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| |
| <div class=WordSection1> |
| |
| <p class=MsoTitle>Forward Lock Converter And Decoder</p> |
| |
| <p class=MsoToc1><span |
| class=MsoHyperlink><a href="#_Toc276471422">1<span style='font-size:12.0pt; |
| line-height:150%;color:windowtext;text-transform:none;font-weight:normal; |
| text-decoration:none'> </span>Introduction<span style='color:windowtext; |
| display:none;text-decoration:none'>. </span><span |
| style='color:windowtext;display:none;text-decoration:none'>3</span></a></span></p> |
| |
| <p class=MsoToc1><span class=MsoHyperlink><a href="#_Toc276471423">2<span |
| style='font-size:12.0pt;line-height:150%;color:windowtext;text-transform:none; |
| font-weight:normal;text-decoration:none'> </span>Overview<span |
| style='color:windowtext;display:none;text-decoration:none'>... </span><span |
| style='color:windowtext;display:none;text-decoration:none'>3</span></a></span></p> |
| |
| <p class=MsoToc1><span class=MsoHyperlink><a href="#_Toc276471424">3<span |
| style='font-size:12.0pt;line-height:150%;color:windowtext;text-transform:none; |
| font-weight:normal;text-decoration:none'> </span>Use Cases<span |
| style='color:windowtext;display:none;text-decoration:none'>. </span><span |
| style='color:windowtext;display:none;text-decoration:none'>4</span></a></span></p> |
| |
| <p class=MsoToc2><span class=MsoHyperlink><span style='font-variant:normal !important; |
| text-transform:uppercase'><a href="#_Toc276471425">3.1<span style='font-size: |
| 12.0pt;line-height:150%;color:windowtext;text-transform:none;text-decoration: |
| none'> </span>Converter<span style='color:windowtext;display:none; |
| text-decoration:none'>. </span><span |
| style='color:windowtext;display:none;text-decoration:none'>4</span></a></span></span></p> |
| |
| <p class=MsoToc3><span class=MsoHyperlink><a href="#_Toc276471426">3.1.1<span |
| style='font-size:12.0pt;line-height:150%;color:windowtext;font-style:normal; |
| text-decoration:none'> </span>Convert Data (Push-Mode Conversion)<span |
| style='color:windowtext;display:none;text-decoration:none'> </span><span |
| style='color:windowtext;display:none;text-decoration:none'>4</span></a></span></p> |
| |
| <p class=MsoToc3><span class=MsoHyperlink><a href="#_Toc276471427">3.1.2<span |
| style='font-size:12.0pt;line-height:150%;color:windowtext;font-style:normal; |
| text-decoration:none'> </span>Convert File (Pull-Mode Conversion)<span |
| style='color:windowtext;display:none;text-decoration:none'> </span><span |
| style='color:windowtext;display:none;text-decoration:none'>6</span></a></span></p> |
| |
| <p class=MsoToc2><span class=MsoHyperlink><span style='font-variant:normal !important; |
| text-transform:uppercase'><a href="#_Toc276471428">3.2<span style='font-size: |
| 12.0pt;line-height:150%;color:windowtext;text-transform:none;text-decoration: |
| none'> </span>Decoder<span style='color:windowtext;display:none; |
| text-decoration:none'>. </span><span |
| style='color:windowtext;display:none;text-decoration:none'>7</span></a></span></span></p> |
| |
| <p class=MsoToc3><span class=MsoHyperlink><a href="#_Toc276471429">3.2.1<span |
| style='font-size:12.0pt;line-height:150%;color:windowtext;font-style:normal; |
| text-decoration:none'> </span>Check Integrity<span style='color:windowtext; |
| display:none;text-decoration:none'>. </span><span |
| style='color:windowtext;display:none;text-decoration:none'>8</span></a></span></p> |
| |
| <p class=MsoToc3><span class=MsoHyperlink><a href="#_Toc276471430">3.2.2<span |
| style='font-size:12.0pt;line-height:150%;color:windowtext;font-style:normal; |
| text-decoration:none'> </span>Get Content Type<span style='color:windowtext; |
| display:none;text-decoration:none'>. </span><span |
| style='color:windowtext;display:none;text-decoration:none'>9</span></a></span></p> |
| |
| <p class=MsoToc3><span class=MsoHyperlink><a href="#_Toc276471431">3.2.3<span |
| style='font-size:12.0pt;line-height:150%;color:windowtext;font-style:normal; |
| text-decoration:none'> </span>Decode File<span style='color:windowtext; |
| display:none;text-decoration:none'>. </span><span |
| style='color:windowtext;display:none;text-decoration:none'>10</span></a></span></p> |
| |
| <p class=MsoToc1><span class=MsoHyperlink><a href="#_Toc276471432">4<span |
| style='font-size:12.0pt;line-height:150%;color:windowtext;text-transform:none; |
| font-weight:normal;text-decoration:none'> </span>Definition of the |
| Internal Forward Lock File Format<span style='color:windowtext;display:none; |
| text-decoration:none'>. </span><span |
| style='color:windowtext;display:none;text-decoration:none'>11</span></a></span></p> |
| |
| <p class=MsoToc2><span class=MsoHyperlink><span style='font-variant:normal !important; |
| text-transform:uppercase'><a href="#_Toc276471433">4.1<span style='font-size: |
| 12.0pt;line-height:150%;color:windowtext;text-transform:none;text-decoration: |
