OpenSSL Command-Line HOWTO

Paul Heinlein

Initial publication: June 13, 2004
Most recent revision: August 6, 2005

The openssl application that ships with the OpenSSL libraries can perform a wide range of crypto operations. This HOWTO provides some cookbook-style recipes for using it.

Table of Contents

How do I find out what OpenSSL version I'm running?
How do I get a list of the available commands?
How do I get a list of available ciphers?
How do I benchmark my system's performance?
How do I benchmark remote connections?
How do I generate a self-signed certificate?
How do I generate a certificate request for VeriSign?
How do I test a new certificate?
How do I retrieve a remote certificate?
How do I extract information from a certificate?
How do I export or import a PKCS#12 certificate?
Certificate Verification
How do I verify a certificate?
What certificate authorities does OpenSSL recognize?
How do I get OpenSSL to recognize/verify a certificate?
Command-line clients and servers
How do I connect to a secure SMTP server?
How do I connect to a secure [whatever] server?
How do I set up an SSL server from the command line?
How do I create an MD5 or SHA1 digest of a file?
How do I sign a digest?
How do I verify a signed digest?
What other kinds of digests are available?
How do I base64-encode something?
How do I simply encrypt a file?
How do I generate an RSA key?
How do I generate a public RSA key?
How do I generate a DSA key?
How do I remove a passphrase from a key?
Password hashes
How do I generate a crypt-style password hash?
How do I generate a shadow-style password hash?
Random data
How do I generate random bits?
How do I verify a signed S/MIME message?
How do I encrypt a S/MIME message?
How do I sign a S/MIME message?
For further reading
Comments welcome


The openssl command-line binary that ships with the OpenSSL libraries can perform a wide range of cryptographic operations. It can come in handy in scripts or for accomplishing one-time command-line tasks.

Documentation for using the openssl application is somewhat scattered, however, so this article aims to provide some practical examples of its use. I assume that you've already got a functional OpenSSL installation and that the openssl binary is in your shell's PATH.

Just to be clear, this article is strictly practical; it does not concern cryptographic theory and concepts. If you don't know what an MD5 sum is, this article won't enlighten you one bit—but if all you need to know is how to use openssl to generate a file sum, you're in luck.

The nature of this article is that I'll be adding new examples incrementally. Check back at a later date if I haven't gotten to the information you need.

How do I find out what OpenSSL version I'm running?

Use the version option.

$ openssl version
OpenSSL 0.9.7d 17 Mar 2004

How do I get a list of the available commands?

There are three built-in options for getting lists of available commands, but none of them provide what I consider useful output. The best thing to do is provide an invalid command (help or -h will do nicely) to get a readable answer.

$ openssl help
openssl:Error: 'help' is an invalid command.

Standard commands
asn1parse      ca             ciphers        crl            crl2pkcs7      
dgst           dh             dhparam        dsa            dsaparam       
enc            engine         errstr         gendh          gendsa         
genrsa         nseq           ocsp           passwd         pkcs12         
pkcs7          pkcs8          rand           req            rsa            
rsautl         s_client       s_server       s_time         sess_id        
smime          speed          spkac          verify         version        

Message Digest commands (see the `dgst' command for more details)
md2            md4            md5            rmd160         sha            

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         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         

What the shell calls “Standard commands” are the main top-level options.

You can use the same trick with any of the subcommands.

$ openssl dgst -h
unknown option '-h'
options are
-c              to output the digest with separating colons
-d              to output debug info
-hex            output as hex dump
-binary         output in binary form
-sign   file    sign digest using private key in file
-verify file    verify a signature using public key in file
-prverify file  verify a signature using private key in file
-keyform arg    key file format (PEM or ENGINE)
-signature file signature to verify
-binary         output in binary form
-engine e       use engine e, possibly a hardware device.
-md5 to use the md5 message digest algorithm (default)
-md4 to use the md4 message digest algorithm
-md2 to use the md2 message digest algorithm
-sha1 to use the sha1 message digest algorithm
-sha to use the sha message digest algorithm
-mdc2 to use the mdc2 message digest algorithm
-ripemd160 to use the ripemd160 message digest algorithm

In more boring fashion, you can consult the OpenSSL man pages.

