Use PutBucketEncryption with an AWS SDK or CLI

    /// <summary>
    /// Set the bucket server side encryption to use AWSKMS with a customer-managed key id.
    /// </summary>
    /// <param name="bucketName">Name of the bucket.</param>
    /// <param name="kmsKeyId">The Id of the KMS Key.</param>
    /// <returns>True if successful.</returns>
    public static async Task<bool> SetBucketServerSideEncryption(string bucketName, string kmsKeyId)
    {
        var serverSideEncryptionByDefault = new ServerSideEncryptionConfiguration
        {
            ServerSideEncryptionRules = new List<ServerSideEncryptionRule>
            {
                new ServerSideEncryptionRule
                {
                    ServerSideEncryptionByDefault = new ServerSideEncryptionByDefault
                    {
                        ServerSideEncryptionAlgorithm = ServerSideEncryptionMethod.AWSKMS,
                        ServerSideEncryptionKeyManagementServiceKeyId = kmsKeyId
                    }
                }
            }
        };
        try
        {
            var encryptionResponse = await _s3Client.PutBucketEncryptionAsync(new PutBucketEncryptionRequest
            {
                BucketName = bucketName,
                ServerSideEncryptionConfiguration = serverSideEncryptionByDefault,
            });
            
            return encryptionResponse.HttpStatusCode == HttpStatusCode.OK;
        }
        catch (AmazonS3Exception ex)
        {
            Console.WriteLine(ex.ErrorCode == "AccessDenied"
                ? $"This account does not have permission to set encryption on {bucketName}, please try again."
                : $"Unable to set bucket encryption for bucket {bucketName}, {ex.Message}");
        }
        return false;
    }

To configure server-side encryption for a bucket

The following put-bucket-encryption example sets AES256 encryption as the default for the specified bucket.

aws s3api put-bucket-encryption \
    --bucket my-bucket \
    --server-side-encryption-configuration '{"Rules": [{"ApplyServerSideEncryptionByDefault": {"SSEAlgorithm": "AES256"}}]}'

This command produces no output.

Example 1: This command enables default AES256 server side encryption with Amazon S3 Managed Keys(SSE-S3) on the given bucket.

$Encryptionconfig = @{ServerSideEncryptionByDefault = @{ServerSideEncryptionAlgorithm = "AES256"}}
Set-S3BucketEncryption -BucketName 'amzn-s3-demo-bucket' -ServerSideEncryptionConfiguration_ServerSideEncryptionRule $Encryptionconfig