Feb 05, 2020 Regarding this, is AES 256 CBC secure? The AES-GCM mode of operation can actually be carried out in parallel both for encryption and decryption. The additional security that this method provides also allows the VPN use only a 128 bit key, whereas AES-CBC typically requires a 256 bit key to be considered secure. I have working encoder/decoder on my php backend with key and vector in plain text. Trying to create a simple POC decoder to pass some secret info from my backend to CF worker via client request.
-->Applies to: SQL Server (all supported versions) Azure SQL Database Azure SQL Managed Instance
This document describes encryption algorithms and mechanisms to derive cryptographic material used in the Always Encrypted feature in SQL Server and Azure SQL Database.
Keys, key stores, and key encryption algorithms
Always Encrypted uses two types of keys: Column master keys and column encryption keys.
A column master key (CMK) is a key encrypting key (for example, a key that is used to encrypt other keys) that is always in a client's control, and is stored in an external key store. An Always Encrypted-enabled client driver interacts with the key store via a CMK store provider, which can be either part of the driver library (a Microsoft/system provider) or part of the client application (a custom provider). Client driver libraries currently include Microsoft key store providers for Windows Certificate Store and hardware security modules (HSMs). For the current list of providers, see CREATE COLUMN MASTER KEY (Transact-SQL). An application developer can supply a custom provider for an arbitrary store.
A column encryption key (CEK), is a content encryption key (for example, a key that is used to protect data) that is protected by a CMK.
All Microsoft CMK store providers encrypt CEKs by using RSA with Optimal Asymmetric Encryption Padding (RSA-OAEP). The key store provider that supports Microsoft Cryptography API: Next Generation (CNG) in .NET Framework (SqlColumnEncryptionCngProvider Class) uses the default parameters specified by RFC 8017 in Section A.2.1. Those default parameters are using a hash function of SHA-1 and a mask generation function of MGF1 with SHA-1. All other key store providers use SHA-256.
Always Encrypted internally uses FIPS 140-2 validated cryptographic modules.
Data Encryption Algorithm
Always Encrypted uses the AEAD_AES_256_CBC_HMAC_SHA_256 algorithm to encrypt data in the database.
AEAD_AES_256_CBC_HMAC_SHA_256 is derived from the specification draft at https://tools.ietf.org/html/draft-mcgrew-aead-aes-cbc-hmac-sha2-05. It uses an Authenticated Encryption scheme with Associated Data, following an Encrypt-then-MAC approach. That is, the plaintext is first encrypted, and the MAC is produced based on the resulting ciphertext.
In order to conceal patterns, AEAD_AES_256_CBC_HMAC_SHA_256 uses the Cipher Block Chaining (CBC) mode of operation, where an initial value is fed into the system named the initialization vector (IV). The full description of the CBC mode can be found at https://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf.
AEAD_AES_256_CBC_HMAC_SHA_256 computes a ciphertext value for a given plaintext value using the following steps.
Step 1: Generating the initialization vector (IV)
Always Encrypted supports two variations of AEAD_AES_256_CBC_HMAC_SHA_256:
Randomized
Deterministic
For randomized encryption, the IV is randomly generated. As a result, each time the same plaintext is encrypted, a different ciphertext is generated, which prevents any information disclosure.
If there's deterministic encryption, the IV isn't randomly generated, but instead it's derived from the plaintext value using the following algorithm:
Where iv_key is derived from the CEK in the following way:
The HMAC value truncation is performed to fit one block of data as needed for the IV.As a result, deterministic encryption always produces the same ciphertext for a given plaintext value, which enables inferring whether two plaintext values are equal by comparing their corresponding ciphertext values. This limited information disclosure allows the database system to support equality comparison on encrypted column values.
Deterministic encryption is more effective in concealing patterns, compared to alternatives, such as using a pre-defined IV value.
Step 2: Computing AES_256_CBC Ciphertext
After computing the IV, the AES_256_CBC ciphertext is generated:
Where the encryption key (enc_key) is derived from the CEK as follows.
Step 3: Computing MAC
Subsequently, the MAC is computed using the following algorithm:
Where:
Step 4: Concatenation
Finally, the encrypted value is produced by concatenating the algorithm version byte, the MAC, the IV, and the AES_256_CBC ciphertext:
Ciphertext Length
The lengths (in bytes) of particular components of AEAD_AES_256_CBC_HMAC_SHA_256 ciphertext are:
versionbyte: 1
MAC: 32
IV: 16
aes_256_cbc_ciphertext:
(FLOOR (DATALENGTH(cell_data)/ block_size) + 1)* block_size, where:block_size is 16 bytes
cell_data is a plaintext value
Therefore, the minimal size of aes_256_cbc_ciphertext is 1 block, which is 16 bytes.
