--- 1/draft-ietf-httpbis-encryption-encoding-03.txt 2016-10-31 17:16:25.099883081 -0700 +++ 2/draft-ietf-httpbis-encryption-encoding-04.txt 2016-10-31 17:16:25.135883974 -0700 @@ -1,18 +1,18 @@ HTTP Working Group M. Thomson Internet-Draft Mozilla -Intended status: Standards Track October 9, 2016 -Expires: April 12, 2017 +Intended status: Standards Track October 31, 2016 +Expires: May 4, 2017 Encrypted Content-Encoding for HTTP - draft-ietf-httpbis-encryption-encoding-03 + draft-ietf-httpbis-encryption-encoding-04 Abstract This memo introduces a content coding for HTTP that allows message payloads to be encrypted. Note to Readers Discussion of this draft takes place on the HTTP working group mailing list (ietf-http-wg@w3.org), which is archived at @@ -30,75 +30,68 @@ Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on April 12, 2017. + This Internet-Draft will expire on May 4, 2017. Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents - 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 + 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3 - 2. The "aesgcm" HTTP Content Coding . . . . . . . . . . . . . . 4 - 3. The Encryption HTTP Header Field . . . . . . . . . . . . . . 6 - 3.1. Encryption Header Field Parameters . . . . . . . . . . . 6 - 3.2. Content Encryption Key Derivation . . . . . . . . . . . . 7 - 3.3. Nonce Derivation . . . . . . . . . . . . . . . . . . . . 8 - 4. Crypto-Key Header Field . . . . . . . . . . . . . . . . . . . 8 - 5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 9 - 5.1. Encryption of a Response . . . . . . . . . . . . . . . . 9 - 5.2. Encryption with Multiple Records . . . . . . . . . . . . 10 - 5.3. Encryption and Compression . . . . . . . . . . . . . . . 10 - 5.4. Encryption with More Than One Key . . . . . . . . . . . . 11 - 6. Security Considerations . . . . . . . . . . . . . . . . . . . 11 - 6.1. Key and Nonce Reuse . . . . . . . . . . . . . . . . . . . 11 - 6.2. Data Encryption Limits . . . . . . . . . . . . . . . . . 12 - 6.3. Content Integrity . . . . . . . . . . . . . . . . . . . . 12 - 6.4. Leaking Information in Headers . . . . . . . . . . . . . 12 - 6.5. Poisoning Storage . . . . . . . . . . . . . . . . . . . . 13 - 6.6. Sizing and Timing Attacks . . . . . . . . . . . . . . . . 13 - 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 - 7.1. The "aesgcm" HTTP Content Coding . . . . . . . . . . . . 13 - 7.2. Encryption Header Fields . . . . . . . . . . . . . . . . 14 - 7.3. The HTTP Encryption Parameter Registry . . . . . . . . . 14 - 7.3.1. keyid . . . . . . . . . . . . . . . . . . . . . . . . 15 - 7.3.2. salt . . . . . . . . . . . . . . . . . . . . . . . . 15 - 7.3.3. rs . . . . . . . . . . . . . . . . . . . . . . . . . 15 - 7.4. The HTTP Crypto-Key Parameter Registry . . . . . . . . . 15 - 7.4.1. keyid . . . . . . . . . . . . . . . . . . . . . . . . 16 - 7.4.2. aesgcm . . . . . . . . . . . . . . . . . . . . . . . 16 - 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 - 8.1. Normative References . . . . . . . . . . . . . . . . . . 16 - 8.2. Informative References . . . . . . . . . . . . . . . . . 17 - Appendix A. JWE Mapping . . . . . . . . . . . . . . . . . . . . 18 - Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 19 - Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19 + 2. The "aes128gcm" HTTP Content Coding . . . . . . . . . . . . . 3 + 2.1. Encryption Content Coding Header . . . . . . . . . . . . 5 + 2.2. Content Encryption Key Derivation . . . . . . . . . . . . 6 + 2.3. Nonce Derivation . . . . . . . . . . . . . . . . . . . . 7 + 3. Crypto-Key Header Field . . . . . . . . . . . . . . . . . . . 7 + 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8 + 4.1. Encryption of a Response . . . . . . . . . . . . . . . . 8 + 4.2. Encryption with Multiple Records . . . . . . . . . . . . 9 + 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 + 5.1. Key and Nonce Reuse . . . . . . . . . . . . . . . . . . . 10 + 5.2. Data Encryption Limits . . . . . . . . . . . . . . . . . 10 + 5.3. Content Integrity . . . . . . . . . . . . . . . . . . . . 10 + 5.4. Leaking Information in Headers . . . . . . . . . . . . . 11 + 5.5. Poisoning Storage . . . . . . . . . . . . . . . . . . . . 11 + 5.6. Sizing and Timing Attacks . . . . . . . . . . . . . . . . 12 + 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 + 6.1. The "aes128gcm" HTTP Content Coding . . . . . . . . . . . 12 + 6.2. Crypto-Key Header Field . . . . . . . . . . . . . . . . . 12 + 6.3. The HTTP Crypto-Key Parameter Registry . . . . . . . . . 12 + 6.3.1. keyid . . . . . . . . . . . . . . . . . . . . . . . . 13 + 6.3.2. aes128gcm . . . . . . . . . . . . . . . . . . . . . . 