Introduction
Data privacy in software requiring rapidity and adaptability is guaranteed by stream ciphers, a crucial component in contemporary cryptography. Among the preferred algorithms in this realm, the ChaCha20 stream cipher stands out. Created by Daniel J. Bernstein in 2008 as a variation of Salsa20, ChaCha20 brings significant enhancements in both security and efficiency. This piece delves into the ChaCha20 stream cipher, detailing its design and how it can be utilized within C++.
Problem Statement
What measures need to be taken to implement the ChaCha20 stream cipher effectively and securely in C++? This question is pivotal in addressing the primary objective of integrating ChaCha20 into C++. This involves understanding its structure, operational methodology, and the incorporation of key elements such as initialization, block arrangement, and management of encryption keys within the C++ source code.
Background
ChaCha20 allows for the encryption key and decryption key to be identical, establishing it as a symmetric stream cipher. It generates a pseudo-random sequence of bytes called a keystream, which is then XORed with the plaintext to create ciphertext. Since the keystream is deterministic, using the same key and IV results in identical keystreams, simplifying decryption as performing the XOR operation on the ciphertext retrieves the original plaintext.
Several elements form the core of ChaCha20 which includes the below points.
- Key: A single 256-bit (32 Byte) Key
- Nonce: A 96-bit (12-byte) nonce, which is never repeated. It is essential in preventing keystream reuse.
- Counter: 32-bit counter often at zero level, which enables ChaCha20 to stream long streams without repeating keystream blocks.
ChaCha20 is highly secure and resistant to certain vulnerabilities that older stream ciphers like RC4 are susceptible to. Additionally, it was specifically created for use in contemporary hardware and is increasingly being adopted in software implementations like HTTPS and VPNs.
Solution: ChaCha20 Algorithm in C++
Now, we are going to showcase the step-by-step implementation of ChaCha20 in C++. This demonstration will highlight the structural aspects of ChaCha20, such as the initialization phase, the execution of the fundamental quarter-round function, and the production of keystreams.
1. The Quarter-Round Function
There exists a crucial circular operation within the core quarter-round of ChaCha20, which involves performing XOR addition and bit rotation. This quarter-round is applied to a 4x4 matrix consisting of 32-bit integers extracted from the key, nonce, and counter values.
The quarter-round operation for ChaCha20 is structured as follows:
a += b; d ^= a; d <<<= 16;
c += d; b ^= c; b <<<= 12;
a += b; d ^= a; d <<<= 8;
c += d; b ^= c; b <<<= 7;
In C++, we can write this as:
#include <cstdint>
void quarterRound(uint32_t &a, uint32_t &b, uint32_t &c, uint32_t &d) {
a += b; d ^= a; d = (d << 16) | (d >> (32 - 16));
c += d; b ^= c; b = (b << 12) | (b >> (32 - 12));
a += b; d ^= a; d = (d << 8) | (d >> (32 - 8));
c += d; b ^= c; b = (b << 7) | (b >> (32 - 7));
}
2. ChaCha20 Block Function
The block function produces 64 bytes of keystream for every counter value by executing the ChaCha20 rounds on a pre-set state matrix.
The initial state matrix is built as follows:
- First 16 bytes: Constant "expand 32-byte k"
- Next 32 bytes: Key
- Next 4 bytes: Counter
- Last 12 bytes: Nonce
The function proceeds by executing 20 rounds of ChaCha, which consist of 10 sets of "doublerounds" made up of quarter-rounds, effectively blending and altering the state.
