Using and Abusing SSL Securty
OR
It must be secure, its so bloody hard to use
(With some apologies to Eric A. Young and
the OpenSSL developers, but not too many)
Patrick Powell
Tue Jun 11 18:09:52 PDT 2002
Introduction
Why add SSL security to LPRng?
a) It's there.
b) Everybody and their dog is using it.
c) It is needed to support IPP
So I got hold of the SSL and TLS book by Eric Rescorla (he may
regret this shameless plug, but be it on his head), and started
reading it. After two weeks and a VERY large bottle of 'Super Strong
No Doze Wakeup Pills' I had kind of figured out what needed to be done.
a) Set up some certs (i.e. - X509 Authentication Certificates).
b) Get some code from some other places and the examples.
c) Read the code, figure out what it was doing, and then
reverse engineer the SSL stuff.
d) File off serial numbers, recode, etc., where necessary to
1) avoid the GNU license curse
2) make it LPRng specific
My references were:
Mod_ssl from the Apache project.
http://www.apache.org
Follow links to Mod_SSL OR get apache2 which has mod_ssl in it.
Stole the organization for certs, as well
as looking at how the Makefile created and installed the various
certificates.
fetchmail
ftp://ftp.ccil.org/pub/esr/fetchmail
http://www.tuxedo.org/~esr/fetchmail
Ummm... this was happenstance, I use fetchmail and it
has the SSL authentication in it.
curl
http://download.sourceforge.net/curl/
Again, I use curl and it has SSL.
The articles by Eric Rescola:
An Introduction to OpenSSL Programming
http://www.rtfm.com/openssl-examples/
wserver, wclient, sclient
And the book: SSL and TLS - Desiging and Building Secure Systems
And, of course, the OpenSSL code, the examples in the code,
the utilities, etc. etc. etc. About 260,000 lines of etc.
Sigh...
WHAT I DID
a) Started with the Eric Rescola articles, and the
examples for wserver, wclient.
b) Added various things to handle getting authentication.
- printing the Subject and Issuer information
- adding directory information for Certificate locations
c) Created new certificates using what I thought would work...
d) Read 260,000 lines of OpenSSL code and heartily cursed
the OpenSSL developers, the OpenSSL coders, and just about
anybody who is associated with the project for NOT putting
in some trace statements OR better error message reporting.
But that is over new, and I have recalled the guys with the
baseball bats.
e) Documented this so that other people can figure out what I did.
HERE IS WHAT YOU NEED TO KNOW
The idea behind SSL is that you create some files (Certificates)
that have various private/public key information in them.
A checksum is calculated over the information, and then the checksum
is 'encrypted' using a private key of some 'signer'. This is attached
to the certficate file... and the whole thing is encoded in the most
obnoxious manner... ANS1 to be exact. This is then EXPANDED into a text
format called PEM, and forms the 'certificate file'.
<aside> Ignore the SSL experts who are frothing over this
cavalier description of the details.
Details, smetails. You listen to them, you be on the No Doze,
big time, pretty quick. Boring... </aside>
Now lets see what we do to validate that a certificate is correct
or from the 'Subject' who is identified in the Certificate. We get
the X509 certificate for the 'signer' (or 'Issuer' in X509 jargon).
Since the public key of the signer is including in the 'signer'
certificate, we can use this to check that the information in the
suspect certificate is valid by using it to decrypt the checksum
information encoded with the private key. If this matches, we have
validated the certificate. (Well, not quite. There are a couple
more gotchas.)
Now we must validate the 'signers' cerificate, which was
in turn signed by another signer, and so we go up the food chain,
I mean 'authentication chain', until we reach Nirvana:
a certificate which is signed by itself (i.e. - root certificate)
or more exactly, a certificate where the 'subject' or the person
identified by the CERT and the 'issuer' or the person who signed the
CERT are the same.
Now lets see how we use this for printing.
Each user and/or print spooler is given a certificate with a corresponding
set of private and secret keys. When a client sends a request to the lpd
server, he signs it using his private key; the lpd spooler gets the request,
and then decodes/checks it using the public key in the users certificate.
The SSL protocol provides a way to:
a) set up an encrypted connection (not our problem)
b) exchange certificate information
(Hmm... need to tell OpenSSL what certs to exchange)
c) validate the certificates (strictly speaking, this X509
stuff, but what the hey...) and hence, authenticate the
end users.
(Need to tell OpenSSL where the certs are).
d) set up and perform encrypted data exchange. (not our problem).
So all we really need to do is set up the CERTIFICATES, tell the
OpenSSL library where they are, and it should do the work for us.