13 + 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 + 7.1. Normative References . . . . . . . . . . . . . . . . . . 13 + 7.2. Informative References . . . . . . . . . . . . . . . . . 14 + Appendix A. JWE Mapping . . . . . . . . . . . . . . . . . . . . 15 + Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 16 + Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16 1. Introduction It is sometimes desirable to encrypt the contents of a HTTP message (request or response) so that when the payload is stored (e.g., with a HTTP PUT), only someone with the appropriate key can read it. For example, it might be necessary to store a file on a server without exposing its contents to that server. Furthermore, that same file could be replicated to other servers (to make it more resistant @@ -131,401 +124,344 @@ Key header field that can be used to convey keying material. 1.1. Notational Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Base64url encoding is defined in Section 2 of [RFC7515]. -2. The "aesgcm" HTTP Content Coding +2. The "aes128gcm" HTTP Content Coding - The "aesgcm" HTTP content coding indicates that a payload has been + The "aes128gcm" HTTP content coding indicates that a payload has been encrypted using Advanced Encryption Standard (AES) in Galois/Counter Mode (GCM) as identified as AEAD_AES_128_GCM in [RFC5116], Section 5.1. The AEAD_AES_128_GCM algorithm uses a 128 bit content encryption key. - When this content coding is in use, the Encryption header field - (Section 3) describes how encryption has been applied. The Crypto- - Key header field (Section 4) can be included to describe how the - content encryption key is derived or retrieved. + Using this content coding requires knowledge of a key. The Crypto- + Key header field (Section 3) can be included to describe how the + content encryption key is derived or retrieved. Keys might be + provided in messages that are separate from those with encrypted + content using Crypto-Key, or provided through external mechanisms. - The "aesgcm" content coding uses a single fixed set of encryption + The "aes128gcm" content coding uses a single fixed set of encryption primitives. Cipher suite agility is achieved by defining a new content coding scheme. This ensures that only the HTTP Accept- Encoding header field is necessary to negotiate the use of encryption. - The "aesgcm" content coding uses a fixed record size. The resulting - encoding is either a single record, or a series of fixed-size - records. The final record, or a lone record, MUST be shorter than - the fixed record size. + The "aes128gcm" content coding uses a fixed record size. The final + encoding consists of a header (see Section 2.1), zero or more fixed + size encrypted records, and a partial record. The partial record + MUST be shorter than the fixed record size. +-----------+ content is rs octets minus padding | data | of between 2 and 65537 octets; +-----------+ the last record is smaller | v +-----+-----------+ add padding to get rs octets; | pad | data | the last record contains +-----+-----------+ up to rs minus 1 octets | v +--------------------+ encrypt with AEAD_AES_128_GCM; | ciphertext | final size is rs plus 16 octets +--------------------+ the last record is smaller The record size determines the length of each portion of plaintext that is enciphered, with the exception of the final record, which is - necessarily smaller. The record size defaults to 4096 octets, but - can be changed using the "rs" parameter on the Encryption header - field. + necessarily smaller. The record size ("rs") is included in the + content coding header (see Section 2.1). AEAD_AES_128_GCM produces ciphertext 16 octets longer than its input plaintext. Therefore, the length of each enciphered record other than the last is equal to the value of the "rs" parameter plus 16 - octets. A receiver MUST fail to decrypt if the final record - ciphertext is less than 18 octets in size. Valid records always + octets. To prevent an attacker from truncating a stream, an encoder + MUST append a record that contains only padding and is smaller than + the full record size if the final record ends on a record boundary. + A receiver MUST fail to decrypt if the final record ciphertext is + less than 18 octets in size or equal to the record size plus 16 (that + is, the size of a full encrypted record). Valid records always contain at least two octets of padding and a 16 octet authentication tag. Each record contains between 2 and 65537 octets of padding, inserted into a record before the enciphered content. Padding consists of a two octet unsigned integer in network byte order, followed that number of zero-valued octets. A receiver MUST fail to decrypt if any padding octet other than the first two are non-zero, or