We are given two files, one is encryption code, the second one is ciphertext.
```
$ cat script.py
from secret import flag, key

f = open('ciphertext.txt', 'w')

p = 1044388881413152506679602719846529545831269060992135009022588756444338172022322690710444046669809783930111585737890362691860127079270495454517218673016928427459146001866885779762982229321192368303346235204368051010309155674155697460347176946394076535157284994895284821633700921811716738972451834979455897010306333468590751358365138782250372269117968985194322444535687415522007151638638141456178420621277822674995027990278673458629544391736919766299005511505446177668154446234882665961680796576903199116089347634947187778906528008004756692571666922964122566174582776707332452371001272163776841229318324903125740713574141005124561965913888899753461735347970011693256316751660678950830027510255804846105583465055446615090444309583050775808509297040039680057435342253926566240898195863631588888936364129920059308455669454034010391478238784189888594672336242763795138176353222845524644040094258962433613354036104643881925238489224010194193088911666165584229424668165441688927790460608264864204237717002054744337988941974661214699689706521543006262604535890998125752275942608772174376107314217749233048217904944409836238235772306749874396760463376480215133461333478395682746608242585133953883882226786118030184028136755970045385534758453247
g = 5
A = 1026312539297800437474663698165859314949881437729617621666434357798219198741950468733395500361477359726152747087790103309627020498122003777642051150130697457594304849673838709900017711265818285080832347734747895550397950729716624922572654209637755195129162139245110756558638081495998280747642920484467428206475906559638681536868548289456924005274209311355030582255692087426910634838198143851507435754029135363794578075936092774722678311786272841489629294721103591751528609388061794369341067986401129462942050916582521451289187645626081017578576190303952351748434876686541368607656026867091583868645619423975306245327421218767449273192101105293424028461698783545171866070124432565063559495566733441286372612161876492134408160732339966921175762866198980795890946054558528891296203285979664329713156129091098226212735763844909789916934266711879564086741733061623347281499025678164709559814150194881622611023214199434022258730549350019749882889143749385314934896284396513061241138504029046053964944026179039768718830854958634216721133676746317913559932277050177463811150719675119168868527853864167729206220819613297736800799391257602899169041109002518019207734013899840092155297682096290489330476118066934735827328128343402508975429994312
d = key
m = int(flag.encode('hex'), 16) % p

B = pow(g, d, p)
k = pow(A, d, p)
c = k * m % p

f.write(str(B) + '\n')
f.write(str(c) + '\n')

f.close()

$ cat ciphertext.txt
1044388881413152506679602719846529545831269060992135009022588756444338172022322690710444046669809783930111585737890362691860127079270495454517218673016928427459146001866885779762982229321192368303346235204368051010309155674155697460347176946394076535157284994895284821633700921811716738972451834979455897010306333468590751358365138782250372269117968985194322444535687415522007151638638141456178420621277822674995027990278673458629544391736919766299005511505446177668154446234882665961680796576903199116089347634947187778906528008004756692571666922964122566174582776707332452371001272163776841229318324903125740713574141005124561965913888899753461735347970011693256316751660678950830027510255804846105583465055446615090444309583050775808509297040039680057435342253926566240898195863631588888936364129920059308455669454034010391478238784189888594672336242763795138176353222845524644040094258962433613354036104643881925238489224010194193088911666165584229424668165441688927790460608264864204237717002054744337988941974661214699689706521543006262604535890998125752275942608772174376107314217749233048217904944409836238235772306749874396760463376480215133461333478395682746608242585133953883882226786118030184028136755970045385534758453246
626538023179183383530326775878874061243537575637678230775990839517480703838547661223043351239918394865532739593994849154430591305198322288328417436892902443505917379006443598061697092201021536603987285163299838045075649512028921956548205676208712226470308113290505059049053554206416767772651512039632936795232724801922443388952796545084060280910673215063573749033668717290815841834685051547255093401042248880082370281195663905731764841136214458769095907569163184276786105979509363239839613385785232307550226120692268670717158539363476974302593377514937470953649913863219081380729197866499068054753726732556988368232198510160183731571741889761632557457157261911256679728707526809880224074940827825962124087677609578885077944156589121586951372962667391498824853627326311588259929756093569512874696625507873168095797130866697040006079191565592914811736694534863664544646201900855732710994613768016495896341014229622129847800138058041138449691835652916517559125407914079964177419733079221280850917496784527992741048638603428068452948455069671414667008770873913118369984179054902139270844669700487715140145038503768704763641223894377872853332484296603268546135216483844722413025955229649646292980986542646384960063297213437076185709400450

```
Encryption that is used here is El Gamal. The objective is simple - if it's implemented correctly, then the ciphertext is secure.We know however that it's not, so let's find a flaw. I used `yafu` to verify that `p` is indeed prime. Moreover, `(p-1)/2` is a prime, therefore the order of generator (`5` in this case) is either `1`, `2`, `(p-1)/2` or `p-1` (it must divide `p-1`. First two are obviously false, third one is too (`pow(5, (p-1)//2, p) != 1`), so we know that `ord_p(5) = p-1`.

It tourns out that `B+1 == p`, - in this case discrete logarithm is easy - `pow(g, key, p) == -1 => pow(g, 2*key, p) == 1 => 2*key == p-1 => key == (p-1)//2`. We decrypt as in standard El Gamal and get the flag.