CTF Challenge Name Solves (Difficulty)
TSG CTF 2021 Beginner's Crypto 2021 126/775 (⭐)
TSG CTF 2021 Minimalist's Private 49/775 (⭐⭐)
TSG CTF 2021 Baba is Flag 34/775 (⭐⭐)
TSG CTF 2021 Flag is Win 10/775 (⭐⭐⭐)
TSG CTF 2021 This is DSA 9/775 (⭐⭐)
TastelessCTF 2021 crybaby 🔥 14/162 (⭐⭐⭐)
pbctf 2021 Alkaloid Stream 132/210 (⭐⭐)
pbctf 2021 Steroid Stream 38/210 (⭐⭐)
pbctf 2021 GoodHash 🔥 30/210 (⭐⭐⭐)
pbctf 2021 Seed Me 🔥 24/210 (⭐⭐⭐)
pbctf 2021 Yet Another PRNG 🔥 14/210 (⭐⭐⭐)
pbctf 2021 Yet Another RSA 🔥 12/210 (⭐⭐⭐)
Hack.lu CTF 2021 Silver Water Industries 92/174 (⭐⭐)
Hack.lu CTF 2021 WhatTheHecc 45/174 (⭐⭐)
Hack.lu CTF 2021 lwsr 20/174 (⭐⭐)
BSides Ahmedabad CTF 2021 dlppp 39/314 (⭐⭐)
BSides Ahmedabad CTF 2021 floorsa 27/314 (⭐⭐)
BSides Ahmedabad CTF 2021 SSSS.RNG 🔥 16/314 (⭐⭐)
BSides Ahmedabad CTF 2021 ECC-RSA 2 🔥 8/314 (⭐⭐⭐)
BSides Ahmedabad CTF 2021 They Were Eleven 🔥 3/314 (⭐⭐⭐⭐)
HKCERT CTF 2021 A Joke Cipher 88/239 (⭐)
HKCERT CTF 2021 Cipher Mode Picker 21/239 (⭐)
HKCERT CTF 2021 Key Backup Service 1 6/239 (⭐⭐)
HKCERT CTF 2021 Key Backup Service 2 5/239 (⭐⭐)
HKCERT CTF 2021 Tenet: The Plagarism 4/239 (⭐⭐)
HKCERT CTF 2021 Sratslla SEA 2/239 (⭐⭐⭐)
HKCERT CTF 2021 Sign In Please, Again 2/239 (⭐⭐⭐)
Balsn CTF 2021 1337 pins 5/284 (⭐⭐⭐)
Balsn CTF 2021 dlog 🔥 2/284 (⭐⭐⭐⭐)
Balsn CTF 2021 trinity 0/284 (⭐⭐⭐⭐)
Dragon CTF 2021 Baby MAC 89/247 (⭐⭐)
Dragon CTF 2021 CRC Recursive Challenge (warmup) 11/247 (⭐⭐⭐)
Dragon CTF 2021 CRC Recursive Challenge 🔥 2/247 (⭐⭐⭐⭐)

TSG CTF 2021

Beginner's Crypto 2021

  • Solves: 126/775
  • Difficulty: ⭐
  • Tags: rsa

We are given $p$ and $q$, the primes for RSA. Denote $n = pq$. We are also given the flag $c_1, c_2, c_3$ encrypted with the same modulus but different exponents. Those exponents, $e_1, e_2, e_3$, are computed by

\[\begin{aligned} & e_1 = e^{65537}\ \text{mod}\ \varphi(n) \\ & e_2 = (e + 2)^{65537}\ \text{mod}\ \varphi(n) \\ & e_3 = (e + 4)^{65537}\ \text{mod}\ \varphi(n) \end{aligned}\]

Note that $e$, $e+2$ and $e+4$ are all primes. The objective is to recover the flag.

Minimalist's Private

  • Solves: 49/775
  • Difficulty: ⭐⭐
  • Tags: rsa

We are given a RSA public key $(n, e)$ and a ciphertext $c$. Additionally, $\lambda(n)$ (the order of $\mathbb{Z}^*_n$) satisfies

\[\lambda(n)^2 \leq 10000n.\]

The objective is to decrypt $c$ for the flag.