a record has more padding than the record size can accommodate. The nonce for each record is a 96-bit value constructed from the record sequence number and the input keying material. Nonce - derivation is covered in Section 3.3. + derivation is covered in Section 2.3. The additional data passed to each invocation of AEAD_AES_128_GCM is a zero-length octet sequence. - A sequence of full-sized records can be truncated to produce a - shorter sequence of records with valid authentication tags. To - prevent an attacker from truncating a stream, an encoder MUST append - a record that contains only padding and is smaller than the full - record size if the final record ends on a record boundary. A - receiver MUST treat the stream as failed due to truncation if the - final record is the full record size. - A consequence of this record structure is that range requests [RFC7233] and random access to encrypted payload bodies are possible - at the granularity of the record size. However, without data from - adjacent ranges, partial records cannot be used. Thus, it is best if - range requests start and end on multiples of the record size, plus - the 16 octet authentication tag size. + at the granularity of the record size. Partial records at the ends + of a range cannot be decrypted. Thus, it is best if range requests + start and end on record boundaries. Selecting the record size most appropriate for a given situation requires a trade-off. A smaller record size allows decrypted octets to be released more rapidly, which can be appropriate for applications that depend on responsiveness. Smaller records also reduce the additional data required if random access into the ciphertext is needed. Applications that depend on being able to pad by arbitrary amounts cannot increase the record size beyond 65537 octets. Applications that don't depending on streaming, random access, or arbitrary padding can use larger records, or even a single record. A larger record size reduces the processing and data overheads. -3. The Encryption HTTP Header Field - - The "Encryption" HTTP header field describes the encrypted content - coding(s) that have been applied to a payload body, and therefore how - those content coding(s) can be removed. - - The "Encryption" header field uses the extended ABNF syntax defined - in Section 1.2 of [RFC7230] and the "parameter" and "OWS" rules from - [RFC7231]. - - Encryption = #encryption_params - encryption_params = [ parameter *( OWS ";" OWS parameter ) ] - - If the payload is encrypted more than once (as reflected by having - multiple content codings that imply encryption), each application of - the content coding is reflected in a separate Encryption header field - value in the order in which they were applied. - - Encryption header field values with multiple instances of the same - parameter name are invalid. - - Servers processing PUT requests MUST persist the value of the - Encryption header field, unless they remove the content coding by - decrypting the payload. +2.1. Encryption Content Coding Header -3.1. Encryption Header Field Parameters + The content coding uses a header block that includes all parameters + needed to decrypt the content (other than the key). The header block + is placed in the body of a message ahead of the sequence of records. - The following parameters are used in determining the content - encryption key that is used for encryption: + +-----------+--------+-----------+---------------+ + | salt (16) | rs (4) | idlen (1) | keyid (idlen) | + +-----------+--------+-----------+---------------+ - keyid: The "keyid" parameter identifies the keying material that is - used. When the Crypto-Key header field is used, the "keyid" - identifies a matching value in that field. The "keyid" parameter - MUST be used if keying material included in an Crypto-Key header - field is needed to derive the content encryption key. The "keyid" - parameter can also be used to identify keys in an application- - specific fashion. + salt: The "salt" parameter comprises the first 16 octets of the + "aes128gcm" content coding header. The same "salt" parameter + value MUST NOT be reused for two different payload bodies that + have the same input keying material; generating a random salt for + every application of the content coding ensures that content + encryption key reuse is highly unlikely. - salt: The "salt" parameter contains a base64url-encoded octets - [RFC7515] that is used as salt in deriving a unique content - encryption key (see Section 3.2). The "salt" parameter MUST be - present, and MUST be exactly 16 octets long when decoded. The - "salt" parameter MUST NOT be reused for two different payload - bodies that have the same input keying