This chapter covers the following topics:
- Introduction to PKI Infrastructure
- Using Digital Signatures to Sign a Tcl Script
- Tcl Script-Failure Scenario
- Scaling Tcl Script Distribution
From the early days of programming, those with nefarious intent have attempted—and in many cases successfully integrated—additional code into programs that have caused serious consequences to the unsuspecting user. Because a Tcl script is a program running on an IOS-based device, additional code added to the script could potentially send passwords via e-mail, erase the saved configurations or IOS images, cause the device to reload periodically, and so on. The capability to protect scripts against this type of attack is paramount!
Cisco IOS Software Release 12.4(15)T introduced a new feature to improve security for Tcl scripts. The purpose of this security feature is to provide a method for signing a Tcl script that proves the script has not been modified since the time it was signed by the author. It relies on techniques and methods of existing public key cryptography libraries.
Introduction to PKI Infrastructure
Public key infrastructure (PKI) is used to provide a scalable method for certificate/key exchange and is commonly used for secure exchange data (confidentiality), to ensure that the data has not been modified in transit (integrity), to authenticate the origin, and for nonrepudiation.
PKI enables you to send messages with confidentiality and sign messages that are guaranteed to be genuine. Confidentiality allows for conducting secret communication via a public channel; however, it is not needed because the Tcl script is not secret and can be viewed by anyone who has access.
Even though confidentiality is not needed, we do require Tcl scripts to have a digital signature to guarantee they are truly genuine and have not been tampered with. PKI enables you to generate a digital signature that is connected to the Tcl script and guarantees it is genuine.
Because PKI is well established, it can be leveraged to provide the security needed for signing Tcl scripts. Many applications already take advantage of PKI and leverage the security it provides, including SSL/HTTPS, IPsec, and S/MIME. You are probably familiar with purchasing items online, filing tax returns online, and software licensing, all of which use PKI and secure protocols.
Using PKI will provide assurance that a script has not been tampered with and give you the confidence to implement a signed script in a production environment.
PKI Prerequisite
To use PKI, a public/private key pair must first be established. Generally, the longer the key, the higher the level of security provided by that key.
A private key is simply a long string of characters, typically 512. As the name indicates, this key is not to be shared and must always be kept private.
The public key is public and distributed to anyone.
The public/private key pairs are simple to generate and the private key is (nearly) impossible to derive from the public key. Although any security algorithm can be broken given enough time and CPU cycles, longer keys make this task infeasible.
Confidentiality with PKI
When sending messages through a public channel, you can use PKI to encrypt the data so that only the intended recipient can decrypt it. The sender of an encrypted message can encrypt the data but cannot decrypt the data after it has been encrypted. The recipient's public key is used to encrypt the data. The data is sent to the recipient, and once it is received is decrypted with the recipient's private key. This process is unidirectional. For two-way encrypted communication to occur, two key pairs are needed. Each sender encrypts data using the correct public key of the intended recipient. This process is also referred to as asymmetric encryption.
As shown in Figure 7-1, Alice will be sending Bob an encrypted message. The process is as follows:
- Step 1. For Alice to send any encrypted information to Bob, she must have Bob's public key.
- Step 2. Alice uses Bob's public key to encrypt the message she wants to send to Bob.
- Step 3. Even if someone captures the information in transit, it is undecipherable without Bob's private key, or at least a tremendous number of CPU cycles used to break the private key.
- Step 4. When Bob receives the information, he uses his own private key to decrypt the original message that Alice sent.
)
Figure 7-1 PKI Infrastructure
Digital Signatures with PKI
Digital signatures enable you to ensure that information is not altered in transit from sender to recipient. You can do so by creating a hash or signature of the original data.
The entire message must remain intact. Even if 1 bit of the data is changed, the entire message integrity is violated. When you sign a check, for example, you are verifying the authenticity with your unique signature. If the amount of the check is changed from your written and numeric values, this represents an obvious violation.
To provide the integrity check, the entire data being sent is run through a hashing function. In simple terms, the hashing function produces a unique result for any given input data, of a known length. Common hashing functions include message digest 4 (MD4), message digest 5 (MD5), and secure hash algorithm (SHA). The hashed data result can be sent using the PKI confidentiality process described earlier. In this way, the message integrity is preserved from sender to recipient.
Figure 7-2 shows how Alice can send a message with a digital signature:
- Step 1. Alice runs the original message through a hash function, creating a unique value.
