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Object Key and Metadata
Each Amazon S3 object has data, a key, and metadata. When you create an object you specify the key name. This key name uniquely identifies the object in the bucket. For example, in Amazon S3 console (see AWS Management Console), when you highlight a bucket, a list of objects in your bucket appear. These names are the object keys. The name for a key is a sequence of Unicode characters whose UTF-8 encoding is at most 1024 bytes long.
Note: If you anticipate that your workload against Amazon S3 will exceed 100 requests per second, follow the Amazon S3 key naming guidelines for best performance. For information, see Request Rate and Performance Considerations.
In addition to the key, each Amazon S3 object has metadata. It is a set of name-value pairs. You can set object metadata at the time you upload it. After you upload the object, you cannot modify object metadata. The only way to modify object metadata is to make copy of the object and set the metadata. For more information, go to PUT Object - Copy in the Amazon Simple Storage Service API Reference. You can use the Amazon S3 management console to update the object metadata but internally it makes an object copy replacing the existing object to set the metadata.There are two kinds of metadata: system metadata and user-defined metadata.
Encryption provides added security for your object data stored in your buckets in Amazon S3. You can encrypt data on your client-side and upload the encrypted data to Amazon S3. In this case, you manage encryption process, the encryption keys, and related tools. Optionally, you might want to use the server-side encryption feature in which Amazon S3 encrypts your object data before saving it on disks in its data centers and decrypts it when you download the objects, freeing you from the tasks of managing encryption, encryption keys, and related tools. You can also use your own encryption keys with the Amazon S3 server-side encryption feature
Server-side encryption encrypts only the object data. Any object metadata is not encrypted.
Instead of using Amazon S3's server-side encryption, you also have the option of encrypting your data before sending it to Amazon S3. You can build your own library that encrypts your objects data on the client side before uploading it to Amazon S3. Optionally, you can use the AWS SDK for Java, which you can use to automatically encrypt your data before uploading it to Amazon S3.
Specifying Encryption Metadata Storage Location : When the Amazon S3 client (using the AmazonS3EncryptionClient class) encrypts data and uploads it to Amazon S3, the encrypted envelope symmetric key is also stored in S3. By default, the encrypted key is stored as user-defined object metadata. After you upload an encrypted object, you can view its properties and see the additional metadata name-value pairs related to encryption. For example, the key name x-amz-meta-x-amz-key and key value equal to the envelope key are set on an client-side encrypted object uploaded to Amazon S3. Optionally, you can also choose to store encryption metadata as an instruction file stored at the same location as the encrypted file. The instruction file will have the same key name as the encrypted data file but with the extension ".instruction" appended. You should use an instruction file when the strength of your encryption key results in a symmetric key that is too big for the object metadata. Metadata should be less than 2 KB. Encryption metadata is either stored as object metadata or an instruction file, but not both.
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