Asymmetric key specs
The following topics provide technical information about the key specs that Amazon KMS supports for asymmetric KMS keys. Information about the SYMMETRIC_DEFAULT key spec for symmetric encryption keys is included for comparison.
Topics
RSA key specs
When you use an RSA key spec, Amazon KMS creates an asymmetric KMS key with an RSA key pair. The private key never leaves Amazon KMS unencrypted. You can use the public key within Amazon KMS, or download the public key for use outside of Amazon KMS.
Warning
When you encrypt data outside of Amazon KMS, be sure that you can decrypt your ciphertext. If you use the public key from a KMS key that has been deleted from Amazon KMS, the public key from a KMS key configured for signing and verification, or an encryption algorithm that is not supported by the KMS key, the data is unrecoverable.
In Amazon KMS, you can use asymmetric KMS keys with RSA key pairs for encryption and decryption, or signing and verification, but not both. This property, known as key usage, is determined separately from the key spec, but you should make that decision before you select a key spec.
Amazon KMS supports the following RSA key specs for encryption and decryption or signing and verification:

RSA_2048

RSA_3072

RSA_4096
RSA key specs differ by the length of the RSA key in bits. The RSA key spec that you
choose might be determined by your security standards or the requirements of your task.
In general, use the largest key that is practical and affordable for your task.
Cryptographic operations on KMS keys with different RSA key specs are priced
differently. For information about Amazon KMS pricing, see Amazon Key Management Service Pricing
RSA key specs for encryption and decryption
When an RSA asymmetric KMS key is used for encryption and decryption, you
encrypt with the public key and decrypt with the private key. When you call the
Encrypt
operation in Amazon KMS for an RSA KMS key, Amazon KMS uses the
public key in the RSA key pair and the encryption algorithm you specify to encrypt
your data. To decrypt the ciphertext, call the Decrypt
operation and
specify the same KMS key and encryption algorithm. Amazon KMS then uses the private key
in the RSA key pair to decrypt your data.
You can also download the public key and use it to encrypt data outside of Amazon KMS.
Be sure to use an encryption algorithm that Amazon KMS supports for RSA KMS keys. To
decrypt the ciphertext, call the Decrypt
function with the same
KMS key and encryption algorithm.
Amazon KMS supports two encryption algorithms for KMS keys with RSA key specs. These
algorithms, which are defined in PKCS #1 v2.2
Supported encryption algorithms for RSA key specs  

Encryption algorithm  Algorithm description  
RSAES_OAEP_SHA_1  PKCS #1 v2.2, Section 7.1. RSA encryption with OAEP Padding using SHA1 for both the hash and in the MGF1 mask generation function along with an empty label.  
RSAES_OAEP_SHA_256  PKCS #1, Section 7.1. RSA encryption with OAEP Padding using SHA256 for both the hash and in the MGF1 mask generation function along with an empty label. 
You cannot configure a KMS key to use a particular encryption algorithm. However, you can use the kms:EncryptionAlgorithm policy condition to specify the encryption algorithms that principals are allowed to use with the KMS key.
To get the encryption algorithms for a KMS key, view the cryptographic configuration of the KMS key in the Amazon KMS console or use the DescribeKey operation. Amazon KMS also provides the key spec and encryption algorithms when you download your public key, either in the Amazon KMS console or by using the GetPublicKey operation.
You might choose an RSA key spec based on the length of the plaintext data that you can encrypt in each request. The following table shows the maximum size, in bytes, of the plaintext that you can encrypt in a single call to the Encrypt operation. The values differ with the key spec and encryption algorithm. To compare, you can use a symmetric encryption KMS key to encrypt up to 4096 bytes at one time.
To compute the maximum plaintext length in bytes for these algorithms, use the
following formula: (key_size_in_bits
/ 8)  (2 *
hash_length_in_bits
/8)  2. For example, for RSA_2048
with SHA256, the maximum plaintext size in bytes is (2048/8)  (2 * 256/8) 2 =
190.
Maximum plaintext size (in bytes) in an Encrypt operation  