How do I get a list of available ciphers?

Use the ciphers option. The ciphers(1) man page is quite helpful.

# list all available ciphers
openssl ciphers -v

# list only TLSv1 ciphers
openssl ciphers -v -tls1

# list only high encryption ciphers (keys larger than 128 bits)
openssl ciphers -v 'HIGH'

# list only high encryption ciphers using the AES algorithm
openssl ciphers -v 'AES+HIGH'


How do I benchmark my system's performance?

The OpenSSL developers have built a benchmarking suite directly into the openssl binary. It's accessible via the speed option. It tests how many operations it can perform in a given time, rather than how long it takes to perform a given number of operations. This strikes me a quite sane, because the benchmarks don't take significantly longer to run on a slow system than on a fast one.

To run a catchall benchmark, run it without any further options.

openssl speed

There are two sets of results. The first reports how many bytes per second can be processed for each algorithm, the second the times needed for sign/verify cycles. Here are the results on an 866MHz Pentium III.

The 'numbers' are in 1000s of bytes per second processed.
type             16 bytes     64 bytes    256 bytes   1024 bytes   8192 bytes
md2                670.66k     1426.54k     1992.36k     2213.89k     2291.03k
mdc2                 0.00         0.00         0.00         0.00         0.00 
md4               6125.45k    21351.38k    60480.17k   111804.42k   149312.30k
md5               5193.06k    18369.35k    53073.32k   100135.94k   135624.02k
hmac(md5)         3041.32k    11279.21k    36399.53k    82535.42k   128502.44k
sha1              4964.28k    15264.32k    36865.37k    56804.01k    67289.09k
rmd160            4668.18k    13871.53k    30900.74k    44740.61k    51590.49k
rc4              80531.08k    90495.68k    96069.21k    97128.79k    95857.32k
des cbc          17984.00k    18767.62k    18894.34k    19037.87k    19049.13k
des ede3          6622.08k     6758.36k     6824.11k     6781.95k     6832.13k
idea cbc             0.00         0.00         0.00         0.00         0.00 
rc2 cbc           7148.38k     7361.83k     7430.14k     7442.77k     7451.99k
rc5-32/12 cbc        0.00         0.00         0.00         0.00         0.00 
blowfish cbc     28184.34k    30013.45k    30568.87k    30514.86k    30743.29k
cast cbc         15924.76k    17093.03k    17436.67k    17531.22k    17487.19k
aes-128 cbc      15880.95k    16398.36k    16509.10k    16560.81k    16569.69k
aes-192 cbc      13653.11k    14207.34k    14279.08k    14312.11k    14333.27k
aes-256 cbc      12320.43k    12614.25k    12673.62k    12717.40k    12678.49k
                  sign    verify    sign/s verify/s
rsa  512 bits   0.0017s   0.0002s    577.1   6452.1
rsa 1024 bits   0.0083s   0.0004s    121.2   2300.5
rsa 2048 bits   0.0484s   0.0014s     20.7    701.2
rsa 4096 bits   0.3252s   0.0050s      3.1    201.5
                  sign    verify    sign/s verify/s
dsa  512 bits   0.0014s   0.0017s    714.0    598.8
dsa 1024 bits   0.0041s   0.0050s    246.5    199.2
dsa 2048 bits   0.0135s   0.0164s     74.0     60.8

You can run any of the algorithm-specific subtests directly.

# test rsa speeds
openssl speed rsa

# do the same test on a two-way SMP system
openssl speed rsa -multi 2

How do I benchmark remote connections?

The s_time option lets you test connection performance. The most simple invocation will run for 30 seconds, use any cipher, and use SSL handshaking to determine number of connections per second, using both new and reused sessions:

openssl s_time -connect

Beyond that most simple invocation, s_time gives you a wide variety of testing options.