Thus, the length of ciphertext, resulting from encrypting a given plaintext values (cell_data), can be calculated using the following formula:
For example:
A 4-byte long int plaintext value becomes a 65-byte long binary value after encryption.
A 2,000-byte long nchar(1000) plaintext values becomes a 2,065-byte long binary value after encryption.
Aes 256 Cbc Key Replacement
The following table contains a complete list of data types and the length of ciphertext for each type.
| Data Type | Ciphertext Length [bytes] |
|---|---|
| bigint | 65 |
| binary | Varies. Use the formula above. |
| bit | 65 |
| char | Varies. Use the formula above. |
| date | 65 |
| datetime | 65 |
| datetime2 | 65 |
| datetimeoffset | 65 |
| decimal | 81 |
| float | 65 |
| geography | N/A (not supported) |
| geometry | N/A (not supported) |
| hierarchyid | N/A (not supported) |
| image | N/A (not supported) |
| int | 65 |
| money | 65 |
| nchar | Varies. Use the formula above. |
| ntext | N/A (not supported) |
| numeric | 81 |
| nvarchar | Varies. Use the formula above. |
| real | 65 |
| smalldatetime | 65 |
| smallint | 65 |
| smallmoney | 65 |
| sql_variant | N/A (not supported) |
| sysname | N/A (not supported) |
| text | N/A (not supported) |
| time | 65 |
| timestamp (rowversion) | N/A (not supported) |
| tinyint | 65 |
| uniqueidentifier | 81 |
| varbinary | Varies. Use the formula above. |
| varchar | Varies. Use the formula above. |
| xml | N/A (not supported) |
.NET Reference
Aes 256 Cbc Key Replacement
For details about the algorithms, discussed in this document, see the SqlAeadAes256CbcHmac256Algorithm.cs, SqlColumnEncryptionCertificateStoreProvider.cs, and SqlColumnEncryptionCertificateStoreProvider.cs files in the .NET Reference.
See Also
Advanced Encryption Standard(AES) is a symmetric encryption algorithm. AES is the industry standard as of now as it allows 128 bit, 192 bit and 256 bit encryption.Symmetric encryption is very fast as compared to asymmetric encryption and are used in systems such as database system. Following is an online tool to generate AES encrypted password and decrypt AES encrypted password. It provides two mode of encryption and decryption ECB and CBC mode. For more info on AES encryption visit this explanation on AES Encryption.
Also, you can find the sample usage screenshot below:
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Aes-256-cbc Key Laravel
Usage Guide
Any plain-text input or output that you enter or we generate is not stored on this site, this tool is provided via an HTTPS URL to ensure that text cannot be stolen.
For encryption, you can either enter the plain text, password, an image file or a .txt file that you want to encrypt. Now choose the block cipher mode of encryption. ECB(Electronic Code Book) is the simplest encryption mode and does not require IV for encryption. The input plain text will be divided into blocks and each block will be encrypted with the key provided and hence identical plain text blocks are encrypted into identical cipher text blocks. CBC mode is highly recommended and it requires IV to make each message unique. If no IV is entered then default will be used here for CBC mode and that defaults to a zero based byte[16].
The AES algorithm has a 128-bit block size, regardless of whether you key length is 256, 192 or 128 bits. When a symmetric cipher mode requires an IV, the length of the IV must be equal to the block size of the cipher. Hence, you must always use an IV of 128 bits (16 bytes) with AES.
AES provides 128 bit, 192 bit and 256 bit of secret key size for encryption. Things to remember here is if you are selecting 128 bits for encryption, then the secret key must be of 16 bits long and 24 and 32 bits for 192 and 256 bits of key size. Now you can enter the secret key accordingly. By default, the encrypted text will be base64 encoded but you have options to select the output format as HEX too.
Similarly, for image and .txt file the encrypted form will be Base64 encoded.
Below is a screenshot that shows a sample usage of this online AES encryption tool.
AES decryption has also the same process. By default it assumes the entered text be in Base64. The input can be Base64 encoded or Hex encoded image and .txt file too. And the final decrypted output will be Base64 string. If the intended output is a plain-text then, it can be decoded to plain-text in-place.
But if the intended output is an image or .txt file then you can use this tool to convert the base64 encoded output to an image.
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