Authentication, TLS Web Client Authentication

A web server will not allow for code signing

CRL Distribution Points lists where the certificates Certificate Revocation List info can be found. Important when certificates need to be revoked.

X509v3 CRL Distribution Points:

    Full Name:

    Full Name:

Each CRL is signed by the CA that issued it. If they were distributed over TLS browsers might face chicken or egg problem.

Certificate Policies indicate the policy under which the cert was issued

X509v3 Certificate Policies: 
    Policy: 2.16.840.1.114412.1.1

Authority Information Access gives the Online Certificate Status Protocol (OCSP) to check for certificate revocation in real time. May also contain a link to the next issuers certificate is found.

These days, server certificates are rarely signed directly by trusted root certificates, which means that users must include one or more intermediate certificates in their configuration

Mistakes are easy to make and will invalidate the certificates. Some clients (e.g., Internet Explorer) will use the information provided in this extension to fix an incomplete certificate chain, but many clients won’t.

Authority Information Access: 
    OCSP - URI:
    CA Issuers - URI:

Subject Alternative Name identifies all the hostnames for which the certificate is valid, if this does not exist it falls back to the CN - Common Name

Key and Certificate Conversion#

Private keys can be stored in a variety of formats:

  • Binary (DER) Certificate: x.509 in its raw form
  • Ascii (PEM) Certificate: base64 encoded DER with -----BEGIN CERTIFICATE----- and -----END CERTIFICATE-----. Usually with one certificate per file.
  • Binary (DER) Key: Private key in raw form
  • Ascii (PEM) key: base64 DER key
  • PKCS#7 certificates: complex format designed for transport of signed or encrypted data. Has .p7b or .p7c extensions and can inclue the entire certificate chain.
  • PKCS#12 (PFX) key and certificate: complex format can store and protect key along wit entire certificate chain. .p12 or .pfx extensions. Common on microsoft.

PEM and DER conversions#

Convert from pem to der

openssl x509 -inform PEM -in fd.pem -outform DER -out fd.der

Convert from der to pem

openssl x509 -inform DER -in fd.der -outform PEM -out fd.pem

More in the book on other conversions


  • Choosing a cipher suite
  • performance measurement of crypto operations

Choosing a Cipher Suite#

A common task on TLS web servers.

In apache, cipher strength may look like this:

SSLHonorCipherOrder On

Get a list of supported ciphers:

openssl ciphers -v 'ALL:COMPLIMENTOFALL'

ECDHE-ECDSA-CHACHA20-POLY1305 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=ChaCha20-Poly1305 Mac=AEAD
ECDHE-RSA-CHACHA20-POLY1305 TLSv1.2 Kx=ECDH     Au=RSA  Enc=ChaCha20-Poly1305 Mac=AEAD
DHE-RSA-CHACHA20-POLY1305 TLSv1.2 Kx=DH       Au=RSA  Enc=ChaCha20-Poly1305 Mac=AEAD
ECDHE-RSA-AES256-SHA384 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AES(256)  Mac=SHA384
ECDHE-ECDSA-AES256-SHA384 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AES(256)  Mac=SHA384

Output contains: suite name, required minimum protocol version, key-exchange algorithm, Authentication Algorithm, Cipher Algorithm and Strength and MAC algorithm

The order you place the algorithms in is important.

You can select algorithms using different criteriaL

openssl ciphers -v HIGH

Sorting by @STRENGTH:

openssl ciphers -v HIGH@STRENGTH

More info on the various keywords in the book

Putting it all together#

  • Use only strong ciphers 128-bits and up
  • Use only suites that provide strong authnetication
  • Do not use suites relying on weak primitives (MD5)
  • Prefer ECDSA over RSA