material; generating a - random salt for every application of the content coding ensures - that content encryption key reuse is highly unlikely. + rs: The "rs" or record size parameter contains an unsigned 32-bit + integer in network byte order that describes the record size in + octets. Note that it is therefore impossible to exceed the + 2^36-31 limit on plaintext input to AEAD_AES_128_GCM. Values + smaller than 3 are invalid. - rs: The "rs" parameter contains a positive decimal integer that - describes the record size in octets. This value MUST be greater - than 1. For the "aesgcm" content coding, this value MUST NOT be - greater than 2^36-31 (see Section 6.2). The "rs" parameter is - optional. If the "rs" parameter is absent, the record size - defaults to 4096 octets. + keyid: The "keyid" parameter can be used to identify the keying + material that is used. When the Crypto-Key header field is used, + the "keyid" identifies a matching value in that field. The + "keyid" parameter MUST be used if keying material included in an + Crypto-Key header field is needed to derive the content encryption + key. The "keyid" parameter can also be used to identify keys in + an application-specific fashion. -3.2. Content Encryption Key Derivation +2.2. Content Encryption Key Derivation In order to allow the reuse of keying material for multiple different HTTP messages, a content encryption key is derived for each message. The content encryption key is derived from the decoded value of the "salt" parameter using the HMAC-based key derivation function (HKDF) described in [RFC5869] using the SHA-256 hash algorithm [FIPS180-4]. - The decoded value of the "salt" parameter is the salt input to HKDF - function. The keying material identified by the "keyid" parameter is - the input keying material (IKM) to HKDF. Input keying material can - either be prearranged, or can be described using the Crypto-Key - header field (Section 4). The extract phase of HKDF therefore - produces a pseudorandom key (PRK) as follows: + The value of the "salt" parameter is the salt input to HKDF function. + The keying material identified by the "keyid" parameter is the input + keying material (IKM) to HKDF. Input keying material can either be + prearranged, or can be described using the Crypto-Key header field + (Section 3). The extract phase of HKDF therefore produces a + pseudorandom key (PRK) as follows: PRK = HMAC-SHA-256(salt, IKM) The info parameter to HKDF is set to the ASCII-encoded string - "Content-Encoding: aesgcm", a single zero octet and an optional - context string: + "Content-Encoding: aes128gcm" and a single zero octet: - cek_info = "Content-Encoding: aesgcm" || 0x00 || context + cek_info = "Content-Encoding: aes128gcm" || 0x00 - Unless otherwise specified, the context is a zero length octet - sequence. Specifications that use this content coding MAY specify - the use of an expanded context to cover additional inputs in the key - derivation. + Note: Concatenation of octet sequences is represented by the "||" + operator. AEAD_AES_128_GCM requires a 16 octet (128 bit) content encryption key (CEK), so the length (L) parameter to HKDF is 16. The second step of HKDF can therefore be simplified to the first 16 octets of a single HMAC: CEK = HMAC-SHA-256(PRK, cek_info || 0x01) -3.3. Nonce Derivation +2.3. Nonce Derivation The nonce input to AEAD_AES_128_GCM is constructed for each record. - The nonce for each record is a 12 octet (96 bit) value is produced - from the record sequence number and a value derived from the input - keying material. + The nonce for each record is a 12 octet (96 bit) value that is + produced from the record sequence number and a value derived from the + input keying material. The input keying material and salt values are input to HKDF with different info and length parameters. The length (L) parameter is 12 octets. The info parameter for the - nonce is the ASCII-encoded string "Content-Encoding: nonce", a single - zero octet and an context: - - nonce_info = "Content-Encoding: nonce" || 0x00 || context + nonce is the ASCII-encoded string "Content-Encoding: nonce", + terminated by a a single zero octet: - The context for nonce derivation is the same as is used for content - encryption key derivation. + nonce_info = "Content-Encoding: nonce" || 0x00 The result is combined with the record sequence number - using exclusive or - to produce the nonce. The record sequence number (SEQ) is a 96-bit unsigned integer in network byte order that starts at zero. Thus, the final nonce for each record is a 12 octet value: NONCE = HMAC-SHA-256(PRK, nonce_info || 0x01) XOR SEQ This nonce construction prevents removal or reordering of records. However, it permits truncation of