# retrieve remote test.html page using only new sessions
openssl s_time -connect -www /test.html -new

# similar, using only SSL v3 and high encryption (see
# ciphers(1) man page for cipher strings)
openssl s_time \
  -connect -www /test.html -new \
  -ssl3 -cipher HIGH

# compare relative performance of various ciphers in
# 10-second tests
for c in $(openssl ciphers -ssl3 RSA); do
  echo $c
  openssl s_time -connect \
    -www / -new -time 10 -cipher $c 2>&1 | \
    grep bytes

If you don't have an SSL-enabled web server available for your use, you can emulate one using the s_server option.

# on one host, set up the server (using default port 4433)
openssl s_server -cert mycert.pem -www

# on second host (or even the same one), run s_time
openssl s_time -connect myhost:4433 -www / -new -ssl3


How do I generate a self-signed certificate?

You'll first need to decide whether or not you want to encrypt your key. Doing so means that the key is protected by a passphrase.

On the plus side, adding a passphrase to a key makes it more secure, so the key is less likely to be useful to someone who steals it. The downside, however, is that you'll have to either store the passphrase in a file or type it manually every time you want to start your web or ldap server.

It violates my normally paranoid nature to say it, but I prefer unencrypted keys, so I don't have to manually type a passphrase each time a secure daemon is started. (It's not terribly difficult to decrypt your key if you later tire of typing a passphrase.)

This example will produce a file called mycert.pem which will contain both the private key and the public certificate based on it. The certificate will be valid for 365 days, and the key (thanks to the -nodes option) is unencrypted.

openssl req \
  -x509 -nodes -days 365 \
  -newkey rsa:1024 -keyout mycert.pem -out mycert.pem

Using this command-line invocation, you'll have to answer a lot of questions: Country Name, State, City, and so on. The tricky question is “Common Name.” You'll want to answer with the hostname or CNAME by which people will address the server. This is very important. If your web server's real hostname is but people will be using to address the box, then use the latter name to answer the “Common Name” question.

Once you're comfortable with the answers you provide to those questions, you can script the whole thing by adding the -subj option. I've included some information about location into the example that follows, but the only thing you really need to include for the certificate to be useful is the hostname (CN).

openssl req \
  -x509 -nodes -days 365 \
  -subj '/C=US/ST=Oregon/L=Portland/' \
  -newkey rsa:1024 -keyout mycert.pem -out mycert.pem

How do I generate a certificate request for VeriSign?

Applying for a certificate signed by a recognized certificate authority like VeriSign is a complex bureaucratic process. You've got to perform all the requisite paperwork before creating a certificate request.

As in the recipe for creating a self-signed certificate, you'll have to decide whether or not you want a passphrase on your private key. The recipe below assumes you don't. You'll end up with two files: a new private key called mykey.pem and a certificate request called myreq.pem.

openssl req \
  -new -newkey rsa:1024 -nodes \
  -keyout mykey.pem -out myreq.pem

If you've already got a key and would like to use it for generating the request, the syntax is a bit simpler.

openssl req -new -key mykey.pem -out myreq.pem

Similarly, you can also provide subject information on the command line.

openssl req \
  -new -newkey rsa:1024 -nodes \
  -subj '/ Dom, Inc./C=US/ST=Oregon/L=Portland' \
  -keyout mykey.pem -out myreq.pem

When dealing with an institution like VeriSign, you need to take special care to make sure that the information you provide during the creation of the certificate request is exactly correct. I know from personal experience that even a difference as trivial as substituting “and” for “&” in the Organization Name will stall the process.

If you'd like, you can double check the signature and information provided in the certificate request.

# verify signature
openssl req -in myreq.pem -noout -verify -key mykey.pem

# check info
openssl req -in myreq.pem -noout -text

Save the key file in a secure location. You'll need it in order to use the certificate VeriSign sends you. The certificate request will typically be pasted into VeriSign's online application form.

How do I test a new certificate?

The s_server option provides a simple but effective testing method. The example below assumes you've combined your key and certificate into one file called mycert.pem.