- Step 2. Alice encrypts the hash value with her private key, resulting in the digital signature.
- Step 3. The signature is attached to the message.
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Figure 7-2 Signing Digital Signatures
Figure 7-3 shows how Bob can verify the integrity of a message.
- Step 1. For Bob to verify that Alice is indeed the originator of the message, Bob must have Alice's public key.
- Step 2. Bob will extract the signature from the message.
- Step 3. The message will be hashed using the same process that Alice used, and a result will be generated.
- Step 4. The signature will also be decrypted using Alice's public key, and a hash value will be generated.
- Step 5. Bob will then compare the message hash value to the signature value. If they match, Bob can be reasonably certain that the message is unaltered.
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Figure 7-3 Verifying Digital Signatures
Using Digital Signatures to Sign a Tcl Script
Two prerequisites must be met to sign a Tcl script:
- A digital certificate must be made available on the router that will perform the signature check of the Tcl script. The digital certificate is stored in the IOS running-configuration and may also be saved in the nonvolatile random-access memory (NVRAM) of the router.
- The Tcl script must have been signed with the private key that matches the public key available in the digital certificate. The signature is provided in a special format and is in plain text directly after the Tcl commands in the script.
If Tcl script signature checking is enabled, different actions can take place when a Tcl script is executed. If the signature of the Tcl script matches the digital certificate, the Tcl script will be executed immediately. If the signature fails to be verified, the following choices are available, depending on the IOS configuration:
- The script can be immediately stopped.
- The script can be allowed to run even though the signature check failed, in a special "safe" Tcl mode. The "safe" TCL mode has a reduced number of keywords available and is thought to be less dangerous than the full Tcl mode.
- The script can be allowed to run normally. This can be used for testing purposes, but would rarely be used in an actual live network. In effect, this turns off the security check.
To digitally sign a script, an IOS image containing the crypto feature set must be used. This means the image name contains the k9 feature set. For example, the following image contains the crypto feature: c7200-adventerprisek9-mz.
The following example details how to correctly sign a Tcl script with a digital signature, using a UNIX host as the certificate authority (CA) server. As an alternative, a CA can also be created using other operating systems or can be hosted commercially.
A CA is a trusted third party that maintains, verifies, enrolls, distributes, and revokes public keys. It is for that very reason that the CA must be secure.
The previous examples assumed that Bob or Alice had the other's public key. But how does Bob know that the key he has is really from Alice? There are a couple of answers to that question:
- Alice and Bob exchanged public keys out-of-band. This works fine in a small environment, but when there are hundreds or thousands of devices, manually exchanging keys becomes difficult.
- A CA is used to maintain all certificates.
This is where a CA really shows its value. The CA maintains the public keys or certificates, usually in an X.509 format. Key exchange is as follows:
- Step 1. Before Bob can verify Alice's public key, he must have the CA public key, which should be exchanged out-of-band.
- Step 2. When Bob needs Alice's public key, he sends a request to the CA.
- Step 3. The CA signs Alice's public key with the CA private key, consequently verifying the origination and sends it to Bob.
- Step 4. Bob uses the CA public key to validate Alice's public key.
You must complete the following steps to sign a Tcl script:
- Step 1. Decide on the final Tcl script contents (myscript).
- Step 2. Generate a public/private key pair.
- Step 3. Generate a certificate with the public key.
- Step 4. Generate a detached S/MIME pkcs7 signature for the script you created (myscript) using the private key.
- Step 5. Modify the format of the signature to match the Cisco style for a signed Tcl script and append it to the end of the script you created (myscript).
Step 1: Decide on the Final Tcl Script Contents (Myscript)
Finalize any last-minute changes needed to the script text file. After the Tcl script has been signed, no more changes may be made.
Step 2: Generate a Public/Private Key Pair
The private key must always be kept private! Failure to do so would allow anyone in possession of the private key to sign Tcl scripts as if they were written by the original author.