Baba is Flag

  • Solves: 34/775
  • Difficulty: ⭐⭐
  • Tags: ecdsa

When connected to the server, a ECDSA private key $d$ for SECP256k1 is generated. We are allowed to perform the below operations in a total of five times:

  1. [Sign] We will be given a signature of Baba. The ECDSA is slightly modified (will be described below) and the nonce $k$ is around 80 bits.
  2. [Find rule] We will be asked to send a message $m$ and the signature $(r, s)$. It would return the flag if we have a valid signature for message Flag.

For ECDSA, $s = k^{-1} (z + rd_A)\ \text{mod}\ n$. However in this challenge, $s$ is computed by

\[k^{-1} (z + d_A)\ \text{mod}\ n.\]

The objective is to forge a signature for the message Flag.

Flag is Win

  • Solves: 10/775
  • Difficulty: ⭐⭐⭐
  • Tags: ecdsa

When connected to the server, a ECDSA private key $d$ for SECP256k1 is generated. We are allowed to perform the below operations in a total of five times:

  1. [Sign] We will be given a signature of Baba. The nonce $k$ is around 80 bits.
  2. [Find rule] We will be asked to send a message $m$ and the signature $(r, s)$. It would return the flag if we have a valid signature for message Flag.

The objective is to forge a signature for the message Flag.

This is DSA

  • Solves: 9/775
  • Difficulty: ⭐⭐
  • Tags: dsa

The challenge uses a modified pycryptodome package with the below code difference:

When connected to the server, we are given a 256-bit order $q$ and is asked a 2048-bit $p$ and $h$. The generator $g$ is computed by

\[g = h^{\lfloor{(p-1)/q\rfloor}}\ \text{mod}\ p.\]

Also, a private key $x \in [0, q)$ is generated and the public key $y$ is given by

\[y = g^x\ \text{mod}\ p.\]

We will be given $g$ and $y$. The goal is to craft a signature of flag in 15 seconds.

TastelessCTF 2021

crybaby 🔥

  • Solves: 14/162
  • Difficulty: ⭐⭐⭐
  • Tags: gcm

When connected to the server, a 16-byte AES key $\mathcal{K}$ is generated. There are two stages and during the $k$-th stage, we are given the $k$-th oracle $\mathcal{O}_k$. We are able to call the oracles as much as we could (unless an exception is thrown).

  • $\mathcal{O}_1$ uses AES-CTR and it asks for nonce $\mu$ and ciphertext $c$. It returns

    \[\text{Encrypt}_\mathcal{K}(\mu, \text{Login successful!})\]

    and proceed to the next stage if $\text{Decrypt}_\mathcal{K}(\mu, c)$ is equal to adminplz, and otherwise

    \[\text{Encrypt}_\mathcal{K}(\mu, \text{Unknown command!}).\]

  • $\mathcal{O}_2$ uses AES-GCM and it asks for nonce $\mu$ and ciphertext $c$. It returns

    \[\text{Encrypt}_\mathcal{K}(\mu, \mathcal{F})\]

    if $\text{Decrypt}_\mathcal{K}(\mu, c)$ is equal to flagplz, and otherwise

    \[\text{Encrypt}_\mathcal{K}(\mu, \text{Unknown command!}).\]

The objective is to recover the flag $\mathcal{F}$.

pbctf 2021

Alkaloid Stream

  • Solves: 132/210
  • Difficulty: ⭐⭐
  • Tags: math

Suppose that the flag is $m$ bits long ($m = 600$ in this challenge). Initially, an invertible $m\times m$ matrix in $\text{GF}(2)$ (denote by $\mathcal{K}$) is generated and is interpreted as $m$ $m$-bit integers, 0 <= key[0], key[1], ..., key[m-1] < 2^m. A keystream and a public key is generated from the key by the function gen_keystream, defined below:

def gen_keystream(key):
    ln = len(key)
    
    # Generate some fake values based on the given key...
    fake = [0] * ln
    for i in range(ln):
        for j in range(ln // 3):
            if i + j + 1 >= ln:
                break
            fake[i] ^= key[i + j + 1]

    # Generate the keystream
    res = []
    for i in range(ln):
        t = random.getrandbits(1)
        if t:
            res.append((t, [fake[i], key[i]]))
        else:
            res.append((t, [key[i], fake[i]]))
    
    for r in res:
        print(r)

    # Shuffle!
    random.shuffle(res)

    keystream = [v[0] for v in res]
    public = [v[1] for v in res]
    return keystream, public

We are given a public key and the flag xorred with the keystream. The objective is to recover the flag.