First, launch the test server on the machine on which the certificate will be used. By default, the server will listen on port 4433; you can alter that using the -accept option.

openssl s_server -cert mycert.pem -www

If the server launches without complaint, then chances are good that the certificate is ready for production use.

You can also point your web browser at the test server, e.g., https://yourserver:4433/. Don't forget to specify the “https” protocol; plain-old “http” won't work. You should see a page listing the various ciphers available and some statistics about your connection. Most modern browsers allow you to examine the certificate as well.

How do I retrieve a remote certificate?

If you combine openssl and sed, you can retrieve remote certificates via a shell one-liner or a simple script.

# usage: [port]

openssl s_client -connect ${REMHOST}:${REMPORT} 2>&1 |\

You'll typically have to press Ctrl+C to close the script, since the remote server is probably waiting for some sort of input.

How do I extract information from a certificate?

An SSL certificate contains a wide range of information: issuer, valid dates, subject, and some hardcore crypto stuff. The x509 subcommand is the entry point for retrieving this information. The examples below all assume that the certificate you want to examine is stored in a file named cert.pem.

Using the -text option will give you the full breadth of information.

openssl x509 -text -in cert.pem

Other options will provide more targeted sets of data.

# who issued the cert?
openssl x509 -noout -in cert.pem -issuer

# to whom was it issued?
openssl x509 -noout -in cert.pem -subject

# for what dates is it valid?
openssl x509 -noout -in cert.pem -dates

# the above, all at once
openssl x509 -noout -in cert.pem -issuer -subject -dates

# what is its hash value?
openssl x509 -noout -in cert.pem -hash

# what is its MD5 fingerprint?
openssl x509 -noout -in cert.pem -fingerprint

How do I export or import a PKCS#12 certificate?

PKCS#12 files can be imported and exported by a number of applications, including Microsoft IIS. They are often associated with the file extension .pfx.

To create a PKCS#12 certificate, you'll need a private key and a certificate. During the conversion process, you'll be given an opportunity to put an “Export Password” (which can be empty, if you choose) on the certificate.

# create a file containing key and self-signed certificate
openssl req \
  -x509 -nodes -days 365 \
  -newkey rsa:1024 -keyout mycert.pem -out mycert.pem

# export mycert.pem as PKCS#12 file, mycert.pfx
openssl pkcs12 -export \
  -out mycert.pfx -in mycert.pem \
  -name "My Certificate"

If someone sends you a PKCS#12 and any passwords needed to work with it, you can export it into standard PEM format.

# export certificate and passphrase-less key
openssl pkcs12 -in mycert.pfx -out mycert.pem -nodes

# same as above, but you'll be prompted for a passphrase for
# the private key
openssl pkcs12 -in mycert.pfx -out mycert.pem

Certificate Verification

Applications linked against the OpenSSL libraries can verify certificates signed by a recognized certificate authority (CA).

How do I verify a certificate?

Use the verify option to verify certificates.

openssl verify cert.pem

If your local OpenSSL installation recognizes the certificate or its signing authority and everything else (dates, signing chain, etc.) checks out, you'll get a simple OK message.

$ openssl verify OK

If anything is amiss, you'll see some error messages with short descriptions of the problem, e.g.,

  • error 10 at 0 depth lookup:certificate has expired. Certificates are typically issued for a limited period of time—usually just one year—and openssl will complain if a certificate has expired.

  • error 18 at 0 depth lookup:self signed certificate. Unless you make an exception, OpenSSL won't verify a self-signed certificate.

What certificate authorities does OpenSSL recognize?

When OpenSSL was built for your system, it was configured with a “Directory for OpenSSL files.” (That's the --openssldir option passed to the configure script, for you hands-on types.) This is the directory that typically holds information about certificate authorities your system trusts.

The default location for this directory is /usr/local/ssl, but most vendors put it elsewhere, e.g., /usr/share/ssl (Red Hat/Fedora), /etc/ssl (Gentoo), /usr/lib/ssl (Debian), or /System/Library/OpenSSL (Macintosh OS X).

