OpenSSL Command-Line HOWTO

Paul Heinlein
First published on June 13, 2004
Last updated on May 3, 2024



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 1.0.1e-fips 11 Feb 2013

You can get much more information with the version -a option.

$ openssl version -a
OpenSSL 1.0.1e-fips 11 Feb 2013
built on: Thu Jul 23 19:06:35 UTC 2015
platform: linux-x86_64
options:  bn(64,64) md2(int) rc4(16x,int) des(idx,cisc,16,int)
          idea(int) blowfish(idx)
-Wall -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions
-fstack-protector --param=ssp-buffer-size=4 -m64 -mtune=generic
OPENSSLDIR: "/etc/pki/tls"
engines:  rdrand dynamic

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           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             ts                verify            version

Message Digest commands (see the `dgst' command for more details)
md2               md4               md5               rmd160
sha               sha1

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              idea
idea-cbc          idea-cfb          idea-ecb          idea-ofb
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          seed-ofb          zlib

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
-r              to output the digest in coreutils format
-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)
-out filename   output to filename rather than stdout
-signature file signature to verify
-sigopt nm:v    signature parameter
-hmac key       create hashed MAC with key
-mac algorithm  create MAC (not neccessarily HMAC)
-macopt nm:v    MAC algorithm parameters or key
-engine e       use engine e, possibly a hardware device.
-md4            to use the md4 message digest algorithm
-md5            to use the md5 message digest algorithm
-ripemd160      to use the ripemd160 message digest algorithm
-sha            to use the sha message digest algorithm
-sha1           to use the sha1 message digest algorithm
-sha224         to use the sha224 message digest algorithm
-sha256         to use the sha256 message digest algorithm
-sha384         to use the sha384 message digest algorithm
-sha512         to use the sha512 message digest algorithm
-whirlpool      to use the whirlpool 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 as 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 2.70GHz Intel Xeon E5.

The 'numbers' are in 1000s of bytes per second processed.
type             16 bytes     64 bytes    256 bytes   1024 bytes   8192 bytes
md2               2540.48k     5184.66k     6989.57k     7651.67k     7872.51k
mdc2                 0.00         0.00         0.00         0.00         0.00
md4              83248.41k   261068.18k   624212.82k   940529.32k  1128846.68k
md5              62411.57k   184768.36k   408835.75k   586930.52k   678061.98k
hmac(md5)        48713.62k   148265.56k   359626.67k   563050.68k   670255.79k
sha1             68829.72k   195087.40k   431001.51k   623344.42k   729505.79k
rmd160           38598.59k    96226.86k   183336.45k   235962.71k   257526.44k
rc4             480093.57k   678565.35k   783765.42k   818297.51k   838205.99k
des cbc          69500.17k    71184.75k    71491.50k    71641.77k    72010.15k
des ede3         26433.63k    26717.01k    26772.99k    26788.18k    26907.57k
idea cbc         95690.28k    99334.17k   100835.40k   100787.54k   100900.86k
seed cbc         76871.40k    77238.46k    77736.50k    77452.97k    77545.47k
rc2 cbc          48984.63k    49589.03k    50188.07k    50103.98k    50066.77k
rc5-32/12 cbc        0.00         0.00         0.00         0.00         0.00
blowfish cbc    122583.30k   129550.92k   130876.67k   131111.94k   131394.22k
cast cbc        109471.38k   114523.31k   115934.46k   116200.45k   116331.86k
aes-128 cbc     128352.23k   138604.76k   141173.42k   142832.25k   142682.79k
aes-192 cbc     107703.93k   114456.79k   117716.65k   118847.36k   118784.00k
aes-256 cbc      93374.87k    99521.51k   101198.51k   101382.49k   101635.41k
camellia-128 cbc    99270.57k   150412.42k   170346.33k   176311.91k   177913.86k
camellia-192 cbc    85896.60k   117356.52k   128556.97k   132759.72k   133425.83k
camellia-256 cbc    87351.27k   117695.15k   128972.03k   132130.47k   133455.87k
sha256           52372.61k   117766.12k   204825.69k   249974.10k   270914.90k
sha512           41278.19k   165820.37k   258298.69k   365981.70k   419864.58k
whirlpool        24803.02k    53047.07k    87593.90k   104570.54k   111159.98k
aes-128 ige     128441.31k   132981.88k   133269.08k   133738.15k   133966.51k
aes-192 ige     107831.37k   111507.07k   111800.66k   112156.67k   112219.48k
aes-256 ige      94382.07k    96351.17k    96750.68k    96958.46k    97446.44k
ghash           888644.92k  1452788.80k  1696788.74k  1763055.96k  1799086.49k
                  sign    verify    sign/s verify/s
rsa  512 bits 0.000049s 0.000004s  20547.1 248266.2
rsa 1024 bits 0.000194s 0.000011s   5146.0  90735.4
rsa 2048 bits 0.001194s 0.000037s    837.3  27277.1
rsa 4096 bits 0.008560s 0.000137s    116.8   7324.5
                  sign    verify    sign/s verify/s
dsa  512 bits 0.000048s 0.000046s  20667.7  21701.8
dsa 1024 bits 0.000113s 0.000126s   8831.9   7951.8
dsa 2048 bits 0.000362s 0.000430s   2762.0   2322.9
                              sign    verify    sign/s verify/s
 256 bit ecdsa (nistp256)   0.0001s   0.0004s   9856.1   2524.4
 384 bit ecdsa (nistp384)   0.0002s   0.0008s   5103.6   1191.7
 521 bit ecdsa (nistp521)   0.0004s   0.0018s   2679.0    550.3
                              op      op/s
 256 bit ecdh (nistp256)   0.0003s   3063.8
 384 bit ecdh (nistp384)   0.0007s   1447.3
 521 bit ecdh (nistp521)   0.0015s    666.2