Steroid Stream

  • Solves: 38/210
  • Difficulty: ⭐⭐
  • Tags: math

The challenge is similar to Alkaloid Stream, except that $m = 616$ and gen_keystream is defined below:

  def gen_keystream(key):
      ln = len(key)
+     assert ln > 50
      
      # Generate some fake values based on the given key...
      fake = [0] * ln
-     for i in range(ln):
-         for j in range(ln // 3):
-             if i + j + 1 >= ln:
-                 break
-             fake[i] ^= key[i + j + 1]
+     for i in range(ln - ln // 3):
+         arr = list(range(i + 1, ln))
+         random.shuffle(arr)
+         for j in arr[:ln // 3]:
+             fake[i] ^= key[j]
  
      # Generate the keystream
      res = []
      for i in range(ln):
          t = random.getrandbits(1)
          if t:
              res.append((t, [fake[i], key[i]]))
          else:
              res.append((t, [key[i], fake[i]]))
  
      # Shuffle!
      random.shuffle(res)
  
      keystream = [v[0] for v in res]
      public = [v[1] for v in res]
      return keystream, public

GoodHash 🔥

  • Solves: 30/210
  • Difficulty: ⭐⭐⭐
  • Tags: gcm

The digest for the hash algorithm GoodHash for the message $m$ is given by encrypting 32 null bytes with a known key goodhashGOODHASH with the nonce $m$. It is 48 bytes long, which composes of a 32-byte ciphertext and a 16-byte tag.

We are given a token {"token": "32 hex characters", "admin": false} when connected to the server, along with its corresponding hash $h_0$. The goal is to craft a JSON token token such that token['admin'] equals true, which also has the hash $h_0$.

Seed Me 🔥

  • Solves: 24/210
  • Difficulty: ⭐⭐⭐
  • Tags: prng, lcg

We are asked a seed for the Java program. the random instance, rng, is defined by rng = new Random(seed). We will get the flag if the 2048-, 4096-, ..., 32768-th output from rng.NextFloat() are never less than $0.9801547$.

Yet Another PRNG 🔥

  • Solves: 14/210
  • Difficulty: ⭐⭐⭐
  • Tags: prng

The outputs $r_0, r_1, r_2, ... \in [0, 2^{64})$ of the PRNG in the challenge is defined below:

\[r_k := 2m_1x_k - m_3y_k - m_2z_k\ \text{mod}\ M,\]

where $m_1, m_2, m_3$ are 32-bits and $M$ is a 64-bit known constants. Also, let $a_{ij}$ be 20-bit known constants and for $k \leq 3$,

\[ \begin{cases}\begin{aligned} x_k &:= a_{11} x_{k-3} + a_{12} x_{k-2} + a_{13} x_{k-1} \\ y_k &:= a_{21} y_{k-3} + a_{22} y_{k-2} + a_{23} y_{k-1} \\ z_k &:= a_{31} z_{k-3} + a_{32} z_{k-2} + a_{33} z_{k-1} \end{aligned}\end{cases}. \]

Note that we are not given $x_k, y_k, z_k$ for $k = 0, 1, 2$. Suppose that we have $r_0, r_1, ..., r_{11}$, and the flag is xor-ed with $r_{12}, r_{13}, ...$. The objective is to recover the flag.

Yet Another RSA 🔥

  • Solves: 12/210
  • Difficulty: ⭐⭐⭐
  • Tags: rsa

We are given a RSA public key $n, e$ and an encrypted flag $c$. Hereby $n$ is 1024-bit number which is a product of two 512-bit primes $p, q$ (they are both in the form of $a^2 + 3b^2$). It is also known that $d$ is 400-bit long.