| none'> </span>Key Derivation<span style='color:windowtext;display:none; |
| text-decoration:none'>.. </span><span |
| style='color:windowtext;display:none;text-decoration:none'>11</span></a></span></span></p> |
| |
| <p class=MsoToc2><span class=MsoHyperlink><span style='font-variant:normal !important; |
| text-transform:uppercase'><a href="#_Toc276471434">4.2<span style='font-size: |
| 12.0pt;line-height:150%;color:windowtext;text-transform:none;text-decoration: |
| none'> </span>Calculation of the Counters<span style='color:windowtext; |
| display:none;text-decoration:none'>. </span><span |
| style='color:windowtext;display:none;text-decoration:none'>12</span></a></span></span></p> |
| |
| <p class=MsoToc1><span class=MsoHyperlink><a href="#_Toc276471435">5<span |
| style='font-size:12.0pt;line-height:150%;color:windowtext;text-transform:none; |
| font-weight:normal;text-decoration:none'> </span>Unit Test Cases<span |
| style='color:windowtext;display:none;text-decoration:none'>. </span><span |
| style='color:windowtext;display:none;text-decoration:none'>12</span></a></span></p> |
| |
| <p class=MsoToc1><span class=MsoHyperlink><a href="#_Toc276471436">6<span |
| style='font-size:12.0pt;line-height:150%;color:windowtext;text-transform:none; |
| font-weight:normal;text-decoration:none'> </span>References<span |
| style='color:windowtext;display:none;text-decoration:none'>. </span><span |
| style='color:windowtext;display:none;text-decoration:none'>12</span></a></span></p> |
| |
| <p class=MsoBodyText></p> |
| |
| </div> |
| |
| <span style='font-size:12.0pt;font-family:"Times New Roman","serif"'><br |
| clear=all style='page-break-before:right'> |
| </span> |
| |
| <div class=WordSection2> |
| |
| <h1><a name="_Toc276471422"></a><a name="_Ref263085474">1<span |
| style='font:7.0pt "Times New Roman"'> </span>Introduction</a></h1> |
| |
| <p class=MsoBodyText>The internal Forward Lock file format is used for encrypting |
| inherently unencrypted OMA DRM version 1 Forward Lock and Combined Delivery |
| files so they can be securely stored on externally accessible file system partitions |
| such as memory stick.</p> |
| |
| <p class=MsoBodyText>Our general strategy is to convert such <i>OMA DRM Message</i> |
| (.dm) files to internal Forward Lock (.fl) files as soon as they are |
| downloaded or otherwise transferred to the phone, and not actually provide any |
| decoders for .dm files.</p> |
| |
| <h1><a name="_Toc276471423">2<span style='font:7.0pt "Times New Roman"'> |
| </span>Overview</a></h1> |
| |
| <p class=MsoBodyText>The <i>Forward Lock Converter</i> converts OMA DRM Message |
| files to the internal file format. The <i>Forward Lock Decoder</i> provides a |
| POSIX-level API for transparent reading and seeking through such a converted |
| file as if it were unencrypted. The API also includes functions for checking a |
| files integrity and getting the MIME type of its embedded content.</p> |
| |
| <p class=MsoBodyText style='margin-bottom:24.0pt'>The converter and decoder are |
| built into two separate libraries, which share common code for random number |
| generation and key encryption in a third library. For test purposes there is |
| also a unit test application. See Figure 1.</p> |
| |
| <p class=MsoBodyText style='page-break-after:avoid'><img width=288 height=364 |
| src="images/image001.gif"></p> |
| |
| <p class=MsoCaption style='margin-top:12.0pt;margin-right:0cm;margin-bottom: |
| 12.0pt;margin-left:0cm'><a name="_Ref262730885">Figure </a>1. Block diagram illustrating the dependencies between the executable modules.</p> |
| |
| <b><span style='font-size:16.0pt;font-family:"Arial","sans-serif"'><br |
| clear=all style='page-break-before:always'> |
| </span></b> |
| |
| <h1><a name="_Toc276471424">3<span style='font:7.0pt "Times New Roman"'> |
| </span>Use Cases</a></h1> |
| |
| <p class=MsoBodyText>This section describes all the use cases for the converter |
| and decoder. It shows the sequence of API calls that should be used to solve |
| these use cases.</p> |
| |
| <h2><a name="_Toc276471425">3.1<span style='font:7.0pt "Times New Roman"'> |
| </span>Converter</a></h2> |
| |
| <p class=MsoBodyText>Through the converter API, conversion can be performed in one |
| of two ways:</p> |
| |
| <p class=MsoBodyText style='margin-left:36.0pt;text-indent:-18.0pt'>1.<span |
| style='font:7.0pt "Times New Roman"'> </span><i>Push-mode |
| conversion</i> is when the client progressively feeds data to the converter as |
| it arrives. This is appropriate when data arrives gradually in chunks, with |
| idle time in between. Consequently, push mode is used for converting files |
| being downloaded through HTTP. See section 3.1.1.</p> |
| |
| <p class=MsoBodyText style='margin-left:36.0pt;text-indent:-18.0pt'>2.<span |
| style='font:7.0pt "Times New Roman"'> </span><i>Pull-mode |