I don't know of any built-in method for identifying the location of this directory, but here's a hack that'll work on Linux systems.

strace openssl verify /some/file 2>&1 | grep cert.pem

On Solaris systems, use truss instead of strace. Either way, you should see a reference to the OpenSSL directory. (Sadly, the ktrace utility found on Mac OS X systems isn't quite powerful enough to do all this work.) Here's a system where it's /etc/ssl:

$ strace openssl verify /some/file 2>&1 | grep cert.pem
open("/etc/ssl/cert.pem", O_RDONLY)     = 3

Within that directory and a subdirectory called certs, you're likely to find one or more of three different kinds of files.

  1. A large file called cert.pem, an omnibus collection of many certificates from recognized certificate authorities like VeriSign and Thawte.

  2. Some small files in the certs subdirectory named with a .pem file extension, each of which contains a certificate from a single CA.

  3. Some symlinks in the certs subdirectory with obscure filenames like 052eae11.0. There is typically one of these links for each .pem file.

    The first part of obscure filename is actually a hash value based on the certificate within the .pem file to which it points. The file extension is just an iterator, since it's theoretically possible that multiple certificates can generate identical hashes.

    On my Gentoo system, for example, there's a symlink named f73e89fd.0 that points to a file named vsignss.pem. Sure enough, the certificate in that file generates a hash the equates to the name of the symlink:

    $ openssl x509 -noout -hash -in vsignss.pem

When an application encounters a remote certificate, it will typically check to see if the cert can be found in cert.pem or, if not, in a file named after the certificate's hash value. If found, the certificate is considered verified.

It's interesting to note that some applications, like Sendmail, allow you to specify at runtime the location of the certificates you trust, while others, like Pine, do not.

How do I get OpenSSL to recognize/verify a certificate?

Put the file that contains the certificate you'd like to trust into the certs directory discussed above. Then create the hash-based symlink. Here's a little script that'll do just that.

# usage: filename [filename ...]

for CERTFILE in $*; do
  # make sure file exists and is a valid cert
  test -f "$CERTFILE" || continue
  HASH=$(openssl x509 -noout -hash -in "$CERTFILE")
  test -n "$HASH" || continue

  # use lowest available iterator for symlink
  for ITER in 0 1 2 3 4 5 6 7 8 9; do
    test -f "${HASH}.${ITER}" && continue
    ln -s "$CERTFILE" "${HASH}.${ITER}"
    test -L "${HASH}.${ITER}" && break

Command-line clients and servers

The s_client and s_server options provide a way to launch SSL-enabled command-line clients and servers. There are other examples of their use scattered around this document, but this section is dedicated solely to them.

In this section, I assume you are familiar with the specific protocols at issue: SMTP, HTTP, etc. Explaining them is out of the scope of this article.

How do I connect to a secure SMTP server?

You can test, or even use, an SSL-enabled SMTP server from the command line using the s_client option.

Secure SMTP servers offer secure connections on up to three ports: 25 (TLS), 465 (SSL), and 587 (TLS). Some time around the 0.9.7 release, the openssl binary was given the ability to use STARTTLS when talking to SMTP servers.

# port 25/TLS; use same syntax for port 587
openssl s_client -connect -starttls smtp

# port 465/SSL
openssl s_client -connect

RFC821 suggests (although it falls short of explicitly specifying) the two characters "<CRLF>" as line-terminator. Most mail agents do not care about this and accept either "<LF>" or "<CRLF>" as line-terminators, but Qmail does not. If you want to comply to the letter with RFC821 and/or communicate with Qmail, use also the -crlf option:

openssl s_client -connect -crlf -starttls smtp

How do I connect to a secure [whatever] server?

Connecting to a different type of SSL-enabled server is essentially the same operation as outlined above. As of the date of this writing, openssl only supports command-line TLS with SMTP servers, so you have to use straightforward SSL connections with any other protocol.

# https: HTTP over SSL
openssl s_client -connect

# ldaps: LDAP over SSL
openssl s_client -connect

# imaps: IMAP over SSL
openssl s_client -connect

# pop3s: POP-3 over SSL
openssl s_client -connect

How do I set up an SSL server from the command line?