OpenSSL has built in benchmarking

Test algorithms:

openssl speed rc4 aes rsa ecdh sha

                    sign    verify    sign/s    verify/s
    rsa  512 bits 0.000696s 0.000040s   1437.6   25296.0
    rsa 1024 bits 0.004225s 0.000200s    236.7   4996.5
    rsa 2048 bits 0.028837s 0.000822s     34.7   1215.9
    rsa 4096 bits 0.196471s 0.003028s      5.1    330.2
                                op      op/s
    160 bit ecdh (secp160r1)   0.0012s    844.2
    192 bit ecdh (nistp192)   0.0011s    923.3
    224 bit ecdh (nistp224)   0.0016s    633.7
    256 bit ecdh (nistp256)   0.0020s    507.0
    384 bit ecdh (nistp384)   0.0051s    196.9
    521 bit ecdh (nistp521)   0.0114s     87.8
    163 bit ecdh (nistk163)   0.0010s   1008.0
    233 bit ecdh (nistk233)   0.0014s    725.0
    283 bit ecdh (nistk283)   0.0031s    327.0
    409 bit ecdh (nistk409)   0.0065s    153.2
    571 bit ecdh (nistk571)   0.0143s     70.0
    163 bit ecdh (nistb163)   0.0011s    932.3
    233 bit ecdh (nistb233)   0.0015s    683.8
    283 bit ecdh (nistb283)   0.0034s    297.4
    409 bit ecdh (nistb409)   0.0072s    138.5
    571 bit ecdh (nistb571)   0.0162s     61.6

It is good because you can upgrade to newer openssl versions and then see a speed enchancement

By default, the speed command uses only a single process - to use mutliple cores use the -multi switch

openssl speed -multi 4 rsa

                     sign    verify    sign/s verify/s
    rsa  512 bits 0.000260s 0.000018s   3844.5  54424.3
    rsa 1024 bits 0.001450s 0.000072s    689.5  13889.0
    rsa 2048 bits 0.009740s 0.000274s    102.7   3652.1
    rsa 4096 bits 0.066480s 0.000996s     15.0   1004.1

The performance is 4 times better than before.

The results show that 102.7 2048-bit signatures - meaning 100 brand new TLS connections per second.

So when you get to servers with high load, TLS connnections performance may become a bottleneck.

Sometimes the speed command is wrong as it does not use the fastest implementation making use of native instructions on the CPU: AES-NI

Creating a Private Certificate Authority#

  • All you need for your own CA is included in openssl
  • Interface is purely command-line, so not user friendly
  • Much better to use a private CA in a development environment than to use self-signed certificates everywhere
  • Client certificates can also increase the security
  • Biggest challenge with a private CA is keeping the infrastructure secure
  • The root key must be kept offline because all security depends on it
  • CRL’s and OCSP responder certificate’s must be refreshed on a regular basis

Features and Limitations#

  • One root CA that is similar in structure to which other subordinate CA’s can be created
  • The root CA should remain offline
  • Subordinate CA will be technically constrained - it is allowed to issue certificates only for allowed hostnames

After setup the root certificate will have to be securely distributed to intended clients. Once the root is in place you can issue client and server certificates.

Creating a root CA:

  1. Root CA Configuration
  2. Directory structure and intialisation of key files
  3. generating the root key and certificate

All certificates will be CA’s according to the basicConstraints nameConstriants limits the allowed hostnames - keeping it safe so they can’t issue arbirary hostnames.

Root CA Configuration#
man config
  • You can choose to make the root certificate valid for 10 years

    man ca

Info in the book about setting up the config

Root CA Directory Structure#
mkdir root-ca
cd root-ca
mkdir certs db private
chmod 700 private
touch db/index
openssl rand -hex 16  > db/serial
echo 1001 > db/crlnumber
  • certs/ - Certificate storage
  • db/ - certificate db (index) and files that hold the next certificate and CRL
  • private/ - stores private keys - one for the CA and one for the OCSP responder. Important that no other user has access to it.
Root CA generation#

Example root_ca.conf:

# Simple Root CA

# The [default] section contains global constants that can be referred to from
# the entire configuration file. It may also hold settings pertaining to more
# than one openssl command.

[ default ]
ca                      = root-ca               # CA name
dir                     = .                     # Top dir

# The next part of the configuration file is used by the openssl req command.
# It defines the CA's key pair, its DN, and the desired extensions for the CA
# certificate.