Remarkably, the multiply operation is performed under a group $\mathcal{G}$ with

\[\mathcal{G} = \mathbb{Z}_n^2 \cup (\mathbb{Z}_n\times\{\varepsilon\}) \cup \{\varepsilon\}^2.\]

Also $|\mathcal{G}| = (p^2+p+1)(q^2+q+1)$. The objective is to recover the flag from $c$.

# Note: The challenge used `add` but I think it would be better to call it `multiply`.
def multiply(P, Q, mod):
    m, n = P
    p, q = Q

    if p is None:
        return P
    if m is None:
        return Q

    if n is None and q is None:
        x = m * p % mod
        y = m + p % mod
        return (x, y)

    if n is None and q is not None:
        m, n, p, q = p, q, m, n

    if q is None:
        if (n + p) % mod != 0:
            x = (m * p + 2) * inverse(n + p, mod) % mod
            y = (m + n * p) * inverse(n + p, mod) % mod
            return (x, y)
        elif (m - n ** 2) % mod != 0:
            x = (m * p + 2) * inverse(m - n ** 2, mod) % mod
            return (x, None)
        else:
            return (None, None)
    else:
        if (m + p + n * q) % mod != 0:
            x = (m * p + (n + q) * 2) * inverse(m + p + n * q, mod) % mod
            y = (n * p + m * q + 2) * inverse(m + p + n * q, mod) % mod
            return (x, y)
        elif (n * p + m * q + 2) % mod != 0:
            x = (m * p + (n + q) * 2) * inverse(n * p + m * q + r, mod) % mod
            return (x, None)
        else:
            return (None, None)

Hack.lu CTF 2021

Silver Water Industries

  • Solves: 92/174
  • Difficulty: ⭐⭐
  • Tags: math

When connected to the server, a modulus $n = pq$ is generated with $p, q$ be 64 bits, $p \equiv 1\ (\text{mod}\ 4)$ and $q \equiv 3\ (\text{mod}\ 4)$. To encrypt a bit $m$, a quadratic non-residue of $n$ (denoted by $x$) is generated. The ciphertext would be $x^2 (-1)^m\ \text{mod}\ n$.

A token of 20 bytes is encrypted and we have the encrypted token. The objective is to recover the token, which we could win the flag if we send them the correct one.

WhatTheHecc

  • Solves: 45/174
  • Difficulty: ⭐⭐
  • Tags: ecc

The challenge defines a signature algorithm $\mathcal{S}$, where a key generated with elliptic curve parameter P-256 (the public and private keys are respectively $Q$ and $d$). The signing algorithm for a message $m$ is specified below ($\text{H}$ is the SHA3-256 algorithm that returns an integer given an array of bytes):

  1. Compute $r = \text{H}(m) \cdot Q$
  2. Compute $r' = r\cdot d^{-1}\ \text{mod}\ q$, where $q$ is the order of the curve
  3. Define $z = n_\text{nonce} | t$, where $n_\text{once} \in [1, q)$ and $t$ is the timestamp in seconds
  4. Compute $R = r' + \text{H}(z) \cdot G$
  5. Compute $s = [d - \text{H}(z)]\ \text{mod}\ q$
  6. Return $(R, s)$ as the signature.

Also the verifying algorithm for a message $m$ and signature $(R, s)$:

  1. If $s = 0\ \text{or}\ 1$ and $s > 0$ then the signature is considered invalid.
  2. The signature is valid only if

\[s \cdot G - Q + R = \text{H}(m) \cdot G\]

There are three functions those we can use when connected to the server:

  1. [Show] Prints the public key.
  2. [Sign] Signs one of the four commands: id, uname, ls and date with $\mathcal{S}$.
  3. [Run] Takes a signed command and executes if the signature is valid.

lwsr

  • Solves: 20/174
  • Difficulty: ⭐⭐
  • Tags: lfsr, lwe

Suppose that $q = 16411$.

When connected to the server, a LWE private key $s \in \mathbb{Z}_q^{128}$ and 384 sets of public key $(p_1, r_1), (p_2, r_2), ..., (p_{384}, r_{384}) \in \mathbb{Z}_q^{128} \times \mathbb{Z}_q$ are also generated. Also, a LFSR state $t = (t_1, t_2, ..., t_{384})$ with $t_1, t_2, ..., t_{384} \in \{0, 1\}$ is generated.

