Weibo's post-censorship system initially checks only for literal keyword strings, but after a user posts a literal blacklisted keyword the server switches to regex/wildcard matching for that user's subsequent posts — catching obfuscations like 'Fa-ccc-lun' that were not blocked before the trigger event. This per-user escalation of pattern matching means keyword obfuscation provides only one-shot protection.

A snapshot from 6 April 2012 shows TOM-Skype had 1,130 censorship keywords and SinaUC had 1,490, with only 21 words in common — all high-frequency stock keywords (e.g., 'falun', 'Epoch Times'). This near-total divergence indicates each company independently compiled its own blacklist rather than distributing from a centralized government source.

§4 evaluation

TOM-Skype's client-downloaded keyword blacklist was updated with current-event-specific terms (protest locations, individual names like Bo Xilai) while SinaUC's lists were not updated with current events and appeared more targeted at spam removal. This correlation between surveillance capability and more timely, politically specific keyword updates suggests censors prioritize maintaining current blacklists on clients that also perform message surveillance.

§4, §5 evaluation

Weibo's search censorship is more aggressive than its post censorship: searching for a keyword returned no results in cases where posting the same keyword was not blocked. The authors hypothesize this asymmetry reflects resource constraints — post censorship requires processing longer, more varied content at high volume, while search censorship is cheaper to apply broadly.

§3, §5 detection

Bigram frequency analysis of Weibo around the December 2011 Wukan village protests (Figure 1) shows censorship of the keyword 'Wukan' was applied proactively before mainstream media coverage and lifted after the government announced a peaceful resolution on 21 December 2011 — demonstrating that censors operate on a news-cycle timescale and use temporary suppression to manage narrative rather than indefinitely blocking topics.

§3, Figure 1, §5 evaluation

During the June 2025 shutdown, Iranian authorities blocked international One-Time Password (OTP) SMS delivery, preventing new sign-ins to foreign secure-messaging platforms and VPN services. This forced users toward government-approved domestic platforms that lack security and privacy protections. The blockade of OTPs effectively weaponized account-recovery flows as a secondary shutdown layer, disproportionately affecting users who needed to activate new circumvention tools during the crisis.

2025-iran-shutdown-measurement §Human Rights Implications port-blockingdpikeyword-filtering

Simple character-level perturbations (English) and homophone substitutions (Chinese), combined with LLM instruction-following prompts directing the model to use word substitutions in its output, successfully bypassed all input and output filters for all 41 input-blocked and 197 output-blocked queries across five major Chinese LLM services (Baidu-Chat, DeepSeek, Doubao, Kimi, Qwen). Every input-blocked query contained at least one keyword combination that alone triggered the filter, confirming keyword-matching rather than semantic classification.

2026-ablove-characterizing §VII-D keyword-filtering

Cross-national experiments conducted from Singapore, South Korea, and Taiwan during February 19–24, 2025 found no variance in blocking implementations, event syntax, or server infrastructure across all five Chinese LLM services. Input blocking was enacted identically in all three international locations, and services connected to the exact same server IP addresses globally — Kimi and Baidu-Chat connected to identical IPs and DeepSeek to the same two addresses across all tested locations.

2026-ablove-characterizing §VIII-A keyword-filtering

Input blocking in Chinese LLM services (DeepSeek, Qwen, Kimi, Doubao) is overwhelmingly consistent: all four services persistently block the exact same queries across all 5 measurement samples in both Simplified and Traditional Chinese. Output blocking is far less consistent, with only 29 out of 349 output-blocked queries blocked across all 5 samples. Baidu-Chat is exceptional: it performs almost no input blocking but instead relies heavily on post-search and output blocking (78.6% of blocks are output-phase).

2026-ablove-characterizing §VI-B keyword-filteringdpi

DeepSeek, Kimi, and Doubao all transmit analytics logs to the same Autonomous System (AS24429, Zhejiang Taobao Network Co., Ltd., a ByteDance/Volcengine subsidiary), with one monitoring endpoint IP directly overlapping between DeepSeek and Doubao. Additionally, all four non-Qwen services maintain connections with servers physically located in China throughout the chat session, transmitting user IDs, session IDs, viewport data, language preferences, and in Baidu-Chat's case, the full query text via URL-encoded CAPTCHA requests.

2026-ablove-characterizing §VI-D keyword-filtering

All five Chinese LLM services transmit partial or complete responses to the client machine even when output blocking is triggered, representing a major information leak. For DeepSeek and Qwen, truncated blocked responses are on average close in token length to full successful responses. For Baidu-Chat, the complete response is transmitted to the client but only partially rendered in the browser UI, with only a word or two visible on screen.

2026-ablove-characterizing §VI-C keyword-filtering