To generate a key pair, you can use the open source project OpenSSL. Executable versions of the OpenSSL are available for download at http://www.openssl.org
Using a UNIX host or similar, run the following command to generate a key pair (this example uses a 2048-byte key):
$ openssl genrsa -out privkey.pem 2048 Generating RSA private key, 2048 bit long modulus ..........................................+++ ...............................................................................+ ++ e is 65537 (0x10001) $
As you can see from the directory, the following file has been created:
$ ls -l total 5 -rw-r--r-- 1 joe mkgroup-l-d 114 May 28 10:23 myscript -rw-r--r-- 1 joe mkgroup-l-d 1679 May 28 10:23 privkey.pem $
The new file is called privkey.pem and contains both the private key and public key. The file needs to be kept in a secure location because it holds the private key.
Next, extract the public key from the key pair file:
$ openssl rsa -in privkey.pem -pubout -out pubkey.pem writing RSA key $
As you can see from the directory, the following file has been created:
$ ls -l total 6 -rw-r--r-- 1 joe mkgroup-l-d 114 May 28 10:23 myscript -rw-r--r-- 1 joe mkgroup-l-d 1679 May 28 10:23 privkey.pem -rw-r--r-- 1 joe mkgroup-l-d 451 May 28 10:25 pubkey.pem $
Now there are two separate files, one that contains the pair of keys (privkey.pem) and another file that contains only the public key (pubkey.pem).
Step 3: Generate a Certificate with the Key Pair
To create a certificate, we must answer a few questions. These answers will be stored along with the certificate, in case any concerns arise later about where the certificate comes from:
$ openssl req -new -x509 -key privkey.pem -out cert.pem -days 1095
You are about to be asked to enter information that will be incorporated into your certificate request. What you are about to enter is what is called a distinguished name (DN). There are quite a few fields, but some may be left blank.
For some fields there will be a default value. If you enter a period (.), the field will be left blank:
Country Name (2 letter code) [AU]:US State or Province Name (full name) [Some-State]:California Locality Name (eg, city) []:San Jose Organization Name (eg, company) [Internet Widgits Pty Ltd]:Acme Inc. Organizational Unit Name (eg, section) []:Central Unit Common Name (eg, YOUR name) []:Joe Email Address []:joe@xyz.net
As you can see from the directory, the following cert.pem file has been added:
$ ls -l total 10 -rw-r--r-- 1 joe mkgroup-l-d 1639 May 28 10:26 cert.pem -rw-r--r-- 1 joe mkgroup-l-d 114 May 28 10:23 myscript -rw-r--r-- 1 joe mkgroup-l-d 1679 May 28 10:23 privkey.pem -rw-r--r-- 1 joe mkgroup-l-d 451 May 28 10:25 pubkey.pem $
The certificate has now been generated in the file cert.pem. This certificate will later be transferred to the IOS router for the router to perform the signature check on the signed Tcl script.
Step 4: Generate a Detached S/MIME pkcs7 Signature for Myscript Using the Private Key
When the script is signed, a new file is generated called myscript.pk7, which contains the signature:
$ cat myscript puts hello puts "argc = $argc" puts "argv = $argv" puts "argv0 = $argv0" puts "tcl_interactive = $tcl_interactive" $ $ openssl smime -sign -in myscript -out myscript.pk7 -signer cert.pem -inkey pr ivkey.pem -outform DER –binary $
The myscript.pk7 file has been added:
$ ls -l myscript.pk7 -rw-r--r-- 1 joe mkgroup-l-d 1856 May 28 10:30 myscript.pk7 $
To validate that the signature matches the myscript certificate we generated earlier, perform the following:
$ openssl smime -verify -in myscript.pk7 -CAfile cert.pem -inform DER -content myscript puts hello puts "argc = $argc" puts "argv = $argv" puts "argv0 = $argv0" Verification successful puts "tcl_interactive = $tcl_interactive" $
The "Verification successful" message indicates that myscript matches the contents of the signature.
Step 5: Modify the Format of the Signature to Match the Cisco Style for Signed Tcl Scripts and Append It to the End of Myscript
Now that a signature for myscript has been generated, we still need to make some formatting changes to put myscript in the correct format for Cisco IOS to understand.
The format of a signed Tcl script is as follows:
Actual Tcl script contents in plain test ... #Cisco Tcl Signature V1.0 #Actual hex data of the signature
The signature portion of myscript is inserted after the hash character (#). Tcl always treats this as a comment. If this script is executed on an IOS router that does not know about Tcl script signature checking, the router will simply ignore these commented lines.