(Ho ho ho... it sounds so simple...)
There are two components to a certificate:
a) the certificate file (name.crt file) itself
b) the private key corresponding to the public key in
the certificate file (name.key file).
Now clearly if the private key was obtained by somebody
then they could impersonate a user. So there are two possibilities:
a) make the private key file readable only by the people that
need it (more on this later).
b) encrypt the private key and then when you need to use it
to sign something, provide a decryption password/key.
So, we have 3 components: the certificate (name.crt), the encrypted
private key (name.key, but encrypted) and a password that we use
to decrypt the private key. But since we do not need to keep the
private key hidden away, we can put the private key and certificate
in the same file:
-----BEGIN RSA PRIVATE KEY-----
Proc-Type: 4,ENCRYPTED
DEK-Info: DES-EDE3-CBC,3EAD3ED0FA436761
Vi5K0olpFfe2ltDpY/7gPM4iW74gYqtO1yEFm1DOhp7Kd8hB5Is6TVuVX78zkTaP
...
j6Z5TX61x4YCHKleFa9nXFC5god/MCYzIHKKep0f4TKWCZcJLR5AyQ==
-----END RSA PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
MIIDGzCCAoSgAwIBAgIBADANBgkqhkiG9w0BAQQFADCBkzELMAkGA1UEBhMCVVMx
...
3VapletoUPtYPvUAAgAg4w28pKWvlVW3tU/CsoHDEw==
-----END CERTIFICATE-----
Which is the convention we adopted for LPRng.
Now lets deal with the problem of certificates. There are
FOUR types of certificates that we need:
CERTS/files
root (/etc/lpd/ssl.crt/ca.crt)
- signer1 (/etc/lpd/ssl.crt/signer1.crt - signed by ca)
- print spoolers (/etc/lpd/ssl.server/h121.crt - signed by signer1 )
- users (${HOME}/.lpd/user1.crt - signed by signer1 )
- signer2 (/etc/lpd/ssl.crt/signer2.crt - signed by ca)
- print spoolers (/etc/lpd/ssl.server/h121.crt - signed by signer2)
- users (${HOME}/.lpd/user2.crt - signed by signer2)
The ROOT cert signs the signer certs, which in turn
can sign print spooler certs and user certs. If you feel
lucky, you can also use the root cert to sign print spoolers
or user certs.
Now, what happens if and when somebody gets hold of the
private key for the root cert or a signing cert? If it is
for the root cert, you are doomed. Reissue all the CERTS.
Start from square 0.
If it is a signing cert, then you can REVOKE it. How do you do this?
You put it into a 'revocation' directory, and then tell OpenSSL
to do its magic and update the CERT information so that it is revoked.
(More on this later).
Finally, how does the certificate information get transferred?
This is a little complicated, and there are a zillion ways to do
this. I have opted to implement and use a very simple method based
on the Apached mod_perl SSL setup.
a) A directory (/etc/lpd/ssl.crt/) containing
all of the signing certificates, including the root certificate
(ca.crt). These will be used by both servers and clients.
Note: due to the OpenSSL implementation, it is necessary
to create a set of symbolic links to these files.
Just to totally confuse things, OpenSSL also supports putting
all of the certificates in a single file. The certificates should
be put in root to leaf order, i.e. - a breadth first walk
of the certificate tree.
printcap/configure option: ssl_ca_path=DIR
configure:
--with-ssl_ca_path=DIR
default ${sysconfdir}/lpd/ssl.crt/
--with-ssl_ca_file=FILE
default - none
Note: See the SSL_CTX_load_verify_locations(
SSL_CTX *ctx, const char *CAfile, const char *CApath);
documentation for the details.
b) A file (/etc/lpd/server.crt/server.crt) containing
the cert that is used by the server and is sent to the
client to identify the server. It should also contain
the private key for the server.
printcap/configure option: ssl_server_cert=FILE
configure:
--with-ssl_server_cert=PATH
default ${sysconfdir}/lpd/server.crt/server.crt
Note: See SSL_CTX_use_certificate_chain_file(
SSL_CTX *ctx, const char *file);
SSL_use_PrivateKey_file(SSL *ssl, char *file,
int type);
documentation for details. Note that this file can
contain multiple certs, but these must be sorted in top
(root CA) to bottom (server) order.
i.e. -
private_key, server cert (additional certs
specified by ssl_ca_path or ssl_ca_file)
OR
root CA cert, signer1 cert, ..., server cert
Note: the private key can be in any position.
c) A file (/etc/lpd/server.crt/server.pwd) containing the
password for the private key in the server cert file.