| conversion</i> is when the converter drives the process and consumes data from |
| the client as it needs it. This is appropriate when the entire file to be |
| converted is readily available. Hence, pull mode is used by the unit test application. |
| See section 3.1.2.</p> |
| |
| <p class=MsoBodyText>Internally, pull-mode conversion is implemented in terms |
| of the API for push-mode conversion.</p> |
| |
| <h3><a name="_Toc276471426"></a><a name="_Ref263085478">3.1.1<span |
| style='font:7.0pt "Times New Roman"'> </span>Convert Data |
| (Push-Mode Conversion)</a></h3> |
| |
| <p class=MsoBodyText>Push-mode conversion is performed as follows (see also Figure 2):</p> |
| |
| <p class=MsoBodyText style='margin-left:36.0pt;text-indent:-18.0pt'>1.<span |
| style='font:7.0pt "Times New Roman"'> </span><span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockConv_OpenSession</span> |
| initializes the output parameter and returns a <i>session ID</i> to be used in |
| subsequent calls to the API. The output parameter is a union of return values |
| whose correct use at any given moment is determined by the API function last |
| called.</p> |
| |
| <p class=MsoBodyText style='margin-left:36.0pt;text-indent:-18.0pt'>2.<span |
| style='font:7.0pt "Times New Roman"'> </span><span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockConv_ConvertData</span> |
| is called repeatedly until no more input data remains. Each call converts the |
| maximum amount of data possible and writes it to the output buffer. The client then |
| writes this data to file.</p> |
| |
| <p class=MsoBodyText style='margin-left:36.0pt;text-indent:-18.0pt'>3.<span |
| style='font:7.0pt "Times New Roman"'> </span><span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockConv_CloseSession</span> |
| cleans up the session and deallocates the output buffer. If all has gone well, a |
| two-part cryptographic signature of the output file is calculated. The client |
| must go back and rewrite part of the file header with this updated signature |
| information.</p> |
| |
| <p class=MsoBodyText>Every time a file is being converted, the converter calls <span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockGlue_GetRandomNumber</span> |
| to generate a new, unique session key. No two converted files look alike, even |
| if the original files are the same.</p> |
| |
| <p class=MsoBodyText><b>Note:</b> The random bytes cannot come from any bare-minimum |
| implementation of the C-library <span style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>rand</span> |
| functionthey must be cryptographically secure. Otherwise, security will be |
| compromised.</p> |
| |
| <p class=MsoBodyText>The session key is encrypted and stored within the |
| converted file. Key encryption is performed using <span style='font-size:10.0pt; |
| font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockGlue_GetEncryptedKeyLength</span> and <span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockGlue_EncryptKey</span>. |
| These two functions, together with the corresponding decryption function (<span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockGlue_DecryptKey</span>), |
| are the integration points where an OEM manufacturer may implement their own |
| key-encryption scheme.</p> |
| |
| <p class=MsoBodyText><b>Note:</b> The key-encryption key must be unique to each |
| device; this is what makes the files forward lockprotected. Ideally, it should |
| be derived from secret hardware parameters, but at the very least it should be |
| persistent from one master reset to the next.</p> |
| |
| <div style='margin-bottom:24.0pt;border:solid windowtext 1.0pt;padding:1.0pt 4.0pt 1.0pt 4.0pt; |
| background:#F2F2F2'> |
| |
| <p class=MsoBodyText style='background:#F2F2F2;border: |
| none;padding:0cm'><b>Note:</b> In the open-source implementation of the <span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>libfwdlock-common</span> |
| library, a random key-encryption key is generated and stored in plaintext in |
| the file system, without being obfuscated in any way (doing so would be futile |
| since the source code is openly available). This key must be kept secret from |
| the user, and shouldnt be possible to extract through backup-and-restore |
| functionality or the like. OEM manufacturers will probably want to implement a |
| truly hardware-based device-unique key.</p> |
| |
| </div> |
| |
| <p class=MsoBodyText style='page-break-after:avoid'><img width=531 height=563 |
| src="images/image002.gif"></p> |
| |
| <p class=MsoCaption style='margin-top:6.0pt;margin-right:0cm;margin-bottom: |
| 12.0pt;margin-left:0cm'><a name="_Ref263085187">Figure </a>2. Converter UC: Convert Data.</p> |
| |
| <b><span style='font-size:13.0pt;font-family:"Arial","sans-serif"'><br |
| clear=all style='page-break-before:always'> |
| </span></b> |
| |
| <h3><a name="_Toc276471427"></a><a name="_Ref263163082">3.1.2<span |