Encryption algorithm  
Key spec  RSAES_OAEP_SHA_1  RSAES_OAEP_SHA_256  
RSA_2048  214  190  
RSA_3072  342  318  
RSA_4096  470  446 
RSA key specs for signing and verification
When an RSA asymmetric KMS key is used for signing and verification, you generate the signature for a message with the private key and verify the signature with the public key.
When you call the Sign
operation in Amazon KMS for an asymmetric
KMS key, Amazon KMS uses the private key in the RSA key pair, the message, and the
signing algorithm you specify, to generate a signature. To verify the signature,
call the Verify operation. Specify
the signature, plus the same KMS key, message, and signing algorithm. Amazon KMS then
uses the public key in the RSA key pair to verify the signature. You can also
download the public key and use it to verify the signature outside of Amazon KMS.
Amazon KMS supports the following signing algorithms for all KMS keys with an RSA key spec. You are required to specify a signing algorithm when you call the Sign and Verify operations. You can choose a different algorithm for each request. When signing with RSA key pairs, RSASSAPSS algorithms are preferred. We include RSASSAPKCS1v1_5 algorithms for compatibility with existing applications.
Supported signing algorithms for RSA key specs  

Signing algorithm  Algorithm description  
RSASSA_PSS_SHA_256  PKCS #1 v2.2, Section 8.1, RSA signature with PSS padding using SHA256 for both the message digest and the MGF1 mask generation function along with a 256bit salt  
RSASSA_PSS_SHA_384  PKCS #1 v2.2, Section 8.1, RSA signature with PSS padding using SHA384 for both the message digest and the MGF1 mask generation function along with a 384bit salt  
RSASSA_PSS_SHA_512  PKCS #1 v2.2, Section 8.1, RSA signature with PSS padding using SHA512 for both the message digest and the MGF1 mask generation function along with a 512bit salt  
RSASSA_PKCS1_V1_5_SHA_256  PKCS #1 v2.2, Section 8.2, RSA signature with PKCS #1v1.5 Padding and SHA256  
RSASSA_PKCS1_V1_5_SHA_384  PKCS #1 v2.2, Section 8.2, RSA signature with PKCS #1v1.5 Padding and SHA384  
RSASSA_PKCS1_V1_5_SHA_512  PKCS #1 v2.2, Section 8.2, RSA signature with PKCS #1v1.5 Padding and SHA512 
You cannot configure a KMS key to use particular signing algorithms. However, you can use the kms:SigningAlgorithm policy condition to specify the signing algorithms that principals are allowed to use with the KMS key.
To get the signing algorithms for a KMS key, view the cryptographic configuration of the KMS key in the Amazon KMS console or by using the DescribeKey operation. Amazon KMS also provides the key spec and signing algorithms when you download your public key, either in the Amazon KMS console or by using the GetPublicKey operation.
Elliptic curve key specs
When you use an elliptic curve (ECC) key spec, Amazon KMS creates an asymmetric KMS key with an ECC key pair for signing and verification. The private key that generates signature never leaves Amazon KMS unencrypted. You can use the public key to verify signatures within Amazon KMS, or download the public key for use outside of Amazon KMS.
Amazon KMS supports the following ECC key specs for asymmetric KMS keys.

Asymmetric NISTrecommended elliptic curve key pairs (signing and verification)

ECC_NIST_P256 (secp256r1)

ECC_NIST_P384 (secp384r1)

ECC_NIST_P521 (secp521r1)


Other asymmetric elliptic curve key pairs (signing and verification)

ECC_SECG_P256K1 (secp256k1
), commonly used for cryptocurrencies.

The ECC key spec that you choose might be determined by your security standards or the requirements of your task. In general, use the curve with the most points that is practical and affordable for your task.
If you're creating an asymmetric KMS key to use with cryptocurrencies, use the ECC_SECG_P256K1 key spec. You can also use this key spec for other purposes, but it is required for Bitcoin, and other cryptocurrencies.
KMS keys with different ECC key specs are priced differently and are subject to
different request quotas. For information about Amazon KMS pricing, see Amazon Key Management Service Pricing
The following table shows the signing algorithms that Amazon KMS supports for each of the ECC key specs. You cannot configure a KMS key to use particular signing algorithms. However, you can use the kms:SigningAlgorithm policy condition to specify the signing algorithms that principals are allowed to use with the KMS key.
Supported signing algorithms for ECC key specs  

