Convert.FromBase64String行为异常，附RSA解密代码求助

Hey, let's figure out why Convert.FromBase64String is acting up with your decrypted content. I'll walk through the most likely problems and how to fix them, based on the code and sample payload you shared.

First, let's recap what your code is doing: you split incoming data into chunks, decrypt each with RSA, concatenate the results into a string sb, then try to convert that Base64 string to bytes—something's breaking here.

Common Culprits & Solutions #

• Invalid Characters Sneaking Into Your Concatenated String
  When you decrypt chunks and mash them together, it's easy to end up with hidden whitespace, null bytes, or weird non-printable characters that Base64 doesn't tolerate. Base64 only accepts A-Z, a-z, 0-9, +, /, and = (for padding). Any other character will throw off Convert.FromBase64String.

  Fix this by cleaning the string first:

  // Strip out any characters that aren't valid Base64
  var cleanSb = new string(sb.Where(c => 
      char.IsLetterOrDigit(c) || c == '+' || c == '/' || c == '=').ToArray());
  // Now use the cleaned string for conversion
  var payloadBytes = Convert.FromBase64String(cleanSb);
  

• Broken Decryption or Chunk Splitting
  If even one chunk fails to decrypt properly (wrong private key, truncated chunk during transmission) or your Split('|') is cutting chunks incorrectly, your final sb will be invalid Base64.

  Debug this with a few quick checks:

  1. Log each individual decrypted chunk before concatenation—make sure each piece looks like part of a valid Base64 string (no garbage characters).
  2. Double-check that globals.serverPrivateKey is correctly loaded (confirm it's in the right format for your Cryptography.RSA.decrypt method—XML, PEM, etc.).
  3. Verify that the original data doesn't contain any | characters within chunks (if it does, your delimiter is bad and you'll split content incorrectly).

• Missing Base64 Padding
  Base64 strings need to be padded to a length divisible by 4 using = characters. If your sb is missing this padding, Convert.FromBase64String might throw an error or return garbage data.

  Add padding manually if needed:

  int paddingNeeded = sb.Length % 4;
  if (paddingNeeded != 0)
  {
      sb += new string('=', 4 - paddingNeeded);
  }
  var payloadBytes = Convert.FromBase64String(sb);
  

• Your Payload is GZIP Compressed
  The sample sb you shared starts with H4sI—that's the Base64 representation of GZIP's magic bytes. If you're expecting plain text, you need to decompress it after converting from Base64:

  var payloadBytes = Convert.FromBase64String(sb);
  using (var gzipStream = new GZipStream(new MemoryStream(payloadBytes), CompressionMode.Decompress))
  using (var reader = new StreamReader(gzipStream))
  {
      var plainText = reader.ReadToEnd();
      Debug.Log("Decompressed content: " + plainText);
  }
  

Quick Debugging Tips #

1. Log the length of sb—valid Base64 strings are always a multiple of 4 (unless padding is missing, which we can fix).
2. Test with your sample sb directly: hardcode that string and run Convert.FromBase64String on it. If it works, the problem is in your decryption or chunk-splitting step, not the conversion itself.

   var testSb = "H4sIAAAAAAAAAG2MywrCQAxF/yXrUqjuulMXUlwI/kGcibU4L5IUkdJ/N1oXCq4mc+69ZwIXUARayNzXifSaRWuXY8Tkpd6TdkrxRFJyEoIKPpENetLBMmPvp53+qjwq1t3S8ySOh6JDTtZqVusX41zwHAjaCwYhk5m7qSBhNAab7c5KhQdnv6axQBTdbVkojzZQRk8/4FHsXlVwJ+Qv91wBk4xBzXs8wPwEOn1v+/wAAAA=";
   var testBytes = Convert.FromBase64String(testSb);
   // This should run without errors—if it does, your decryption flow is the issue
   

内容的提问来源于stack exchange，提问作者Martin Kørvel