the tail of the sequence (see Section 2 for how this is avoided). -4. Crypto-Key Header Field +3. Crypto-Key Header Field A Crypto-Key header field can be used to describe the input keying - material used in the Encryption header field. + material used by the "aes128gcm" content coding. + + Ordinarily, this header field will not appear in the same message as + the encrypted content. Including the encryption key with the + encrypted payload reduces the value of using encryption to a somewhat + complicated checksum. However, the Crypto-Key header field could be + used in one message to provision keys for other messages. The Crypto-Key header field uses the extended ABNF syntax defined in Section 1.2 of [RFC7230] and the "parameter" and "OWS" rules from [RFC7231]. - Crypto-Key = #crypto_key_params - crypto_key_params = [ parameter *( OWS ";" OWS parameter ) ] + Crypto-Key = #crypto-key-params + crypto-key-params = [ parameter *( OWS ";" OWS parameter ) ] keyid: The "keyid" parameter corresponds to the "keyid" parameter in - the Encryption header field. + the content coding. - aesgcm: The "aesgcm" parameter contains the base64url-encoded octets - [RFC7515] of the input keying material for the "aesgcm" content - coding. + aes128gcm: The "aes128gcm" parameter contains the base64url-encoded + octets [RFC7515] of the input keying material for the "aes128gcm" + content coding. Crypto-Key header field values with multiple instances of the same - parameter name are invalid. + parameter name in a single crypto-key-params production are invalid. The input keying material used by the key derivation (see - Section 3.2) can be determined based on the information in the + Section 2.2) can be determined based on the information in the Crypto-Key header field. The value or values provided in the Crypto-Key header field is valid only for the current HTTP message unless additional information indicates a greater scope. Alternative methods for determining input keying material MAY be defined by specifications that use this content coding. This - document only defines the use of the "aesgcm" parameter which + document only defines the use of the "aes128gcm" parameter which describes an explicit key. - The "aesgcm" parameter MUST decode to at least 16 octets in order to - be used as input keying material for "aesgcm" content coding. + The "aes128gcm" parameter MUST decode to at least 16 octets in order + to be used as input keying material for "aes128gcm" content coding. -5. Examples +4. Examples This section shows a few examples of the encrypted content coding. Note: All binary values in the examples in this section use base64url encoding [RFC7515]. This includes the bodies of requests. Whitespace and line wrapping is added to fit formatting constraints. -5.1. Encryption of a Response +4.1. Encryption of a Response Here, a successful HTTP GET response has been encrypted using input - keying material that is identified by a URI. + keying material that is identified by the string "a1". The encrypted data in this example is the UTF-8 encoded string "I am the walrus". The input keying material is included in the Crypto-Key header field. The content body contains a single record only and is shown here using base64url encoding for presentation reasons. HTTP/1.1 200 OK Content-Type: application/octet-stream Content-Length: 33 - Content-Encoding: aesgcm - Encryption: keyid="a1"; salt="vr0o6Uq3w_KDWeatc27mUg" - Crypto-Key: keyid="a1"; aesgcm="csPJEXBYA5U-Tal9EdJi-w" + Content-Encoding: aes128gcm + Crypto-Key: aes128gcm=B33e_VeFrOyIHwFTIfmesA - VDeU0XxaJkOJDAxPl7h9JD5V8N43RorP7PfpPdZZQuwF + 9Y1iaZMzICC05DO3y8dWiAAAopoAzpM9l8LHdpDaO9C-UvT4kttTI_edSsHv1o5b + lWZ5mBYL Note that the media type has been changed to "application/octet- stream" to avoid exposing information about the content. + Alternatively (and equivalently), the Content-Type header field can + be omitted. -5.2. Encryption with Multiple Records +4.2. Encryption with Multiple Records This example shows the same encrypted message, but split into records of 10 octets each. The first record includes a single additional octet of padding, which causes the end of the content to align with a record boundary, forcing the creation of a third record that contains only padding. HTTP/1.1 200 OK Content-Length: 70 - Content-Encoding: aesgcm - Encryption: keyid="a1"; salt="4pdat984KmT9BWsU3np0nw"; rs=10 - Crypto-Key: keyid="a1"; aesgcm="BO3ZVPxUlnLORbVGMpbT1Q" - - uzLfrZ4cbMTC6hlUqHz4NvWZshFlTN3o2RLr6FrIuOKEfl2VrM_jYgoiIyEo - Zvc-ZGwV-RMJejG4M6ZfGysBAdhpPqrLzw - -5.3. Encryption and Compression - - In this example, a response is first compressed, then encrypted. - Note that this particular encoding might compromise confidentiality - if the contents could be influenced by an attacker. - - HTTP/1.1 200 OK - Content-Type: text/html - Content-Encoding: gzip, aesgcm - Transfer-Encoding: chunked - Encryption: keyid="me@example.com"; - salt="m2hJ_NttRtFyUiMRPwfpHA" - - [encrypted payload] - -5.4. Encryption with More Than One Key - - Here, a PUT request has been encrypted twice with different input - keying material; decrypting twice is necessary to read the content. - The outer layer of encryption uses a 1200 octet record size. - - PUT /thing HTTP/1.1 - Host: storage.example.com - Content-Type: application/http - Content-Encoding: aesgcm, aesgcm - Content-Length: 1235 - Encryption: keyid="mailto:me@example.com"; - salt="NfzOeuV5USPRA-n_9s1Lag", - keyid="bob/keys/123"; - salt="bDMSGoc2uobK_IhavSHsHA"; rs=1200 + Content-Encoding: aes128gcm + Crypto-Key: keyid="a1"; aes128gcm="BO3ZVPxUlnLORbVGMpbT1Q" - [encrypted payload] + _lgOPHdbKmIaLnZC7_8huQAAAAoCYTGkQWUSYylMKzMduBHDCFDwL2oODx8nkh0n + uOTNrh48DaWSm02DiQPzQAOGe6xRAeBj588hH6jQRTh_szFRS2Nwx9Aeuiic -6. Security Considerations +5. Security Considerations This mechanism assumes the presence of a key management framework that is used to manage the distribution of keys between valid senders and receivers. Defining key management is part of composing this mechanism into a larger application, protocol, or framework. Implementation of cryptography - and key management in particular - can be difficult. For instance, implementations need to account for the potential for exposing keying material on side channels, such as might be exposed by the time it takes to perform a given operation. The requirements for a good implementation of cryptographic algorithms can change over time. -6.1. Key and Nonce Reuse +5.1. Key and Nonce Reuse Encrypting different plaintext with the same content encryption key and nonce in AES-GCM is not safe [RFC5116]. The scheme defined here uses a fixed progression of nonce values. Thus, a new content encryption key is needed for every application of the content coding. Since input keying material can be reused, a unique "salt" parameter is needed to ensure a content encryption key is not reused. If a content encryption key is reused - that is, if input keying material and salt are reused - this could expose the plaintext and the authentication key, nullifying the protection offered by encryption. Thus, if the same input keying material is reused, then the salt parameter MUST be unique each time. This ensures that the content encryption key is not reused. An implementation SHOULD generate a random salt parameter for every message; a counter could achieve the same result. -6.2. Data Encryption Limits +5.2. Data Encryption Limits There are limits to the data that AEAD_AES_128_GCM can encipher. The - maximum record size is 2^36-31 [RFC5116]. In order to preserve a - 2^-40 probability of indistinguishability under chosen plaintext - attack (IND-CPA), the total amount of plaintext that can be - enciphered MUST be less than 2^44.5 blocks [AEBounds]. + maximum value for the record size is limited by the size of the "rs" + field in the header (see Section 2.1), which ensures that the 2^36-31 + limit for a single application of AEAD_AES_128_GCM is not reached + [RFC5116]. In order to preserve a 2^-40 probability of + indistinguishability under chosen plaintext attack (IND-CPA), the + total amount of plaintext that can be enciphered MUST be less than + 2^44.5 blocks of 16 octets [AEBounds]. If rs is a multiple of 16 octets, this means 398 terabytes can be encrypted safely, including padding. However, if the record size is - a multiple of 16 octets, the total amount of data that can be safely - encrypted is reduced. The worst case is a record size of 3 octets, - for which at most 74 terabytes of plaintext can be encrypted, of - which at least two-thirds is padding. + not a multiple of 16 octets, the total amount of data that can be + safely encrypted is reduced proportionally. The worst case is a + record size of 3 octets, for which at most 74 terabytes of plaintext + can be encrypted, of which at least two-thirds is padding. -6.3. Content Integrity +5.3. Content Integrity This mechanism only provides content origin authentication. The authentication tag only ensures that an entity with access to the content encryption key produced the encrypted data. Any entity with the content encryption key can therefore produce content that will be accepted as valid. This includes all recipients of the same HTTP message. Furthermore, any entity that is able to modify both the Encryption @@ -523,25 +459,26 @@ This mechanism only provides content origin authentication. The authentication tag only ensures that an entity with access to the