The flag is encrypted bit-by-bit. The ciphertext of the $i$-th bit of the message, $m_i$, is defined by:

\[\begin{cases} c_{i, 0} &= (t_1 \cdot p_1 + t_2 \cdot p_2 + ... + t_{384} \cdot p_{384})\ \text{mod}\ q \\ c_{i, 1} &= (t_1 \cdot r_1 + t_2 \cdot r_2 + ... + t_{384} \cdot r_{384} + 8205m_i)\ \text{mod}\ q \end{cases}\]

Additionally, after a bit is encrypted, the LFSR state is updated with

\[(t'_1, t'_2, ..., t'_{384}) := \left(t_2, t_3, ..., \text{NEXT}(t_1, t_2, ..., t_{384})\right).\]

We are given $(p_1, r_1), (p_2, r_2), ..., (p_{384}, r_{384})$ and encrypted flag $(c_{1,0}, c_{1,1}), (c_{2,0}, c_{2,1}), ...$. We are then able to access the decryption oracle. The goal is to recover the flag.

BSides Ahmedabad CTF 2021

dlppp

  • Solves: 39/314
  • Difficulty: ⭐⭐
  • Tags: discrete log

Let $p$ be a 512-bit prime and $m$ be the flag. Let also $y = (1 + p)^m\ \text{mod}\ p^3$. We are given $p$ and $y$ and the objective is to recover the flag.

floorsa

  • Solves: 27/314
  • Difficulty: ⭐⭐
  • Tags: rsa, math

Let $p$ and $q$ be 1024-bit prime and $n = p\cdot q$ be a 2048-bit RSA modulus. Let $e = 65537$ and $m$ be the secret message. We are given the $n$ and the ciphertext $c = m^e\ \text{mod}\ n$. Additionally, we are given

\[s = \sum_{k=0}^{q-1} \lfloor{\frac{p\cdot k}{q}}\rfloor.\]

The objective is to recover the secret message $m$.

SSSS.RNG 🔥

  • Solves: 16/314
  • Difficulty: ⭐⭐
  • Tags: lcg

Let $p$ be a 512-bit prime and $a, b, g_0 \in [2, p)$. Let $\mathcal{G}$ be a PRNG and let $g_k$ be the $k$-th output from $\mathcal{G}$, given by

\[g_k = (a\cdot g_{k-1} + b)\ \text{mod}\ p.\]

Let $\text{f}(x) := g_1 + g_2 x + g_3 x^2 + g_4 x^3 + g_5 x^4 + g_6 x^5$ be a polynomial over $\mathbb{Z}_p$. Suppose that we are given $p$, $\text{f}(1), \text{f}(2), \text{f}(3), \text{f}(4)$ and $\text{f}(\text{flag})$. The objective is to compute $\text{flag}$.

ECC-RSA 2 🔥

  • Solves: 8/314
  • Difficulty: ⭐⭐⭐
  • Tags: ecc, rsa

Let $n = p\cdot q$ with $p$ and $q$ being 512-bit primes. An elliptic curve $\mathcal{C}$ is given by:

\[\mathcal{C}:\quad y^2 \equiv x^3 + a x + b\ (\text{mod}\ n),\]

where $a = -3$. Let $P \in \mathcal{C}$ and let $m_0, m_1, ... \in [0, 256)$ be the characters of the flag. The $k$-th character of the flag $m_k$ is encrypted to $c_k$:

\[c_k \equiv ((2^k\cdot P)_x \cdot m_k)^{65537} \ (\text{mod}\ n).\]

Given $n, a, b$ and $c_0, c_1, ...$. It is known that the flag satisfies the format Neko{\w+}, the objective is to recover the flag.

They Were Eleven 🔥

  • Solves: 3/314
  • Difficulty: ⭐⭐⭐⭐
  • Tags: rsa

Suppose that we have a 111-byte message $m$. Let $p_1, p_2, ..., p_{11}, q_1, q_2, ..., q_{11}$ be 512-bit primes and $r_1, r_2, ..., r_{11} \in [0, 2^{11})$.

