GACTF oldmodem writeup


题目描述

old modem (bell 202)
China:
https://pan.baidu.com/s/184Trg9M94uVSekGycaAR_w (密码:5mp2)
Overseas:
https://drive.google.com/drive/folders/1T94OrcveHAZTmTCwaVCojLXYlJc3lL3f?usp=sharing

Writeup

首先,modem是调制解调器(猫)的意思,Google发现bell 202是一种标准,完成这些信息收集后,正式开始
下载文件,文件无后缀,放入Kali
首先file命令看下文件类型
得到信息oldmodem: Zip archive data, at least v2.0 to extract
所以解压unzip oldmodem
解压后得到一个疑似音频类文件

Archive:  oldmodem
inflating: encoded

GACTF oldmodem writeup
再次file命令查看格式得到信息encoded: RIFF (little-endian) data, WAVE audio, Microsoft PCM, 16 bit, mono 48000 Hz
由于是kali里,所以就不再拖入Windows使用winhex查看hex了,直接使用xxd命令

xxd encoded | head -n 10

GACTF oldmodem writeup
确定encode文件是一个WAV文件,修改后缀,尝试播放,声音很杂,联想开头收集的信息,尝试从帽上入手
使用的工具:minimodem
kali可通过apt 命令直接安装apt-get install minimodem
查看软件手册
正好就有所谓的bell 202
GACTF oldmodem writeup
-r 指定读取模式
-f 选择读取的文件
1200 指定Bell202 1200 bps
输入如下

minimodem -r -f encoded 1200
### CARRIER 1200 @ 1200.0 Hz ###
The Bell 202 modem was an early (1976) modem standard developed by the Bell System. It specifies audio frequency-shift keying (AFSK) to encode and transfer data at a rate of 1200 bits per second, half-duplex (i.e. transmission only in one direction at a time). These signalling protocols, also used in third-party modems, are referred to generically as Bell 202 modulation, and any device employing it as Bell-202-compatible.

Bell 202 AFSK uses a 1200 Hz tone for mark (typically a binary 1) and 2200 Hz for space (typically a binary 0).
In North America, Bell 202 AFSK modulation is used to transmit Caller ID information over POTS lines in the public telephone network. It is also employed in some commercial settings.

In addition, Bell 202 is the basis for the most commonly used physical layer for the HART Communication Protocol - a communication protocol widely used in the process industries.

Surplus Bell 202 modems were used by amateur radio operators to construct the first packet radio stations, despite its low signalling speed. A modified Bell 202 AFSK modulation, a common physical layer for AX.25, remains the standard for amateur VHF operation in most areas. Notably, Automatic Packet Reporting System (APRS) transmissions are encoded this way on VHF. On HF, APRS uses Bell 103 modulation.

The Bell 202 standard was adopted around 1980 as the communications standard for subsea oil and gas production control systems, pioneered by the then FSSL (Ferranti Subsea Systems Ltd.) Controls, a spin-out company from the former TRW - Ferranti joint venture in the UK. This modulation standard was retained until around 2000, when it was superseded by faster FSK and PSK modulation methods, although it is still utilised for extension of existing control systems that are already configured for this technique.

The 202 standard permitted useful techniques such as multi-dropping of slave modems to allow multiple nodes to be connected to the host via a single modem channel. Other techniques have included superposition of signal on power conductors, and distances in excess of 80 km were achieved in subsea applications using these techniques. This has been enhanced through the use of Manchester encoding over the FSK link, to provide simple Modulo-2 RZ (return to Zero) bit error detection and suppression improvement over these long distances.

Here is the flag: GACTF{9621827f-a41b-4f27-8d72-9e0b77415a4f}
### NOCARRIER ndata=2423 confidence=4.397 ampl=0.997 bps=1200.00 (rate perfect) ###

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GACTF oldmodem writeup

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