[keycloak-user] Encrypted SAML response

[Pål Oliver Kristiansen]

Hi!

We have set up Keycloak as an IDP with a F5 BigIP APM as a SAML SP.
Everything works well out of the box, but when we activate encryption of
assertions, we get an error on the
BigIP side.

So in order to investigate the situation, I'm trying to understand the
process of SAML assertion response encryption.
And I am a bit confused when reading the Keycloak documentation and
comparing that to what I see in the Keycloak admin console.
Here is what I mean; in the documentation (
https://keycloak.github.io/docs/userguide/keycloak-server/html/saml.html),
activating
assertion response encryption is described like this:

"Encrypt Assertions
Encrypt assertions in SAML documents with the realm's private key. The AES
algorithm is used with a key size of 128 bits."

But in the admin console (under clients) it is described like this in the
tooltip:

"Encrypt Assertions
Should SAML assertions be encrypted with client's public key using AES?"

So basically, which key is used for encrypting the assertion response?

I have inspected the actual HTTP post and it looks something like this:

<samlp:Response>
...
  <saml:EncryptedAssertion>
    <xenc:EncryptedData xmlns:xenc="http://www.w3.org/2001/04/xmlenc#"
Type="http://www.w3.org/2001/04/xmlenc#Element">
      <xenc:EncryptionMethod Algorithm="
http://www.w3.org/2001/04/xmlenc#aes128-cbc"/>
        <ds:KeyInfo xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
          <xenc:EncryptedKey>
            <xenc:EncryptionMethod Algorithm="
http://www.w3.org/2001/04/xmlenc#rsa-1_5"/>
            <xenc:CipherData>
              <xenc:CipherValue>---[block of cipher
text]---</xenc:CipherValue>
            </xenc:CipherData>
          </xenc:EncryptedKey>
        </ds:KeyInfo>
      <xenc:CipherData>
        <xenc:CipherValue>---[block of cipher text]---</xenc:CipherValue>
      </xenc:CipherData>
    </xenc:EncryptedData>
  </saml:EncryptedAssertion>
</samlp:Response>

So it at least looks like the key to decrypt the encrypted response is
included. It is encrypted using RSA, but with which key?
My understanding was something like this:

1. The encryption of a SAML assertion response is done using AES-128, with
some key.
2. The key is added alongside the encrypted assertion response and
encrypted using the SP's public key.
3. The SP receives the encrypted assertion response and the encrypted key,
decrypts the key using its' private key and uses
this decrypted key to decrypt the actual assertion response.

Is this generally correct?

The error we get from BigIP is this: "failed to process encrypted
assertion, error: RSA decrypt"
Which leads me to think that maybe the decryption of the included key did
not work out.
Any ideas or tips to how to approach this?

Thanks!
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