This file should to be 600, owned by the LPD server user.
Note: See the SSL_CTX_set_default_passwd_cb for details.
The password is read from the file.
printcap/configure option: ssl_server_passwd=FILE
--with-ssl_server_passwd=PATH
default ${sysconfdir}/lpd/server.crt/server.crt
d) For user authentication to the server, users will need to
specify a certificate and password. This can be by using
values in default files or
${HOME}/.lpr/ssl.crt/ - signing/root certificates
(if not present, then ${sysconfdir}/lpd/ssl.crt/ is used)
Environment variable: LPR_CA_PATH
${HOME}/.lpr/client.crt - client cert and key
Environment variable: LPR_SSL_CERT
${HOME}/.lpr/client.pwd - file containing client password
Environment variable: LPR_SSL_PASSWORD
SETTING UP CERTIFICATE AUTHORITY
There are several types of certificate files:
- CA root (self signed)
- signing certs (signed by CA or by signing cert)
- user certs (used only by client programs)
- server certs (used by lpd server AND by lpd server when
forwarding to a remote queue)
This is managed as follows.
1. signing certs are in a directory (or a file)
default: /etc/lpd/ssl.ca
2. certificates are created in a working directory
default: /etc/lpd/ssl.certs
3. you usually want to have a set of easy to use defaults.
The lprng_certs script supports this.
usage: lprng_certs option
init - make directory structure
newca - make new root CA and defaults
defaults - set new default values
encrypt keyfile - set or change password on private key file
gen - generate user, server, or signing cert
verify cert* - verify certs
certs can be path or user-XX.csr values
CREATE DIRECTORY STRUCTURE
Use: lprng_certs init
- creates directories for lpd server
CREATE CA ROOT CERT
Use: lprng newca
- sets up the directory structure, then sets a set of default values
- creates the CA root certificate and key file.
Example:
#> lprng_certs newca
lprng_certs -- LPRng SSL Certificate Management
Copyright (c) 2002 Patrick Powell
Based on CCA by Ralf S. Engelschall
(Copyright (c) 1998-2001 Ralf S. Engelschall, All Rights Reserved.)
WARNING: /etc/lpd/ssl.ca/ca.crt already exists! Do you want to overwrite it? [N/y] Y
INITIALIZATION - SET DEFAULTS
...
______________________________________________________________________
STEP 1: Generating RSA private key for CA (1024 bit)
______________________________________________________________________
STEP 2: Generating X.509 certificate signing request for CA
______________________________________________________________________
STEP 3: Generating X.509 certificate for CA signed by itself
______________________________________________________________________
RESULT:
/etc/lpd/ssl.ca/ca.crt:
/C=US/ST=California/L=San Diego/O=Astart/OU=Certificate Authority/CN=Astart
[EMAIL PROTECTED]
error 18 at 0 depth lookup:self signed certificate
OK
______________________________________________________________________
STEP 4. Enrypting RSA private key with a pass phrase for security
The contents of the certificate key file (the generated private key)
should be echo kept secret, especially so if it is used to sign
Certificates or for User authentication.
SSL experts strongly recommend you to encrypt the key file with
a Triple-DES cipher and a Pass Phrase. When using LPRng, you provide
the password via a file or file descriptor specified by an environent
variable, i.e. - SSL_PASSWORD_FILE or SSL_PASSWORD_FD, or in the
${HOME}/.ssl_password file.
The LPD server uses the ssl_server_password_file option to specify
the location of a file containing the password.
See the LPRng HOWTO for details, or the printcap(5) man page.
key file is /etc/lpd/ssl.ca/ca.key
Encrypt the private key now? [Y/n]: y
Fine, you're using an encrypted private key to sign CERTS.
______________________________________________________________________
STEP 5: Combine CERT and KEY file
Generate single CERT and KEY file? [Y/n] y
Use the following commands to examine the CERT and KEY files:
openssl x509 -text -in /etc/lpd/ssl.ca/ca.crt
openssl rsa -text -in /etc/lpd/ssl.ca/ca.crt
CREATE A USER/SERVER CERT
- creates a certificate with the appropriate entries for use
as a signing, server (lpd), or client (user) certificate.
Example:
#> lprng_certs newca
lprng_certs -- LPRng SSL Certificate Management
Copyright (c) 2002 Patrick Powell
Based on CCA by Ralf S. Engelschall
(Copyright (c) 1998-2001 Ralf S. Engelschall, All Rights Reserved.)
