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Real World Uses For OpenSSL

Aug 14, 2003 Atlanta Linux Enthusiasts

What is SSL and TLS?

  • The SSL protocols define a set of rules about when to send and not send messages
  • SSL provides confidentiality, authentication and message integrity
  • Utilizes cryptographically sound message digests and symmetric encryption algorithms
  • Provides the ability to resume sessions (SSL session IDs allow this)
  • SSLv1 and SSLv2 were developed by Netscape
  • SSLv3 and TLSv1 devloped by a community
  • Defines two roles: client and server

How Does SSL Work?

  • SSL works on a message passing paradigm
  • Client initates connection, proposing ciphers and digest algorithms (specified in ClientHello)
  • Server selects the cipher/MD/SSL version to be used (specified in ServerHello)
  • ChangeCipherSpec is used to enable the negotiated security services

What is an x.509 Certificate?

  • Digital equivalent of a drivers license
  • Attests to the truth of an individual or Organization
  • Usually issued by a certificate authority

Come in three flavors: Personal, Code signing and Web Site certificates

  • CA certificates are special, the issuer and subject are the same
  • Certificate Authorities certify their own identity
  • Digital certificates contain a variety of information, including:
  • Version
  • Serial Number
  • Issuer
  • Subject
  • Algorithm Identifier
  • Extensions
  • Signature
  • Period Of Validity
  • Subject's Public Key
  • Subject Unique ID
  • Issuer Unique ID

What is a Certificate Authority?

  • Establishes trust (You trust the big CAS, don't you? )
  • When a CA “ signs” your certificate, they are vouching that you are who you say you are
  • ( Verisign accidentally issued two certs to a hacker claiming to be MSFT, oops )
  • Two forms of Certificate Authorities: Private and Public
    • Public Certificate Authorities (Verisign, Thawte, etc.) issue certficates to the world
    • Private Certificate Authorities (OpenCA, Entrust CA) issue certificates to an organization
  • Certificate delegation/chaining allows a CA to delegate signing capabilities

What Tools Are Available To Work With SSL And TLS?

OpenSSL

  • Set of opensource cryptographic libraries
  • Implements SSLv2, SSLv3 and TLSv1
  • Support for OCSP Version 1 and 2
  • Support Certificate Revocation Lists
  • APIs and CLI can be used to encrypt and sign data
  • If you have a recent version of Linux or FreeBSD/OpenBSD, openssl is included
  • Configurable via a single configuration file (Default openssl.cnf)
  • Can be configured to use hardware cryptographic accelerators
  • Previously known as SSLeay (Eric A. Young was original maintainer)
  • Current version as of this writing is 0.9.7b
  • You can find the software at: openssl.org

Stunnel (Universal Secure Tunnel)

  • Works as a wrapper for insecure services
  • Adds wire-level security for POP3, IMAP, HTTP and NNTP and SMTP
  • Supports client and server certificates
  • Stunnel version 4.X uses a runtime configuration file
  • Supports chroot() jails
  • Current version as of this writing is 4.04
  • You can find the software at: stunnel.org

SSL Dump

  • SSLv3/TLS network protocol analyzer
  • Packet Capture (PCAP) is used to intercept packets on the wire
  • Works on most UNIX platforms (FreeBSD, Linux, Solaris, HP-UX, etc)
  • Provides facilities to view SSL messages, application data and state changes
  • Syntax is similar to tcpdump
  • The current version as of this writing is 0.9b3
  • You can find the software at: rtfm.com/ssldump

Fun Things To Do With OpenSSL And Friends

Creating RSA & DSA Keys

  • “ genrsa” and “ gendsa” allow you to create RSA and DSA private keys - “ rsa” and “ dsa” allow you to extract public keys from the private key - “ rsautl” allows you to encrypt, decrypt and sign data (it is not intended for this purpose) - Examples:

Generate a set of DSA parameters

  • openssl dsaparam -out dsaparam 2048

Generate a DSA key

  • openssl gendsa -out dsaprivkey.pem -des3 dsaparam

Print the DSA public key associated with the private key above

  • openssl dsa -in dsaprivkey.pem -pubout -out dsapubkey.pem

Generate an RSA private key and encrypt it with 3DES

  • openssl genrsa -des3 -out rsa.key 2048

Print the public key associated with the RSA private key

  • openssl rsa -in rsa.key -pubout

Creating X.509 Certificates

  • openssl allows you to create CSRs and x.509 certificates
  • Support DSA and RSA keys
  • Supports PEM and DER formatted certificates
  • nodes can be used to strip passwords from private keys (not recommended!)
  • Default certificate fields can be specified in openssl.cnf
  • Example:
openssl req -x509 -outform PEM -keyform PEM -keyout cert.key -days 500 \
  -out cert.crt -newkey rsa:2048

Enter PEM pass phrase:
Verifying - Enter PEM pass phrase:
Country Name (2 letter code) [GB]:US
State or Province Name (full name) [Berkshire]:Georgia
Locality Name (eg, city) [Newbury]:Atlanta
Organization Name (eg, company) [My Company Ltd]: H8x0R Computing
Organizational Unit Name (eg, section) []: IT
Common Name (eg, your name or your server's hostname) []: www.h8x0r.com
Email Address []: postmaster@h8x0r.com

Printing The Contents Of A Certificate

  • The “x509” and “req” options allow you to print certificates
  • Individual options to print issuer, serial number, hash, subject, fingerprint etc. etc.
  • Examples:

