search

certnanny / sscep

SSCEP is a command line client for the SCEP protocol
Other
174 stars 91 forks source link

readme

SSCEP - Simple SCEP client for Unix

Copyright (c) Jarkko Turkulainen 2003. All rights reserved.

See the file COPYING for licensing information.

WHAT IS SSCEP?

SSCEP is a client-only implementation of the SCEP (Cisco System's Simple Certificate Enrollment Protocol). SSCEP is designed for OpenBSD's isakmpd, but it will probably work with any Unix system with a recent compiler and OpenSSL toolkit libraries installed.

WHAT SCEP?

(From the Cisco Systems White Paper):

SCEP is a PKI communication protocol which leverages existing technology by using PKCS#7 and PKCS#10. SCEP is the evolution of the enrollment protocol developed by Verisign, Inc. for Cisco Systems, Inc. It now enjoys wide support in both client and CA implementations.

The goal of SCEP is to support the secure issuance of certificates to network devices in a scalable manner, using existing technology whenever possible. The protocol supports the following operations:

Certificate and CRL access can be achieved by using the LDAP protocol, or by using the query messages defined in SCEP.

SSCEP FEATURES

Currently, SSCEP implements:

There's no LDAP support, and probably there will never be (that's why it is simple).

SSCEP has been tested successfully against the following CA products:

(*) by default, subjectAltName extensions are dropped from certificate

(**) only DNS subjectAltName allowed (demo mode)

(***) demo requires to use /C=US/O=Entrust

HOW TO COMPILE

The program should compile on the following systems:

In general, two build systems are supported:

Prerequisites:

On Ubuntu you may use:

sudo apt-get install autoconf automake libtool pkg-config libssl-dev

Unix:

To compile run: $ make

To generate the configure script when checking out from github source:

$ ./bootstrap.sh

To compile from a tarball created with 'make dist'

$ ./configure
$ make
$ make install

To build an RPM package from the tarball do

cp sscep-*.tar.gz ~/rpmbuild/SOURCES
rpmbuild -ba scripts/sscep.spec

Windows:

  1. Download and install:

  2. Start the CMake GUI, select Where is the source code and Where to put the binaries (it could be the same), then Configure and Generate the project files.

  3. Start the Visual Studio, open the generated Solution (sscep.sln) and build the project. Then copy the sscep binary (Debug or Release) and configuration file sscep.conf somewhere.

macOS:

Install a few packages from Homebrew:

$ brew install autoconf automake libtool pkg-config openssl

To generate the configure script when checking out from github source:

$ glibtoolize
$ aclocal
$ automake -a -c -f
$ autoreconf

Set PKG_CONFIG_PATH and then the usual will work:

$ export PKG_CONFIG_PATH="/usr/local/opt/openssl@1.1/lib/pkgconfig"
$ ./configure
$ make
$ make install

HOW TO USE

Running the command "sscep" without any arguments should give you a list of arguments and command line options.

$ ./sscep

sscep version 0.10.x

Usage: ./sscep OPERATION [OPTIONS]

Available OPERATIONs are
  getca             Get CA/RA certificate(s)
  enroll            Enroll certificate
  getcert           Query certificate
  getcrl            Query CRL
  getcaps           Query SCEP capabilities

General OPTIONS
  -u <url>          SCEP server URL
  -p <host:port>    Use proxy server at host:port
  -g <engine>       Use the given cryptographic engine
  -f <file>         Use configuration file
  -c <file>         CA certificate file or '-n' suffixed files (write if OPERATION is getca)
  -E <name>         PKCS#7 encryption algorithm (des|3des|blowfish|aes[128]|aes192|aes256)
  -S <name>         PKCS#7 signature algorithm (md5|sha1|sha224|sha256|sha384|sha512)
  -W <secs>         Wait for connectivity, up to <secs> seconds
  -v                Verbose output (for debugging the configuration)
  -d                Debug output (more verbose, for debugging the implementation)

OPTIONS for OPERATION getca are
  -i <string>       CA identifier string
  -F <name>         Fingerprint algorithm (md5|sha1|sha224|sha256|sha384|sha512)