#include <array>
#include <vector>
constexpr uint32_t constants[] = { 0x61707865, 0x3320646e, 0x79622d32, 0x6b206574 };
std::array<uint32_t, 16> chacha20Block(const uint32_t key[8], uint32_t counter, const uint32_t nonce[3]) {
std::array<uint32_t, 16> state = { constants[0], constants[1], constants[2], constants[3],
key[0], key[1], key[2], key[3],
key[4], key[5], key[6], key[7],
counter, nonce[0], nonce[1], nonce[2] };
std::array<uint32_t, 16> workingState = state;
for (int i = 0; i < 10; ++i) {
// Apply the ChaCha20 quarter-rounds in "column" then "diagonal" orders
quarterRound(workingState[0], workingState[4], workingState[8], workingState[12]);
quarterRound(workingState[1], workingState[5], workingState[9], workingState[13]);
quarterRound(workingState[2], workingState[6], workingState[10], workingState[14]);
quarterRound(workingState[3], workingState[7], workingState[11], workingState[15]);
quarterRound(workingState[0], workingState[5], workingState[10], workingState[15]);
quarterRound(workingState[1], workingState[6], workingState[11], workingState[12]);
quarterRound(workingState[2], workingState[7], workingState[8], workingState[13]);
quarterRound(workingState[3], workingState[4], workingState[9], workingState[14]);
}
// Add the original state to the working state to produce the keystream block
for (int i = 0; i < 16; ++i) {
workingState[i] += state[i];
}
return workingState;
}
3. Encrypting Data with ChaCha20
With the block feature in place, encryption now involves creating the keystream and XORing it with the plaintext information.
std::vector<uint8_t> chacha20Encrypt(const std::vector<uint8_t> &plaintext, const uint32_t key[8], uint32_t counter, const uint32_t nonce[3]) {
std::vector<uint8_t> ciphertext(plaintext.size());
for (size_t i = 0; i < plaintext.size(); i += 64) {
auto block = chacha20Block(key, counter++, nonce);
for (size_t j = 0; j < 64 && i + j < plaintext.size(); ++j) {
uint8_t keystream_byte = (block[j / 4] >> (8 * (j % 4))) & 0xFF;
ciphertext[i + j] = plaintext[i + j] ^ keystream_byte;
}
}
return ciphertext;
}
Program 1:
#include <cstdint>
#include <array>
#include <vector>
#include <iostream>
// ChaCha20 constants
constexpr uint32_t constants[] = { 0x61707865, 0x3320646e, 0x79622d32, 0x6b206574 };
// Quarter-round function for ChaCha20
void quarterRound(uint32_t &a, uint32_t &b, uint32_t &c, uint32_t &d) {
a += b; d ^= a; d = (d << 16) | (d >> (32 - 16));
c += d; b ^= c; b = (b << 12) | (b >> (32 - 12));
a += b; d ^= a; d = (d << 8) | (d >> (32 - 8));
c += d; b ^= c; b = (b << 7) | (b >> (32 - 7));
}
// ChaCha20 block function that produces a 64-byte keystream block
std::array<uint32_t, 16> chacha20Block(const uint32_t key[8], uint32_t counter, const uint32_t nonce[3]) {
std::array<uint32_t, 16> state = {
constants[0], constants[1], constants[2], constants[3],
key[0], key[1], key[2], key[3],
key[4], key[5], key[6], key[7],
counter, nonce[0], nonce[1], nonce[2]
};
std::array<uint32_t, 16> workingState = state;
for (int i = 0; i < 10; ++i) {
// Column rounds
quarterRound(workingState[0], workingState[4], workingState[8], workingState[12]);
quarterRound(workingState[1], workingState[5], workingState[9], workingState[13]);
quarterRound(workingState[2], workingState[6], workingState[10], workingState[14]);
quarterRound(workingState[3], workingState[7], workingState[11], workingState[15]);
// Diagonal rounds
quarterRound(workingState[0], workingState[5], workingState[10], workingState[15]);
quarterRound(workingState[1], workingState[6], workingState[11], workingState[12]);
quarterRound(workingState[2], workingState[7], workingState[8], workingState[13]);
quarterRound(workingState[3], workingState[4], workingState[9], workingState[14]);
}
for (int i = 0; i < 16; ++i) {
workingState[i] += state[i];
}
return workingState;
}
// ChaCha20 encryption/decryption function
std::vector<uint8_t> chacha20Encrypt(const std::vector<uint8_t> &input, const uint32_t key[8], uint32_t counter, const uint32_t nonce[3]) {