The s_server option allows you to set up an SSL-enabled server from the command line, but it's I wouldn't recommend using it for anything other than testing or debugging. If you need a production-quality wrapper around an otherwise insecure server, check out Stunnel instead.

The s_server option works best when you have a certificate; it's fairly limited without one.

# the -www option will sent back an HTML-formatted status page
# to any HTTP clients that request a page
openssl s_server -cert mycert.pem -www

# the -WWW option "emulates a simple web server. Pages will be
# resolved relative to the current directory." This example
# is listening on the https port, rather than the default
# port 4433
openssl s_server -accept 443 -cert mycert.pem -WWW


Generating digests with the dgst option is one of the more straightforward tasks you can accomplish with the openssl binary. Producing digests is done so often, as a matter of fact, that you can find special-use binaries for doing the same thing.

How do I create an MD5 or SHA1 digest of a file?

Digests are created using the dgst option.

# MD5 digest
openssl dgst -md5 filename

# SHA1 digest
openssl dgst -sha1 filename

The MD5 digests are indentical to those created with the widely available md5sum command, though the output formats differ.

$ openssl dgst -md5 foo-2.23.tar.gz
MD5(foo-2.23.tar.gz)= 81eda7985e99d28acd6d286aa0e13e07
$ md5sum foo-2.23.tar.gz
81eda7985e99d28acd6d286aa0e13e07  foo-2.23.tar.gz

The same is true for SHA1 digests and the output of the sha1sum application.

$ openssl dgst -sha1 foo-2.23.tar.gz
SHA1(foo-2.23.tar.gz)= e4eabc78894e2c204d788521812497e021f45c08
$ sha1sum foo-2.23.tar.gz
e4eabc78894e2c204d788521812497e021f45c08  foo-2.23.tar.gz

How do I sign a digest?

If you want to ensure that the digest you create doesn't get modified without your permission, you can sign it using your private key. The following example assumes that you want to sign the SHA1 sum of a file called foo-1.23.tar.gz.

# signed digest will be foo-1.23.tar.gz.sha1
openssl dgst -sha1 \
  -sign mykey.pem
  -out foo-1.23.tar.gz.sha1 \

How do I verify a signed digest?

To verify a signed digest you'll need the file from which the digest was derived, the signed digest, and the signer's public key.

# to verify foo-1.23.tar.gz using foo-1.23.tar.gz.sha1
# and pubkey.pem
openssl dgst -sha1 \
  -verify pubkey.pem \
  -signature foo-1.23.tar.gz.sha1 \

What other kinds of digests are available?

Use the built-in list-message-digest-commands option to get a list of the digest types available to your local OpenSSL installation.

openssl list-message-digest-commands


How do I base64-encode something?

Use the enc -base64 option.

# send encoded contents of file.txt to stdout
openssl enc -base64 -in file.txt

# same, but write contents to file.txt.enc
openssl enc -base64 -in file.txt -out file.txt.enc

It's also possible to do a quick command-line encoding of a string value:

$ echo "encode me" | openssl enc -base64

Note that echo will silently attach a newline character to your string. Consider using its -n option if you want to avoid that situation, which could be important if you're trying to encode a password or authentication string.

$ echo -n "encode me" | openssl enc -base64

Use the -d (decode) option to reverse the process.

$ echo "ZW5jb2RlIG1lCg==" | openssl enc -base64 -d
encode me

How do I simply encrypt a file?

Simple file encryption is probably better done using a tool like GPG. Still, you may have occasion to want to encrypt a file without having to build or use a key/certificate structure. All you want to have to remember is a password. It can nearly be that simple—if you can also remember the cipher you employed for encryption.

To choose a cipher, consult the enc(1) man page. More simply (and perhaps more accurately), you can ask openssl for a list in one of two ways.

