Just default CPA

import requests as r
import sys
import re
import base64

URL = "http://167.71.246.232:8080/crypto.php"
flag = ''
b = 16
data = 'a'*15

while True:

    d = {'text_to_encrypt': data, 'do_encrypt':'Encrypt'}

    block = r.post(URL, data=d).text
    block = re.search(r'<b>(.*?)</b>', block).group(1)

    base64_bytes = block.encode('ascii')
    block = base64.b64decode(base64_bytes)


    for i in range(0x20, 0x80):
        if chr(i) in '&#': continue
        d = {'text_to_encrypt': data+flag+chr(i), 'do_encrypt':'Encrypt'}

        resp = r.post(URL, data=d).text
        resp = re.search(r'<b>(.*?)</b>', resp).group(1)

        base64_bytes = resp.encode('ascii')
        resp = base64.b64decode(base64_bytes)

        sys.stdout.write('\r'+flag+chr(i))
        sys.stdout.flush()

        if block[:b] == resp[:b]:
            data = data[1:]
            flag += chr(i)
            if len(flag) == 16:
                b += 16
                data = 'a'*15
            break
    else:
        print "\r" + flag + " \nDONE"
        break

flag{b4d_bl0cks_for_g0nks}