Posts tagged gnutls

Certificates, OpenSSL in depth and GnuTLS

posted on 2014-07-10 14:37:52

This post should give an overview on the most used OpenSSL commands, and how SSL/TLS/X.509 in general works.

Since this post was written a long time ago, it might get revisited in the future. But this will be a major overhaul, so this will not happen in the near future either.

But there will come some ascii art on a schematic PKI in general, the section about the filenames will get cleaned up as well as the openssl section.

post vocabulary and some notes

The most used terms are abbreviated in the following.

PK = Private Key
C = Certificate
CSR = Certificate Signing Request
CA = Certificate Authority

Usually this seems way harder than it is in reality, once you get the hang of it. Hardest part is to understand which file belonging to which server is needed for the current step.


Some more abbreviations first:

SSL : Secure Sockets Layer
TLS : Transport Layer Security
X.509 : Public Key Infrastructure (PKI) and Priviledge Management Infrastructure (PMI) standard by the "International Telecommunication Union Telecommunication Standardization Sector" (ITU-T).

SSL and its successor TLS, which includes SSL, are protocols for encrypting internet communication. The C infrastructure setup is defined in the X.509 standard. That is why these acronyms are popping up in any discussion about this topic.

On a sidenote, a more general equation:


Since this post is focused on usability, the techniques in question that are used in a PKI or PMI are of no concern here.

The C chain looks usually like this: (intermeadiates can, but need not exist)

  1. Root C
  2. Intermediate C
  3. C

The last C is the one issued by the CA where you subitted your CSR to.

Only if all C's are present and used correctly, SSL checking tools (See here or here.) will tell you your C's are set up accordingly.

File types

There exist a bunch of file types, you have to be able to differentiate.

file types

.key : private key file (PK), but that's just a convention
.csr : certificate signing request (CSR)
.crt : certificate (C)
.cer : certificate (C), Microsoft used this naming scheme earlier

For .pem and .der files, see next section.

PK.key, CSR.csr, C.crt are kind of placeholders for your actual filenames in the following sections. A good naming scheme would be subdomain_domain_tld-year, without dots. Dots happen to either not work or cause other problems. Appending the year your C was issued helps with distinguishing in case you renew a certain certificate.

containers and encodings

Containers are used for grouping together C's (and) into a single file.

.pem: ascii / base64 encoded container
.der: container in binary format

The extension hints at the encoding being used, for the container. A container usually consists of the set of all C's (the entire trust chain), and can optionally also contain the PK.

All the files from the section before can be in PEM or DER format, IIRC!

For more information on the Distinguished Encoding Rules (DER) or the Privacy-enhanced Electronic Mail (PEM), just click these links.


PK / CSR generation

For usage with Certificate Authorities (CA's)

Generate a PK and a CSR:

openssl req -out CSR.csr -new -newkey rsa:2048 -nodes -keyout PK.key

If you already have an existing PK and just need a CSR:

openssl req -out CSR.csr -key PK.key -new

Create a new CSR for an existing C:

openssl x509 -x509toreq -in C.crt -out CSR.csr -signkey PK.key

Complete self-signed certificate

Generation of a self-signed (ss) C, based on a newly generated PK with a term of validity of one year (365 days):

openssl req -x509 -nodes -days 365 -newkey rsa:2048 -keyout PK.key -out C.crt

ss-C's for https are still better than traffic over plain http, but for private websites for example, StartSSL Certificates provide C's for free. Free as in 'no money needed'.

convert PEM to DER

openssl x509 -in C.crt -outform der -out C.der

convert DER to PEM

openssl x509 -in C.crt -inform der -outform pem -out C.pem

viewing PEM encoded files containing a C

For debugging reasons, this might actually be the most used command.

openssl x509 -in C.pem -text -noout
openssl x509 -in C.crt -text -noout
openssl x509 -in C.cer -text -noout

This will not work on a single PK file.


Get it:

apt-get install gnutls-bin -y



Instead of the openssl tool suite, this is actually self-explanatory.


In the following, keyfiles are called .key extension-wise, but that is just a name differentiation. They are in reality just .pem files, too, but with this practice files are easier to differentiate.

generate PK's (private keys)

certtool --generate-privkey --outfile PK.key --rsa

Use --dsa or --ecc flags if you want to change the used cryptosystem.

generate CSR's (certificate signing requests)

certtool --generate-request --load-privkey PK.key --outfile CSR.pem

generate C (certificate) from CSR (certificate signing request)

Usually this is a CA_C.pem, a CA certificate.

certtool --generate-certificate --load-ca-privkey CA_PK.key --load-ca-certificate CA_C.pem --load-request CSR.pem --outfile C.pem

generate C (certificate) from PK (private key), lacking a CSR

certtool --generate-certificate --load-ca-privkey CA_PK.key --load-ca-certificate CA_C.pem --load-privkey PK.key --outfile C.pem

generate a self-signed C (certificate), the fast way

certtool --generate-privkey --outfile CA_PK.key --rsa
certtool --generate-self-signed --load-privkey CA_PK.key --outfile CA_C.pem

Here's a one-liner to copy-paste:

certtool --generate-privkey --outfile CA_PK.key --rsa && certtool --generate-self-signed --load-privkey CA_PK.key --outfile CA_C.pem

create a .p12 / pkcs #12 container file

A .p12 file includes all three part usually needed on the server side:

  • CA certificate

  • server PK

  • server C

    certtool --to-p12 --load-ca-certificate CA_C.pem --load-privkey PK.key --load-certificate C.pem --outfile CONTAINER.p12 --outder

show certificate information

certtool --certificate-info --infile C.pem

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