content encryption key produced the encrypted data. Any entity with the content encryption key can therefore produce content that will be accepted as valid. This includes all recipients of the same HTTP message. Furthermore, any entity that is able to modify both the Encryption header field and the HTTP message body can replace the contents. + Without the content encryption key or the input keying material, modifications to or replacement of parts of a payload body are not possible. -6.4. Leaking Information in Headers +5.4. Leaking Information in Headers Because only the payload body is encrypted, information exposed in header fields is visible to anyone who can read the HTTP message. This could expose side-channel information. For example, the Content-Type header field can leak information about the payload body. There are a number of strategies available to mitigate this threat, depending upon the application's threat model and the users' @@ -557,168 +494,111 @@ in other representations, etc.), omit the relevant headers, and/ or normalize them. In the case of Content-Type, this could be accomplished by always sending Content-Type: application/octet- stream (the most generic media type), or no Content-Type at all. 3. If it is considered sensitive information and it is not possible to convey it elsewhere, encapsulate the HTTP message using the application/http media type (Section 8.3.2 of [RFC7230]), encrypting that as the payload of the "outer" message. -6.5. Poisoning Storage +5.5. Poisoning Storage This mechanism only offers encryption of content; it does not perform authentication or authorization, which still needs to be performed (e.g., by HTTP authentication [RFC7235]). This is especially relevant when a HTTP PUT request is accepted by a server; if the request is unauthenticated, it becomes possible for a third party to deny service and/or poison the store. -6.6. Sizing and Timing Attacks +5.6. Sizing and Timing Attacks Applications using this mechanism need to be aware that the size of encrypted messages, as well as their timing, HTTP methods, URIs and so on, may leak sensitive information. This risk can be mitigated through the use of the padding that this mechanism provides. Alternatively, splitting up content into segments and storing the separately might reduce exposure. HTTP/2 [RFC7540] combined with TLS [RFC5246] might be used to hide the size of individual messages. -7. IANA Considerations +6. IANA Considerations -7.1. The "aesgcm" HTTP Content Coding +6.1. The "aes128gcm" HTTP Content Coding - This memo registers the "aesgcm" HTTP content coding in the HTTP + This memo registers the "aes128gcm" HTTP content coding in the HTTP Content Codings Registry, as detailed in Section 2. - o Name: aesgcm + o Name: aes128gcm + o Description: AES-GCM encryption with a 128-bit content encryption key o Reference: this specification -7.2. Encryption Header Fields - - This memo registers the "Encryption" HTTP header field in the - Permanent Message Header Registry, as detailed in Section 3. - - o Field name: Encryption - - o Protocol: HTTP - - o Status: Standard - - o Reference: this specification - - o Notes: +6.2. Crypto-Key Header Field This memo registers the "Crypto-Key" HTTP header field in the - Permanent Message Header Registry, as detailed in Section 4. + Permanent Message Header Registry, as detailed in Section 3. o Field name: Crypto-Key o Protocol: HTTP o Status: Standard o Reference: this specification o Notes: -7.3. The HTTP Encryption Parameter Registry - - This memo establishes a registry for parameters used by the - "Encryption" header field under the "Hypertext Transfer Protocol - (HTTP) Parameters" grouping. The "Hypertext Transfer Protocol (HTTP) - Encryption Parameters" registry operates under an "Specification - Required" policy [RFC5226]. - - Entries in this registry are expected to include the following - information: - - o Parameter Name: The name of the parameter. - - o Purpose: A brief description of the purpose of the parameter. - - o Reference: A reference to a specification that defines the - semantics of the parameter. - - The initial contents of this registry are: - -7.3.1. keyid - - o Parameter Name: keyid - - o Purpose: Identify the key that is in use. - - o Reference: this document - -7.3.2. salt - - o Parameter Name: salt - - o Purpose: Provide a source of entropy for derivation of a content - encryption key. This value is mandatory. - - o Reference: this document - -7.3.3. rs - - o Parameter Name: rs - - o Purpose: The size of the encrypted records. - - o Reference: this document - -7.4. The HTTP Crypto-Key Parameter Registry +6.3. The HTTP Crypto-Key Parameter Registry This memo establishes a registry for