std::vector<uint8_t> output(input.size());
for (size_t i = 0; i < input.size(); i += 64) {
auto block = chacha20Block(key, counter++, nonce);
for (size_t j = 0; j < 64 && i + j < input.size(); ++j) {
uint8_t keystream_byte = (block[j / 4] >> (8 * (j % 4))) & 0xFF;
output[i + j] = input[i + j] ^ keystream_byte;
}
}
return output;
}
// Example usage of ChaCha20 for encryption and decryption
int main() {
// 256-bit (32-byte) key for ChaCha20
uint32_t key[8] = { 0x03020100, 0x07060504, 0x0B0A0908, 0x0F0E0D0C,
0x13121110, 0x17161514, 0x1B1A1918, 0x1F1E1D1C };
// 96-bit (12-byte) nonce
uint32_t nonce[3] = { 0x00000000, 0x4A000000, 0x00000000 };
// Sample plaintext (example)
std::vector<uint8_t> plaintext = { 'H', 'e', 'l', 'l', 'o', ',', ' ', 'W', 'o', 'r', 'l', 'd', '!' };
// Encrypting the plaintext
auto ciphertext = chacha20Encrypt(plaintext, key, 1, nonce);
std::cout << "Ciphertext: ";
for (auto byte : ciphertext) {
std::cout << std::hex << (int)byte << " ";
}
std::cout << std::endl;
// Decrypting the ciphertext (ChaCha20 encryption is symmetric)
auto decrypted = chacha20Encrypt(ciphertext, key, 1, nonce);
std::cout << "Decrypted text: ";
for (auto byte : decrypted) {
std::cout << (char)byte;
}
std::cout << std::endl;
return 0;
}
Output:
Ciphertext: 6a 2a 3d 9f 2f 37 f9 b6 40 ac 4b b 99
Decrypted text: Hello, World!
Explanation:
- Initialization: Incorporates AES constants and the pre-computed values of S-box and Rcon.
- Critical Expansion: This is the process of deriving a key schedule from the original 256-bit key that can be used for all rounds of AES.
- Encryption Steps: We implement AddRoundKey, SubBytes, ShiftRows and MixColumns, which are thin transformations on the AES state. Encryption makes use of CBC mode to XOR each plain text block with the preceding ciphertext block.
- Decryption Steps: It takes the opposite sequence to that of the encryption steps. AES Encrypt/Decrypt Block: Transforms individual blocks consisting of rounds in AES.
- CBC Mode: join the encryption and decryption block using the initialization vector.
- We implement AddRoundKey, SubBytes, ShiftRows and MixColumns, which are thin transformations on the AES state.
- Encryption makes use of CBC mode to XOR each plain text block with the preceding ciphertext block.
- It takes the opposite sequence to that of the encryption steps.
- AES Encrypt/Decrypt Block: Transforms individual blocks consisting of rounds in AES.
Program 2:
#include <cstdint>
#include <vector>
#include <array>
#include <iostream>
#include <iomanip>
#include <cstring>
// ChaCha20 constants
constexpr std::array<uint32_t, 4> constants = { 0x61707865, 0x3320646e, 0x79622d32, 0x6b206574 };
// Quarter-round function for ChaCha20
void quarterRound(uint32_t &a, uint32_t &b, uint32_t &c, uint32_t &d) {
a += b; d ^= a; d = (d << 16) | (d >> (32 - 16));
c += d; b ^= c; b = (b << 12) | (b >> (32 - 12));
a += b; d ^= a; d = (d << 8) | (d >> (32 - 8));
c += d; b ^= c; b = (b << 7) | (b >> (32 - 7));
}
// ChaCha20 block function for generating keystream
std::array<uint32_t, 16> chacha20Block(const uint32_t key[8], uint32_t counter, const uint32_t nonce[3]) {
std::array<uint32_t, 16> state = {
constants[0], constants[1], constants[2], constants[3],
key[0], key[1], key[2], key[3],
key[4], key[5], key[6], key[7],
counter, nonce[0], nonce[1], nonce[2]
};
std::array<uint32_t, 16> workingState = state;
for (int i = 0; i < 10; ++i) {
// Column rounds
quarterRound(workingState[0], workingState[4], workingState[8], workingState[12]);
quarterRound(workingState[1], workingState[5], workingState[9], workingState[13]);
quarterRound(workingState[2], workingState[6], workingState[10], workingState[14]);
quarterRound(workingState[3], workingState[7], workingState[11], workingState[15]);
// Diagonal rounds
quarterRound(workingState[0], workingState[5], workingState[10], workingState[15]);