OPTIONS for OPERATION enroll are
  -k <file>         Private key file
  -r <file>         Certificate request file
  -K <file>         Signature private key file, use with -O
  -O <file>         Signature certificate (used instead of self-signed)
  -l <file>         Write enrolled certificate in file
  -e <file>         Use different CA cert for encryption
  -L <file>         Write selfsigned certificate in file
  -t <secs>         Polling interval in seconds
  -T <secs>         Max polling time in seconds
  -n <count>        Max number of GetCertInitial requests
  -R                Resume interrupted enrollment

OPTIONS for OPERATION getcert are
  -k <file>         Signature private key file
  -l <file>         Signature local certificate file
  -O <file>         Issuer Certificate of the certificate to query (requires -s)
  -s <number>       Certificate serial number (decimal)
  -w <file>         Write certificate in file

OPTIONS for OPERATION getcrl are
  -k <file>         Signature private key file
  -l <file>         Signature local certificate file
  -O <file>         Certificate to get the CRL for (reads issuer and serial)
  -s <number>       Certificate serial number (decimal)
  -w <file>         Write CRL in file

SSCEP also supports configuration via a configuration file (-f). This is the recommended way to configure SSCEP and all the examples in below assume that you have done so.

All configuration options are key-value pairs separated with the equal sign and grouped into sections:

[section]
Key = Value

Quotation marks are optional - they are needed only if the value contains space characters (space or tab). Quotation marks inside the value string must be escaped using a backslash:

Key = "Value \"containing quotation marks\""

Comment lines (lines starting with '#') and empty lines are discarded.

Here are the available configuration file keys and example values:

Key Explanation Example Command options
URL URL of the SCEP server. http://example.com/scep -u
CACertFile Single CA certificate file, or multiple CA certificates suffixed with -0, -1, ... to write (getca) or to choose from (all other operations). ./ca.crt -c
CAIdentifier Some CAs require you to define this. mydomain.com -i
CertReqFile Certificate request file created with mkrequest. ./local.csr -r
EncAlgorithm PKCS#7 encryption algorithm. Available algorithms are des, 3des, blowfish, aes/aes128, aes192 and aes256. NOTE: SCEP provides no mechanism to "negotiate" the algorithm - even if you send 3des, reply might be des (same thing applies to SigAlgorithm). -E
EncCertFile If your CA/RA uses a different certificate for encyption and signing, define this. CACertFile is used for verifying the signature. ./enc.crt -e
SignCertFile Instead of creating a self-signed certificate from the new key pair use an already existing certficate/key to sign the SCEP request. If the "old" certificate and key is used, the CA can verify that the holder of the private key for an existing certificate re-enrolls for a renewal certificate, allowing for automatic approval of the request. Requires specification of the corresponding SignKeyFile (-K). ./sig.crt -O
SignKeyFile See SignCertFile. Specifies the corresponding private key. ./sig.key -K
FingerPrint Display fingerprint algorithm. Available algorithms are md5, sha1, sha224, sha256, sha384 and sha512. Default is the best from getcacaps, or md5. -F
GetCertFile Write certificate asquired via getcert operation. ./cert.crt -w
GetCertSerial Certificate serial number. Define this for getcert. The value is defined as a decimal number. 12 -s
GetCrlFile Write CRL to file. ./crl.crl -w
LocalCertFile Write successfully enrolled certificate. ./local.crt -l
MaxPollCount Max number of GetCertInitial requests. 50 -n
MaxPollTime Max polling time in seconds. 28800 -T
PollInterval Poll periodically for pending certificate. 60 -t
PrivateKeyFile Private key created with mkrequest. ./local.key -k
Proxy Use HTTP proxy at host:port. localhost:8080 -p
SelfSignedFile Write optionally the selfsigned certificate in file (needed in SCEP transaction). ./selfsigned.crt -L
SigAlgorithm PKCS#7 signature algorithm. Available algorithms are md5, sha1, sha224, sha256, sha384 and sha512. Default is the best from getcacaps, or md5. -S
Verbose Verbose output? Answer "yes" or "no" -v
Debug Debug output? Answer "yes" or "no". -d

The actual enrollment is done with the following procedure:

STEP 1 - Gather information

STEP 2 - Make certificate request and key

Before the enrollment can take place, sscep needs a private key file and the corresponding X.509 certificate request in PKCS#10 format.