| style='font:7.0pt "Times New Roman"'> </span>Convert File |
| (Pull-Mode Conversion)</a></h3> |
| |
| <p class=MsoBodyText>Pull-mode conversion is performed by calling <span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_ConvertFile</span> |
| with the filename, unless there is need for a specialized <span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>read</span> function, in |
| which case <span style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_ConvertOpenFile</span> |
| should be used directly instead. See Figure 3.</p> |
| |
| <p class=MsoBodyText style='margin-bottom:24.0pt'>Internally, <span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_ConvertFile</span> |
| calls <span style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_ConvertOpenFile</span>. |
| The latter then proceeds with the conversion using the push-mode API, acting as |
| the client in the previous use case; see section 3.1.1.</p> |
| |
| <p class=MsoBodyText style='page-break-after:avoid'><img width=531 height=731 |
| src="images/image003.gif"></p> |
| |
| <p class=MsoCaption style='margin-top:6.0pt;margin-right:0cm;margin-bottom: |
| 12.0pt;margin-left:0cm'><a name="_Ref263085208">Figure </a>3. Converter UC: Convert File.</p> |
| |
| <b><i><span style='font-size:14.0pt;font-family:"Arial","sans-serif"'><br |
| clear=all style='page-break-before:always'> |
| </span></i></b> |
| |
| <h2><a name="_Toc276471428">3.2<span style='font:7.0pt "Times New Roman"'> |
| </span>Decoder</a></h2> |
| |
| <p class=MsoBodyText>The decoder API allows the client to do the following:</p> |
| |
| <p class=MsoBodyText style='margin-left:36.0pt;text-indent:-18.0pt'>1.<span |
| style='font:7.0pt "Times New Roman"'> </span>Check |
| the integrity of an internal Forward Lock file, i.e., detect whether it has |
| been manipulated in any way; see section 3.2.1.</p> |
| |
| <p class=MsoBodyText style='margin-left:36.0pt;text-indent:-18.0pt'>2.<span |
| style='font:7.0pt "Times New Roman"'> </span>Get |
| the MIME type of the embedded content (the original MIME type before DRM protection |
| was applied); see section 3.2.2.</p> |
| |
| <p class=MsoBodyText style='margin-left:36.0pt;text-indent:-18.0pt'>3.<span |
| style='font:7.0pt "Times New Roman"'> </span>Decode |
| the file by random access, i.e., read and seek through it in an arbitrary |
| manner; see section 3.2.3.</p> |
| |
| <p class=MsoBodyText>All subsequent operations on a file first require it to be |
| opened. Opening a file returns a <i>file descriptor</i>a handle to be used in |
| these subsequent operations.</p> |
| |
| <p class=MsoBodyText>If the filename is known, an internal Forward Lock file |
| can be opened using <span style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_open</span>. |
| If only the file descriptor of an already open file is available, a decoding |
| session can instead be initialized using <span style='font-size:10.0pt; |
| font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_attach</span>.</p> |
| |
| <p class=MsoBodyText>Internally, <span style='font-size:10.0pt;font-family: |
| "Lucida Console","DejaVu Sans Mono"'>FwdLockFile_open</span> calls <span style='font-size:10.0pt; |
| font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_attach</span>. For efficiency |
| reasons, <span style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_attach</span> |
| therefore assumes that the file position is at the beginning of the file when |
| the function gets called. A client who calls it directly must make sure that |
| this assumption holds.</p> |
| |
| <p class=MsoBodyText>When a file is being attached, the session key stored in |
| the file during conversion is decrypted using <span style='font-size:10.0pt; |
| font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockGlue_GetEncryptedKeyLength</span> and <span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockGlue_DecryptKey</span>, |
| in order to set up for decoding and integrity checking.</p> |
| |
| <p class=MsoBodyText>For just getting the content type, however, retrieving the |
| session key would strictly speaking not be necessary, so there is an |
| opportunity here to optimize for that if it proves necessary later.</p> |
| |
| <p class=MsoBodyText>Symmetrical to <span style='font-size:10.0pt;font-family: |
| "Lucida Console","DejaVu Sans Mono"'>FwdLockFile_open</span> and <span style='font-size:10.0pt; |
| font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_attach</span>, there are also functions |
| for closing a file or detaching from it:</p> |
| |
| <p class=MsoBodyText style='margin-left:36.0pt;text-indent:-18.0pt'>1.<span |
| style='font:7.0pt "Times New Roman"'> </span>If |
| it was opened with <span style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_open</span> |
| it should be closed with <span style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_close</span>.</p> |
| |
