The blog post explores methods to extract content from an LCP-protected ePub file, primarily for archiving or personal use. It details the challenges posed by LCP's encryption and license validation, and walks through a technical process involving inspecting the ePub's structure, locating the encrypted content, and ultimately decrypting it using the user's own credentials and a modified version of Adobe's Digital Editions library. The author emphasizes this is for educational purposes only and discourages any copyright infringement. While acknowledging potential legal and ethical concerns, the post frames the process as a way to reclaim control over purchased digital content and ensure future accessibility.
This blog post by Keith Packard details his exploration into decrypting an ePub protected by the Readium LCP (Library Content Protection) Digital Rights Management (DRM) scheme. He begins by outlining his motivations, driven by the desire to access his legally purchased ebooks on any device without vendor lock-in, emphasizing the importance of interoperability and the right to personal use of purchased digital content.
Packard meticulously describes his step-by-step process. He initiates his investigation by examining the structure of the protected ePub, which is essentially a ZIP archive. He notes the presence of an META-INF/license.lcpl file, indicative of LCP protection, and a .lcpenc file extension on the encrypted XHTML content files within the ePub. He then delves into analyzing the license.lcpl file, which is an XML document containing the license information and a crucial encrypted key. He identifies the encryption algorithm used as AES-128-CBC and highlights the presence of a user key, content key, and a hint to derive the latter from the former.
The post goes on to describe how Packard reverse-engineered the decryption process. He leverages the open-source Readium LCP client code, specifically focusing on a JavaScript implementation. By carefully examining the JavaScript functions, particularly those involved in key derivation and decryption, he manages to understand the necessary cryptographic operations. He explains his understanding of how the user key, obtained from the license acquisition process, is used to derive the content key via a key derivation function (KDF). He further explains how this content key is subsequently used to decrypt the individual XHTML files comprising the ePub’s content.
While acknowledging successful decryption of the content key, Packard emphasizes that his work is not yet complete. The post concludes by stating that he has yet to implement the actual decryption of the XHTML content files, leaving this as a future step. He also mentions the need to understand how to acquire the user key legitimately through a proper LCP license server interaction, which he has not yet explored in this particular post. He expresses optimism about the feasibility of achieving full decryption and emphasizes his commitment to documenting his findings for educational purposes and to advocate for open access to legally purchased content.
Summary of Comments ( 3 )
https://news.ycombinator.com/item?id=43378627
HN commenters generally express skepticism towards the robustness of LCP "protection," viewing it as a minor speedbump rather than a genuine barrier. Several point out that determined users can always access content through methods like disabling JavaScript or using developer tools. One commenter mentions DeDRM tools as an existing solution for bypassing such restrictions, while others suggest that the real protection lies in social pressure and legal consequences, not technical measures. The feasibility of converting ePubs to PDF and then extracting text is also discussed. Overall, the sentiment is that DRM ultimately harms accessibility and legitimate users more than pirates.
The Hacker News post "Extracting content from an LCP "protected" ePub" has generated several comments discussing the effectiveness and ethics of LCP (Limited Content Protection) for ebooks.
One commenter points out the inherent weakness of DRM, stating that if a device can render the content, it can be captured. They argue that DRM only inconveniences legitimate users while dedicated pirates will always find a way around it. This comment highlights the common sentiment that DRM is a futile effort in the long run.
Another commenter dives into the technicalities, explaining that LCP isn't designed to prevent copying, but rather to associate a specific decryption key with a user or device. They describe how LCP uses a server to deliver keys, and how the described method intercepts this communication to obtain the key. This clarifies the actual function of LCP and how the exploit bypasses it.
A further comment expands on the limitations of LCP, mentioning that it doesn't protect against screen scraping or even simply printing to PDF. This emphasizes the vulnerability of the protection scheme and how easily it can be circumvented by relatively simple methods.
The discussion also touches on the legal aspects, with one commenter noting that circumventing DRM, even poorly implemented DRM like LCP, might still be illegal depending on jurisdiction. This provides a counterpoint to the purely technical discussion and highlights the legal ramifications of such actions.
Another commenter criticizes Adobe's implementation of LCP, calling it "pathetic" and suggesting that it provides a false sense of security to publishers. They express frustration with the apparent lack of effort in implementing a robust protection scheme.
Finally, some comments focus on the underlying issue of control versus access. One user argues that publishers fear losing control over their content, while another contends that what publishers really want is the ability to track and control access to the content. This highlights a philosophical difference in approaches to digital content distribution.
In summary, the comments express a general skepticism towards LCP's effectiveness as a DRM solution. They highlight the inherent limitations of DRM, discuss the technical aspects of the exploit, and touch upon the legal and philosophical implications of digital content protection. Several commenters express frustration with the perceived weakness of LCP and the seeming futility of DRM in general.