Formats of encrypted data and signatures - Key Management Service

Data encryption output of Encryption SDK


Data encryption output	Component	Description
Message header	Version	The current version is 1.
	Algorithm	For more information, see Algorithms.
	Data key list	The data key list contains at least one data key. The information of each data key consists of the following parts: The Alibaba Cloud Resource Name (ARN) of the customer master key (CMK) that is used to encrypt the data key. An ARN includes the region ID, user ID, and CMK ID, and is presented in the format of `acs:kms:RegionId:UserId:key/CmkId`. The ciphertext of the data key encrypted by using the primary version of the specified CMK. The ciphertext is the value of the CipherBlob parameter that is returned by the GenerateDataKey operation.
	EncryptionContext	The encryption context data that is used as additional authentication data for symmetric encryption algorithms.
	Initialization vector for header authentication	The initialization vector that is used to compute header authentication information. The value is a random number.
	Header authentication information	The system computes the header authentication information based on Galois Message Authentication Code (GMAC). If verification fails, an error is returned, indicating that the format of the encryption message is invalid.
Message body	Initialization vector	An initialization vector is an input value with a fixed length. In most cases, it is a random number or pseudo-random number.
	Ciphertext data	The ciphertext returned after data is encrypted.
	Authentication data	The authentication data returned when Galois/Counter Mode (GCM) is used. Authentication data is used to verify data integrity. If the verification fails, a decryption failure is reported.

The following table describes algorithm information in a message header.


No.	Algorithm information	Algorithm	Working mode	Length of key (bit)	Length of initialization vector (byte)
1	AES_GCM_NOPADDING_128	AES	GCM	128	12
2	AES_GCM_NOPADDING_256	AES	GCM	256	12
3	AES_CBC_NOPADDING_128	AES	CBC	128	16
4	AES_CBC_NOPADDING_256	AES	CBC	256	16
5	AES_CBC_PKCS5_128	AES	CBC	128	16
6	AES_CBC_PKCS5_256	AES	CBC	256	16
7	AES_CTR_NOPADDING_128	AES	CTR	128	16
8	AES_CTR_NOPADDING_256	AES	CTR	256	16
9	SM4_GCM_NOPADDING_128	SM4	GCM	128	16
10	SM4_CBC_NOPADDING_128	SM4	CBC	128	16
11	SM4_CBC_PKCS5_128	SM4	CBC	128	16
12	SM4_CTR_NOPADDING_128	SM4	CTR	128	16

Note Only AES_GCM_NOPADDING_128, AES_GCM_NOPADDING_256, and SM4_GCM_NOPADDING_128 contain authentication data, which is 16 bytes in length.

Format definition of data encryption output

Data encryption output is encoded in ASN.1. The following code shows the format of data encryption output in the ASN.1 syntax:

EncryptionMessage ::== SEQUENCE {
    encryptionHead        EncryptionHead           --Message header
    encryptionBody        EncryptionBody           --Message body
}

EncryptionHead ::== SEQUENCE {
    version               INTEGER                  --Version
    algorithm             INTEGER                  --Algorithm
    encryptedDataKeys     SET EncryptedDataKey     --Data key list
    encryptionContext     SET EncryptionContext    --Encryption context
    headerIv              OCTECT STRING            --Initialization vector for header authentication
    headerAuthTag         OCTECT STRING            --Header authentication information
}

EncryptionBody ::== SEQUENCE{
    iv                    OCTECT STRING            --Initialization vector
    cipherText            OCTECT STRING            --Ciphertext data
    authTag               OCTECT STRING            --GCM authentication information
}

EncryptedDataKey ::== SEQUENCE {
    cmkArn                OCTECT STRING            --ARN of the KMS CMK
    encryptedDataKey      OCTECT STRING            --Ciphertext of the data key
}

EncryptionContext ::== SEQUENCE {
    key                   OCTECT STRING
    value                 OCTECT STRING
}

Example of data encryption output

SEQUENCE (2 elem)
  SEQUENCE (6 elem)
    INTEGER 1                                                       // Version
    INTEGER 2                                                       // Algorithm
    SET (2 elem)                                                    // Data key list
      SEQUENCE (2 elem)
        OCTET STRING (77 byte) acs:kms:cn-beijing:1540355698xxxxx:key/2fad5f44-9573-4f28-8956-xxxx…
        OCTET STRING (108 byte) 36613739356232362D626163642xxxx262642D383630612D323563313839316131663…
      SEQUENCE (2 elem)
        OCTET STRING (77 byte) acs:kms:cn-hangzhou:1540355698xxxxx:key/f6d61352-82bb-450a-b105-xxxx…
        OCTET STRING (108 byte) 62623630646439352D343165302xxxx237382D616233332D356262636136643633643…
    SET (5 elem)                                                    // EncryptionContext set
      SEQUENCE (2 elem)
        OCTET STRING (11 byte) encryption
        OCTET STRING (7 byte) context
      SEQUENCE (2 elem)
        OCTET STRING (7 byte) is not
        OCTET STRING (6 byte) secret
      SEQUENCE (2 elem)
        OCTET STRING (9 byte) but adds
        OCTET STRING (15 byte) useful metadata
      SEQUENCE (2 elem)
        OCTET STRING (18 byte) that can help you
        OCTET STRING (17 byte) be confident that
      SEQUENCE (2 elem)
        OCTET STRING (26 byte) the data you are handling
        OCTET STRING (23 byte) is what you think it is
    OCTET STRING (12 byte) E66C1CE19C79F3FBCD62858D                  // Initialization vector for header authentication
    OCTET STRING (16 byte) CEEC46C65670E82CD78028AC0104D083          // Header authentication information
  SEQUENCE (3 elem)                                                  // Encrypted message
    OCTET STRING (12 byte) EF49E2CBB768A7AD0FB0FE20                  // Initialization vector
    OCTET STRING (13 byte) 89A4AB43CD793F7711767C491A                // Ciphertext data
    OCTET STRING (16 byte) 2E93DA019B7A6507155BA3AA252750E3          // Authentication data

Length of data encryption output

(108 bytes + 77 bytes) × Number of CMKs

Note The ARN of a CMK is 108 bytes in length. The value of the CipherBlob parameter returned by the GenerateDataKey operation is 77 bytes in length.
Length of EncryptionContext
Length of ASN.1-encoded data: 30 bytes

Note The length of ASN.1-encoded data is 30 bytes.
Length of ciphertext data
Initialization vector
Authentication information

Key Management Service:Formats of encrypted data and signatures

Formats of encrypted data

Signature format