Why my teammate, Angelboy, is so angry? http://52.196.144.8:8080/

$ java -version java version "1.7.0_111" OpenJDK Runtime Environment (IcedTea 2.6.7) (7u111-2.6.7-0ubuntu0.14.04.3) OpenJDK 64-Bit Server VM (build 24.111-b01, mixed mode) 

The page gives us its source, a Java class file. Decompiling it, we find that the page lets us guess a byte of a secret key, but only after we solve a md5 captcha, increasing in difficulty as we recover more and more bytes. When we guess the last byte correctly, the page also gives us a flag encrypted with the md5 hash of our ip + secret key.

After we recovered the secret key, we found that decryption didn't work. Apparently, Java's conversion from byte arrays to strings is weird. Running decryption routines under the version of Java given in the hint correctly decrypted the flag.

Captcha solving script: captcha.py