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Synchronous client for accessing Amazon S3.

Appendix: Amazon S3 client-side encryption meta information


This appendix summarizes the current crypto related meta information associated with an S3 object encrypted using SDK client-side encryption. In SDK's terminology, these meta information are classified as "user metatdata". In general, the meta information is just a map of key/value pairs. Physically they are persisted either as metadata of the encrypted S3 object, or as a separate S3 object called an "instruction file" (which is just a JSON file in plaintext.) For all references to the Cipher Algorithm Names, Modes, and Padding, such as "AES/CBC/PKCS5Padding", please refer to Oracle's Java™ Cryptography Architecture Standard Algorithm Name Documentation for more details.

S3 metadata vs instruction file

Note when a key/value pair is stored as "user metadata" as part of the S3 metadata, the SDK always adds an additional prefix of x-amz-meta- to the key name. In contrast, the SDK doesn't add any prefix when the key names are stored in an instruction file. (The reason has to do with how the http headers are handled.) For example, x-amz-key would get stored as x-amz-meta-x-amz-key in S3 metadata. In contrast, x-amz-key would simply be stored as is (ie without the prefix) in an instruction file. All the keys described below are considered "user metadata" by the SDK.

S3 v1 crypto meta information (supported by all SDK's)

For non-KMS client side encryption, this v1 crypto meta information is used to support the "Encryption Only" crypto mode (which involves the use of AES/CBC/PKCS5Padding for content encryption, and AES/ECB for encrypting the one-time randomly generated data key). For KMS client-side encryption, the v2 crypto meta information is used.

x-amz-keyContent encrypting key (cek) in encrypted form, base64 encoded. The cek is randomly generated per S3 object, and is always an AES 256-bit key. The corresponding cipher is always "AES/CBC/PKCS5Padding".
x-amz-ivRandomly generated IV (per S3 object), base64 encoded
x-amz-matdesc Customer provided material description in JSON (UTF8) format. Used to identify the client-side master key (ie used to encrypt/wrap the generated content encrypting key).
x-amz-unencrypted-content-lengthUnencrypted content length (optional but should be specified whenever possible).

S3 v2 crypto meta information (currently supported by the Java SDK)

This v2 crypto meta information is used to support both authenticated encryption (which involves the use of AES/GCM/NoPadding for content encryption, and key wrapping for the one-time randomly generated data key), and KMS client-side encryption (which can either be encryption-only or authenticated encryption.)

x-amz-key-v2CEK in key wrapped form. This is necessary so that the S3 encryption client that doesn't recognize the v2 format will not mistakenly decrypt S3 object encrypted in v2 format.
x-amz-ivRandomly generated IV (per S3 object), base64 encoded. (Same as v1.)
x-amz-matdescCustomer provided material description in JSON format. (Same as v1). Used to identify the client-side master key.
  • For KMS client side encryption, the KMS Customer Master Key ID is stored as part of the material description, x-amz-matdesc, under the key-name "kms_cmk_id".
x-amz-unencrypted-content-lengthUnencrypted content length. (optional but should be specified whenever possible. Same as v1.)
x-amz-wrap-algKey wrapping algorithm used.
  • Supported values: "AESWrap", "RSA/ECB/OAEPWithSHA-256AndMGF1Padding", "kms"
  • No standard key wrapping is used if this meta information is absent
  • Always set to "kms" if KMS is used for client-side encryption
x-amz-cek-algContent encryption algorithm used.
  • Supported values: "AES/GCM/NoPadding", "AES/CBC/PKCS5Padding"
  • Default to "AES/CBC/PKCS5Padding" if this key is absent.
x-amz-tag-lenTag length (in bits) when AEAD is in use.
  • Only applicable if AEAD is in use. This meta information is absent otherwise, or if KMS is in use.
  • Supported value: "128"

KMS Integration for client-side encryption

  1. All client-side KMS protected S3 objects are stored in v2 crypto meta information format. However, the key wrapping algorithm, x-amz-wrap-alg is always set to "kms".
  2. The KMS Customer Master Key ID is currently stored as part of the material description, x-amz-matdesc, under the key-name "kms_cmk_id".

Sample S3 crypto meta information for KMS client-side encryption

CryptoMode: Encryption Only
 x-amz-iv: xqOHh+M6y0UogwG5SHJM3Q==
 x-amz-unencrypted-content-length: 32768
 x-amz-cek-alg: AES/CBC/PKCS5Padding
 x-amz-wrap-alg: kms
 x-amz-matdesc: {"kms_cmk_id":"d3fd2273-4ca0-4da5-b1c2-c89742ec6a26"}
CryptoMode: Authenticated Encryption
 x-amz-key-v2: CiAJ2GqwpJRnt4izujwoalC2YdNRNSMEslG5rw3pEI+J8hKnAQEBAwB4CdhqsKSUZ7eIs7o8KGpQtmHTUTUjBLJRua8N6RCPifIAAAB+MHwGCSqGSIb3DQEHBqBvMG0CAQAwaAYJKoZIhvcNAQcBMB4GCWCGSAFlAwQBLjARBAytFVyW4I6QmwcPK1ECARCAO1VzoM6KxA9DHyNMm/BEbbjle2vLA7IY7f9OkUoSqrdxgKxpNID9QaL/7928m1Djtld0bJpHefjm89C
 x-amz-iv: VZBksyij6DhrUMye
 x-amz-unencrypted-content-length: 32768
 x-amz-cek-alg: AES/GCM/NoPadding
 x-amz-wrap-alg: kms
 x-amz-matdesc: {"kms_cmk_id":"d3fd2273-4ca0-4da5-b1c2-c89742ec6a26"}
 x-amz-tag-len: 128

S3 metadata of an instruction file

The following key is always included as part of the S3 metadata of the instruction file (which is just an S3 object). Since the SDK treats this key as "user metadata", the key name is always prefixed with x-amz-meta- when physically stored.

x-amz-crypto-instr-filethe presence of this key is used to indicate this S3 object is an instruction file (of an associated client-side encrypted S3 object).
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