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
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.
Use the version
option.
$ openssl version
OpenSSL 0.9.7d 17 Mar 2004
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
x509
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 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.
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'
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
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 remote.host:443
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 remote.host:443 -www /test.html -new # similar, using only SSL v3 and high encryption (see # ciphers(1) man page for cipher strings) openssl s_time \ -connect remote.host:443 -www /test.html -new \ -ssl3 -cipher HIGH # compare relative performance of various ciphers in # 10-second tests IFS=":" for c in $(openssl ciphers -ssl3 RSA); do echo $c openssl s_time -connect remote.host:443 \ -www / -new -time 10 -cipher $c 2>&1 | \ grep bytes echo done
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
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 mybox.mydomain.com
but people will be using www.mydomain.com
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/CN=www.madboa.com' \ -newkey rsa:1024 -keyout mycert.pem -out mycert.pem
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 '/CN=www.mydom.com/O=My 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.
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.
If you combine openssl and sed, you can retrieve remote certificates via a shell one-liner or a simple script.
#!/bin/sh # # usage: retrieve-cert.sh remote.host.name [port] # REMHOST=$1 REMPORT=${2:-443} openssl s_client -connect ${REMHOST}:${REMPORT} 2>&1 |\ sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p'
You'll typically have to press Ctrl+C to close the script, since the remote server is probably waiting for some sort of input.
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
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
Applications linked against the OpenSSL libraries can verify certificates signed by a recognized certificate authority (CA).
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 remote.site.pem
remote.site.pem: 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.
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.
A large file called cert.pem
, an omnibus collection of many certificates from
recognized certificate authorities like VeriSign and Thawte.
Some small files in the certs
subdirectory named with a .pem
file extension, each of which contains a certificate from a single CA.
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
f73e89fd
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.
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.
#!/bin/sh # # usage: certlink.sh 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 done done
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.
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 remote.host:25 -starttls smtp # port 465/SSL openssl s_client -connect remote.host:465
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 remote.host:25 -crlf -starttls smtp
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 remote.host:443 # ldaps: LDAP over SSL openssl s_client -connect remote.host:636 # imaps: IMAP over SSL openssl s_client -connect remote.host:993 # pop3s: POP-3 over SSL openssl s_client -connect remote.host:995
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.
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
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 \ foo-1.23.tar.gz
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 \ foo-1.23.tar.gz
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
ZW5jb2RlIG1lCg==
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
ZW5jb2RlIG1l
Use the -d
(decode) option to reverse the process.
$ echo "ZW5jb2RlIG1lCg==" | openssl enc -base64 -d
encode me
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
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
Use the rsa
option to produce a public version of your private RSA key.
openssl rsa -in mykey.pem -pubout
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
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
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.
You can generate a new hash quite simply:
$ openssl passwd MySecret
8E4vqBR4UOYF.
If you know an existing password's “salt,” you can duplicate the hash.
$ openssl passwd -salt 8E MySecret
8E4vqBR4UOYF.
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
$1$sXiKzkus$haDZ9JpVrRHBznY5OxB82.
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
$1$sXiKzkus$haDZ9JpVrRHBznY5OxB82.
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.
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 failure:pk7_doit.c:804: 23016:error:21075069:PKCS7 routines:PKCS7_verify:signature failure:pk7_smime.c:265:
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
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 <you@youraddress.com>' \ -to 'Her Fullname <her@heraddress.com>' \ -subject 'My encrypted reply' |\ sendmail her@heraddress.com
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 <you@youraddress.com>' \ -to 'Her Fullname <her@heraddress.com>' \ -subject 'My encrypted reply' |\ sendmail her@heraddress.com
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).
Comments and suggestions about this document are appreciated and can be addressed to the author at <heinlein@madboa.com>
.
This article is licensed under a Creative Commons License.