xmlenc.pl

/*  Part of SWI-Prolog

    Author:        Matt Lilley
    E-mail:        thetrime@gmail.com
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    Copyright (c)  2004-2016, SWI-Prolog Foundation
                              VU University Amsterdam
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*/


:-module(xmlenc,
         [ decrypt_xml/4,   % +EncryptedXML, -DecryptedXML, :KeyCallback, +Options
           load_certificate_from_base64_string/2 % +Base64String, -Certificate
         ]).
:- autoload(library(base64),[base64/2]).
:- autoload(library(crypto),
	    [crypto_data_decrypt/6,rsa_private_decrypt/4,hex_bytes/2]).
:- autoload(library(error),[existence_error/2,domain_error/2]).
:- autoload(library(lists),[append/3]).
:- autoload(library(sgml),[load_structure/3]).
:- autoload(library(ssl),[load_certificate/2]).
:- autoload(library(uri),[uri_components/2]).
:- autoload(library(http/http_open),[http_open/3]).

:- meta_predicate
    decrypt_xml(+, -, 3, +).

/** <module> XML encryption library

This library is a partial implementation of the XML encryption standard.
It implements the _decryption_ part, which is needed by SAML clients.

@see https://www.w3.org/TR/xmlenc-core1/
@see https://en.wikipedia.org/wiki/Security_Assertion_Markup_Language
*/

% These are the 4 mandatory block cipher algorithms
% (actually aes-192-cbc is not mandatory, but it is easy to support)
ssl_algorithm('http://www.w3.org/2001/04/xmlenc#tripledes-cbc', 'des3',         8).
ssl_algorithm('http://www.w3.org/2001/04/xmlenc#aes128-cbc',    'aes-128-cbc', 16).
ssl_algorithm('http://www.w3.org/2001/04/xmlenc#aes256-cbc',    'aes-256-cbc', 32).
ssl_algorithm('http://www.w3.org/2001/04/xmlenc#aes192-cbc',    'aes-192-cbc', 24).

%!  decrypt_xml(+DOMIn, -DOMOut, :KeyCallback, +Options) is det.
%
%   @arg KeyCallback may be called as follows:
%           - call(KeyCallback, name,        KeyName,         Key)
%           - call(KeyCallback, public_key,  public_key(RSA), Key)
%           - call(KeyCallback, certificate, Certificate,     Key)

decrypt_xml([], [], _, _):- !.
decrypt_xml([element(ns(_, 'http://www.w3.org/2001/04/xmlenc#'):'EncryptedData',
                     Attributes, EncryptedData)|Siblings],
            [Decrypted|NewSiblings], KeyCallback, Options) :-
    !,
    decrypt_element(Attributes, EncryptedData, Decrypted, KeyCallback, Options),
    decrypt_xml(Siblings, NewSiblings, KeyCallback, Options).

decrypt_xml([element(Tag, Attributes, Children)|Siblings],
            [element(Tag, Attributes, NewChildren)|NewSiblings], KeyCallback, Options) :-
    !,
    decrypt_xml(Children, NewChildren, KeyCallback, Options),
    decrypt_xml(Siblings, NewSiblings, KeyCallback, Options).
decrypt_xml([Other|Siblings], [Other|NewSiblings], KeyCallback, Options):-
    decrypt_xml(Siblings, NewSiblings, KeyCallback, Options).

%!   decrypt_element(+Attributes,
%!                   +EncryptedData,
%!                   -DecryptedElement,
%!                   +Options).
%
%    Decrypt an EncryptedData element  with   Attributes  and  child
%    EncryptedData DecryptedElement will either be an element/3 term
%    or a string as dictacted by   the Type attribute in Attributes.
%    If Attributes does not contain a  Type attribute then we assume
%    it is a string

:-meta_predicate(decrypt_element(+, +, -, 3, +)).

decrypt_element(Attributes, EncryptedData, Decrypted, KeyCallback, Options):-
    XENC = ns(_, 'http://www.w3.org/2001/04/xmlenc#'),
    (  memberchk(element(XENC:'CipherData', _, CipherData), EncryptedData)
    -> true
    ;  existence_error(cipher_data, EncryptedData)
    ),
    % The Type attribute is not mandatory. However, 3.1 states that
    % "Without this information, the decryptor will be unable to automatically restore the XML document to its original cleartext form."
    (  memberchk('Type'=Type, Attributes)
    -> true
    ;  Type = 'http://www.w3.org/2001/04/xmlenc#Content'
    ),

    % First of all, determine the algorithm used to encrypt the data
    determine_encryption_algorithm(EncryptedData, Algorithm, IVSize),