Print the contents on an x509 certificate

  • openssl x509 -in cert.crt -text

Print the Subject Field on a certificate

  • openssl x509 -subject -in cert.crt -noout

Print the Issuer of the certificate

  • openssl x509 -issuer -in cert.crt -noout

Print the date the certificate will expire

  • openssl x509 -enddate -in cert.crt -noout

Print out the contents of a server certificate

  • openssl s_client -connect www.giddie-up.net:443 -showcerts

Becoming A Certificate Authority

  • Create supporting files and directories
  • Create a key pair for signing certificates
  • Protect your private key with your life
  • Example:

Setup directories and files

mkdir /etc/ca
mkdir /etc/ca/certs
mkdir /etc/ca/keys
chmod 700 /etc/ca /etc/ca/keys
echo '1' > /etc/ca/serial.txt
touch /etc/ca/index.txt

Create your certifcate and self-sign it

  • openssl req -x509 -newkey rsa:2048 -out cacert.pem -outform PEM - -keyout cakey.pem -keyform PEM

Generate a certificate signing request

  • openssl req -newkey rsa:1024 -keyout testkey.pem -keyform PEM - -out testreq.pem -outform PEM

Sign the CSR

  • openssl ca -in testreq.pem

Revoking Certifcates

  • Allows your CA to revoke certs who have lost keys or certs - Examples:

Revoke a bad certificate

  • openssl ca -revoke testcert.pem

Generate a CRL to post to our website/OCSP

  • openssl ca -gencrl -out certs.crl

Encrypting And Decrypting Data

  • Supports various symmetric ciphers (AES, 3DES, Blowfish, RC4, Cast5)
  • Encrypted data can be Base64 encoded for easy trasport
  • Supports CBC, CFB, ECB and OFB modes of encryption
  • Data can be left as 0s and 1s for easy storage
  • Example:

Encrypt /etc/services

  • openssl enc -in /etc/services -out /tmp/services.enc.blfsh -base64 -blowfish

Use a horrible encryption algorithm so others can view my data

  • openssl enc -in Finances.xls -out Finances.xls.enc.rc4 -rc4-40

Let's base64 encode /etc/services so I can send it to someone

  • openssl enc -in file.bin -out file.bin.b64 -base64

Decrypt a blowfish encrypted file

  • openssl enc -d -in services.enc.blfsh -blowfish -base64

Generating Message Digests

  • Generates MD5, SHA1 and RIPEMD160 checksums
  • Can produce colon delimited checksums for easy parsing
  • Can verify a checksum using an existing public key
  • Can verify a checksum using a private key
  • Can sign a message digest with your private key
  • Can output checksum as binary or hex
  • Example:

Generate a SHA1 checksum of /etc/services

  • openssl dgst sha1 /etc/services

Generate an MD5 checksum of /etc/services

  • openssl md5 /etc/services

Generate a RIPEMD160 Checksum of /etc/services

  • openssl rmd160 /etc/services

Testing An SSL-Enabled Server

  • s_server option can mimic an SSL-enabled server
  • s_client option can mimic an SSL client
  • Supports SSLv2/SSLv3 and TLSv1
  • Allows for client and server side certificates
  • s_client can be used to make sure your web servers are up and running
  • Example:
#!/bin/sh

HOST="localhost"
OPENSSL_BINARY= "/usr/local/ssl/bin/openssl"
PORT="443"
LOG_ERROR_MESSAGE="apache_connect_failure"
TMP=/tmp/connect.$$
OPT=” s_client -tls1 -quiet”

${OPENSSL_BINARY} ${OPT} -connect ${HOST}:${PORT} > ${TMP} 2>&1 << EOF
GET / HTTP/1.0
EOF

if egrep "Server:" ${TMP} > /dev/null; then
    :
else
    logger -p daemon.notice "WEB_SERVER_ERROR (${LOG_ERROR_MESSAGE}):
    Could not connect to ${HOST} on TCP Port ${PORT}"
fi

rm -f ${TMP}

Securing Insecure Services With Stunnel

  • “[]” defines the service to be used with this connection
  • “client” specifies if this is a server or client
  • “accept” specifies the host and port to listen on
  • “connect” is the remote end of the connection to connect to upon an accept()
  • Logfiles can be specified with the “output” directive
  • “Cafile” can be used with “verify” to validate certificates
  • Server certificates can be specified with the “cert” directive
  • “setuid” and “setgid” can be used to change the UID/GID the server runs as
  • Run the Stunnel Daemon:
$ /usr/sbin/stunnel /etc/stunnel/stunnel-client.conf

$ cat /etc/stunnel/stunnel-client.conf

client=yes
Cafile=/etc/stunnel/cacert.pem
pid=/var/run/stunnel.pid
output = /var/log/stunnel.log
[imaps]
accept=127.0.0.1:imap
connect=imap.prefetch.net:imaps

Debugging SSL and TLS Problems

  • ssldump can dump messages, state changes and application data
  • Traffic can be isolated by port, src/dst IP, and network
  • Example:

Dump the SSL messages and state changes

  • ssldump -t -a -A -i eth0 host zoom

Dump out all SSL traffic destined for zoom, including application data

  • ssldump -d -t -a -A -k privatekey.pem -p password -i eth0 host zoom

Learn More

SSL and TLS Essentials Practical Network Security with OpenSSL