Key spec  Signing algorithm  Algorithm description  
ECC_NIST_P256  ECDSA_SHA_256  NIST FIPS 1864, Section 6.4, ECDSA signature using the curve specified by the key and SHA256 for the message digest.  
ECC_NIST_P384  ECDSA_SHA_384  NIST FIPS 1864, Section 6.4, ECDSA signature using the curve specified by the key and SHA384 for the message digest.  
ECC_NIST_P521  ECDSA_SHA_512  NIST FIPS 1864, Section 6.4, ECDSA signature using the curve specified by the key and SHA512 for the message digest.  
ECC_SECG_P256K1  ECDSA_SHA_256  NIST FIPS 1864, Section 6.4, ECDSA signature using the curve specified by the key and SHA256 for the message digest. 
SM2 key spec (China Regions only)
The SM2 key spec is an elliptic curve key spec defined within the GM/T series of
specifications published by China's Office of State Commercial Cryptography
Administration (OSCCA)
Unlike the ECC key spec, you can use an SM2 KMS key for signing and verification, or encryption and decryption. You must specify the key usage when you create the KMS key, and you cannot change it after the key is created.
Amazon KMS supports the following SM2 encryption and signing algorithms:
 SM2PKE encryption algorithm
SM2PKE is an elliptic curve based encryption algorithm defined by OSCCA in GM/T 0003.42012.
 SM2DSA signing algorithm
SM2DSA is an elliptic curve based signing algorithm defined by OSCCA in GM/T 0003.22012. SM2DSA requires a distinguishing ID that is hashed with the SM3 hashing algorithm and then combined with the message, or message digest, that you passed to Amazon KMS. This concatenated value is then hashed and signed by Amazon KMS.
Offline operations with SM2 (China Regions only)
You can download the public key of your SM2 key pair for use in offline operations, that is, operations outside of Amazon KMS. However, when using your SM2 public key offline, you may need to manually perform extra conversions and calculations. SM2DSA operations may require you to provide a distinguishing ID or calculate a message digest. SM2PKE encrypt operations may require you to convert the raw ciphertext output to a format Amazon KMS can accept.
To help you with these operations, the SM2OfflineOperationHelper
class for Java has methods that perform the tasks for you. You can use this helper
class as a model for other cryptographic providers.
Important
The SM2OfflineOperationHelper
reference code is designed to be compatible with
Bouncy Castle
Offline verification with SM2 key pairs (China Regions only)
To verify a signature outside of Amazon KMS with an SM2 public key, you must specify
the distinguishing ID. When you pass a raw message, MessageType:RAW
, to the Sign API, Amazon KMS uses the default
distinguishing ID, 1234567812345678
, defined by OSCCA in GM/T 00092012. You cannot specify your own
distinguishing ID within Amazon KMS.
However, if you are generating a message digest outside of Amazon, you can specify your own distinguishing ID,
then pass the message digest, MessageType:DIGEST
, to Amazon KMS to sign. To
do this, change the DEFAULT_DISTINGUISHING_ID
value in the
SM2OfflineOperationHelper
class. The distinguishing ID you specify can be any string
up to 8,192 characters long. After Amazon KMS signs the message
digest, you need either the message digest or the message and the distinguishing
ID used to compute the digest to verify it offline.
SM2OfflineOperationHelper
class
Within Amazon KMS, the raw ciphertext conversions and SM2DSA message digest
calculations occur automatically. Not all cryptographic providers implement SM2 in the
same way. Some libraries, like OpenSSLSM2OfflineOperationHelper
class
with libraries, like Bouncy
Castle
The SM2OfflineOperationHelper
class provides methods for the following offline
operations:
 Message digest calculation
To generate a message digest offline that you can use for offline verification, or that you can pass to Amazon KMS to sign, use the
calculateSM2Digest
method. ThecalculateSM2Digest
method generates a message digest with the SM3 hashing algorithm. The GetPublicKey API returns your public key in binary format. You must parse the binary key into a Java PublicKey. Provide the parsed public key with the message. The method automatically combines your message with the default distinguishing ID,1234567812345678
, but you can set your own distinguishing ID by changing theDEFAULT_DISTINGUISHING_ID