This can be created using the mkrequest script, or manually by openssl. Create an request.cnf, such as:

oid_section        = new_oids

[ req ]
default_bits       = 2048
default_keyfile    = local.key
encrypt_key        = no

distinguished_name = req_dn
attributes         = req_attributes
req_extensions     = req_ext

[ new_oids ]
certTemplateName   = 1.3.6.1.4.1.311.20.2

[ req_dn ]
0.domainComponent  = org
1.domainComponent  = OpenXPKI
2.domainComponent  = Test Deployment
commonName         = device

[ req_attributes ]

[ req_ext ]
basicConstraints   = critical, CA:FALSE
keyUsage           = critical, digitalSignature, keyEncipherment
extendedKeyUsage   = serverAuth, clientAuth
subjectAltName     = @alt_names
certTemplateName   = ASN1:UTF8String:pc-client

[ alt_names ]
DNS.1 = www.example.com
DNS.2 = example.com

To create a key and a request named local.key and local.csr run:

$ openssl req -new -config request.cnf -out local.csr

You can automate this process using the mkrequest shell script. Edit the DN variables in the mkrequest file if you need. When ready, make the request:

$ mkrequest -ip 172.30.0.1
Generating RSA private key, 1024 bit long modulus
..............++++++
...++++++
e is 65537 (0x10001)
Using configuration from .4018client.cnf

This also writes key and request named local.key and local.csr (you can change the "local" with variable PREFIX in mkrequest).

If the CA supports automatic enrollment, you may supply the password in cert request:

$ mkrequest -ip 172.30.0.1 password

STEP 3 - Get CA certificate

$ ./sscep getca -u http://example.com/scep -c ca.crt
./sscep: requesting CA certificate
./sscep: valid response from server
./sscep: MD5 fingerprint: 1D:3C:4C:DF:99:73:B8:FB:B4:EE:C4:56:A9:7C:37:A3
./sscep: CA certificate written as ca.crt

NOTE: it is very important to make sure that the CA certificate is really what you think it is. The security of the whole protocol depends on that!! This is why the fingerprint is printed on terminal - you should check that from your CA. You can check the fingerprint any time with command

$ openssl x509 -in ca.crt -noout -fingerprint

If the CA sends a certificate chain, sscep writes all certificates in the order it founds them in reply and names them with an integer suffix (-number) appended to CACertFile.

$ ./sscep getca -u http://example.com/scep -c ca.crt
./sscep: requesting CA certificate
./sscep: valid response from server
./sscep: found certificate with
  subject: /C=FI/O=klake.org/CN=klake.org VPN RA
  issuer: /C=FI/O=klake.org/CN=klake.org VPN CA
  usage: Digital Signature, Non Repudiation
  MD5 fingerprint: 7A:92:84:2A:6F:EE:28:14:F9:69:D8:9D:61:34:B5:67
./sscep: certificate written as ca.crt-0
./sscep: found certificate with
  subject: /C=FI/O=klake.org/CN=klake.org VPN CA
  issuer: /C=FI/O=klake.org/CN=klake.org VPN CA
  usage: Digital Signature, Non Repudiation, Certificate Sign, CRL Sign
  MD5 fingerprint: A5:CE:94:5C:96:77:94:E8:F5:31:AB:D5:31:18:1D:E1
./sscep: certificate written as ca.crt-1

SSCEP prints out issuer, subject, key usage and md5/sha1 fingerprint for each certificate it founds. This information might help you to decide what certificate to use as -c and (optionally) -e in subsequent operations.

Some CAs may give you a three (or more) certificates, the root CA(s) plus different RA certificates for encryption and signing. If that's your case, you have to define encryption certificate with command line option (-e). Probably it is the certificate with key usage "Key Encipherment".

You may also use the base name (e.g., ca.crt) of all certificates and rely on an automated certificate selection. The system loads all available certificates (ca.crt-0, ca.crt-1, ...) and then:

  1. Tries to find a certificate that:
    • Is at the end of the received chain, i.e., do not sign other certificate.
    • Has key usage "Digital Signature" (for -c) or "Key Encipherment" (for -e), or does not have any key usage defined.
  2. If no such key is found, selects the first certificate in the chain, which is usually the right certificate anyway.

