XML Encryption Implementation Experience

Takeshi Imamura

Tokyo Research Laboratory

IBM Research

imamu@jp.ibm.com

Design (1/4)

l Steps by the spec

Design (1/4)

l Steps by the spec

Design (2/4)

l Approaches

Ø Application-level implementation

² DOM-based, SAX-based, ...

Ø Parser-level implementation

² XNI (Xerces Native Interface) -based, ...

Design (3/4)

l Steps by application-level implementation

Design (4/4)

l Steps by parser-level implementation

Status

l DOM-based implementation

l Environment

Ø Java 2 SDK 1.3

Ø Java Cryptography Extension (JCE) 1.2

Ø Xerces Java Parser 1.2

l Supported algorithms

Ø 3DES, RSA-v1.5, base64

l Supported data to be encrypted

Ø XML element, XML element content, arbitrary binary data

l Reference

Ø IBM alphaWorks

² http://www.alphaworks.ibm.com/tech/xmlsecuritysuite

Code Fragment for Encryption

// 0. Already given

Element elem = ...; // Element to be encrypted

Key key = ...; // Key named "key"

AlgorithmFactory algFac = ...; // Factory for algorithm implementations

// 1. Create <EncryptedData> as template

EncryptionMethod em = new EncryptionMethod();

em.setAlgorithm(EncryptionMethod.TRIPLE_DES_CBC);

KeyName kn = new KeyName();

kn.setValue("key");

KeyInfo ki = new KeyInfo();

ki.addKeyId(kn);

EncryptedData ed = new EncryptedData();

ed.setType(EncryptedData.ELEMENT);

ed.setEncryptionMethod(em);

ed.setKeyInfo(ki);

Element encData = ed.createElement(elem.getOwnerDocument());

// 2. Create and set up encryption context

EncryptionContext encCont = new EncryptionContext();

encCont.addData(elem, false, encData);

encCont.setKey(key);

encCont.setAlgorithmFactory(algFac);

// 3. Perform encryption

encCont.encrypt();

Created <EncryptedData>

<EncryptedData

xmlns="http://www.w3.org/2001/04/xmlenc#"

Type="http://www.w3.org/2001/04/xmlenc#Element">

<EncryptionMethod

Algorithm="http://www.w3.org/2001/04/xmlenc#3des-cbc" />

<KeyInfo

xmlns="http://www.w3.org/2000/09/xmldsig#">

<KeyName>key</KeyName>

</KeyInfo>

</EncryptedData>

Code Fragment for Decryption

// 0. Already given

Element encData = ...; // <EncryptedData> to be decrypted

KeyInfoResolver kiRes = ...;

// Resolver from <KeyInfo> to key

AlgorithmFactory algFac = ...;

// Factory for algorithm implementations

// 1. Create and set up decryption context

DecryptionContext decCont = new DecryptionContext();

decCont.addEncryptedData(encData);

decCont.setKeyInfoResolver(kiRes);

decCont.setAlgorithmFactory(algFac);

// 2. Perform decryption

decCont.decrypt();

Sample Program - XMLCipher

l XML element is being encrypted with key stored in keystore

l All parameters are provided using configuration file, which consists of:

Ø <data> for input file, type of element to be encrypted, and output file

Ø <template> for template specifying encryption algorithm and key (used only for encryption)

Ø <keyinfo> for keystore name, keystore password, key alias, and key password

Challenges (1/2)

1. How to obtain octet sequence corresponding to DOM tree in encryption

Ø Serialize DOM tree >> representation not preserved, e.g.,

² Attributes' order

² Whitespaces in attribute value

² Quotation marks

² ...

Ø Extract octet sequence from XML document

Challenges (2/2)

2. How to obtain DOM tree corresponding to octet sequence in decryption*

Ø Place octet sequence in place of <EncryptedData> and then re-parse the whole XML document >> high cost

Ø Parse octet sequence in context of <EncryptedData>

<?xml version="1.0"?>

<!DOCTYPE p:root (reference to external subset) [

<!ELEMENT p:root ANY>

(internal subset)

]>

<p:root xmlns:p="(some namespace)"

(namespace declarations)

(xml:lang declaration)

(xml:space declaration)

(xml:base declaration)>

(octet sequence)

</p:root>

*Do not occur for parser-level implementation

Questions (1/2)

1. Spec says in Section 4.1 "Encryption"

Ø If the data to be encrypted is an XML element or XML element content, the octet sequence is an UTF-8 encoded string representation of the element or its content ...

Ø If the data being encrypted is an XML element or XML element content, the unencrypted data is removed and replaced with the new XML structure ...

Questions (2/2)

2. Spec says in Section 4.3 "XML Encryption"

Ø If the application wishes to canonicalize or encode/compress the data in an XML packaging format, the application needs to marchal the XML accordingly and identify the resulting type with optional the EncryptedData Type attribute. ...

Ø Element 'http://www.w3.org/2001/04/xmlenc#Element'

"[39] element ::= EmptyElemTag | STag content ETag"