value.
 Verify
To verify a signature offline, use the
offlineSM2DSAVerify
method. TheofflineSM2DSAVerify
method uses the message digest calculated from the specified distinguishing ID, and original message you provide to verify the digital signature. The GetPublicKey API returns your public key in binary format. You must parse the binary key into a Java PublicKey. Provide the parsed public key with the original message and the signature you want to verify. For more details, see Offline verification with SM2 key pairs.
 Encrypt
To encrypt plaintext offline, use the
offlineSM2PKEEncrypt
method. This method ensures the ciphertext is in a format Amazon KMS can decrypt. TheofflineSM2PKEEncrypt
method encrypts the plaintext, and then converts the raw ciphertext produced by SM2PKE to the ASN.1 format. The GetPublicKey API returns your public key in binary format. You must parse the binary key into a Java PublicKey. Provide the parsed public key with the plaintext that you want to encrypt.If you're unsure whether you need to perform the conversion, use the following OpenSSL operation to test the format of your ciphertext. If the operation fails, you need to convert the ciphertext to the ASN.1 format.
openssl asn1parse inform DER in
ciphertext.der
By default, the SM2OfflineOperationHelper
class uses the default distinguishing ID, 1234567812345678
, when generating message digests for SM2DSA operations.
package com.amazon.kms.utils; import javax.crypto.BadPaddingException; import javax.crypto.Cipher; import javax.crypto.IllegalBlockSizeException; import javax.crypto.NoSuchPaddingException; import java.io.IOException; import java.math.BigInteger; import java.nio.ByteBuffer; import java.nio.charset.StandardCharsets; import java.security.InvalidKeyException; import java.security.MessageDigest; import java.security.NoSuchAlgorithmException; import java.security.NoSuchProviderException; import java.security.PrivateKey; import java.security.PublicKey; import org.bouncycastle.crypto.CryptoException; import org.bouncycastle.jce.interfaces.ECPublicKey; import java.util.Arrays; import org.bouncycastle.asn1.ASN1EncodableVector; import org.bouncycastle.asn1.ASN1Integer; import org.bouncycastle.asn1.DEROctetString; import org.bouncycastle.asn1.DERSequence; import org.bouncycastle.asn1.gm.GMNamedCurves; import org.bouncycastle.asn1.x9.X9ECParameters; import org.bouncycastle.crypto.CipherParameters; import org.bouncycastle.crypto.params.ParametersWithID; import org.bouncycastle.crypto.params.ParametersWithRandom; import org.bouncycastle.crypto.signers.SM2Signer; import org.bouncycastle.jcajce.provider.asymmetric.util.ECUtil; public class SM2OfflineOperationHelper { // You can change the DEFAULT_DISTINGUISHING_ID value to set your own distinguishing ID, // the DEFAULT_DISTINGUISHING_ID can be any string up to 8,192 characters long. private static final byte[] DEFAULT_DISTINGUISHING_ID = "1234567812345678".getBytes(StandardCharsets.UTF_8); private static final X9ECParameters SM2_X9EC_PARAMETERS = GMNamedCurves.getByName("sm2p256v1"); // ***calculateSM2Digest*** // Calculate message digest public static byte[] calculateSM2Digest(final PublicKey publicKey, final byte[] message) throws NoSuchProviderException, NoSuchAlgorithmException { final ECPublicKey ecPublicKey = (ECPublicKey) publicKey; // Generate SM3 hash of default distinguishing ID, 1234567812345678 final int entlenA = DEFAULT_DISTINGUISHING_ID.length * 8; final byte [] entla = new byte[] { (byte) (entlenA & 0xFF00), (byte) (entlenA & 0x00FF) }; final byte [] a = SM2_X9EC_PARAMETERS.getCurve().getA().getEncoded(); final byte [] b = SM2_X9EC_PARAMETERS.getCurve().getB().getEncoded(); final byte [] xg = SM2_X9EC_PARAMETERS.getG().getXCoord().getEncoded(); final byte [] yg = SM2_X9EC_PARAMETERS.getG().getYCoord().getEncoded(); final byte[] xa = ecPublicKey.getQ().getXCoord().getEncoded(); final byte[] ya = ecPublicKey.getQ().getYCoord().getEncoded(); final byte[] za = MessageDigest.getInstance("SM3", "BC") .digest(ByteBuffer.allocate(entla.length + DEFAULT_DISTINGUISHING_ID.length + a.length + b.length + xg.length + yg.length + xa.length + ya.length).put(entla).put(DEFAULT_DISTINGUISHING_ID).put(a).put(b).put(xg).put(yg).put(xa).put(ya) .array()); // Combine hashed distinguishing ID with original message to generate final digest return