# see the list under the 'Cipher commands' heading
openssl -h

# or get a long list, one cipher per line
openssl list-cipher-commands

After you choose a cipher, you'll also have to decide if you want to base64-encode the data. Doing so will mean the encrypted data can be, say, pasted into an email message. Otherwise, the output will be a binary file.

# encrypt file.txt to file.enc using 256-bit AES in CBC mode
openssl enc -aes-256-cbc -salt -in file.txt -out file.enc

# the same, only the output is base64 encoded for, e.g., e-mail
openssl enc -aes-256-cbc -a -salt -in file.txt -out file.enc

To decrypt file.enc you or the file's recipient will need to remember the cipher and the passphrase.

# decrypt binary file.enc
openssl enc -d -aes-256-cbc -in file.enc

# decrypt base64-encoded version
openssl enc -d -aes-256-cbc -a -in file.enc


How do I generate an RSA key?

Use the genrsa option.

# default 512-bit key, sent to standard output
openssl genrsa

# 1024-bit key, saved to file named mykey.pem
openssl genrsa -out mykey.pem 1024

# same as above, but encrypted with a passphrase
openssl genrsa -des3 -out mykey.pem 1024

How do I generate a public RSA key?

Use the rsa option to produce a public version of your private RSA key.

openssl rsa -in mykey.pem -pubout

How do I generate a DSA key?

Building DSA keys requires a parameter file, and DSA verify operations are slower than their RSA counterparts, so they aren't as widely used as RSA keys.

If you're only going to build a single DSA key, you can do so in just one step using the dsaparam subcommand.

# key will be called dsakey.pem
openssl dsaparam -noout -out dsakey.pem -genkey 1024

If, on the other hand, you'll be creating several DSA keys, you'll probably want to build a shared parameter file before generating the keys. It can take a while to build the parameters, but once built, key generation is done quickly.

# create parameters in dsaparam.pem
openssl dsaparam -out dsaparam.pem 1024

# create first key
openssl gendsa -out key1.pem dsaparam.pem

# and second ...
openssl gendsa -out key2.pem dsaparam.pem

How do I remove a passphrase from a key?

Perhaps you've grown tired of typing your passphrase every time your secure daemon starts. You can decrypt your key, removing the passphrase requirement, using the rsa or dsa option, depending on the signature algorithm you chose when creating your private key.

If you created an RSA key and it is stored in a standalone file called key.pem, then here's how to output a decrypted version of the same key to a file called newkey.pem.

# you'll be prompted for your passphrase one last time
openssl rsa -in key.pem -out newkey.pem

Often, you'll have your private key and public certificate stored in the same file. If they are stored in a file called mycert.pem, you can construct a decrypted version called newcert.pem in two steps.

# you'll need to type your passphrase once more
openssl rsa -in mycert.pem -out newcert.pem
openssl x509 -in mycert.pem >>newcert.pem

Password hashes

Using the passwd option, you can generate password hashes that interoperate with traditional /etc/passwd files, newer-style /etc/shadow files, and Apache password files.

How do I generate a crypt-style password hash?

You can generate a new hash quite simply:

$ openssl passwd MySecret

If you know an existing password's “salt,” you can duplicate the hash.

$ openssl passwd -salt 8E MySecret

How do I generate a shadow-style password hash?

Newer Unix systems use a more secure MD5-based hashing mechanism that uses an eight-character salt (as compared to the two-character salt in traditional crypt()-style hashes). Generating them is still straightforward using the -1 option:

$ openssl passwd -1 MySecret

The salt in this format consists of the eight characters between the second and third dollar signs, in this case sXiKzkus. So you can also duplicate a hash with a known salt and password.

$ openssl passwd -1 -salt sXiKzkus MySecret

Random data

How do I generate random bits?

Use the rand option to generate binary or base64-encoded data.

# write 128 random bits of base64-encoded data to stdout
openssl rand -base64 128

# write 1024 bits of binary random data to a file
openssl rand -out random-data.bin 1024

# seed openssl with semi-random bytes from browser cache
cd $(find ~/.mozilla/firefox -type d -name Cache)
openssl rand -rand $(find . -type f -printf '%f:') -base64 1024


S/MIME is a standard for sending and receiving secure MIME data, especially in e-mail messages. Automated S/MIME capabilities have been added to quite a few e-mail clients, though openssl can provide command-line S/MIME services using the smime option.