| <p class=MsoBodyText style='margin-left:36.0pt;text-indent:-18.0pt'>2.<span |
| style='font:7.0pt "Times New Roman"'> </span>If |
| it was attached with <span style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_attach</span> |
| it should be detached with <span style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_detach</span>.</p> |
| |
| <b><span style='font-size:13.0pt;font-family:"Arial","sans-serif"'><br |
| clear=all style='page-break-before:always'> |
| </span></b> |
| |
| <h3><a name="_Ref263163099"></a><a name="_Toc276471429">3.2.1<span |
| style='font:7.0pt "Times New Roman"'> </span>Check Integrity</a></h3> |
| |
| <p class=MsoBodyText>There are three methods for checking the integrity of an |
| internal Forward Lock file, in whole or in part (see also Figure 4):</p> |
| |
| <p class=MsoBodyText style='margin-left:36.0pt;text-indent:-18.0pt'>1.<span |
| style='font:7.0pt "Times New Roman"'> </span><span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_CheckDataIntegrity</span>, |
| which checks the integrity of the encrypted content data.</p> |
| |
| <p class=MsoBodyText style='margin-left:36.0pt;text-indent:-18.0pt'>2.<span |
| style='font:7.0pt "Times New Roman"'> </span><span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_CheckHeaderIntegrity</span>, |
| which checks the integrity of the file header, including the content type and |
| other fields not currently supported but reserved for future use.</p> |
| |
| <p class=MsoBodyText style='margin-left:36.0pt;text-indent:-18.0pt'>3.<span |
| style='font:7.0pt "Times New Roman"'> </span><span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_CheckIntegrity</span>, |
| which internally calls first <span style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_CheckHeaderIntegrity</span> |
| and then <span style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_CheckDataIntegrity</span>.</p> |
| |
| <p class=MsoBodyText style='margin-bottom:24.0pt'><span style='font-size:10.0pt; |
| font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_CheckHeaderIntegrity</span> is |
| generally much faster than <span style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_CheckDataIntegrity</span>, |
| whose running time is directly proportional to the size of the file.</p> |
| |
| <p class=MsoBodyText style='page-break-after:avoid'><img width=543 height=575 |
| src="images/image004.gif"></p> |
| |
| <p class=MsoCaption style='margin-top:6.0pt;margin-right:0cm;margin-bottom: |
| 12.0pt;margin-left:0cm'><a name="_Ref263163308">Figure </a>4. Decoder UC: Check Integrity.</p> |
| |
| <b><span style='font-size:13.0pt;font-family:"Arial","sans-serif"'><br |
| clear=all style='page-break-before:always'> |
| </span></b> |
| |
| <h3><a name="_Toc276471430"></a><a name="_Ref263163117">3.2.2<span |
| style='font:7.0pt "Times New Roman"'> </span>Get Content Type</a></h3> |
| |
| <p class=MsoBodyText style='margin-bottom:24.0pt'><span style='font-size:10.0pt; |
| font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_GetContentType</span> returns a |
| read-only reference to an ASCII string containing the MIME type of the |
| embedded content. This reference is valid as long as the file is kept open. |
| Clients who need access to the content type after closing the file should make |
| a copy of the string. See Figure 5 below.</p> |
| |
| <p class=MsoBodyText style='page-break-after:avoid'><img width=543 height=488 |
| src="images/image005.gif"></p> |
| |
| <p class=MsoCaption style='margin-top:6.0pt;margin-right:0cm;margin-bottom: |
| 12.0pt;margin-left:0cm'><a name="_Ref263163392">Figure </a>5. Decoder UC: Get Content Type.</p> |
| |
| <b><span style='font-size:13.0pt;font-family:"Arial","sans-serif"'><br |
| clear=all style='page-break-before:always'> |
| </span></b> |
| |
| <h3><a name="_Toc276471431"></a><a name="_Ref263163137">3.2.3<span |
| style='font:7.0pt "Times New Roman"'> </span>Decode File</a></h3> |
| |
| <p class=MsoBodyText>After opening an internal Forward Lock file (or attaching |
| to an already open one), it can be transparently read from as if it were |
| unencrypted. Any number of calls to read data from the current file position or |
| set it to a new one (which is what <span style='font-size:10.0pt;font-family: |
| "Lucida Console","DejaVu Sans Mono"'>lseek</span> does) can be made in any order; this is what we |
| call <i>random access</i>. See Figure 6.</p> |
| |
| <p class=MsoBodyText>The Forward Lock Decoder versions of the <span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>read</span>, <span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>lseek</span>, and <span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>close</span> functions |
| have the exact same signatures as their POSIX counterparts. So, for example, |
| the call <span style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_lseek(fd, |
| 0, SEEK_END)</span> returns the size of the embedded content data, i.e., the |