    % There are now two tasks remaining, and they seem like they ought to be quite simple, but unfortunately they are not
    % First, we must determine the key used to encrypt the message
    determine_key(EncryptedData, Key, KeyCallback, Options),

    % Then, we must determine what the encrypted data even IS
    % If the message includes a CipherValue then this is straightfoward - the encrypted data is the base64-encoded child
    % of this element.
    (  memberchk(element(XENC:'CipherValue', _, CipherValueElement), CipherData)
    -> base64_element(CipherValueElement, CipherValueWithIV),
           string_codes(CipherValueWithIV, CipherValueWithIVCodes),
           length(IVCodes, IVSize),
           append(IVCodes, CipherCodes, CipherValueWithIVCodes),
           string_codes(IV, IVCodes),
           string_codes(CipherText, CipherCodes),
           length(CipherValueWithIVCodes, _),
           crypto_data_decrypt(CipherText, Algorithm, Key, IV, DecryptedStringWithPadding, [padding(none), encoding(octet)])
    ;  memberchk(element(XENC:'CipherReference', CipherReferenceAttributes, CipherReference), CipherData)->
           % However, it is allowed to include CipherReference instead. This is an arbitrary URI and a list of transforms to convert the
           % data identified by that URI into the raw octets that represent the encrypted data
           % The URI attribute of the CipherReference element is mandatory
           memberchk('URI'=CipherURI, CipherReferenceAttributes),
           % The transforms attribute is optional, though.
           (  memberchk(element('Transforms', _, Transforms), CipherReference)
           -> true
           ;  Transforms = []
           ),
           uri_components(CipherURI, uri_components(Scheme, _, _, _, _)),
           (  ( Scheme == 'http' ; Scheme == 'https')
              % FIXME: URI may not be an *absolute* URL
           ->  with_output_to(string(RawCipherValue),
                          setup_call_cleanup(http_open(CipherURI, HTTPStream, []),
                                             copy_stream_data(HTTPStream, current_output),
                                             close(HTTPStream)))
           ;  domain_error(resolvable_uri, CipherURI)
           ),
           apply_ciphertext_transforms(RawCipherValue, Transforms, CipherValue),
           sub_string(CipherValue, 0, IVSize, _, IV),
           sub_string(CipherValue, IVSize, _, 0, CipherText),
           crypto_data_decrypt(CipherText, Algorithm, Key, IV, DecryptedStringWithPadding, [padding(none), encoding(octet)])
    ),
    % The XML-ENC padding scheme does not comply with RFC-1423. This has been noted a few times by people trying to write
    % XML-ENC decryptors backed by OpenSSL, which insists on compliance. The only recourse we have is to disable padding entirely
    % and do it in our application
    xmlenc_padding(DecryptedStringWithPadding, DecryptedString),
    % Now that we have the decrypted data, we can decide whether to turn it into an element or leave it as
    % content
    (  Type == 'http://www.w3.org/2001/04/xmlenc#Element'
    -> setup_call_cleanup(open_string(DecryptedString, StringStream),
                          load_structure(StringStream, [Decrypted], [dialect(xmlns), keep_prefix(true)]),
                          close(StringStream))
    ;  Decrypted = DecryptedString
    ).

xmlenc_padding(DecryptedStringWithPadding, DecryptedString):-
    string_length(DecryptedStringWithPadding, _),
    string_codes(DecryptedStringWithPadding, Codes),
    append(_, [LastCode], Codes),
    length(Padding, LastCode),
    append(DecryptedCodes, Padding, Codes),
    !,
    string_codes(DecryptedString, DecryptedCodes).

apply_ciphertext_transforms(CipherValue, [], CipherValue):- !.
apply_ciphertext_transforms(_, [_AnythingElse|_], _):-
    % FIXME: Not implemented
    throw(error(implementation_missing('CipherReference transforms are not implemented', _))).

:- meta_predicate determine_key(+,-,3,+).
determine_key(EncryptedData, Key, KeyCallback, Options):-
    DS = ns(_, 'http://www.w3.org/2000/09/xmldsig#'),
    (  memberchk(element(DS:'KeyInfo', _, KeyInfo), EncryptedData)
    -> true
    ;  % Technically the KeyInfo is not mandatory. However, without a key we cannot decrypt
           % so raise an error. In the future Options could contain a key if it is agreed upon
           % by some other channel
           existence_error(key_info, EncryptedData)
    ),
    resolve_key(KeyInfo, Key, KeyCallback, Options).