Note that the documentation in the smime(1) man page includes a number of good examples.

How do I verify a signed S/MIME message?

It's pretty easy to verify a signed message. Use your mail client to save the signed message to a file. In this example, I assume that the file is named msg.txt.

openssl smime -verify -in msg.txt

If the sender's certificate is signed by a certificate authority trusted by your OpenSSL infrastructure, you'll see some mail headers, a copy of the message, and a concluding line that says Verification successful.

If the messages has been modified by an unauthorized party, the output will conclude with a failure message indicating that the digest and/or the signature doesn't match what you received:

Verification failure
23016:error:21071065:PKCS7 routines:PKCS7_signatureVerify:digest
23016:error:21075069:PKCS7 routines:PKCS7_verify:signature

Likewise, if the sender's certificate isn't recognized by your OpenSSL infrastructure, you'll get a similar error:

Verification failure
9544:error:21075075:PKCS7 routines:PKCS7_verify:certificate verify
error:pk7_smime.c:222:Verify error:self signed certificate

Most e-mail clients send a copy of the public certificate in the signature attached to the message. From the command line, you can view the certificate data yourself. You'll use the smime -pk7out option to pipe a copy of the PKCS#7 certificate back into the pkcs7 option. It's oddly cumbersome but it works.

openssl smime -pk7out -in msg.txt | \
openssl pkcs7 -text -noout -print_certs

If you'd like to extract a copy of your correspondent's certificate for long-term use, use just the first part of that pipe.

openssl smime -pk7out -in msg.txt -out her-cert.pem

At that point, you can either integrate it into your OpenSSL infrastructure or you can save it off somewhere for special use.

openssl smime -verify -in msg.txt -CAfile /path/to/her-cert.pem

How do I encrypt a S/MIME message?

Let's say that someone sends you her public certificate and asks that you encrypt some message to her. You've saved her certificate as her-cert.pem. You've saved your reply as my-message.txt.

To get the default—though fairly weak—RC2-40 encryption, you just tell openssl where the message and the certificate are located.

openssl smime her-cert.pem -encrypt -in my-message.txt

If you're pretty sure your remote correspondent has a robust SSL toolkit, you can specify a stronger encryption algorithm like triple DES:

openssl smime her-cert.pem -encrypt -des3 -in my-message.txt

By default, the encrypted message, including the mail headers, is sent to standard output. Use the -out option or your shell to redirect it to a file. Or, much trickier, pipe the output directly to sendmail.

openssl smime her-cert.pem \
  -encrypt \
  -des3 \
  -in my-message.txt \
  -from 'Your Fullname <>' \
  -to 'Her Fullname <>' \
  -subject 'My encrypted reply' |\

How do I sign a S/MIME message?

If you don't need to encrypt the entire message, but you do want to sign it so that your recipient can be assured of the message's integrity, the recipe is similar to that for encryption. The main difference is that you need to have your own key and certificate, since you can't sign anything with the recipient's cert.

openssl smime \
  -sign \
  -signer /path/to/your-cert.pem \
  -in my-message.txt \
  -from 'Your Fullname <>' \
  -to 'Her Fullname <>' \
  -subject 'My encrypted reply' |\

For further reading

Though it takes time to read them all and figure out how they relate to one another, the OpenSSL man pages are the best place to start: asn1parse(1), ca(1), ciphers(1), config(5), crl(1), crl2pkcs7(1), dgst(1), dhparam(1), dsa(1), dsaparam(1), enc(1), gendsa(1), genrsa(1), nseq(1), ocsp(1), openssl(1), passwd(1), pkcs12(1), pkcs7(1), pkcs8(1), rand(1), req(1), rsa(1), rsautl(1), s_client(1), s_server(1), s_time(1), sess_id(1), smime(1), speed(1), spkac(1), verify(1), version(1), x509(1).

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