** exception error: bad argument
     in function  crypto:block_crypt_nif/4
        called as crypto:block_crypt_nif(aes_256_ecb,<<"CAFE08F852D1E968EBF173067E9B4E03">>,
                                         <<15,0,202,90,40,145,123,195,50,251,134,69,162,53,183,10,
                                           75,154,223,89,217,4,161,139,81,137,...>>,
                                         false)
     in call from ecrkme:decrypt_flag/1 (ecrkme.erl, line 42)
     in call from ecrkme:check_pass/0 (ecrkme.erl, line 15)
     in call from ecrkme:main/0 (ecrkme.erl, line 10)

proplists:get_value(ciphers, crypto:supports()).       
[des3_cbc,des_ede3,des3_cbf,des3_cfb,aes_cbc,aes_cbc128,
 aes_cbc256,aes_cfb8,aes_cfb128,aes_ctr,aes_ecb,aes_gcm,
 aes_ccm,aes_ige256,des_cbc,des_cfb,des_ecb,blowfish_cbc,
 blowfish_cfb64,blowfish_ofb64,blowfish_ecb,rc2_cbc,rc4,
 chacha20_poly1305,chacha20]

1> ecrkme:main().                                  
Calculating challenge...
Congrats! Your flag: 
        This-is-Erl4ng-R3v3<пропущено>

You do not need to do reverse engineering. Our code is open:
        https://github.com/rbkmoney