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SSL(8) System Manager's Manual SSL(8)

ssl — details for libssl and libcrypto

This document describes some of the issues relating to the use of the OpenSSL libssl and libcrypto libraries. This document is intended as an overview of what the libraries do, and what uses them.

The SSL libraries (libssl and libcrypto) implement the SSL version 3 and TLS version 1 protocols. SSL and TLS are most commonly used by the HTTPS protocol for encrypted web transactions, as can be done with httpd(8). The libcrypto library is also used by various programs such as ssh(1), sshd(8), and isakmpd(8).

OpenBSD uses the arandom(4) device as the default source for random data when needed by the routines in libcrypto and libssl. If the arandom(4) device does not exist or is not readable, many of the routines will fail. This is most commonly seen by users as the RSA routines failing in applications such as ssh(1) and httpd(8).

It is important to remember when using a random data source for certificate and key generation that the random data source should not be visible by people who could duplicate the process and come up with the same result. You should ensure that nobody who you don't trust is in a position to read the same random data used by you to generate keys and certificates. The arandom(4) device ensures that no two users on the same machine will see the same data. See openssl(1) for more information on how to use different sources of random data.

The most common uses of SSL/TLS will require you to generate a server certificate, which is provided by your host as evidence of its identity when clients make new connections. The certificates reside in the /etc/ssl directory, with the keys in the /etc/ssl/private directory.

Private keys can be encrypted using AES and a passphrase to protect their integrity should the encrypted file be disclosed. However, it is important to note that encrypted server keys mean that the passphrase needs to be typed in every time the server is started. If a passphrase is not used, you will need to be absolutely sure your key file is kept secure.

To support HTTPS transactions in httpd(8) you will need to generate an RSA certificate.

# openssl genrsa -out /etc/ssl/private/server.key 2048

Or, if you wish the key to be encrypted with a passphrase that you will have to type in when starting servers

# openssl genrsa -aes256 -out /etc/ssl/private/server.key 2048

The next step is to generate a Certificate Signing Request (CSR) which is used to get a Certificate Authority (CA) to sign your certificate. To do this use the command:

# openssl req -new -key /etc/ssl/private/server.key \
  -out /etc/ssl/private/server.csr

This server.csr file can then be given to a Certificate Authority who will sign the key.

You can also sign the key yourself, using the command:

# openssl x509 -sha256 -req -days 365 \
  -in /etc/ssl/private/server.csr \
  -signkey /etc/ssl/private/server.key \
  -out /etc/ssl/server.crt

With /etc/ssl/server.crt and /etc/ssl/private/server.key in place, you should be able to start httpd(8) with the -DSSL flag, enabling HTTPS transactions with your machine on port 443.

You will most likely want to generate a self-signed certificate in the manner above along with your certificate signing request to test your server's functionality even if you are going to have the certificate signed by another Certificate Authority. Once your Certificate Authority returns the signed certificate to you, you can switch to using the new certificate by replacing the self-signed /etc/ssl/server.crt with the certificate signed by your Certificate Authority, and then restarting httpd(8).

Generating a DSA certificate involves several steps. First, generate parameters for DSA keys. The following command will generate 1024-bit keys:

# openssl dsaparam 1024 -out dsa1024.pem

Once you have the DSA parameters generated, you can generate a CSR and unencrypted private key using the command:

# openssl req -nodes -newkey dsa:dsa1024.pem \
  -out /etc/ssl/dsacert.csr -keyout /etc/ssl/private/dsakey.pem

To generate an encrypted private key, you would use:

# openssl req -newkey dsa:dsa1024.pem \
  -out /etc/ssl/dsacert.csr -keyout /etc/ssl/private/dsakey.pem

This server.csr file can then be given to a CA who will sign the key.

You can also sign the key yourself, using the command:

# openssl x509 -sha256 -req -days 365 \
  -in /etc/ssl/private/dsacert.csr \
  -signkey /etc/ssl/private/dsacert.key \
  -out /etc/ssl/dsacert.crt

First, generate parameters for ECDSA keys. The following command will use a NIST/SECG curve over a 384-bit prime field:

# openssl ecparam -out ec-secp384r1.pem -name secp384r1

Once you have the ECDSA parameters generated, you can generate a CSR and unencrypted private key using the command:

# openssl req -nodes -newkey ec:ec-secp384r1.pem \
  -keyout /etc/ssl/private/eccert.key -new \
  -out /etc/ssl/private/eccert.csr

To generate an encrypted private key, you would use:

# openssl req -newkey ec:ec-secp384r1.pem \
  -keyout /etc/ssl/private/eccert.key -new \
  -out /etc/ssl/private/eccert.csr

This eccert.csr file can then be given to a CA who will sign the key.

You can also sign the key yourself, using the command:

# openssl x509 -sha256 -req -days 365 \
  -in /etc/ssl/private/eccert.csr \
  -signkey /etc/ssl/private/eccert.key \
  -out /etc/ssl/eccert.crt

By default, sendmail(8) expects both the keys and certificates to reside in /etc/mail/certs, not in the /etc/ssl directory. The default paths may be overridden in the sendmail.cf file. See starttls(8) for information on configuring sendmail(8) to use SSL/TLS.

openssl(1), ssh(1), ssl(3), arandom(4), httpd(8), isakmpd(8), rc(8), sendmail(8), sshd(8), starttls(8)

Prior to Sept 21, 2000, there were problems shipping fully functional implementations of these protocols, as such shipment would include shipping the United States. RSA Data Security Inc (RSADSI) held the patent on the RSA algorithm in the United States, and because of this, free implementations of RSA were difficult to distribute and propagate. (The RSA patent was probably more effective at preventing the adoption of widespread international integrated crypto than the much maligned ITAR restrictions were.) Prior to OpenBSD 2.8, these libraries shipped without the RSA algorithm -- all such functions were stubbed to fail. Since RSA is a key component of SSL version 2, this meant that SSL version 2 would not work at all. SSL version 3 and TLS version 1 allow for the exchange of keys via mechanisms that do not involve RSA, and would work with the shipped version of the libraries, assuming both ends could agree to a cipher suite and key exchange that did not involve RSA. Likewise, the SSH1 protocol in ssh(1) uses RSA, so it was similarly encumbered.

For instance, another typical alternative is DSA, which is not encumbered by commercial patents (and lawyers).

The HTTPS protocol used by web browsers (in modern incarnations) allows for the use of SSL version 3 and TLS version 1, which in theory allows for encrypted web transactions without using RSA. Unfortunately, all the popular web browsers buy their cryptographic code from RSADSI. Predictably, RSADSI would prefer that web browsers used their patented algorithm, and thus their libraries do not implement any non-RSA cipher and keying combination. The result of this was that while the HTTPS protocol allowed for many cipher suites that did not require the use of patented algorithms, it was very difficult to use these with the popular commercially available software. Prior to version 2.8, OpenBSD allowed users to download RSA enabled versions of the shared libssl and libcrypto libraries which allowed users to enable full function without recompiling the applications. This method is now no longer needed, as the fully functional libraries ship with the system. However, this entire debacle is worth remembering when choosing software and vendors.

Due to multiple flaws in the protocol, SSL version 2 was disabled in OpenBSD 5.2. Users and programs should use SSL version 3 or TLS version 1 instead.

This document first appeared in OpenBSD 2.5.

March 14, 2013 OpenBSD-5.4