ClearPass: Credential Caching and Replay
To enable single sign-on into some legacy applications it may be necessary to provide them with the actual password. While such approach inevitably increases security risk, at times this may be a necessary evil in order to integrate applications with CAS.
Usage Warning!ClearPass is turned off by default. No applications will be able to obtain the user credentials unless ClearPass is explicitly turned on by the below configuration. Think VERY CAREFULLY before turning on this feature, as it MUST be the last resort in getting an integration to work...maybe not even then.
Overview
CAS is able to issue the credential password directly in the CAS validation response. This previously was handled via a proxy authentication sequence and obtaining a proxy-granting ticket for the ClearPass service and was necessary in order to establish trust between the client application and the CAS server. This document describes the configuration that can be applied in order to receive the credential password as an attribute in the CAS validation response.
In order to successfully establish trust between the
CAS server and the application, private/public key pairs are generated by the client application and then
the public key distributed and configured inside CAS. CAS will use the public key to encrypt the credential
password and will issue a new attribute <credential> in the validation response, only if the service is authorized to receive it.
Note that the return of the credential is only carried out by the CAS validation response, provided the client
application issues a request to the /p3/serviceValidate endpoint (or /p3/proxyValidate). Other means of
returning attributes to CAS, such as SAML1 will not support the additional returning of this value.
Also note that CAS by default attempts to encrypt the cached credential in memory via its own pre-generated keys for signing and encryption. When the attribute is to be released to the application, CAS will internally decode the credential first and then will attempt to encrypt it again this time using the service’s public key credentials.
ClearPass via Proxying!CAS no longer supports retrieving the credential via the proxying mechanism. Applications that intend to obtain the credential need to be updated to account for the following approach described here.
Cache Credential
Enable the caching and capturing of the credential in CAS properties.
The following settings and properties are available from the CAS configuration catalog:
cas.clearpass.cache-credential=false
Enable clearpass and allow CAS to cache credentials.
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cas.clearpass.crypto.encryption.key=
The encryption key is a JWT whose length is defined by the encryption key size setting. This setting supports the Spring Expression Language.
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cas.clearpass.crypto.signing.key=
The signing key is a JWT whose length is defined by the signing key size setting. This setting supports the Spring Expression Language.
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cas.clearpass.crypto.alg=
The signing/encryption algorithm to use.
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cas.clearpass.crypto.enabled=true
Whether crypto operations are enabled.
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cas.clearpass.crypto.encryption.key-size=512
The encryption key size.
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cas.clearpass.crypto.signing.key-size=512
The signing key size.
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cas.clearpass.crypto.strategy-type=ENCRYPT_AND_SIGN
Control the cipher sequence of operations. The accepted values are:
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cas.clearpass.crypto.encryption.key=
The encryption key is a JWT whose length is defined by the encryption key size setting. This setting supports the Spring Expression Language.
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cas.clearpass.crypto.signing.key=
The signing key is a JWT whose length is defined by the signing key size setting. This setting supports the Spring Expression Language.
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cas.clearpass.crypto.alg=
The signing/encryption algorithm to use.
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cas.clearpass.crypto.enabled=true
Whether crypto operations are enabled.
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cas.clearpass.crypto.encryption.key-size=512
The encryption key size.
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cas.clearpass.crypto.signing.key-size=512
The signing key size.
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cas.clearpass.crypto.strategy-type=ENCRYPT_AND_SIGN
Control the cipher sequence of operations. The accepted values are:
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This CAS feature is able to accept signing and encryption crypto keys. In most scenarios if keys are not provided, CAS will auto-generate them. The following instructions apply if you wish to manually and beforehand create the signing and encryption keys.
Note that if you are asked to create a JWK of a certain size for the key, you are to use the following set of commands to generate the token:
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wget https://raw.githubusercontent.com/apereo/cas/master/etc/jwk-gen.jar
java -jar jwk-gen.jar -t oct -s [size]
The outcome would be similar to:
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{
"kty": "oct",
"kid": "...",
"k": "..."
}
The generated value for k needs to be assigned to the relevant CAS settings. Note that keys generated via
the above algorithm are processed by CAS using the Advanced Encryption Standard (AES) algorithm which is a
specification for the encryption of electronic data established by the U.S. National Institute of Standards and Technology.
