Trying to decrypt soap call with EncryptedXML but getting "Unable to retrieve the decryption key"

Try using this to get the private key:

X509Certificate2 cert = new X509Certificate2("filename.pfx", "password", X509KeyStorageFlags.Exportable);
//cert.PrivateKey.ToString();

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Unfortunately I'm getting the same error.  I believe the error is not getting the key but this line:

I'm not sure "KeyIdentifier" is the right parameter.  

exml.AddKeyNameMapping("KeyIdentifier", privateKeyRSAA);

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That's just a string that handles the name.

Your issue is here:
var privateKeyRSAA = (RSACryptoServiceProvider)clientCertEnc.PrivateKey;

https://msdn.microsoft.com/en-us/library/system.security.cryptography.xml.encryptedxml.addkeynamemapping(v=vs.110).aspx

The second parameter takes the algorithm variable as the parameter which should then decrypt the XML for you.

I'm a bit lost.  Okay so I currently have this:

X509Certificate2 clientCertEnc = new X509Certificate2(@"C:\certificates\xxxx.pfx", "xxxx", X509KeyStorageFlags.Exportable);
var rsaKey = (RSACryptoServiceProvider)clientCertEnc.PrivateKey;

var exml = new EncryptedXml(doc);
exml.AddKeyNameMapping("KeyIdentifier", rsaKey);
exml.DecryptDocument();

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Why is rsaKey wrong?

From the link it says the second parameter is:
The symmetric key to map to keyName

If you look at the last post you can see they are doing the same thing I am.  

https://stackoverflow.com/questions/29849107/telling-dotnet-which-private-key-to-use-to-decrypt-incoming-xml-documents

How are you encrypting the XML?

I'm not the client is encrypting it on their with a public key.   I'm trying to decrypt it (the encrypted key) on my end with the private key.

Basically I'm getting these two values from the soap body:
The encrypted key is  = Header > Security > EncryptedKey > CipherData > CipherValue. So it would be ANCElFZ5v....==
The body is   Body > EncryptedData > CipherData > CipherValue. So it would be 3kESnJnhc8K.....

The first step is to decrypt the encrypted key. Then the second step is to use the  decrypted encrypted key to decrypt the soap body.

This is how the soap body looks like:

<s:Envelope 
        xmlns:s="http://www.w3.org/2003/05/soap-envelope" 
        xmlns:u="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd">
        <s:Header>
            <VsDebuggerCausalityData 
                xmlns="http://schemas.microsoft.com/vstudio/diagnostics/servicemodelsink">uIDPo4tYpt6X40FEk+VSAe5mc8MAAAAAP497cBuXfk+uFIOY80O0iuLtIW56q7hLktgVYPhbnHMACQAA
            </VsDebuggerCausalityData>
            <o:Security s:mustUnderstand="1" 
                xmlns:o="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd">
                <o:BinarySecurityToken u:Id="uuid-10490fb0-8ee0-4a4c-a8db-77242c9a3b7f-2" ValueType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-x509-token-profile-1.0#X509v3" EncodingType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-soap-message-security-1.0#Base64Binary">MIIF+TCCBOGgAwIBAgIQIWv3OdE866kXP/....t</o:BinarySecurityToken>
                <e:EncryptedKey Id="_0" 
                    xmlns:e="http://www.w3.org/2001/04/xmlenc#">
                    <e:EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#rsa-oaep-mgf1p">
                        <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" 
                            xmlns="http://www.w3.org/2000/09/xmldsig#" />
                    </e:EncryptionMethod>
                    <KeyInfo 
                        xmlns="http://www.w3.org/2000/09/xmldsig#">
                        <o:SecurityTokenReference>
                            <o:KeyIdentifier ValueType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-x509-token-profile-1.0#X509SubjectKeyIdentifier" EncodingType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-soap-message-security-1.0#Base64Binary">j0ZKFBmTz3Kj0cQ82rq63MYAR+0=</o:KeyIdentifier>
                        </o:SecurityTokenReference>
                    </KeyInfo>
                    <e:CipherData>
                        <e:CipherValue>ANCElFZ5v....==</e:CipherValue>
                    </e:CipherData>
                    <e:ReferenceList>
                        <e:DataReference URI="#_2" />
                    </e:ReferenceList>
                </e:EncryptedKey>
                <Signature 
                    xmlns="http://www.w3.org/2000/09/xmldsig#">
                    <SignedInfo>
                        <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
                        <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" />
                        <Reference URI="#_1">
                            <Transforms>
                                <Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
                            </Transforms>
                            <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
                            <DigestValue>akiomlPdi6j1h6r9NDqmh9G1GD0=</DigestValue>
                        </Reference>
                    </SignedInfo>
                    <SignatureValue>LIjqWD/BXsoA0XNR7hv...==</SignatureValue>
                    <KeyInfo>
                        <o:SecurityTokenReference>
                            <o:Reference ValueType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-x509-token-profile-1.0#X509v3" URI="#uuid-10490fb0-8ee0-4a4c-a8db-77242c9a3b7f-2" />
                        </o:SecurityTokenReference>
                    </KeyInfo>
                </Signature>
            </o:Security>
        </s:Header>
        <s:Body u:Id="_1">
            <e:EncryptedData Id="_2" Type="http://www.w3.org/2001/04/xmlenc#Content" 
                xmlns:e="http://www.w3.org/2001/04/xmlenc#">
                <e:EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#aes128-cbc" />
                <e:CipherData>
                    <e:CipherValue>3kESnJnhc8K.....</e:CipherValue>
                </e:CipherData>
            </e:EncryptedData>
        </s:Body>
    </s:Envelope>

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see if this helps you:
https://gist.github.com/yaronn/6765830

They are encrypting it with a public key for which you have the private key . . . correct?

Thank you for the article.  I seen several articles by Yaronn.  I did what he suggested.  

This post talks about it:
https://www.experts-exchange.com/questions/29094171/Decrypt-encrypted-key-from-SOAP-message.html

Going to be jumping over on that message and requesting to delete this one.