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 -sha256 \
  -newkey rsa:2048 -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 -sha256 \
  -subj '/C=US/ST=Oregon/L=Portland/' \
  -newkey rsa:2048 -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 -sha256 -newkey rsa:2048 -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 -sha256 -newkey rsa:2048 -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]

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

You can, in turn, pipe that information back to openssl to do things like check the dates on all your active certificates.

for CERT in \ \ \
  echo |\
  openssl s_client -connect ${CERT} 2>/dev/null |\
  openssl x509 -noout -subject -dates

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 extensions .pfx and .p12.

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 -sha256 -nodes -days 365 \
  -newkey rsa:2048 -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

Web-based consoles for certificate authorities will often compel you to encrypt keys prior to downloading a requested certificate. At the same time, you might not want to install a PKCS#12 file with an encrypted key on a given server. Also you might need to change the “alias” or “name” of a key-certificate pair within the installed file.

Here’s a simple pipe that decrypts the key in a downloaded PFX file and changes the name/alias of the certificate and writes them all to a new PFX/P12 file. The first part of the pipe operation will prompt you to provide the password you set on the downloaded file.

# in this example, the key in the .pfx file is encrypted, while the
# key in the resulting .p12 file is not
openssl pkcs12 -nodes -in | \
openssl pkcs12 -out -export -name "ApacheTomcat"

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).

Use the version option to identify which directory (labeled OPENSSLDIR) your installation uses.

openssl version -d

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 web server using SNI?

The shortage of IPv4 addresses prompted the development of the HTTP 1.1 standard so a single IP address could host multiple name-based virtual servers.

Later, that same shortage of addresses led to the development of the Server Name Indication (SNI) extension of the TLS protocol. When using SNI, the client sends the hostname it wants to contact during the TLS negotiation. An SNI-enabled server is then able to offer the certificate with the matching hostname for the client to verify.

SNI is enabled in openssl by specifying the -servername option.

openssl s_client -connect -servername

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. I’ve seen several systems on which the OpenSSL dgst(1) man page does not accurately report the digest functions available via the local openssl binary. I suggest running openssl dgst -h to see which digests are actually available.

# MD5 digest
openssl dgst -md5 filename

# SHA1 digest
openssl dgst -sha1 filename

# SHA256 digest
openssl dgst -sha256 filename

The MD5 digests are identical 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 SHA256 sum of a file called foo-1.23.tar.gz.

# signed digest will be foo-1.23.tar.gz.sha1
openssl dgst -sha256 \
  -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 -sha256 \
  -verify pubkey.pem \
  -signature foo-1.23.tar.gz.sha1 \

How do I create an Apache digest password entry?

Apache’s HTTP digest authentication feature requires a special password format. Apache ships with the htdigest utility, but it will only write to a file, not to standard output. When working with remote users, it’s sometimes nice for them to be able to generate a password hash on a machine they trust and then mail it for inclusion in your local password database.

The format of the password database is relatively simple: a colon-separated list of the username, authorization realm (specified by the Apache AuthName directive), and an MD5 digest of those two items and the password. Below is a script that duplicates the output of htdigest, except that the output is written to standard output. It takes advantage of the dgst option’s ability to read from standard input.


echo "Create an Apache-friendly Digest Password Entry"
echo "-----------------------------------------------"

# get user input, disabling tty echoing for password
read -p "Enter username: " UNAME
read -p "Enter Apache AuthName: " AUTHNAME
read -s -p "Enter password: " PWORD; echo

printf "\n%s:%s:%s\n" \
  "$UNAME" \
  $(printf "${UNAME}:${AUTHNAME}:${PWORD}" | openssl dgst -md5)

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

Like the list in the dgst(1) man page, this list may be outdated. Let the buyer beware!