MessageDigest.getInstance("SM3", "BC") .digest(ByteBuffer.allocate(za.length + message.length).put(za).put(message) .array()); } // ***offlineSM2DSAVerify*** // Verify digital signature with SM2 public key public static boolean offlineSM2DSAVerify(final PublicKey publicKey, final byte [] message, final byte [] signature) throws InvalidKeyException { final SM2Signer signer = new SM2Signer(); CipherParameters cipherParameters = ECUtil.generatePublicKeyParameter(publicKey); cipherParameters = new ParametersWithID(cipherParameters, DEFAULT_DISTINGUISHING_ID); signer.init(false, cipherParameters); signer.update(message, 0, message.length); return signer.verifySignature(signature); } // ***offlineSM2PKEEncrypt*** // Encrypt data with SM2 public key public static byte[] offlineSM2PKEEncrypt(final PublicKey publicKey, final byte [] plaintext) throws NoSuchPaddingException, NoSuchAlgorithmException, NoSuchProviderException, InvalidKeyException, BadPaddingException, IllegalBlockSizeException, IOException { final Cipher sm2Cipher = Cipher.getInstance("SM2", "BC"); sm2Cipher.init(Cipher.ENCRYPT_MODE, publicKey); // By default, Bouncy Castle returns raw ciphertext in the c1c2c3 format final byte [] cipherText = sm2Cipher.doFinal(plaintext); // Convert the raw ciphertext to the ASN.1 format before passing it to AWS KMS final ASN1EncodableVector asn1EncodableVector = new ASN1EncodableVector(); final int coordinateLength = (SM2_X9EC_PARAMETERS.getCurve().getFieldSize() + 7) / 8 * 2 + 1; final int sm3HashLength = 32; final int xCoordinateInCipherText = 33; final int yCoordinateInCipherText = 65; byte[] coords = new byte[coordinateLength]; byte[] sm3Hash = new byte[sm3HashLength]; byte[] remainingCipherText = new byte[cipherText.length  coordinateLength  sm3HashLength]; // Split components out of the ciphertext System.arraycopy(cipherText, 0, coords, 0, coordinateLength); System.arraycopy(cipherText, cipherText.length  sm3HashLength, sm3Hash, 0, sm3HashLength); System.arraycopy(cipherText, coordinateLength, remainingCipherText, 0,cipherText.length  coordinateLength  sm3HashLength); // Build standard SM2PKE ASN.1 ciphertext vector asn1EncodableVector.add(new ASN1Integer(new BigInteger(1, Arrays.copyOfRange(coords, 1, xCoordinateInCipherText)))); asn1EncodableVector.add(new ASN1Integer(new BigInteger(1, Arrays.copyOfRange(coords, xCoordinateInCipherText, yCoordinateInCipherText)))); asn1EncodableVector.add(new DEROctetString(sm3Hash)); asn1EncodableVector.add(new DEROctetString(remainingCipherText)); return new DERSequence(asn1EncodableVector).getEncoded("DER"); } }
SYMMETRIC_DEFAULT key spec
The default key spec, SYMMETRIC_DEFAULT, is the key spec for symmetric encryption
KMS keys. When you select the Symmetric key type and the
Encrypt and decrypt key usage in the Amazon KMS console, it selects the
SYMMETRIC_DEFAULT
key spec. In the CreateKey operation, if you don't specify a
KeySpec
value, SYMMETRIC_DEFAULT is selected. If you don't have a reason to
use a different key spec, SYMMETRIC_DEFAULT is a good choice.
SYMMETRIC_DEFAULT currently represents AES256GCM, a symmetric algorithm based on
Advanced Encryption Standard
Data encrypted under AES256GCM is protected now and in the future. Cryptographers
consider this algorithm to be quantum resistant.
Theoretical future, largescale quantum computing attacks on ciphertexts created under
256bit AESGCM keys reduce
the effective security of the key to 128 bits
The only exception in China Regions, where SYMMETRIC_DEFAULT represents a 128bit symmetric key that uses SM4 encryption. You can only create a 128bit SM4 key within China Regions. You cannot create a 256bit AESGCM KMS key in China Regions.
You can use a symmetric encryption KMS key in Amazon KMS to encrypt, decrypt, and reencrypt data, and to protect generated data keys and data key pairs. Amazon services that are integrated with Amazon KMS use symmetric encryption KMS keys to encrypt your data at rest. You can import your own key material into a symmetric encryption KMS key and create symmetric encryption KMS keys in custom key stores. For a table comparing the operations that you can perform on symmetric and asymmetric KMS keys, see Comparing Symmetric and Asymmetric KMS keys.
For technical details about Amazon KMS and symmetric encryption keys, see Amazon Key Management Service Cryptographic Details.