:- meta_predicate resolve_key(+,-,3,+).

resolve_key(Info, Key, KeyCallback, Options):-
    % EncryptedKey
    XENC = 'http://www.w3.org/2001/04/xmlenc#',
    memberchk(element(ns(_, XENC):'EncryptedKey', _KeyAttributes, EncryptedKey), Info),
    !,
    % The EncryptedKey is slightly different to EncryptedData. For a start, the algorithms used to decrypt the
    % key are orthogonal to those used for EncryptedData. However we can recursively search for the keys then
    % decrypt them using the different algorithms as needed
    memberchk(element(ns(_, XENC):'EncryptionMethod', MethodAttributes, EncryptionMethod), EncryptedKey),
    memberchk('Algorithm'=Algorithm, MethodAttributes),

    % Now find the KeyInfo
    determine_key(EncryptedKey, PrivateKey, KeyCallback, Options),

    memberchk(element(ns(_, XENC):'CipherData', _, CipherData), EncryptedKey),
    memberchk(element(ns(_, XENC):'CipherValue', _, CipherValueElement), CipherData),
    base64_element(CipherValueElement, CipherValue),
    (  Algorithm == 'http://www.w3.org/2001/04/xmlenc#rsa-oaep-mgf1p'
    -> rsa_private_decrypt(PrivateKey, CipherValue, Key, [encoding(octet), padding(pkcs1_oaep)])
    ;  Algorithm == 'http://www.w3.org/2009/xmlenc11#rsa-oaep',
           memberchk(element(ns(_, 'http://www.w3.org/2009/xmlenc11#'):'MGF', MGFAttributes, _), EncryptionMethod),
           memberchk('Algorithm'='http://www.w3.org/2009/xmlenc11#mgf1sha1', MGFAttributes)   % This is just the same as rsa-oaep-mgf1p!
    -> rsa_private_decrypt(PrivateKey, CipherValue, Key, [encoding(octet), padding(pkcs1_oaep)])
    ;  Algorithm == 'http://www.w3.org/2001/04/xmlenc#rsa-1_5'
    -> rsa_private_decrypt(PrivateKey, CipherValue, Key, [encoding(octet), padding(pkcs1)])
    ;  domain_error(key_transport, Algorithm)
    ).
resolve_key(KeyInfo, _Key, _KeyCallback, _Options):-
    % AgreementMethod. FIXME: Not implemented
    XENC = ns(_, 'http://www.w3.org/2001/04/xmlenc#'),
    memberchk(element(XENC:'AgreementMethod', _KeyAttributes, _AgreementMethod), KeyInfo),
    !,
    throw(not_implemented).
% Additionally, we are allowed to use any elements from XML-DSIG
resolve_key(KeyInfo, Key, KeyCallback, _Options):-
    % KeyName. Use the callback with type=name and hint=KeyName
    DS = ns(_, 'http://www.w3.org/2000/09/xmldsig#'),
    memberchk(element(DS:'KeyName', _KeyAttributes, [KeyName]), KeyInfo),
    !,
    call(KeyCallback, name, KeyName, Key).
resolve_key(KeyInfo, _Key, _KeyCallback, _Options):-
    % RetrievalMethod. FIXME: Not implemented
    DS = ns(_, 'http://www.w3.org/2000/09/xmldsig#'),
    memberchk(element(DS:'RetrievalMethod', _KeyAttributes, _RetrievalMethod), KeyInfo),
    !,
    throw(not_implemented).
resolve_key(KeyInfo, Key, KeyCallback, _Options):-
    % KeyValue.
    DS = ns(_, 'http://www.w3.org/2000/09/xmldsig#'),
    memberchk(element(DS:'KeyValue', _KeyAttributes, KeyValue), KeyInfo),
    !,
    (  memberchk(element(DS:'RSAKeyValue', _, RSAKeyValue), KeyInfo)
    -> memberchk(element(DS:'Modulus', _, [ModulusBase64]), RSAKeyValue),
           memberchk(element(DS:'Exponent', _, [ExponentBase64]), RSAKeyValue),
           base64_to_hex(ModulusBase64, Modulus),
           base64_to_hex(ExponentBase64, Exponent),
           call(KeyCallback, public_key, public_key(rsa(Modulus, Exponent, -, -, -, -, -, -)), Key)
    ;  memberchk(element(DS:'DSAKeyValue', _, _DSAKeyValue), KeyInfo)
    -> throw(error(not_implemented(dsa_key), _)) % FIXME: Not implemented
    ;  existence_error(usable_key_value, KeyValue)
    ).
resolve_key(KeyInfo, Key, KeyCallback, _Options):-
    % X509Data.
    DS = ns(_, 'http://www.w3.org/2000/09/xmldsig#'),
    memberchk(element(DS:'X509Data', _, X509Data), KeyInfo),
    memberchk(element(DS:'X509Certificate', _, [X509Certificate]), X509Data),
    !,
    load_certificate_from_base64_string(X509Certificate, Certificate),
    call(KeyCallback, certificate, Certificate, Key).
resolve_key(KeyInfo, _Key, _KeyCallback, _Options):-
    % PGPData. FIXME: Not implemented
    DS = ns(_, 'http://www.w3.org/2000/09/xmldsig#'),
    memberchk(element(DS:'PGPData', _KeyAttributes, _PGPData), KeyInfo),
    !,
    throw(not_implemented).
resolve_key(KeyInfo, _Key, _KeyCallback, _Options):-
    % SPKIData. FIXME: Not implemented
    DS = ns(_, 'http://www.w3.org/2000/09/xmldsig#'),
    memberchk(element(DS:'SPKIData', _KeyAttributes, _SPKIData), KeyInfo),
    !,
    throw(not_implemented).
resolve_key(KeyInfo, _Key, _KeyCallback, _Options):-
    % MgmtData. FIXME: Not implemented
    DS = ns(_, 'http://www.w3.org/2000/09/xmldsig#'),
    memberchk(element(DS:'MgmtData', _KeyAttributes, _SPKIData), KeyInfo),
    !,
    throw(not_implemented).
resolve_key(Info, _, _, _):-
    % The XML-ENC standard allows for arbitrary other means of transmitting keys in application-specific
    % protocols. This is not supported here, though. In the future a callback could be provided in Options
    % to obtain the key information from a KeyInfo structure.
    existence_error(usable_key, Info).