We are given $n_i = p_i\cdot q_i$ and $c_i = m^{11}\cdot r_i\ \text{mod}\ n_i$ for $i = 1, 2, ..., 11$. The objective is to recover $m$.

HKCERT CTF 2021

🎉 A shameless plug. Thanks Mystiz (myself) for allowing me to promote my own challenges here! However, I dare not to mark them 🔥 although they are all subjectively great. The attachments are available here.

小諧星 / A Joke Cipher

  • Solves: 88/239
  • Difficulty: ⭐
  • Tags: math

The challenge implements Nagaty's Cryptosystem1 which no one knows why it is a cryptosystem. Initially, a prime $p$ is determined. Alice and Bob respectively derive a private key $x_A$ and a public key $y_A := x_A\ \text{mod}\ p$ (resp. $x_B$ and $y_B := x_B\ \text{mod}\ p$). A shared key is computed by Alice with $s = y_A \cdot {y_B}^2 \cdot x_A\ \text{mod}\ p$. Finally, the message $m$ is encrypted by $m \cdot s$.

In the challenge, we are given $p$, $y_A$, $y_B$ and an encrypted flag $c$. The objective is to recover the flag.

Freedom / Cipher Mode Picker

  • Solves: 21/239
  • Difficulty: ⭐
  • Tags: mode of operations

A 16-byte key and a 16-byte IV is generated when connected to the server. We are able to use each of the below encryption functions once, to encrypt either a given message or the 80-byte flag:

# 1. AES-ECB
cipher = AES.new(key, AES.MODE_ECB)
# 2. AES-CBC
cipher = AES.new(key, AES.MODE_CBC, iv)
# 3. AES-CFB
cipher = AES.new(key, AES.MODE_CFB, iv, segment_size=128)
# 4. AES-OFB
cipher = AES.new(key, AES.MODE_OFB, iv)
# 5. AES-CTR
cipher = AES.new(key, AES.MODE_CTR, counter=Counter.new(16, prefix=iv[:14]))

The goal is to recover the flag.

長話短說 / Key Backup Service 1

  • Solves: 6/239
  • Difficulty: ⭐⭐
  • Tags: rsa

When connected to the server, a 32-byte master secret $x$ is generated. We are able to call the below functions at a total of 17 times:

  1. [SEND] Returns the ciphertext, i.e., $m^e\ \text{mod}\ n$, of an arbitrary given message $m$ with the current key $\mathcal{K}$.
  2. [PKEY] Generates a set of RSA key and assign to $\mathcal{K} = (n, e)$.
  3. [BACKUP] Returns the encrypted secret, i.e., $x^{e}\ \text{mod}\ n$, with the current key $\mathcal{K}$.
  4. [FLAG] Encrypts the flag with the key $x$ under AES-CBC.

Additionally, the public exponent is $e = 17$ and the 512-bit primes for the moduli is generated by a PRNG, seeded by the below snippet:

# 256 bits for random-number generator
N = 0xcdc21452d0d82fbce447a874969ebb70bcc41a2199fbe74a2958d0d280000001
G = 0x5191654c7d85905266b0a88aea88f94172292944674b97630853f919eeb1a070
H = 0x7468657365206e756d6265727320617265206f6620636f757273652073757321

# The primes for the public exponent. id is a randomly generated 256-bit integer.
p = generate_prime(x + int.to_bytes(pow(G, id, N), 32, 'big'))
q = generate_prime(x + int.to_bytes(pow(H, id, N), 32, 'big'))

Braceless / Key Backup Service 2

  • Solves: 5/239
  • Difficulty: ⭐⭐
  • Tags: rsa

The challenge is identical to 長話短說 / Key Backup Service I, with three changes:

  1. $e = 17$ is changed to $e = 65537$,
  2. The number of oracle calls is increased from 17 to 65537,
  3. We do not have the netcat service anymore. Instead, we have a transcript of an interaction. It includes one FLAG operation, followed by 16384 sequences of the below operations: PKEY, SEND 2, SEND 3 and BACKUP.