| size of the original file before DRM protection.</p> |
| |
| <p class=MsoBodyText style='margin-bottom:24.0pt'>Moreover, <span |
| style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>FwdLockFile_open</span> |
| is like regular POSIX <span style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'>open</span> |
| except it takes only the filename as a parameteraccess is always read-only.</p> |
| |
| <p class=MsoBodyText style='page-break-after:avoid'><img width=543 height=522 |
| src="images/image006.gif"></p> |
| |
| <p class=MsoCaption style='margin-top:6.0pt;margin-right:0cm;margin-bottom: |
| 12.0pt;margin-left:0cm'><a name="_Ref263166303">Figure </a>6. Decoder UC: Decode File.</p> |
| |
| <b><span style='font-size:16.0pt;font-family:"Arial","sans-serif"'><br |
| clear=all style='page-break-before:always'> |
| </span></b> |
| |
| <h1><a name="_Toc276471432">4<span style='font:7.0pt "Times New Roman"'> |
| </span>Definition of the Internal Forward Lock File Format</a></h1> |
| |
| <p class=MsoBodyText style='margin-bottom:12.0pt'>The inner structure of an internal |
| Forward Lock file is defined in Table 1 below.</p> |
| |
| <table class=MsoNormalTable border=1 cellspacing=0 cellpadding=0 |
| style='border-collapse:collapse;border:none'> |
| <tr> |
| <td width=111 valign=top style='width:83.4pt;border:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'><b>Offset [bytes]</b></p> |
| </td> |
| <td width=96 valign=top style='width:72.0pt;border:solid windowtext 1.0pt; |
| border-left:none;padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'><b>Size [bytes]</b></p> |
| </td> |
| <td width=361 valign=top style='width:270.85pt;border:solid windowtext 1.0pt; |
| border-left:none;padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'><b>Description</b></p> |
| </td> |
| </tr> |
| <tr> |
| <td width=111 valign=top style='width:83.4pt;border:solid windowtext 1.0pt; |
| border-top:none;padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>0</p> |
| </td> |
| <td width=96 valign=top style='width:72.0pt;border-top:none;border-left:none; |
| border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>4</p> |
| </td> |
| <td width=361 valign=top style='width:270.85pt;border-top:none;border-left: |
| none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>The file signature (so-called |
| <i>magic number</i>): a four-character code consisting of the letters |
| F-W-L-K.</p> |
| </td> |
| </tr> |
| <tr> |
| <td width=111 valign=top style='width:83.4pt;border:solid windowtext 1.0pt; |
| border-top:none;padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>4</p> |
| </td> |
| <td width=96 valign=top style='width:72.0pt;border-top:none;border-left:none; |
| border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>1</p> |
| </td> |
| <td width=361 valign=top style='width:270.85pt;border-top:none;border-left: |
| none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>Version number (0 for the |
| first version).</p> |
| </td> |
| </tr> |
| <tr> |
| <td width=111 valign=top style='width:83.4pt;border:solid windowtext 1.0pt; |
| border-top:none;padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>5</p> |
| </td> |
| <td width=96 valign=top style='width:72.0pt;border-top:none;border-left:none; |
| border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>1</p> |
| </td> |
| <td width=361 valign=top style='width:270.85pt;border-top:none;border-left: |
| none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>Indicates the subformat:</p> |
| <p class=MsoNormal style='page-break-after:avoid'><i>0x00 Forward Lock</i></p> |
| <p class=MsoNormal style='page-break-after:avoid'><i>0x01 Combined Delivery</i></p> |
| </td> |
| </tr> |
| <tr> |
| <td width=111 valign=top style='width:83.4pt;border:solid windowtext 1.0pt; |
| border-top:none;padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>6</p> |
| </td> |
| <td width=96 valign=top style='width:72.0pt;border-top:none;border-left:none; |
| border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>1</p> |
| </td> |
| <td width=361 valign=top style='width:270.85pt;border-top:none;border-left: |
| none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>Usage restriction flags (prohibitions |
| against usage as ringtone or as wallpaper and screen saver). Also indicates |
| if the file is bound to a specific SIM card.</p> |
| <p class=MsoNormal style='page-break-after:avoid'><i>0x00 No usage |
| restrictions</i></p> |
| <p class=MsoNormal style='page-break-after:avoid'><i>0x01 Ringtone usage |
| prohibited</i></p> |
| <p class=MsoNormal style='page-break-after:avoid'><i>0x02 Screen usage |
| prohibited</i></p> |
| <p class=MsoNormal style='page-break-after:avoid'><i>0x80 Bound to SIM</i></p> |
| <p class=MsoNormal style='page-break-after:avoid'>(Any number of these may be |
| OR-ed together.)</p> |
| </td> |
| </tr> |
| <tr> |
