# Step by step to generate sample self-signed X.509 certificate chain and sign data with PKCS7 structure

This readme demonstrates how to generate 3-layer X.509 certificate chain (RootCA -> IntermediateCA -> SigningCert) with OpenSSL commands, and user MUST set a UNIQUE Subject Name ("Common Name") on these three different certificates.

## How to generate a self-signed X.509 certificate chain via OPENSSL
* Set OPENSSL environment.

NOTE: Below steps are required for Windows. Linux may already have the OPENSSL environment correctly.

    set OPENSSL_HOME=c:\home\openssl\openssl-[version]
    set OPENSSL_CONF=%OPENSSL_HOME%\apps\openssl.cnf

When a user uses OpenSSL (req or ca command) to generate the certificates, OpenSSL will use the openssl.cnf file as the configuration data (can use “-config path/to/openssl.cnf” to describe the specific config file).

The user need check the openssl.cnf file, to find your CA path setting, e.g. check if the path exists in [ CA_default ] section.

    [ CA_default ]
        dir = ./demoCA              # Where everything is kept

You may need the following steps for initialization:

    rd ./demoCA /S/Q
    mkdir ./demoCA
    echo.>./demoCA/index.txt
    echo 01 > ./demoCA/serial
    mkdir ./demoCA/newcerts

OpenSSL will apply the options from the specified sections in openssl.cnf when creating certificates or certificate signing requests. Make sure your configuration in openssl.cnf is correct and rational for certificate constraints.
The following sample sections were used when generating test certificates in this readme.
    ...
    [ req ]
    default_bits        = 2048
    default_keyfile     = privkey.pem
    distinguished_name  = req_distinguished_name
    attributes          = req_attributes
    x509_extensions     = v3_ca       # The extensions to add to the self signed cert
    ...
    [ v3_ca ]
    # Extensions for a typical Root CA.
    subjectKeyIdentifier=hash
    authorityKeyIdentifier=keyid:always,issuer
    basicConstraints = critical,CA:true
    keyUsage = critical, digitalSignature, cRLSign, keyCertSign
    ...
    [ v3_intermediate_ca ]
    # Extensions for a typical intermediate CA.
    subjectKeyIdentifier = hash
    authorityKeyIdentifier = keyid:always,issuer
    basicConstraints = critical, CA:true
    keyUsage = critical, digitalSignature, cRLSign, keyCertSign
    ...
    [ usr_cert ]
    # Extensions for user end certificates.
    basicConstraints = CA:FALSE
    nsCertType = client, email
    subjectKeyIdentifier = hash
    authorityKeyIdentifier = keyid,issuer
    keyUsage = critical, nonRepudiation, digitalSignature, keyEncipherment
    extendedKeyUsage = clientAuth, emailProtection
    ...

* Generate the certificate chain:

NOTE: User MUST set a UNIQUE "Common Name" on the different certificate

1) Generate the Root Pair:

Generate a root key:

    openssl genrsa -aes256 -out TestRoot.key 2048

Generate a self-signed root certificate:

    openssl req -extensions v3_ca -new -x509 -days 3650 -key TestRoot.key -out TestRoot.crt
    openssl x509 -in TestRoot.crt -out TestRoot.cer -outform DER
    openssl x509 -inform DER -in TestRoot.cer -outform PEM -out TestRoot.pub.pem

2) Generate the Intermediate Pair:

Generate the intermediate key:

    openssl genrsa -aes256 -out TestSub.key 2048

Generate the intermediate certificate:

    openssl req -new -days 3650 -key TestSub.key -out TestSub.csr
    openssl ca -extensions v3_intermediate_ca -in TestSub.csr -days 3650 -out TestSub.crt -cert TestRoot.crt -keyfile TestRoot.key
    openssl x509 -in TestSub.crt -out TestSub.cer -outform DER
    openssl x509 -inform DER -in TestSub.cer -outform PEM -out TestSub.pub.pem

3) Generate User Key Pair for Data Signing:

Generate User key:

    openssl genrsa -aes256 -out TestCert.key 2048

Generate User certificate:

    openssl req -new -days 3650 -key TestCert.key -out TestCert.csr
    openssl ca -extensions usr_cert -in TestCert.csr -days 3650 -out TestCert.crt -cert TestSub.crt -keyfile TestSub.key
    openssl x509 -in TestCert.crt -out TestCert.cer -outform DER
    openssl x509 -inform DER -in TestCert.cer -outform PEM -out TestCert.pub.pem

Convert Key and Certificate for signing. Password is removed with -nodes flag for convenience in this sample.

    openssl pkcs12 -export -out TestCert.pfx -inkey TestCert.key -in TestCert.crt
    openssl pkcs12 -in TestCert.pfx -nodes -out TestCert.pem

* Verify Data Signing & Verification with new X.509 Certificate Chain

1) Sign a Binary File to generate a detached PKCS7 signature:

    openssl smime -sign -binary -signer TestCert.pem -outform DER -md sha256 -certfile TestSub.pub.pem -out test.bin.p7 -in test.bin

2) Verify PKCS7 Signature of a Binary File:

    openssl smime -verify -inform DER -in test.bin.p7 -content test.bin -CAfile TestRoot.pub.pem -out test.org.bin