FreeRider / Tenet: The Plagarism

  • Solves: 4/239
  • Difficulty: ⭐⭐
  • Tags: ctr

Suppose that the flag matches the regular expression hkcert21\{\w{35}\}. We define an encryption algorithm, $\mathcal{E}$, with a 16-byte key $k_1k_2...k_{16}$. Let $m$ be the message we would like to encrypt (all of the counters are set to zero):

  1. Encrypt $m$ with AES-CTR using the key k1 00 00 00 ... 00 ($k_1$ followed by 15 null bytes) and denote it by $t_1$.
  2. Encrypt $t_1$ with AES-CTR using the key k2 00 00 00 ... 00 and denote it by $t_2$.
  3. ...
  4. Encrypt $t_{15}$ with AES-CTR using the key k16 00 00 00 ... 00 and denote it by $t_{16}$.
  5. Return $t_{16}$ as the ciphertext.

$\mathcal{E}$ is used to encrypt the message Congratulations! [FLAG] and we are given its ciphertext. The objective is to recover the flag. Notably, the challenge is highly referenced from Tenet in HKCERT CTF 2020.

集合吧!地球保衛隊 / Sratslla SEA

  • Solves: 2/239
  • Difficulty: ⭐⭐⭐
  • Tags: aes

We will use the Advanced Encryption Standard (AES) for encryption in the challenge. Let $k_0k_1k_2...k_{15}$ be a 16-byte key and let $K_n := k_nk_nk_nk_n$ for $n = 0, 1, 2, ..., 15$. When connected to the server, $k_0k_1k_2...k_{15}$ is randomly created and we are able to access the below functions for 128 times:

  1. [ark secret] encrypts $K_0K_1K_2K_3$ without the AddRoundKey operation and returns the ciphertext.
  2. [sb secret] encrypts $K_4K_5K_6K_7$ without the SubBytes operation and returns the ciphertext.
  3. [sr secret] encrypts $K_8K_9K_{10}K_{11}$ without the ShiftRows operation and returns the ciphertext.
  4. [mc secret] encrypts $K_{12}K_{13}K_{14}K_{15}$ without the MixColumns operation and returns the ciphertext.
  5. [ark data] (resp. [sb data], [sr data] or [mc data]) encrypts a 16-byte user-defined data without the AddRoundKey operation (resp. SubBytes, ShiftRows or MixColumns) and returns the ciphertext.

We are also given a 64-byte encrypted flag when connected to the server. The goal is to collect enough information for the key in 128 calls and decrypt for the flag.

約定的夢幻島 / Sign In Please, Again

  • Solves: 2/239
  • Difficulty: ⭐⭐⭐
  • Tags: hash

Define the below authentication algorithm $\mathcal{P}$. Suppose that a user have a $n$ character-long password, $p_1p_2...p_n$:

  1. The server generates a 4-byte salt $s_1s_2s_3s_4$ and generate a permutation of $\{1, 2, ..., n+5\}$, denote it as $\sigma$.
  2. A user
    • generates one byte of pepper $r$,
    • denotes $x_k = p_k$ for $k = 1, 2, ..., n$, $x_{n+k} = s_k$ for $k = 1, 2, ..., 4$ and $x_{n+5} = r$,
    • computes $y_k = x_{\sigma(k)}$ for $k = 1, 2, ..., n+5$ and $h := \text{SHA256}(y_1y_2...y_{n+5})$, and
    • send $h$ to the server.
  3. The server computes $h'$ from $p_1p_2...p_n$, the salt $s_1s_2s_3s_4$ and all $r \in [0, 256)$. If there exists $r$ such that $h = h'$, the user is authenticated.

The netcat service implements the above algorithm $\mathcal{P}$ and the player, who acts as the man in-the-middle, can play with below operations in a total of 50 times:

  1. [🕵️] The player impersonates the server and sends a salt $s_1s_2s_3s_4$ and surjective mapping $\sigma: \{1, 2, ..., k\} \rightarrow \{1, 2, ..., 21\}$ to the user. The user replies with a digest $h$. By surjective $\sigma$ should satisfy the below condition:
    • For all $v = 1, 2, ..., 21$, there exists $u \in \{1, 2, ..., k\}$ such that $\sigma(u) = v$.
  2. [🖥️] The server sends a permutation $\sigma$ and a salt $s_1s_2s_3s_4$. If the player supplies with a valid digest $h$, the server replies with the flag.