[ req ]
default_bits            = 2048                  # RSA key size
encrypt_key             = yes                   # Protect private key
default_md              = sha1                  # MD to use
utf8                    = yes                   # Input is UTF-8
string_mask             = utf8only              # Emit UTF-8 strings
prompt                  = no                    # Don't prompt for DN
distinguished_name      = ca_dn                 # DN section
req_extensions          = ca_reqext             # Desired extensions

[ ca_dn ]
0.domainComponent       = "org"
1.domainComponent       = "simple"
organizationName        = "Simple Inc"
organizationalUnitName  = "Simple Root CA"
commonName              = "Simple Root CA"

[ ca_reqext ]
keyUsage                = critical,keyCertSign,cRLSign
basicConstraints        = critical,CA:true
subjectKeyIdentifier    = hash

# The remainder of the configuration file is used by the openssl ca command.
# The CA section defines the locations of CA assets, as well as the policies
# applying to the CA.

[ ca ]
default_ca              = root_ca               # The default CA section

[ root_ca ]
certificate             = $dir/ca/$ca.crt       # The CA cert
private_key             = $dir/ca/$ca/private/$ca.key # CA private key
new_certs_dir           = $dir/ca/$ca           # Certificate archive
serial                  = $dir/ca/$ca/db/$ # Serial number file
crlnumber               = $dir/ca/$ca/db/$ # CRL number file
database                = $dir/ca/$ca/db/$ca.db # Index file
unique_subject          = no                    # Require unique subject
default_days            = 3652                  # How long to certify for
default_md              = sha1                  # MD to use
policy                  = match_pol             # Default naming policy
email_in_dn             = no                    # Add email to cert DN
preserve                = no                    # Keep passed DN ordering
name_opt                = ca_default            # Subject DN display options
cert_opt                = ca_default            # Certificate display options
copy_extensions         = none                  # Copy extensions from CSR
x509_extensions         = signing_ca_ext        # Default cert extensions
default_crl_days        = 365                   # How long before next CRL
crl_extensions          = crl_ext               # CRL extensions

# Naming policies control which parts of a DN end up in the certificate and
# under what circumstances certification should be denied.

[ match_pol ]
domainComponent         = match                 # Must match ''
organizationName        = match                 # Must match 'Simple Inc'
organizationalUnitName  = optional              # Included if present
commonName              = supplied              # Must be present

[ any_pol ]
domainComponent         = optional
countryName             = optional
stateOrProvinceName     = optional
localityName            = optional
organizationName        = optional
organizationalUnitName  = optional
commonName              = optional
emailAddress            = optional

# Certificate extensions define what types of certificates the CA is able to
# create.

[ root_ca_ext ]
keyUsage                = critical,keyCertSign,cRLSign
basicConstraints        = critical,CA:true
subjectKeyIdentifier    = hash
authorityKeyIdentifier  = keyid:always

[ signing_ca_ext ]
keyUsage                = critical,keyCertSign,cRLSign
basicConstraints        = critical,CA:true,pathlen:0
subjectKeyIdentifier    = hash
authorityKeyIdentifier  = keyid:always

# CRL extensions exist solely to point to the CA certificate that has issued
# the CRL.

[ crl_ext ]
authorityKeyIdentifier  = keyid:always

Generate key and CSR

openssl req -new -config root-ca.conf -out root-ca.csr -keyout private/root-ca.key

Create a self-signed certificate

openssl ca -selfsign -config root-ca.conf -in root-ca.csr -out root-ca.crt -extensions ca_ext

The index file db/index is plaintext and contains certificate information one per line.

V    240706115345Z        1001    unknown    /C=GB/O=Example/CN=Root CA
  • Status flag: V - valid, R - revoked, E - expired
  • expiration date: YYMMDDHHMMSSZ
  • Revocation date
  • Serial number: Hex
  • File location, unknown if not known
  • Distinguished Name

Root CA Operations#

More info in the book

Testing with OpenSSL#

Due to the large number of protocol features and implementation quirks, it’s sometimes difficult to determine the exact configuration and features of secure servers

Connnecting to SSL Services#

Supply a hostname and a port

openssl s_client -connect

You will get output then get to send HTTP requests, type:


We know the TLS layer is working.