Currently, SSCEP doesn't verify the CA/RA certificate chain. You can do it manually with OpenSSL: 

$ openssl verify -CAfile ca.crt-1 ca.crt-0
ca.crt-0: OK

NOTE: In case of multiple CA/RA certificates, the actual CA (the one who signs your certificate) might not be the same as the CA/RA you are dealing with. Keep this in mind when installing the CA cert in /etc/isakmpd/ca.

STEP 4 - Make enrollment

You need to supply URL (-u), CACertFile (-c), PrivateKeyFile (-k), CertReqFile (-r) and output LocalCertFile (-l). PrivateKeyFile is the key generated in step 2 (local.key), CertReqFile is the request (local.csr) and LocalCertFile is where the enrolled certificate will be written once ready.

If your CA/RA have different certificates for encryption and signing, and you do not want to use the auto-selection mechanism, you must provide also the encryption certificate EncCertFile (-e).

Normally, the enrollment looks like this:

$ ./sscep enroll -u http://example.com/scep -c ca.crt -k local.key -r local.csr -l local.crt
./sscep: sending certificate request
./sscep: valid response from server
./sscep: pkistatus: PENDING
./sscep: requesting certificate (#1)
./sscep: valid response from server
./sscep: pkistatus: PENDING
./sscep: requesting certificate (#2)
./sscep: valid response from server
./sscep: pkistatus: PENDING
....
./sscep: requesting certificate (#NNN)
./sscep: valid response from server
./sscep: pkistatus: SUCCESS
./sscep: certificate written as ./local.crt

First message sent is PKCSReq, that's where your request goes. Then the CA writes request down and sends reply PENDING, indicating that the certificate is not signed yet. SSCEP polls periodically for the certificate by sending GetCertInitial messages until the CA returns SUCCESS. The polling interval can be adjusted with PollInterval (-t). You can interrupt the process any time and start again using "sscep enroll ..". You should use the command line option (-R) when you continue (resume) the interrupted enrollment.

If the CA is configured for automatic enrollment (and your request includes the challenge password), it returns SUCCESS as a first reply. Otherwise, the enrollment requires manual signing and authentication (perhaps a phone call).

STEP 5 - Certificate renewal

The SCEP allows to use the existing certificate (issued by the CA) to authenticate a renewal request. In this context, the SCEP request with the new public key is signed with the old certificate and key (instead of using a self-signed certificate created from the new key pair).

If you want to renew the certificate created previously (local.crt), you follow the enrollment procedure as described before, but supply the current (old) key and certificate as SignKeyFile (-K) and SignCertFile (-O).

$ ./sscep enroll -u http://example.com/scep -c ca.crt -K local.key -O local.crt \
                 -k new.key -r new.csr -l new.crt

The actual behaviour of the SCEP server depends on the CA policy and on the capabilities of the SCEP server (not all servers implement this feature, using the existing certificate with an older SCEP server may or may not work, depending on implementation).

Note: For example, OpenXPKI is capable of automatically approving SCEP requests signed with the already existing key pair.

STEP 6 - Use certificate

Install local.key, local.crt and ca.crt in the isakmpd default locations and you are ready to go! Default locations are

Private key /etc/isakmpd/private/local.key Certificate /etc/isakmpd/certs/local.crt CA certificate /etc/isakmpd/ca/ca.crt

And pay attention to CA certificate if your enrollment was done via RA server. openssl verify -CAfile ca.crt local.crt is your friend here.

STEP 7 - Check out revocation list (optional)

You need your enrolled certificate for this step.

$ ./sscep getcrl -f sscep.conf
./sscep: requesting crl
./sscep: valid response from server
./sscep: pkistatus: SUCCESS
./sscep: CRL written as ./crl.crl

CREDITS

I'd like to thank the following people for providing me feedback:

Fiel Cabral, Manuel Gil Perez

OpenSSL toolkit made this possible.

I would also like to thank OpenSCEP project for it's great software, reading the source code helped me understand the protocol. Unfortunately, it's license is too restrictive for my use.