How to recognise cryptographic algorithms by just looking at them? For each algorithm there is a list of bullet points, sometimes a high-level idea of how the function may look.

AES

Curve25519

void curve25519(out, scalar, point) {
  uint8_t   e[32];
  ...
  for (pos = 254; pos >= 0; --pos) {
    // EC sub, add, mul, sqr operations
  }
}

• >> 0x19 in the multiplication operation.

ChaCha20

Key: initial state includes 16-byte constant “expand 32-byte k”, 32-byte key, 4-byte counter, 12-byte nonce (in that order)

Quater Round:

a += b; d ^= a; d <<<= 16;
c += d; b ^= c; b <<<= 12;
a += b; d ^= a; d <<<=  8;
c += d; b ^= c; b <<<=  7;

Double Round:

// Odd round
QR(0, 4,  8, 12) // column 1
QR(1, 5,  9, 13) // column 2
QR(2, 6, 10, 14) // column 3
QR(3, 7, 11, 15) // column 4
// Even round
QR(0, 5, 10, 15) // diagonal 1 (main diagonal)
QR(1, 6, 11, 12) // diagonal 2
QR(2, 7,  8, 13) // diagonal 3
QR(3, 4,  9, 14) // diagonal 4

• As name suggests, 20 rounds (but there are reduced round variants)
• Commonly implemented as 10 iterations of double round
• “expand 32-byte k” constant
• First rotations are by 16, 12, 8, 7

ChaCha20-Poly1305

• As name suggests, 20 rounds (but there are reduced round variants)
• 256bit key, 96bit nonce
• ciphertext gets 16 byte tag