The signature must be converted to a hex format instead of binary:
$ xxd -ps myscript.pk7 > myscript.hex $
The directory listing shows that the file was created:
$ ls -l myscript.hex -rw-r--r-- 1 joe mkgroup-l-d 3774 May 28 10:42 myscript.hex $
Next, a helper script is used to place the #Cisco Tcl Signature V1.0 and the # characters in the new signature file.
You can show the contents of the file by using the cat command:
$ cat my_append
#!/usr/bin/expect
set my_first {#Cisco Tcl Signature V1.0}
set newline {}
set my_file [lindex $argv 0]
set my_new_file ${my_file}_sig
set my_new_handle [open $my_new_file w]
set my_handle [open $my_file r]
puts $my_new_handle $newline
puts $my_new_handle $my_first
foreach line [split [read $my_handle] "\n"] {
set new_line {#}
append new_line $line
puts $my_new_handle $new_line
}
close $my_new_handle
close $my_handle
$
Initiate the helper script using the following syntax:
$ ./my_append myscript.hex $
The directory listing shows the myscript.hex and myscript.hex_sig files:
$ ls -l myscript.hex* -rw-r--r-- 1 joe mkgroup-l-d 3774 May 28 10:42 myscript.hex -rw-r--r-- 1 joe mkgroup-l-d 3865 May 28 10:56 myscript.hex_sig $
Lastly, the signature file and the script file must be concatenated:
$ cat myscript myscript.hex_sig > myscript.tcl $
The directory listing shows that the file was created:
$ ls -l myscript.tcl -rw-r--r-- 1 joe mkgroup-l-d 3979 May 28 10:58 myscript.tcl $
The signed Tcl script has finally been generated (myscript.tcl)!
The following script combines many of the preceding steps and will help to automate the process:
#!/bin/sh
# the next line restarts using tclsh exec tclsh "$0" "$@"
proc PrintUsageInfo {} {
puts {usage: signme input_file [-c cert_file] [ -k privkey_file]} }
set cert_file cert.pem
set privkey_file privkey.pem
if {$argc == 0} {
PrintUsageInfo
exit -1
}
set state flag
set cnt 0
foreach arg $argv {
switch -- $state {
flag {
switch -glob -- $arg {
\-c {
set state cert
}
\-k {
set state key
}
default {
if {$cnt == 0} {
set filename $arg
} else {
PrintUsageInfo
exit -1
}
}
}
}
cert {
set cert_file $arg
set state flag
}
key {
set privkey_file $arg
set state flag
}
}
incr cnt
}
if {![string equal $state flag]} {
PrintUsageInfo
exit -1
}
if {[catch {set commented_signed_hex [exec openssl smime -sign -in $filename -signer $cert_file -inkey $privkey_file -outform DER -binary | xxd -ps | sed s/^/#/ ]} err]} {
puts stderr "Error signing $filename - $err"
exit -1
}
set signature_tag "\n#Cisco Tcl Signature V1.0"
if {[catch {set fd [open $filename a+]} err]} {
puts stderr "Cannot open $filename - $err"
exit -1
}
puts $fd $signature_tag
puts $fd $commented_signed_hex
close $fd
puts "$filename signed successfully."
exit
To take advantage of the newly signed script, the IOS device must be configured with a certificate.
