<p>Decrypts ciphertext that was encrypted by a KMS key using any of the following operations:</p> <ul> <li> <p> <a>Encrypt</a> </p> </li> <li> <p> <a>GenerateDataKey</a> </p> </li> <li> <p> <a>GenerateDataKeyPair</a> </p> </li> <li> <p> <a>GenerateDataKeyWithoutPlaintext</a> </p> </li> <li> <p> <a>GenerateDataKeyPairWithoutPlaintext</a> </p> </li> </ul> <p>You can use this operation to decrypt ciphertext that was encrypted under a symmetric encryption KMS key or an asymmetric encryption KMS key. When the KMS key is asymmetric, you must specify the KMS key and the encryption algorithm that was used to encrypt the ciphertext. For information about asymmetric KMS keys, see <a href="https://docs.aws.amazon.com/kms/latest/developerguide/symmetric-asymmetric.html">Asymmetric KMS keys</a> in the <i>Key Management Service Developer Guide</i>.</p> <p>The <code>Decrypt</code> operation also decrypts ciphertext that was encrypted outside of KMS by the public key in an KMS asymmetric KMS key. However, it cannot decrypt symmetric ciphertext produced by other libraries, such as the <a href="https://docs.aws.amazon.com/encryption-sdk/latest/developer-guide/">Amazon Web Services Encryption SDK</a> or <a href="https://docs.aws.amazon.com/AmazonS3/latest/dev/UsingClientSideEncryption.html">Amazon S3 client-side encryption</a>. These libraries return a ciphertext format that is incompatible with KMS.</p> <p>If the ciphertext was encrypted under a symmetric encryption KMS key, the <code>KeyId</code> parameter is optional. KMS can get this information from metadata that it adds to the symmetric ciphertext blob. This feature adds durability to your implementation by ensuring that authorized users can decrypt ciphertext decades after it was encrypted, even if they've lost track of the key ID. However, specifying the KMS key is always recommended as a best practice. When you use the <code>KeyId</code> parameter to specify a KMS key, KMS only uses the KMS key you specify. If the ciphertext was encrypted under a different KMS key, the <code>Decrypt</code> operation fails. This practice ensures that you use the KMS key that you intend.</p> <p>Whenever possible, use key policies to give users permission to call the <code>Decrypt</code> operation on a particular KMS key, instead of using &amp;IAM; policies. Otherwise, you might create an &amp;IAM; policy that gives the user <code>Decrypt</code> permission on all KMS keys. This user could decrypt ciphertext that was encrypted by KMS keys in other accounts if the key policy for the cross-account KMS key permits it. If you must use an IAM policy for <code>Decrypt</code> permissions, limit the user to particular KMS keys or particular trusted accounts. For details, see <a href="https://docs.aws.amazon.com/kms/latest/developerguide/iam-policies.html#iam-policies-best-practices">Best practices for IAM policies</a> in the <i>Key Management Service Developer Guide</i>.</p> <p>Applications in Amazon Web Services Nitro Enclaves can call this operation by using the <a href="https://github.com/aws/aws-nitro-enclaves-sdk-c">Amazon Web Services Nitro Enclaves Development Kit</a>. For information about the supporting parameters, see <a href="https://docs.aws.amazon.com/kms/latest/developerguide/services-nitro-enclaves.html">How Amazon Web Services Nitro Enclaves use KMS</a> in the <i>Key Management Service Developer Guide</i>.</p> <p>The KMS key that you use for this operation must be in a compatible key state. For details, see <a href="https://docs.aws.amazon.com/kms/latest/developerguide/key-state.html">Key states of KMS keys</a> in the <i>Key Management Service Developer Guide</i>.</p> <p> <b>Cross-account use</b>: Yes. If you use the <code>KeyId</code> parameter to identify a KMS key in a different Amazon Web Services account, specify the key ARN or the alias ARN of the KMS key.</p> <p> <b>Required permissions</b>: <a href="https://docs.aws.amazon.com/kms/latest/developerguide/kms-api-permissions-reference.html">kms:Decrypt</a> (key policy)</p> <p> <b>Related operations:</b> </p> <ul> <li> <p> <a>Encrypt</a> </p> </li> <li> <p> <a>GenerateDataKey</a> </p> </li> <li> <p> <a>GenerateDataKeyPair</a> </p> </li> <li> <p> <a>ReEncrypt</a> </p> </li> </ul>

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