求助：Anonymous struct未在命名类型内问题及DES算法使用困惑（DevC++ v5.11）

Hey there, let's tackle your two issues one by one—both are totally manageable once you break them down!

First off, this error crops up when you try to declare an anonymous struct (a struct without a formal name) in a scope where the compiler expects a named type. Here are straightforward fixes with examples:

• Wrap the anonymous struct in a named type
  If you were using an anonymous struct directly in a global scope or function, either nest it inside a named struct/union or use typedef to give it an alias. For example:

  // Wrong approach (triggers the error)
  struct {
      int x;
      int y;
  } point;
  
  // Correct: Use typedef to assign a name to the struct
  typedef struct {
      int x;
      int y;
  } Point;
  Point point;
  
  // Or nest it inside a named container if your use case calls for it
  struct Shape {
      struct { // This anonymous struct is now valid inside the named Shape type
          int width;
          int height;
      } dimensions;
      int color;
  };
  

• Skip anonymous structs entirely
  Just give the struct a proper name from the start—this also makes your code more readable for others (and future you):

  struct Point {
      int x;
      int y;
  };
  struct Point point;
  

DES can feel intimidating at first, but breaking it into small, testable chunks will make it click. Let's start with core concepts, then move to actionable steps for Dev-C++:

Simplify the DES Concept First #

At its heart, DES is a symmetric block cipher that transforms 64-bit plaintext into 64-bit ciphertext using a 64-bit key (only 56 bits are actually used for encryption). It runs through 16 rounds of these core steps:

1. Initial Permutation (IP): Rearranges the bits of the input plaintext.
2. Round Key Generation: Splits the original key into 16 unique 48-bit round keys.
3. Round Function: Mixes the plaintext with the current round key via expansion, S-box substitution, and permutation.
4. Final Permutation (FP): Reverses the initial permutation to produce the final ciphertext.

Actionable Steps in Dev-C++ #

• Start with a minimal, commented DES implementation
  Grab a simplified DES code snippet (avoid overly complex libraries) and paste it into Dev-C++. Look for code that splits each DES step into separate functions (e.g., initial_permutation(), generate_round_keys(), round_function()). This lets you test each component individually.

• Use Dev-C++'s debugger to step through code
  Debugging is your best friend here:

  • Set breakpoints at the start of each permutation or round function.
  • Inspect variables (like the plaintext bits after initial permutation) to confirm they match standard DES test vectors (you can find these easily—for example, plaintext "0123456789ABCDEF" with key "133457799BBCDFF1" should produce a specific, documented ciphertext).

• Break down the round function into sub-steps
  The round function is the most complex part—test each sub-step separately:

  1. Expansion: Take the 32-bit right half of the plaintext and expand it to 48 bits.
  2. XOR with Round Key: Combine the expanded bits with the current 48-bit round key.
  3. S-box Substitution: Replace 6-bit chunks with 4-bit values using the standard DES S-boxes.
  4. P-box Permutation: Rearrange the 32 bits output from the S-boxes.

• Test encryption first, then decryption
  Get encryption working with a known test vector first, then implement decryption (which uses the round keys in reverse order) and verify you can recover the original plaintext.

内容的提问来源于stack exchange，提问作者Dima Faust