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

If you’d like to avoid typing a passphrase every time you encrypt or decrypt a file, the openssl(1) man page provides the details under the heading “PASS PHRASE ARGUMENTS.” The format of the password argument is fairly simple.

# provide password on command line
openssl enc -aes-256-cbc -salt -in file.txt \
  -out file.enc -pass pass:mySillyPassword

# provide password in a file
openssl enc -aes-256-cbc -salt -in file.txt \
  -out file.enc -pass file:/path/to/secret/password.txt


How do I interpret SSL error messages?

Poking through your system logs, you see some error messages that are evidently related to OpenSSL or crypto:

sshd[31784]: error: RSA_public_decrypt failed: error:0407006A:lib(4):func(112):reason(106)
sshd[770]: error: RSA_public_decrypt failed: error:0407006A:lib(4):func(112):reason(106)

The first step to figure out what’s going wrong is to use the errstr option to intrepret the error code. The code number is found between “error:” and “:lib”. In this case, it’s 0407006A.

$ openssl errstr 0407006A
error:0407006A:rsa routines:RSA_padding_check_PKCS1_type_1:block type is not 01

If you’ve got a full OpenSSL installation, including all the development documentation, you can start your investigation there. In this example, the RSA_padding_add_PKCS1_type_1(3) man page will inform you that PKCS #1 involves block methods for signatures. After that, of course, you’d need to pore through your application’s source code to identify when it would expect be receiving those sorts of packets.


How do I generate an RSA key?

Use the genrsa option.

# default 1024-bit key, sent to standard output
openssl genrsa

# 2048-bit key, saved to file named mykey.pem
openssl genrsa -out mykey.pem 2048

# same as above, but encrypted with a passphrase
openssl genrsa -des3 -out mykey.pem 2048

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 create an elliptic curve key?

Routines for working with elliptic curve cryptography were added to OpenSSL in version 0.9.8. Generating an EC key involves the ecparam option.

openssl ecparam -out key.pem -name prime256v1 -genkey

# openssl can provide full list of EC parameter names suitable for
# passing to the -name option above:
openssl ecparam -list_curves

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

Prime numbers

Current cryptographic techniques rely heavily on the generation and testing of prime numbers, so it’s no surprise that the OpenSSL libraries contain several routines dealing with primes. Beginning with version 0.9.7e (or so), the prime option was added to the openssl binary.

How do I test whether a number is prime?

Pass the number to the prime option. Note that the number returned by openssl will be in hex, not decimal, format.

$ openssl prime 119054759245460753
1A6F7AC39A53511 is not prime

You can also pass hex numbers directly.

$ openssl prime -hex 2f
2F is prime

How do I generate a set of prime numbers?

Starting with OpenSSL version 1.0.0, the openssl binary can generate prime numbers of a specified length:

$ openssl prime -generate -bits 64
$ openssl prime -generate -bits 64 -hex

If you’re using a version of OpenSSL older than 1.0.0, you’ll have to pass a bunch of numbers to openssl and see what sticks. The seq utility is useful in this capacity.

# define start and ending points
for N in $(seq $AQUO $ADQUEM); do
  # use bc to convert hex to decimal
  openssl prime $N | awk '/is prime/ {print "ibase=16;"$1}' | bc

Random data

How do I generate random data?

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

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

# write 1024 bytes 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

On a Unix box with a /dev/urandom device and a copy of GNU head, or a recent version of BSD head, you can achieve a similar effect, often with better entropy:

# get 32 bytes from /dev/urandom and base64 encode them
head -c 32 /dev/urandom | openssl enc -base64

You can get a wider variety of characters than what’s offered using Base64 encoding by using strings:

# get 32 bytes from /dev/random, grab printable characters, and
# strip whitespace. using echo and the shell's command substitution
# will nicely strip out newlines.
echo $(head -c 32 /dev/random | strings -1) | sed 's/[[:space:]]//g'

Make sure you know the trade-offs between the random and urandom devices before relying on them for truly critical entropy. Consult the random(4) man page on Linux and BSD systems, or random(7D) on Solaris, for further information.


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 signed 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), ec(1), ecparam(1), enc(1), errstr(1), gendsa(1), genpkey(1), genrsa(1), nseq(1), ocsp(1), openssl(1), passwd(1), pkcs12(1), pkcs7(1), pkcs8(1), pkey(1), pkeyparam(1), pkeyutl(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), ts(1), tsget(1), verify(1), version(1), x509(1), x509v3_config(5).

Comments welcome

This document has been online for well over a decade. Much of its development is due to my own curiosity, but several key improvements have come via unsolicited suggestions from readers. So let me say explicitly that comments and suggestions about this document are appreciated and can be addressed to the author at