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Key Management Service:Key types and specifications

Last Updated:Sep 24, 2024

Key Management Service (KMS) supports common symmetric and asymmetric keys and ensures the security of encrypted data. This topic describes the key specifications in KMS by key type.

Key specifications supported for symmetric encryption

Symmetric encryption is commonly implemented to protect sensitive data. You can use the same key to encrypt and decrypt data. KMS keeps the key material of a symmetric key confidential to prevent the symmetric key from being used by unauthorized users or stolen. This ensures the security of encrypted data.

Key specifications

In most cases, symmetric keys are used for data encryption. The following table describes the key specifications that are supported by different types of symmetric keys.

Key type

Key specification

Data encryption mode

Padding mode

Default key

Aliyun_AES_256

GCM (default)

N/A.

Software-protected key

Aliyun_AES_256

GCM (default)

N/A.

Hardware-protected key

  • Aliyun_AES_256

  • Aliyun_AES_192

  • Aliyun_AES_128

GCM (default), electronic codebook (ECB), and cipher block chaining (CBC)

  • If the GCM mode is used, no padding modes are used.

  • If the ECB or CBC mode is used, the padding mode is PKCS7_PADDING or NO_PADDING.

External Key

Aliyun_AES_256

GCM (default), ECB, and CBC

  • If the GCM mode is used, no padding modes are used.

  • If the ECB or CBC mode is used, the padding mode is PKCS7_PADDING or NO_PADDING.

Note

If you call the CreateKey operation to create a key but do not configure KeySpec, KMS creates a symmetric key whose key specification is Aliyun_AES_256.

Technical specifications

Key specifications supported for asymmetric encryption

An asymmetric key consists of a public key and a private key that are mathematically related to each other. The public key is available for all users, but the private key can be used only by the owner or trusted users.

Note

After you create an asymmetric key in KMS, you can obtain the public key in the KMS console or by calling the GetPublicKey operation. KMS ensures the security of the private key and does not support export of private keys by calling operations.

In most cases, asymmetric key pairs are used for signing and verification or to encrypt and transmit a small amount of confidential information, such as symmetric keys. You can specify the key purpose when you create an asymmetric key.

  • If the key purpose is ENCRYPT/DECRYPT, you can call the Encrypt operation to encrypt data by using the public key and call the Decrypt operation to decrypt data by using the private key.

  • If the key purpose is SIGN/VERIFY, you can call the Sign operation to generate a digital signature by using the private key and call the Verify operation to verify the signature by using the public key.

Note

Asymmetric encryption is computationally intensive. In most cases, asymmetric cryptographic operations are used to encrypt data in small pieces such as securely distributing symmetric keys.

Key specifications

The following table describes the key specifications that are supported by different types of asymmetric keys.

Key type

Key specification

Supported encryption algorithm

Supported signature algorithm

Software-protected key

RSA_2048 and RSA_3072

RSAES_OAEP_SHA_256 (default value)

RSA_PSS_SHA_256 (default) and RSA_PKCS1_SHA_256

EC_P256 and EC_P256K

ECIES_DH_SHA_1_XOR_HMAC (default value)

ECDSA_SHA_256 (default value)

Hardware-protected key

RSA_2048, RSA_3072, and RSA_4096

RSAES_OAEP_SHA_256 (default value)

RSA_PSS_SHA_256 (default) and RSA_PKCS1_SHA_256

EC_P256 and EC_P256K

ECIES_DH_SHA_1_XOR_HMAC (default value)

ECDSA_SHA_256 (default value)

Technical specifications

Encryption algorithm description

  • RSAES_OAEP_SHA_256: the RSA encryption algorithm that uses MGF1 and SHA-256 in the RSAES-OAEP padding mode as defined in PKCS #1 in RFC 3447.

  • ECIES_DH_SHA_1_XOR_HMAC: follows SEC 1: Elliptic Curve Cryptography, Version 2.0, uses Elliptic-curve Diffie–Hellman (ECDH) for key agreement, uses key derivation function 2 (KDF2) with SHA-1 for key derivation, uses HMAC-SHA-1 as the Message Authentication Code (MAC) algorithm, and uses XOR for symmetric encryption.

Signature algorithm description

  • RSA_PSS_SHA_256: uses the SHA-256 algorithm to calculate hash values and uses MGF1 with SHA-256 to calculate signatures based on the RSASSA-PSS algorithm defined in PKCS #1 in RFC 3447.

  • RSA_PKCS1_SHA_256: uses the SHA-256 algorithm to calculate hash values and calculate signatures based on the RSASSA-PKCS1-v1_5 algorithm defined in PKCS #1 in RFC 3447.

  • ECDSA_SHA_256: uses the Elliptic Curve Digital Signature Algorithm (ECDSA) to calculate signatures and SHA-256 to calculate hash values.