fish2018/pansou · error

填充长度非法

Error message

填充长度非法

What it means

removeMobilePadding reads the final byte as the padding size and rejects values that are <= 0 or exceed the data length. This means the decrypted data does not carry a valid padding trailer — a strong sign of wrong decryption (bad key/IV/mode) or corrupted ciphertext.

Solutions

1. Verify the decryption key/IV match the server's current values (key rotation).
2. Dump the last byte of decrypted data to see the actual value and compare with the expected padding scheme.
3. Confirm both sides use the same padding convention; align client with server's scheme.
4. Ensure the full ciphertext arrived (check Content-Length vs bytes read).

Example fix

// before
plain, err := decryptMobilePayload(payload, staleKey)
// after
key := fetchCurrentKey() // avoid stale rotated key
plain, err := decryptMobilePayload(payload, key)

last := data[len(data)-1]
if last == 0 || int(last) > len(data) {
    return fmt.Errorf("data not padded with expected scheme")
}

plain, err := removeMobilePadding(decrypted)
if err != nil {
    return refreshKeyAndRetry(decrypted) // likely key/IV drift
}

When it happens

Trigger: decryptMobilePadding's output's last byte is 0 or larger than len(data): decrypting with the wrong key, wrong cipher mode/IV, or feeding already-decrypted data through decryption again.

Common situations: Server rotated its encryption key but client kept the old one; mismatched padding scheme (client expects custom padding, server used PKCS#7 with different byte layout); ciphertext truncated in transit.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• ciphertext too short
• 密文长度不足
• ciphertext too short
• 空响应无法去填充
• 填充校验失败

AI-assisted analysis of fish2018/pansou@beaa561337 (2026-09-07). Data as JSON: /api/errors/4acd1fe07a1158e7. Report an issue: GitHub.

Appendix: source

Thrown at service/check_mobile_crypto.go:109

}

func addMobilePadding(data []byte, blockSize int) []byte {
	paddingSize := blockSize - len(data)%blockSize
	padding := make([]byte, paddingSize)
	for index := range padding {
		padding[index] = byte(paddingSize)
	}
	return append(data, padding...)
}

func removeMobilePadding(data []byte) ([]byte, error) {
	if len(data) == 0 {
		return nil, fmt.Errorf("空响应无法去填充")
	}

	paddingSize := int(data[len(data)-1])
	if paddingSize <= 0 || paddingSize > len(data) {
		return nil, fmt.Errorf("填充长度非法")
	}

	for index := len(data) - paddingSize; index < len(data); index++ {
		if data[index] != byte(paddingSize) {
			return nil, fmt.Errorf("填充校验失败")
		}
	}

	return data[:len(data)-paddingSize], nil
}

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