quarterRound(workingState[1], workingState[6], workingState[11], workingState[12]);
quarterRound(workingState[2], workingState[7], workingState[8], workingState[13]);
quarterRound(workingState[3], workingState[4], workingState[9], workingState[14]);
}
for (int i = 0; i < 16; ++i) {
workingState[i] += state[i];
}
return workingState;
}
// ChaCha20 encryption function
std::vector<uint8_t> chacha20Encrypt(const std::vector<uint8_t> &input, const uint32_t key[8], uint32_t counter, const uint32_t nonce[3]) {
std::vector<uint8_t> output(input.size());
for (size_t i = 0; i < input.size(); i += 64) {
auto block = chacha20Block(key, counter++, nonce);
for (size_t j = 0; j < 64 && i + j < input.size(); ++j) {
uint8_t keystream_byte = (block[j / 4] >> (8 * (j % 4))) & 0xFF;
output[i + j] = input[i + j] ^ keystream_byte;
}
}
return output;
}
// HChaCha20 function, commonly used for key derivation
std::array<uint32_t, 8> hchacha20(const uint32_t key[8], const uint32_t nonce[4]) {
std::array<uint32_t, 16> state = {
constants[0], constants[1], constants[2], constants[3],
key[0], key[1], key[2], key[3],
key[4], key[5], key[6], key[7],
nonce[0], nonce[1], nonce[2], nonce[3]
};
for (int i = 0; i < 10; ++i) {
// Column rounds
quarterRound(state[0], state[4], state[8], state[12]);
quarterRound(state[1], state[5], state[9], state[13]);
quarterRound(state[2], state[6], state[10], state[14]);
quarterRound(state[3], state[7], state[11], state[15]);
// Diagonal rounds
quarterRound(state[0], state[5], state[10], state[15]);
quarterRound(state[1], state[6], state[11], state[12]);
quarterRound(state[2], state[7], state[8], state[13]);
quarterRound(state[3], state[4], state[9], state[14]);
}
return { state[0], state[1], state[2], state[3], state[12], state[13], state[14], state[15] };
}
// Poly1305 one-time authentication tag generation
std::array<uint8_t, 16> poly1305Auth(const std::vector<uint8_t>& msg, const uint8_t key[32]) {
std::array<uint8_t, 16> tag = {0}; // Placeholder: This requires an actual Poly1305 implementation.
// Implement Poly1305 message authentication here if needed, as this is critical for ChaCha20-Poly1305.
return tag;
}
// ChaCha20-Poly1305 AEAD encryption
std::pair<std::vector<uint8_t>, std::array<uint8_t, 16>> chacha20Poly1305Encrypt(
const std::vector<uint8_t>& plaintext, const uint32_t key[8], const uint32_t nonce[3], const std::vector<uint8_t>& aad) {
// Encrypt plaintext
std::vector<uint8_t> ciphertext = chacha20Encrypt(plaintext, key, 1, nonce);
// Calculate Poly1305 tag
uint8_t polyKey[32] = {0}; // Generate Poly1305 key using first 256 bits of keystream block
auto poly1305Tag = poly1305Auth(ciphertext, polyKey);
return { ciphertext, poly1305Tag };
}
int main() {
// 256-bit key for ChaCha20
uint32_t key[8] = { 0x03020100, 0x07060504, 0x0B0A0908, 0x0F0E0D0C, 0x13121110, 0x17161514, 0x1B1A1918, 0x1F1E1D1C };
// 96-bit nonce (12 bytes)
uint32_t nonce[3] = { 0x00000000, 0x4A000000, 0x00000000 };
// Additional authenticated data (AAD)
std::vector<uint8_t> aad = { 0x00, 0x01, 0x02, 0x03, 0x04, 0x05 };
// Sample plaintext
std::vector<uint8_t> plaintext = { 'H', 'e', 'l', 'l', 'o', ',', ' ', 'W', 'o', 'r', 'l', 'd', '!' };
// Encrypt using ChaCha20-Poly1305
auto [ciphertext, tag] = chacha20Poly1305Encrypt(plaintext, key, nonce, aad);
// Output ciphertext
std::cout << "Ciphertext: ";
for (auto byte : ciphertext) {
std::cout << std::hex << std::setw(2) << std::setfill('0') << (int)byte << " ";
}
std::cout << std::endl;
// Output Poly1305 tag
std::cout << "Tag: ";
for (auto byte : tag) {
std::cout << std::hex << std::setw(2) << std::setfill('0') << (int)byte << " ";
}
std::cout << std::endl;
return 0;
}
Output:
Ciphertext: 6a 2a 3d 9f 2f 37 f9 b6 40 ac 4b 0b 99
Tag: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Explanation:
- Quarter Round and Block Functions: The quarterRound and chacha20Block functions perform ChaCha20's 20 rounds of permutation, which issues a keystream of 64 bytes.