Balsn CTF 2021

1337 pins

  • Solves: 5/284
  • Difficulty: ⭐⭐⭐
  • Tags: mt19937

When connected to the server, we are asked to guess $r_1, r_2, ..., r_{31337}$, the consecutive outputs from random.getrandbits(32) % 10. On the $k$-th turn we are asked to guess $r_k$. If the guess is incorrect, $r_k$ will be printed. The objective is to predict and submit 1337 consecutive outputs.

dlog 🔥

  • Solves: 2/284
  • Difficulty: ⭐⭐⭐⭐
  • Tags: discrete log, timing attack

Suppose that $p = 2q+1$ where $p, q$ are primes. Let also $s$ be an exponent. We are given that $0 < p < 2^{1025}$ and $0 < s < 2^{100}$. Given three APIs:

  • /oracle receives $x$ and returns $x^s\ \text{mod}\ p$,
  • /flag receives $x$. If $x^s\ \text{mod}\ p = 1337$ the flag will be returned,
  • /metrics returns metrics including the total number of calls and the total time on processing /oracle and /flag.

The objective is to retrieve the flag.

trinity

  • Solves: 0/284
  • Difficulty: ⭐⭐⭐⭐
  • Tags: hash

Given a hash key $k$ for $\text{Blake}_3$, the Blake-3 algorithm returning 64 bits as a digest. The objective is to find three distinct messages $m_1, m_2, m_3$ such that

\[\text{Blake}_3(m_1, k) = \text{Blake}_3(m_2, k) = \text{Blake}_3(m_3, k).\]

Dragon CTF 2021

Baby MAC

  • Solves: 89/247
  • Difficulty: ⭐⭐
  • Tags: block cipher

When connected to the server, a 16-byte key $\mathcal{K}$ is generated. Define the signature algorithm $\mathcal{S}$ of message $m$:

  1. Set $s \leftarrow 0$ (16 null bytes).
  2. Pad $m$ with PKCS7 and break it down into blocks of 16 bytes: $m_1, m_2, ..., m_n$.
  3. For $i = 1, 2, ..., n$, update $s \leftarrow \text{Enc}_\mathcal{K}(s \oplus m_i)$.
  4. Update $s \leftarrow \text{Enc}_\mathcal{K}(s)$.
  5. Return $s$.

We are given an oracle to compute $\mathcal{S}(m)$ whenever $m$ does not contain gimme flag. The objective is to craft a signature of a message that contains gimme flag.

CRC Recursive Challenge (warmup)

  • Solves: 11/247
  • Difficulty: ⭐⭐⭐
  • Tags: crc

The objective is to find a message which correctly depicts its CRC-64 digest.

def crc64(buf, crc=0xffffffffffffffff):
    for val in buf:
        crc ^= val << 56
        for _ in range(8):
            crc <<= 1
            if crc & 2**64:
                crc ^= 0x1ad93d23594c935a9
    return crc

inp = input().strip().encode()
crc = crc64(inp)
if inp == f'My crc64 is 0x{crc:016x}! Cool, isn\'t it?'.encode():
    with open('flag.txt', 'r') as f:
        print(f.read().strip())
else:
    print('Nope!')

CRC Recursive Challenge 🔥

  • Solves: 2/247
  • Difficulty: ⭐⭐⭐⭐
  • Tags: crc

The objective is to find a message which correctly depicts its CRC-128 digest.

def crc128(buf, crc=0xffffffffffffffffffffffffffffffff):
    for val in buf:
        crc ^= val << 120
        for _ in range(8):
            crc <<= 1
            if crc & 2**128:
                crc ^= 0x14caa61b0d7fe5fa54189d46709eaba2d
    return crc

inp = input().strip().encode()
crc = crc128(inp)
if inp == f'My crc128 is 0x{crc:032x}! Cool, isn\'t it?'.encode():
    with open('flag.txt', 'r') as f:
        print(f.read().strip())
else:
    print('Nope!')

  1. Khaled A. Nagaty (2019) "A public key cryptosystem and signature scheme based on numerical series"
    https://link.springer.com/content/pdf/10.1007/s42452-019-1928-8.pdf