Configuration Metadata
The collection of configuration properties listed in this section are automatically generated from the CAS source and components that contain the actual field definitions, types, descriptions, modules, etc. This metadata may not always be 100% accurate, or could be lacking details and sufficient explanations.
Be Selective
This section is meant as a guide only. Do NOT copy/paste the entire collection of settings into your CAS configuration; rather pick only the properties that you need. Do NOT enable settings unless you are certain of their purpose and do NOT copy settings into your configuration only to keep them as reference. All these ideas lead to upgrade headaches, maintenance nightmares and premature aging.
YAGNI
Note that for nearly ALL use cases, declaring and configuring properties listed here is sufficient. You should NOT have to explicitly massage a CAS XML/Java/etc configuration file to design an authentication handler, create attribute release policies, etc. CAS at runtime will auto-configure all required changes for you. If you are unsure about the meaning of a given CAS setting, do NOT turn it on without hesitation. Review the codebase or better yet, ask questions to clarify the intended behavior.
Naming Convention
Property names can be specified in very relaxed terms. For instance cas.someProperty, cas.some-property, cas.some_property are all valid names. While all
forms are accepted by CAS, there are certain components (in CAS and other frameworks used) whose activation at runtime is conditional on a property value, where
this property is required to have been specified in CAS configuration using kebab case. This is both true for properties that are owned by CAS as well as those
that might be presented to the system via an external library or framework such as Spring Boot, etc.
Note
When possible, properties should be stored in lower-case kebab format, such as cas.property-name=value.
The only possible exception to this rule is when naming actuator endpoints; The name of the
actuator endpoints (i.e. ssoSessions) MUST remain in camelCase mode.
Settings and properties that are controlled by the CAS platform directly always begin with the prefix cas. All other settings are controlled and provided
to CAS via other underlying frameworks and may have their own schemas and syntax. BE CAREFUL with
the distinction. Unrecognized properties are rejected by CAS and/or frameworks upon which CAS depends. This means if you somehow misspell a property definition
or fail to adhere to the dot-notation syntax and such, your setting is entirely refused by CAS and likely the feature it controls will never be activated in the
way you intend.
Validation
Configuration properties are automatically validated on CAS startup to report issues with configuration binding, specially if defined CAS settings cannot be recognized or validated by the configuration schema. Additional validation processes are also handled via Configuration Metadata and property migrations applied automatically on startup by Spring Boot and family.
Indexed Settings
CAS settings able to accept multiple values are typically documented with an index, such as cas.some.setting[0]=value. The index [0] is meant to be
incremented by the adopter to allow for distinct multiple configuration blocks.
Create Keys
The keypair must be generated by the application itself that wishes to obtain the user credential. The public key is shared with CAS. The private key is used by the application to decrypt the credential.
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openssl genrsa -out private.key 1024
openssl rsa -pubout -in private.key -out public.key -inform PEM -outform DER
openssl pkcs8 -topk8 -inform PEM -outform DER -nocrypt -in private.key -out private.p8
Note that while 1024 is perfectly fine, you may need to adjust the key size to larger values such
as 4096 in order to allow CAS to encrypt credentials that are too long in length.
Register Public Key
Once you have received the public key from the client application owner, it must be first registered inside the CAS server’s service registry. The service that holds the public key above must also be authorized to receive the password as an attribute for the given attribute release policy of choice.
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{
"@class" : "org.apereo.cas.services.CasRegisteredService",
"serviceId" : "^https://.+",
"attributeReleasePolicy" : {
"@class" : "org.apereo.cas.services.ReturnAllowedAttributeReleasePolicy",
"authorizedToReleaseCredentialPassword" : true
},
"publicKey" : {
"@class" : "org.apereo.cas.services.RegisteredServicePublicKeyImpl",
"location" : "classpath:public.key",
"algorithm" : "RSA"
}
}
Decrypt the Password
Once the client application has received the credential attribute in the CAS validation response, it can decrypt
it via its own private key. Since the attribute is base64 encoded by default, it needs to be decoded first before
decryption can occur. Here’s a sample code snippet:
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var attributes = ...
varencodedPsw = (String) attributes.get("credential");
/* Use the private.key file generated above. */
var privateKey = ...
var cipher = Cipher.getInstance(privateKey.getAlgorithm());
var cred64 = decodeBase64(encodedPsw);
cipher.init(Cipher.DECRYPT_MODE, privateKey);
var cipherData = cipher.doFinal(cred64);
return new String(cipherData);