- Encryption Function: The chacha20Encrypt function takes plaintext as data as input and encrypts it by XORing with ChaCha20 keystream.
- HChaCha20 Function: The hchacha20 function allows optional derivation of subkeys for use if required.
- Poly1305 Authentication Tag: The poly1305Auth function would use Poly1305 to authenticate the ciphertext, but it only uses a placeholder array in this case.
- ChaCha20-Poly1305 Encryption: The chacha20Poly1305Encrypt function can first encrypt with ChaCha20 and use Poly1305 to generate an authentication tag.
ChaCha20 Vs Other Algorithms
1. ChaCha20 vs. AES
Nevertheless, significant distinctions exist in terms of their construction, structure, and intended application, setting them apart from the widely utilized ChaCha20 and AES encryption algorithms, which are currently highly favored in cryptographic operations.
Key Differences:
- Speed: ChaCha20 is more performant on platforms that do not include AES hardware acceleration (AES-NI), such as phones and low-powered IoT devices.
- Timing Attack Resistance: Due to its architecture, ChaCha20 is timing attack resistant, unlike AES, which can be if sufficient measures are not employed.
- Ease of Implementation: ChaCha20 is more ergonomic to implement safely than AES since there is no padding and modes of operation.
Use Scenarios:
- ChaCha20 is predominantly employed in mobile, compact, and efficient applications, VPN services (such as WireGuard), and protocols resistant to timing attacks.
- AES serves as the default choice for the majority of applications primarily due to its hardware compatibility, rather than being the optimal solution, particularly in the context of banking and corporate systems.
2. ChaCha20 vs. Salsa20
As previously stated, ChaCha20 is a variant of Salsa20 that includes enhancements and adjustments, crafted by the cryptographer Daniel J. Bernstein.
Key Contrasts:
- Nonce Length: ChaCha20 provides a 96-bit nonce, aiming to reduce the chances of nonce repetition compared to Salsa20, which has a 64-bit nonce.
- Dissemination: Theoretically, ChaCha20 achieves superior dissemination, ensuring a more effective spread of input bits in the output compared to Salsa20, enhancing its security features.
Use Cases:
- ChaCha20: In most scenarios, ChaCha20 is favored over Salsa20 due to its enhanced security features.
- Salsa20: The usage of Salsa20 is diminishing in favor of ChaCha20. Nonetheless, it persists in certain environments prioritizing speed and managing nonce uniqueness.
3. ChaCha20 vs Other Stream Ciphers (for instance, RC4)
ChaCha20 is also applied in conjunction with other legacy cipher options like RC4.
Key Contrasts:
- Security: The discontinuation of RC4 is due to various reasons, including vulnerabilities in its keystream, rendering it unsuitable for contemporary applications. In contrast, ChaCha20 does not exhibit these weaknesses.
- Performance: While both algorithms are efficient, ChaCha20 outperforms RC4, especially on modern hardware and energy-efficient devices, offering superior speed and security.
Conclusion:
In summary, the arrangement of this operational C++ realization of ChaCha20, as outlined in this document, incorporates the core procedures that delineate the encryption process of ChaCha20 in a similar fashion, encompassing the setup, the quarter-round operations, and the block functionality. This comprehension also empowers software engineers to recognize the robustness, efficiency, and rationale behind the suitability of ChaCha20 for real-world cryptographic tasks. The significance of ChaCha20 in various contemporary applications and protocols does indeed pave the way for its application, and now, with this C++ implementation available, it can be effortlessly leveraged as a cryptographic asset.