| <td width=111 valign=top style='width:83.4pt;border:solid windowtext 1.0pt; |
| border-top:none;padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>7</p> |
| </td> |
| <td width=96 valign=top style='width:72.0pt;border-top:none;border-left:none; |
| border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>1</p> |
| </td> |
| <td width=361 valign=top style='width:270.85pt;border-top:none;border-left: |
| none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>Length of the MIME content |
| type (<i>k</i>).</p> |
| </td> |
| </tr> |
| <tr> |
| <td width=111 valign=top style='width:83.4pt;border:solid windowtext 1.0pt; |
| border-top:none;padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>8</p> |
| </td> |
| <td width=96 valign=top style='width:72.0pt;border-top:none;border-left:none; |
| border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'><i>k</i></p> |
| </td> |
| <td width=361 valign=top style='width:270.85pt;border-top:none;border-left: |
| none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>The MIME content type |
| (ASCII-encoded without null-character termination).</p> |
| </td> |
| </tr> |
| <tr> |
| <td width=111 valign=top style='width:83.4pt;border:solid windowtext 1.0pt; |
| border-top:none;padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>8+<i>k</i></p> |
| </td> |
| <td width=96 valign=top style='width:72.0pt;border-top:none;border-left:none; |
| border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'><i>l </i>= 0 or 16</p> |
| </td> |
| <td width=361 valign=top style='width:270.85pt;border-top:none;border-left: |
| none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>If the subformat is |
| Combined Delivery, this field contains the auto-generated content ID (16 bytes). |
| If not, this field is zero-size.</p> |
| </td> |
| </tr> |
| <tr> |
| <td width=111 valign=top style='width:83.4pt;border:solid windowtext 1.0pt; |
| border-top:none;padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>8+<i>k</i>+<i>l</i></p> |
| </td> |
| <td width=96 valign=top style='width:72.0pt;border-top:none;border-left:none; |
| border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'><i>m </i>= 0 or 9</p> |
| </td> |
| <td width=361 valign=top style='width:270.85pt;border-top:none;border-left: |
| none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>If the file is bound to a |
| specific SIM card, this field contains the 9-byte packed IMSI number. If not, |
| this field is zero-size.</p> |
| </td> |
| </tr> |
| <tr> |
| <td width=111 valign=top style='width:83.4pt;border:solid windowtext 1.0pt; |
| border-top:none;padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>8+<i>k</i>+<i>l</i>+<i>m</i></p> |
| </td> |
| <td width=96 valign=top style='width:72.0pt;border-top:none;border-left:none; |
| border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'><i>n</i> ≥ 16</p> |
| </td> |
| <td width=361 valign=top style='width:270.85pt;border-top:none;border-left: |
| none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>The encrypted session key, the |
| first sixteen bytes of which are also used as the CTR-mode <i>nonce</i> (similar |
| to the CBC-mode <i>initialization vector</i>).</p> |
| </td> |
| </tr> |
| <tr> |
| <td width=111 valign=top style='width:83.4pt;border:solid windowtext 1.0pt; |
| border-top:none;padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>8+<i>k</i>+<i>l</i>+<i>m</i>+<i>n</i></p> |
| </td> |
| <td width=96 valign=top style='width:72.0pt;border-top:none;border-left:none; |
| border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>20</p> |
| </td> |
| <td width=361 valign=top style='width:270.85pt;border-top:none;border-left: |
| none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>Data signaturethe SHA-1 |
| HMAC of the encrypted content data.</p> |
| </td> |
| </tr> |
| <tr> |
| <td width=111 valign=top style='width:83.4pt;border:solid windowtext 1.0pt; |
| border-top:none;padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>28+<i>k</i>+<i>l</i>+<i>m</i>+<i>n</i></p> |
| </td> |
| <td width=96 valign=top style='width:72.0pt;border-top:none;border-left:none; |
| border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>20</p> |
| </td> |
| <td width=361 valign=top style='width:270.85pt;border-top:none;border-left: |
| none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>Header signaturethe SHA-1 |
| HMAC of all the fields above, including the encrypted session key and data |
| signature.</p> |
| </td> |
| </tr> |
| <tr> |
| <td width=111 valign=top style='width:83.4pt;border:solid windowtext 1.0pt; |
| border-top:none;padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>48+<i>k</i>+<i>l</i>+<i>m</i>+<i>n</i></p> |
| </td> |
| <td width=96 valign=top style='width:72.0pt;border-top:none;border-left:none; |
| border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'><i><to the end of the |
| file></i></p> |
| </td> |