Parts of the Cert#

Server Certificate info

depth=1 C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3
verify error:num=20:unable to get local issuer certificate
verify return:0

If you have certificate validation required and there is a self-signed certificate in the chain:

self signed certificate in certificate chain

You would want to point s_client to the trusted certificate CA, eg:

openssl s_client -connect -CAfile /etc/ssl/certs/ca-certificates.crt

Next the certificates are presented in the order in which they are delivered:

Certificate chain
0 s:/
i:/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3
1 s:/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3
i:/O=Digital Signature Trust Co./CN=DST Root CA X3

First line is the subject, second line is the issuer.

Next part is the cserver certificate:

Server certificate
issuer=/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3

whenever you see a long string on numbers it means that OpenSSL does not know the object identifier (OID)

Information about the TLS protocol

No client certificate CA names sent
SSL handshake has read 3417 bytes and written 444 bytes
New, TLSv1/SSLv3, Cipher is ECDHE-RSA-AES256-GCM-SHA384
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
    Protocol  : TLSv1.2
    Cipher    : ECDHE-RSA-AES256-GCM-SHA384
    Session-ID: 8B6F4472535450C0BEF7DEC8831C769188D946585868F49EE748D71283D5865A
    Master-Key: AC39C620EAA804CD3554AA7949CB076211A4E851F651E1FA264C0F96973F74A012E59165C16DAD58B71749536B1F3A2F
    TLS session ticket lifetime hint: 300 (seconds)
    TLS session ticket:
    0000 - c3 06 75 ff 57 44 fb d1-96 7a 3a 84 71 13 d4 e5   ..u.WD...z:.q...

    Start Time: 1592554496
    Timeout   : 300 (sec)
    Verify return code: 0 (ok)

The most important thing here is tht TLS version and cipher suite used:

Protocol  : TLSv1.2
Cipher    : ECDHE-RSA-AES256-GCM-SHA384

You also have a session ID and ticket for resuming and maintaining state client side

Testing Protocol Upgrades#

When used with HTTP, TLS wraps the entire plaintext communication to form HTTPS

Other protocols start as plaintext and are then upgraded.

Supported protocols: smtp, pop3, imap, ftp and xmpp

To test that kind of protocol you would use the -starttls switch:

openssl s_client -connect -starttls smtp

Extracting Remote Certificates#

When you connect to a remote secure server using s_client, it will dump the server’s PEM encoded certificate to standard output

You can write the certificate to a file:

echo | openssl s_client -connect 2>&1 | sed --quiet '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' >

Explicitly testing a protocol#

Openssl will always choose the best that both client and server can support

openssl s_client -connect -tls1_2

openssl s_client -connect -tls1_3

Test is a server supports a specific cipher suite#

openssl s_client -connect -cipher RC4-SHA

no peer certificate available
No client certificate CA names sent
SSL handshake has read 7 bytes and written 100 bytes
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated


SNI is a TLS extension that enables use of more than one certificate on the same IP endpoint.

Makes virtual scure hosting possible

openssl s_client -connect -servername

Testing Cerfiticate reuse, OCSP Revocation in the book

Testing OCSP Stapling#

OCSP stapling is an optional feature allowing a server certificate to be accompanied by an OCSP response that proves its validity.

OCSP stapling is requested using the -status switch

echo | openssl s_client -connect -status

Checking CRL Certficate Revocation List, Testing Renegotiation

Testing for the BEAST vulnerability#

echo | openssl s_client -connect -cipher 'ALL:!RC4' -no_ssl2 -no_tls1_1 -no_tls1_2

Testing for Heartbleed in the book

TLS Deployment Best Practices#

Obtaining a comprehensive understanding of the SSL/TLS and PKI landscape requires a lot of time and dedication. In my experience, most people don’t need to know everything, but it’s tricky to find the small bits that they do need to know

Best practices are in the book