After logging in to the IOS device and establishing access to configuration mode, complete the following steps:
- Step 1. Configure and enroll in a trust point using the cert.pem file. At the prompt, paste the certificate into the terminal. That is, paste the contents of the cert.pem file beginning after "-----BEGIN CERTIFICATE-----" and ending before "-----END CERTIFICATE-----".":
PE11(config)#crypto pki trustpoint TCLSecurity PE11(ca-trustpoint)#enrollment terminal PE11(ca-trustpoint)#crypto pki authenticate TCLSecurity Enter the base 64 encoded CA certificate. End with a blank line or the word "quit" on a line by itself -----BEGIN CERTIFICATE----- MIIEjDCCA3SgAwIBAgIJANOb35p5QONbMA0GCSqGSIb3DQEBBQUAMIGKMQswCQYD VQQGEwJVUzETMBEGA1UECBMKQ2FsaWZvcm5pYTERMA8GA1UEBxMIU2FuIEpvc2Ux EjAQBgNVBAoTCUFjbWUgSW5jLjEVMBMGA1UECxMMQ2VudHJhbCBVbml0MQwwCgYD VQQDEwNKb2UxGjAYBgkqhkiG9w0BCQEWC2pvZUB4eXoubmV0MB4XDTA5MDUyODE3 MjY1OVoXDTEyMDUyNzE3MjY1OVowgYoxCzAJBgNVBAYTAlVTMRMwEQYDVQQIEwpD YWxpZm9ybmlhMREwDwYDVQQHEwhTYW4gSm9zZTESMBAGA1UEChMJQWNtZSBJbmMu MRUwEwYDVQQLEwxDZW50cmFsIFVuaXQxDDAKBgNVBAMTA0pvZTEaMBgGCSqGSIb3 DQEJARYLam9lQHh5ei5uZXQwggEiMA0GCSqGSIb3DQEBAQUAA4IBDwAwggEKAoIB AQDXjtFzWDXyHftgy7i75HczyvAFh10E2oB/tTC9WA5mih2L8ZMGTu+705LYP0E+ TlhVadastpYSEEVPOrdWiUuqLIoFKV7LE6KsEcKTuGRQp0tEGhfQrPyBuCcpuzO5 FZv7mpCJMvhXzW/wioAvFLE4vuXHHdAhsdK2dD1nOHmljvsx3hJ+Us6PKTnU1BNU HpSReM6T9hH321Wakt9D4Q+qXW6T3IE2pD6tzvTZouLKXD7BMXjoNjMe6vIzlwmY b7E2Txwui6YtPcJK15pRcl1+DozT9iGj43ps6glAIUfjtjCPEoQBblWeqNAHVYWn WDP4FXg9H7z4xjocDuKJm+bBAgMBAAGjgfIwge8wHQYDVR0OBBYEFOb3HxpTyQcw 7YH4JwE2rdZUx4HnMIG/BgNVHSMEgbcwgbSAFOb3HxpTyQcw7YH4JwE2rdZUx4Hn oYGQpIGNMIGKMQswCQYDVQQGEwJVUzETMBEGA1UECBMKQ2FsaWZvcm5pYTERMA8G A1UEBxMIU2FuIEpvc2UxEjAQBgNVBAoTCUFjbWUgSW5jLjEVMBMGA1UECxMMQ2Vu dHJhbCBVbml0MQwwCgYDVQQDEwNKb2UxGjAYBgkqhkiG9w0BCQEWC2pvZUB4eXou bmV0ggkA05vfmnlA41swDAYDVR0TBAUwAwEB/zANBgkqhkiG9w0BAQUFAAOCAQEA F+W1JWf56IJPjYT0f2MForE3/gsKMgUvMh+kyf4Fcgvdh4WuKUEwTVBHpHglOYyL XfNZe6ILf9e3SgmXsqJOwAu/qK8d5uMwZ4d8TVoZqN1QmJPhvBcp7WZS8EvMVAWU vwo8SUgDUY1QzXPa5R333T0k1Vo+wxc7c4zftH/gbbqrGGgP5EAlXKvX75Z/dafv d/jPl4DniOlwz54ieRwjRU7B9w80Oa8EQeGRnsuNBcXRYqNoHJMRQK2xelqBL//8 10TEQeAoN3WjHBkqLXjf6HasnhfnwoNpNEn+ni5xN5uigbmuBzS1TCeevve/Y0ix NnU3fSYpIOnb1tZLBbYY7A== -----END CERTIFICATE----- Certificate has the following attributes: Fingerprint MD5: 856A9FF2 23AF24B0 8422B4FC 1E9E4153 Fingerprint SHA1: 35248814 47468190 4A1A3B6C 9D60C2A8 0B99BB0C % Do you accept this certificate? [yes/no]: yes Trustpoint CA certificate accepted. % Certificate successfully importedVerify that the trust point was accepted correctly:PE11#show crypto pki trustpoints Trustpoint TCLSecurity: Subject Name: e=joe@xyz.net cn=Joe ou=Central Unit o=Acme Inc. l=San Jose st=California c=US Serial Number: 0x0D39BDF9A7940E35B Certificate configured.It looks correct because we see the expected information that was entered when we generated the certificate. - Step 2. Configure the IOS device to require that all Tcl scripts be verified against the certificate before running. In this example, if a script does not pass the security check, it will not be allowed to execute:
PE11(config)#scripting tcl trustpoint name TCLSecurity PE11(config)#scripting tcl securemode PE11(config)#scripting tcl trustpoint untrusted terminateAll Tcl scripts that are run on the router must now perform a signature check. Alternatives for executing scripts include the following:- Execute: Run the script even if the Tcl script fails verification
- Safe-execute: Execute the script in safe mode when the Tcl script fails verification.