| <td width=361 valign=top style='width:270.85pt;border-top:none;border-left: |
| none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt; |
| padding:0cm 5.4pt 0cm 5.4pt'> |
| <p class=MsoNormal style='page-break-after:avoid'>The content data encrypted |
| using 128-bit AES in CTR mode.</p> |
| </td> |
| </tr> |
| </table> |
| |
| <p class=MsoCaption style='margin-top:6.0pt;margin-right:0cm;margin-bottom: |
| 12.0pt;margin-left:0cm;page-break-after:avoid'><a name="_Ref151269206">Table </a>1. Definition of the fields of an internal Forward Lock file.</p> |
| |
| <p class=MsoBodyText>As of now, neither Combined Delivery nor usage |
| restrictions (including SIM binding) are supported. These fields are reserved |
| for future use.</p> |
| |
| <h2><a name="_Toc276471433">4.1<span style='font:7.0pt "Times New Roman"'> |
| </span>Key Derivation</a></h2> |
| |
| <p class=MsoBodyText>The session key consists of sixteen bytes fetched from a |
| cryptographically secure random number generator. From the session key, two |
| separate keys are derived: one used for encryption, the other for signing.</p> |
| |
| <p class=MsoBodyText>The encryption key is the output from encrypting the |
| 16-byte all-zero input block {0, 0,
, 0} using 128-bit AES with the random session |
| key as the key. The signing key is the output from encrypting the 16-byte input |
| block {1, 0,
, 0} the same way. The keys so derived will be cryptographically |
| independent from each other.</p> |
| |
| <p class=MsoBodyText>The session key is encrypted using a hardware-dependent |
| key-encryption key unique to each device. The encrypted session key is stored |
| inside the file, and its first sixteen bytes are also used as the <i>nonce</i> |
| for the CTR-mode encryption of the content data.</p> |
| |
| <h2><a name="_Toc276471434">4.2<span style='font:7.0pt "Times New Roman"'> |
| </span>Calculation of the Counters</a></h2> |
| |
| <p class=MsoBodyText>Using CTR (counter) mode, a block cipher such as AES can |
| be turned into a stream cipher. The process of encryption and decryption is |
| well defined in [1], except for the specifics of the calculation of the |
| counters. For the internal Forward Lock file format, the counters are |
| calculated as follows:</p> |
| |
| <p class=MsoBodyText style='margin-left:36.0pt;text-indent:-18.0pt'>1.<span |
| style='font:7.0pt "Times New Roman"'> </span>The |
| nonce is interpreted as a 128-bit unsigned integer in little-endian format.</p> |
| |
| <p class=MsoBodyText style='margin-left:36.0pt;text-indent:-18.0pt'>2.<span |
| style='font:7.0pt "Times New Roman"'> </span>The |
| zero-based block sequence number (also a little-endian unsigned integer) is |
| added modulo 2<sup>128</sup> to the nonce to produce the counter for a given |
| block.</p> |
| |
| <h1><a name="_Toc276471435">5<span style='font:7.0pt "Times New Roman"'> |
| </span>Unit Test Cases</a></h1> |
| |
| <p class=MsoBodyText>Unit test cases for the converter and decoder come in two |
| varieties:</p> |
| |
| <p class=MsoBodyText style='margin-left:36.0pt;text-indent:-18.0pt'>1.<span |
| style='font:7.0pt "Times New Roman"'> </span><i>Black-box</i> |
| test cases aim to verify that you get sensible results from malformed or |
| tricky input data.</p> |
| |
| <p class=MsoBodyText style='margin-left:36.0pt;text-indent:-18.0pt'>2.<span |
| style='font:7.0pt "Times New Roman"'> </span><i>White-box</i> |
| test cases aim to maximize code coverage using knowledge of code internals.</p> |
| |
| <p class=MsoBodyText>The black-box test cases are dependent on a specifically |
| designed set of input files found in the <span style='font-size:10.0pt; |
| font-family:"Lucida Console","DejaVu Sans Mono"'>forward-lock/internal-format/test/res</span> |
| directory in the repository. For tests variants of the software, these input |
| files will be automatically installed in the file system image during build.</p> |
| |
| <p class=MsoBodyText>Run the test cases from the ADB shell command line as |
| follows:</p> |
| |
| <p class=MsoNormal style='margin-top:0cm;margin-right:0cm;margin-bottom:6.0pt; |
| margin-left:21.55pt'><span style='font-size:10.0pt;font-family:"Lucida Console","DejaVu Sans Mono"'># |
| gtest_fwdlock</span></p> |
| |
| <p class=MsoBodyText>If all black-box but no white-box test cases fail, the |
| input files probably cant be found in the working directory.</p> |
| |
| <h1><a name="_Toc276471436">6<span style='font:7.0pt "Times New Roman"'> |
| </span>References</a></h1> |
| |
| <p class=MsoBodyText style='margin-left:28.9pt;text-indent:-28.9pt'>[1]<span |
| style='font:7.0pt "Times New Roman"'> |
| </span><a |
| href="http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf">Dworkin, |
| Morris: Recommendation for Block Cipher Modes of OperationMethods and |
| Techniques, NIST Special Publication 800-38A, December 2001.</a><a |
| name="_Ref151269073"></a></p> |
| |
| </div> |
| |
| </body> |
| |
| </html> |