base64_to_hex(Base64, Hex):-
    base64(Raw, Base64),
    atom_codes(Raw, Codes),
    hex_bytes(Hex0, Codes),
    string_upper(Hex0, Hex).


determine_encryption_algorithm(EncryptedData, Algorithm, IVSize):-
    XENC = ns(_, 'http://www.w3.org/2001/04/xmlenc#'),
    (  memberchk(element(XENC:'EncryptionMethod', EncryptionMethodAttributes, _), EncryptedData)
    -> % This is a mandatory attribute
           memberchk('Algorithm'=XMLAlgorithm, EncryptionMethodAttributes),
           (  ssl_algorithm(XMLAlgorithm, Algorithm, IVSize)
           -> true
           ; domain_error(block_cipher, XMLAlgorithm)
           )
        % In theory the EncryptionMethod is optional. In pracitse though, if the method is not supplied we
        % cannot decrypt the data. In the future we could support encryption_algorithm/1 as an option to
        % decrypt_element/3 but for now raise an exception
    ; existence_error(encryption_method, EncryptedData)
    ).

base64_element([CipherValueElement], CipherValue):-
    atom_codes(CipherValueElement, Base64Codes),
    delete_newlines(Base64Codes, TrimmedCodes),
    string_codes(Trimmed, TrimmedCodes),
    base64(CipherValue, Trimmed).

delete_newlines([], []):- !.
delete_newlines([13|As], B):- !, delete_newlines(As, B).
delete_newlines([10|As], B):- !, delete_newlines(As, B).
delete_newlines([A|As], [A|B]):- !, delete_newlines(As, B).



%!	load_certificate_from_base64_string(+String, -Certificate) is det.
%
%	Loads a certificate from a string, adding newlines and header
%       where appropriate so that OpenSSL 1.0.1+ will be able to parse it

load_certificate_from_base64_string(UnnormalizedData, Certificate):-
    normalize_space(codes(Codes), UnnormalizedData),
    % Break into 64-byte chunks
    chunk_certificate(Codes, Chunks),
    atomics_to_string(["-----BEGIN CERTIFICATE-----"|Chunks], '\n', CompleteCertificate),
    setup_call_cleanup(open_string(CompleteCertificate, StringStream),
                       load_certificate(StringStream, Certificate),
                       close(StringStream)).

chunk_certificate(Codes, [Chunk|Chunks]):-
    length(ChunkCodes, 64),
    append(ChunkCodes, Rest, Codes),
    !,
    string_codes(Chunk, ChunkCodes),
    chunk_certificate(Rest, Chunks).
chunk_certificate([], ["-----END CERTIFICATE-----\n"]):- !.
chunk_certificate(LastCodes, [LastChunk, "-----END CERTIFICATE-----\n"]):-
    string_codes(LastChunk, LastCodes).