如何离线解密SQL Server AEAD_AES_256_CBC_HMAC_SHA_256加密值及手动解密Always Encrypted密文

Absolutely, you can manually decrypt values encrypted with SQL Server Always Encrypted using the AEAD_AES_256_CBC_HMAC_SHA_256 algorithm—especially since you already hold the necessary encryption keys. Offline decryption is totally feasible for your use case to mitigate Azure availability risks, and here's a step-by-step breakdown to make it happen:

Short answer: Yes. This algorithm uses a combination of AES-256-CBC for encryption and HMAC-SHA-256 for data integrity (an Authenticated Encryption with Associated Data, or AEAD, scheme). As long as you have the plaintext Column Encryption Key (CEK), you can implement the decryption logic yourself without relying on SQL Server or Azure services.

Prerequisites You’ll Need #

• Plaintext Column Encryption Key (CEK): If your CEK was stored encrypted (e.g., wrapped by a Column Master Key, CMK), you first need to decrypt it using your CMK (e.g., a local RSA private key if you're not using Azure Key Vault).
• Encrypted data blob: The raw binary value from your encrypted SQL Server column.
• A programming language with cryptographic libraries (we’ll use C# with .NET’s built-in APIs as an example, but Python/Java work too).

Understand the Encrypted Data Structure #

SQL Server formats AEAD_AES_256_CBC_HMAC_SHA_256 encrypted data into a fixed sequence:

• First 16 bytes: Random initialization vector (IV) for AES-CBC
• Next 32 bytes: HMAC-SHA-256 authentication tag (verifies data integrity)
• Remaining bytes: The AES-256-CBC encrypted plaintext

Additionally, the CEK used for this algorithm is a 512-bit (64-byte) key split into two parts:

• First 32 bytes: AES-256 encryption key
• Last 32 bytes: HMAC-SHA-256 authentication key

Step-by-Step Decryption Process (C# Example) #

Here’s a concrete implementation that handles both integrity verification and decryption:

using System;
using System.Security.Cryptography;

public static byte[] DecryptAlwaysEncryptedValue(byte[] encryptedData, byte[] cekPlaintext)
{
    // Split the 512-bit CEK into AES and HMAC keys
    byte[] aesKey = new byte[32];
    byte[] hmacKey = new byte[32];
    Buffer.BlockCopy(cekPlaintext, 0, aesKey, 0, 32);
    Buffer.BlockCopy(cekPlaintext, 32, hmacKey, 0, 32);

    // Extract IV, auth tag, and ciphertext from the encrypted blob
    byte[] iv = new byte[16];
    byte[] authTag = new byte[32];
    byte[] ciphertext = new byte[encryptedData.Length - 16 - 32];
    
    Buffer.BlockCopy(encryptedData, 0, iv, 0, 16);
    Buffer.BlockCopy(encryptedData, 16, authTag, 0, 32);
    Buffer.BlockCopy(encryptedData, 48, ciphertext, 0, ciphertext.Length);

    // Verify data integrity with HMAC-SHA-256
    using (var hmac = new HMACSHA256(hmacKey))
    {
        byte[] dataToVerify = new byte[iv.Length + ciphertext.Length];
        Buffer.BlockCopy(iv, 0, dataToVerify, 0, iv.Length);
        Buffer.BlockCopy(ciphertext, 0, dataToVerify, iv.Length, ciphertext.Length);
        
        byte[] computedTag = hmac.ComputeHash(dataToVerify);
        if (!CryptographicOperations.FixedTimeEquals(computedTag, authTag))
        {
            throw new CryptographicException("Authentication tag mismatch - data may be tampered with or corrupted.");
        }
    }

    // Decrypt the AES-256-CBC ciphertext
    using (var aes = Aes.Create())
    {
        aes.Key = aesKey;
        aes.IV = iv;
        aes.Mode = CipherMode.CBC;
        aes.Padding = PaddingMode.PKCS7; // SQL Server uses PKCS7 padding

        using (var decryptor = aes.CreateDecryptor(aesKey, iv))
        {
            return decryptor.TransformFinalBlock(ciphertext, 0, ciphertext.Length);
        }
    }
}

Key Notes for Offline Decryption #

• CEK Security: Handle the plaintext CEK with extreme care—avoid writing it to disk, use memory protection APIs (like SecureString in .NET) where possible, and never transmit it over untrusted networks.
• Integrity First: Always verify the HMAC tag before decrypting. Skipping this step leaves you vulnerable to tampered or corrupted data.
• Cross-Language Support: If you prefer Python, use libraries like pycryptodome to replicate the same logic: split the CEK, extract the IV/tag/ciphertext, verify the HMAC, then decrypt with AES-CBC.
• CMK Decryption: If your CEK is wrapped by a CMK (e.g., an RSA key), use your CMK's private key to first decrypt the wrapped CEK into its plaintext form before proceeding with the above steps.

内容的提问来源于stack exchange，提问作者sglogowski