- Step 3. After copying the script myscript.tcl to disk0: of the IOS device, you can initiate it using the following command:
PE11#tclsh disk0:myscript.tcl hello argc = 0 argv = argv0 = unix:myscript.tcl tcl_interactive = 0
Tcl Script-Failure Scenario
In the event the script has been modified, the signature will detect that there was a change and prevent it from executing.
The following example shows that the script was modified and consequently forbidden from being executed.
The first line of the script has been changed from "puts hello" to "puts hellox," and the file has been copied to the IOS device as myscript-changed1char.tcl. Attempting to run the script elicits the following response:
PE11#tclsh disk0:myscript-changed1char.tcl Invalid Signature PE11# *May 28 19:45:28.115: %SYS-6-SCRIPTING_TCL_INVALID_OR_MISSING_SIGNATURE: tcl signing validation failed on script signed with trustpoint name TCLSecurity, cannot run the signed TCL script.
As you can see from the preceding output, the Tcl script security is a valuable feature for protecting the contents of a Tcl script. If any portion of the contents of the Tcl script has been modified by anyone, from the time the script was initially written to the time it is run on the router, the change will be detected and the script will be forbidden from executing.
For smaller company networks, it might be acceptable to have a network administrator manually install the certificate in all routers that need to run the script. The certificate is copied to a local storage such as slot0: or disk0: or any other valid file system attached to the router. In addition, copies of the Tcl script can also copied to these local storage devices attached to the router.
To deploy scripts in a larger network, take advantage of the capability of IOS software to use a TFTP server as a repository and allow all IOS devices to download Tcl scripts from the TFTP server.
Scaling Tcl Script Distribution
In the next example, a central TFTP server has been deployed in the network using an IP address of 192.168.1.81. In this case, the IOS device downloads the Tcl script across the network using TFTP. The signature checking is still performed, thus retaining a high level of security while using an unsecure transfer protocol:
PE11#tclsh tftp://192.168.1.81/myscript.tcl Loading myscript.tcl from 192.168.1.81 (via Serial2/0.111): ! [OK - 4046 bytes] hello argc = 0 argv = argv0 = tftp://192.168.1.81/myscript.tcl tcl_interactive = 0
If the script has been modified, we receive the same results as before:
PE11#tclsh tftp://192.168.1.81/myscript-changed1char.tcl Loading myscript-changed1char.tcl from 192.168.1.81 (via Serial2/0.111): ! [OK - 4047 bytes] Invalid Signature *Mar 27 23:40:35.543: %SYS-6-SCRIPTING_TCL_INVALID_OR_MISSING_SIGNATURE: tcl signing validation failed on script signed with trustpoint name TCLSecurity, cannot run the signed TCL script.
The public certificate is stored locally in the router's NVRAM and consequently is taking up valuable memory space. You may find yourself in a situation where the IOS device is running out of memory. To minimize the impact on NVRAM, you can use the service compress-config command to reduce the certificate's overhead on NVRAM:
PE11(config)#service compress-config PE11(config)#end PE11#write Warning: Attempting to overwrite an NVRAM configuration previously written by a different version of the system image. Overwrite the previous NVRAM configuration?[confirm] *Mar 27 23:47:45.323: %SYS-5-CONFIG_I: Configured from console by console [confirm] Building configuration... Compressed configuration from 5689 bytes to 3535 bytes[OK]
Instead of using a central TFTP server as a repository for Tcl scripts, you can use other protocols to transmit the Tcl Scripts from a central server, including FTP, RCP, SCP, HTTP, and HTTPS.
Summary
Ensuring that Tcl scripts have not been modified is critical to maintaining the integrity of the network infrastructure. If a script is changed, the ramifications could be disastrous! Fortunately, digitally signing a Tcl script is not a difficult task and offers a mechanism from which you can be confident in the integrity of the script.
References
SUN public key infrastructure overview: http://www.sun.com/blueprints/0801/publickey.pdf
OpenSSL documentation: http://www.openssl.org/docs/apps/openssl.html
Cygwin home page: http://www.cygwin.com/
Signed Tcl scripts: http://www.cisco.com/en/US/docs/ios/12_4t/netmgmt/configuration/guide/sign_tcl.html