parameters used by the "Crypto- Key" header field under the "Hypertext Transfer Protocol (HTTP) Parameters" grouping. The "Hypertext Transfer Protocol (HTTP) Crypto-Key Parameters" operates under an "Specification Required" policy [RFC5226]. Entries in this registry are expected to include the following information: o Parameter Name: The name of the parameter. o Purpose: A brief description of the purpose of the parameter. o Reference: A reference to a specification that defines the semantics of the parameter. The initial contents of this registry are: -7.4.1. keyid +6.3.1. keyid o Parameter Name: keyid o Purpose: Identify the key that is in use. o Reference: this document -7.4.2. aesgcm +6.3.2. aes128gcm - o Parameter Name: aesgcm + o Parameter Name: aes128gcm o Purpose: Provide an explicit input keying material value for the - aesgcm content coding. + aes128gcm content coding. o Reference: this document -8. References +7. References -8.1. Normative References +7.1. Normative References [FIPS180-4] Department of Commerce, National., "NIST FIPS 180-4, Secure Hash Standard", March 2012, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, @@ -745,21 +625,21 @@ [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014, . [RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May 2015, . -8.2. Informative References +7.2. Informative References [AEBounds] Luykx, A. and K. Paterson, "Limits on Authenticated Encryption Use in TLS", March 2016, . [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, "OpenPGP Message Format", RFC 4880, DOI 10.17487/RFC4880, November 2007, . @@ -785,66 +665,69 @@ [RFC7516] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)", RFC 7516, DOI 10.17487/RFC7516, May 2015, . [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, May 2015, . - [XMLENC] Eastlake, D., Reagle, J., Imamura, T., Dillaway, B., and - E. Simon, "XML Encryption Syntax and Processing", W3C - REC , December 2002, . + [XMLENC] Eastlake, D., Reagle, J., Hirsch, F., Roessler, T., + Imamura, T., Dillaway, B., Simon, E., Yiu, K., and M. + Nystroem, "XML Encryption Syntax and Processing", W3C + Recommendation REC-xmlenc-core1-20130411 , January 2013, + . Appendix A. JWE Mapping - The "aesgcm" content coding can be considered as a sequence of JSON - Web Encryption (JWE) objects [RFC7516], each corresponding to a + The "aes128gcm" content coding can be considered as a sequence of + JSON Web Encryption (JWE) objects [RFC7516], each corresponding to a single fixed size record that includes leading padding. The following transformations are applied to a JWE object that might be expressed using the JWE Compact Serialization: o The JWE Protected Header is fixed to the value { "alg": "dir", "enc": "A128GCM" }, describing direct encryption using AES-GCM with a 128-bit content encryption key. This header is not transmitted, it is instead implied by the value of the Content- Encoding header field. o The JWE Encrypted Key is empty, as stipulated by the direct encryption algorithm. o The JWE Initialization Vector ("iv") for each record is set to the exclusive or of the 96-bit record sequence number, starting at zero, and a value derived from the input keying material (see - Section 3.3). This value is also not transmitted. + Section 2.3). This value is also not transmitted. - o The final value is the concatenated JWE Ciphertext and the JWE - Authentication Tag, both expressed without base64url encoding. + o The final value is the concatenated header, JWE Ciphertext, and + JWE Authentication Tag, all expressed without base64url encoding. The "." separator is omitted, since the length of these fields is known. - Thus, the example in Section 5.1 can be rendered using the JWE + Thus, the example in Section 4.1 can be rendered using the JWE Compact Serialization as: eyAiYWxnIjogImRpciIsICJlbmMiOiAiQTEyOEdDTSIgfQ..31iQYc1v4a36EgyJ. - VDeU0XxaJkOJDAxPl7h9JD4.VfDeN0aKz-z36T3WWULsBQ + AM6TPZfCx3aQ2jvQvlL0-JLb.21Mj951Kwe_WjluVZnmYFgs Where the first line represents the fixed JWE Protected Header, an empty JWE Encrypted Key, and the algorithmically-determined JWE Initialization Vector. The second line contains the encoded body, split into JWE Ciphertext and JWE Authentication Tag. Appendix B. Acknowledgements Mark Nottingham was an original author of this document. The following people provided valuable input: Richard Barnes, David - Benjamin, Peter Beverloo, Mike Jones, Stephen Farrell, Adam Langley, - John Mattsson, Eric Rescorla, and Jim Schaad. + Benjamin, Peter Beverloo, JR Conlin, Mike Jones, Stephen Farrell, + Adam Langley, John Mattsson, Julian Reschke, Eric Rescorla, Jim + Schaad, and Magnus Westerlund. Author's Address Martin Thomson Mozilla Email: martin.thomson@gmail.com