CERTIFICATE GENERATION
What type of certificate? User/Server/Signing Authority/Help? [u/s/a/H] u
Create in '/etc/lpd/ssl.certs' [return for yes, or specify directory] y
CERT name 'user-01'? [return for yes, or specify name] papowell
CERT name 'papowell'? [return for yes, or specify name]
Creating papowell in /etc/lpd/ssl.certs
Sign with Certificate '/etc/lpd/ssl.ca/ca.crt' [return for yes, or specify cert file]
Private key in /etc/lpd/ssl.ca/ca.crt
Generating user Certificate [papowell]
______________________________________________________________________
STEP 1: Generating RSA private key for user (1024 bit)
______________________________________________________________________
STEP 2: Generating X.509 certificate signing request for user
User Certificate Validity in days [default 365]
______________________________________________________________________
STEP 3: Generating X.509 certificate signed by own CA
______________________________________________________________________
RESULT:
/etc/lpd/ssl.certs/papowell.crt: OK
______________________________________________________________________
STEP 4. Enrypting RSA private key with a pass phrase for security
The contents of the certificate key file (the generated private key)
should be echo kept secret, especially so if it is used to sign
Certificates or for User authentication.
SSL experts strongly recommend you to encrypt the key file with
a Triple-DES cipher and a Pass Phrase. When using LPRng, you provide
the password via a file or file descriptor specified by an environent
variable, i.e. - SSL_PASSWORD_FILE or SSL_PASSWORD_FD, or in the
${HOME}/.ssl_password file.
The LPD server uses the ssl_server_password_file option to specify
the location of a file containing the password.
See the LPRng HOWTO for details, or the printcap(5) man page.
key file is /etc/lpd/ssl.certs/papowell.key
Encrypt the private key now? [Y/n]: y
Fine, you're using an encrypted private key to sign CERTS.
______________________________________________________________________
STEP 5: Combine CERT and KEY file
Generate single CERT and KEY file? [Y/n] y
Use the following commands to examine the CERT and KEY files:
openssl x509 -text -in /etc/lpd/ssl.certs/papowell.crt
openssl rsa -text -in /etc/lpd/ssl.certs/papowell.crt
CREATE A SIGNING CERT
lprng_certs -- LPRng SSL Certificate Management
Copyright (c) 2002 Patrick Powell
Based on CCA by Ralf S. Engelschall
(Copyright (c) 1998-2001 Ralf S. Engelschall, All Rights Reserved.)
CERTIFICATE GENERATION
What type of certificate? User/Server/Signing Authority/Help? [u/s/a/H] a
Create in '/etc/lpd/ssl.ca' [return for yes, or specify directory]
CERT name 'signer-02'? [return for yes, or specify name]
Creating signer-02 in /etc/lpd/ssl.ca
Sign with Certificate '/etc/lpd/ssl.ca/ca.crt' [return for yes, or specify cert file]
Private key in /etc/lpd/ssl.ca/ca.crt
Generating signer Certificate [signer-02]
______________________________________________________________________
STEP 1: Generating RSA private key for signer (1024 bit)
______________________________________________________________________
STEP 2: Generating X.509 certificate signing request for signer
User Certificate Validity in days [default 365]
______________________________________________________________________
STEP 3: Generating X.509 certificate signed by own CA
______________________________________________________________________
RESULT:
/etc/lpd/ssl.ca/signer-02.crt: OK
______________________________________________________________________
STEP 4. Enrypting RSA private key with a pass phrase for security
The contents of the certificate key file (the generated private key)
should be echo kept secret, especially so if it is used to sign
Certificates or for User authentication.
SSL experts strongly recommend you to encrypt the key file with
a Triple-DES cipher and a Pass Phrase. When using LPRng, you provide
the password via a file or file descriptor specified by an environent
variable, i.e. - SSL_PASSWORD_FILE or SSL_PASSWORD_FD, or in the
${HOME}/.ssl_password file.
The LPD server uses the ssl_server_password_file option to specify
the location of a file containing the password.
See the LPRng HOWTO for details, or the printcap(5) man page.
key file is /etc/lpd/ssl.ca/signer-02.key
Encrypt the private key now? [Y/n]:
Fine, you're using an encrypted private key to sign CERTS.
______________________________________________________________________
STEP 5: Combine CERT and KEY file
Generate single CERT and KEY file? [Y/n]
Use the following commands to examine the CERT and KEY files:
openssl x509 -text -in /etc/lpd/ssl.ca/signer-02.crt
openssl rsa -text -in /etc/lpd/ssl.ca/signer-02.crt
- creates a certificate with the appropriate entries for use
as a signing, server (lpd), or client (user) certificate.
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