feat: parse encrypted isphere packet logs

internal/isphere/logcodec/codec.go

@@ -0,0 +1,52 @@
+package logcodec
+
+import (
+	"crypto/cipher"
+	"crypto/des"
+	"encoding/base64"
+	"errors"
+	"fmt"
+)
+
+var (
+	policyKey = []byte("hyhccdtm")
+	policyIV  = []byte{0x12, 0x34, 0x56, 0x78, 0x90, 0xAB, 0xCD, 0xEF}
+)
+
+func DecryptLine(ciphertextBase64 string) (string, error) {
+	ciphertext, err := base64.StdEncoding.DecodeString(ciphertextBase64)
+	if err != nil {
+		return "", fmt.Errorf("decode base64 log line: %w", err)
+	}
+	if len(ciphertext) == 0 || len(ciphertext)%des.BlockSize != 0 {
+		return "", fmt.Errorf("ciphertext length %d is not a positive DES block multiple", len(ciphertext))
+	}
+
+	block, err := des.NewCipher(policyKey)
+	if err != nil {
+		return "", fmt.Errorf("create DES cipher: %w", err)
+	}
+	plain := make([]byte, len(ciphertext))
+	cipher.NewCBCDecrypter(block, policyIV).CryptBlocks(plain, ciphertext)
+	plain, err = unpadPKCS7(plain, des.BlockSize)
+	if err != nil {
+		return "", err
+	}
+	return string(plain), nil
+}
+
+func unpadPKCS7(data []byte, blockSize int) ([]byte, error) {
+	if len(data) == 0 || len(data)%blockSize != 0 {
+		return nil, errors.New("pkcs7 data is empty or not block aligned")
+	}
+	pad := int(data[len(data)-1])
+	if pad == 0 || pad > blockSize || pad > len(data) {
+		return nil, fmt.Errorf("invalid pkcs7 padding length %d", pad)
+	}
+	for i := len(data) - pad; i < len(data); i++ {
+		if int(data[i]) != pad {
+			return nil, errors.New("invalid pkcs7 padding bytes")
+		}
+	}
+	return data[:len(data)-pad], nil
+}