--- 1/draft-ietf-httpbis-encryption-encoding-01.txt 2016-06-29 19:16:38.599975833 -0700 +++ 2/draft-ietf-httpbis-encryption-encoding-02.txt 2016-06-29 19:16:38.643976936 -0700 @@ -1,18 +1,18 @@ HTTP Working Group M. Thomson Internet-Draft Mozilla -Intended status: Standards Track March 20, 2016 -Expires: September 21, 2016 +Intended status: Standards Track June 29, 2016 +Expires: December 31, 2016 Encrypted Content-Encoding for HTTP - draft-ietf-httpbis-encryption-encoding-01 + draft-ietf-httpbis-encryption-encoding-02 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 @@ -30,82 +30,83 @@ 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 September 21, 2016. + This Internet-Draft will expire on December 31, 2016. 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 - 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3 + 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4 2. The "aesgcm" HTTP Content Encoding . . . . . . . . . . . . . 4 - 3. The Encryption HTTP Header Field . . . . . . . . . . . . . . 5 + 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 . . . . . . . . . . . . . . . . . . . . 7 + 3.3. Nonce Derivation . . . . . . . . . . . . . . . . . . . . 8 4. Crypto-Key Header Field . . . . . . . . . . . . . . . . . . . 8 4.1. Explicit Key . . . . . . . . . . . . . . . . . . . . . . 9 4.2. Diffie-Hellman . . . . . . . . . . . . . . . . . . . . . 9 - 4.3. Pre-shared Authentication Secrets . . . . . . . . . . . . 10 + 4.3. Pre-shared Authentication Secrets . . . . . . . . . . . . 11 5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 11 - 5.1. Successful GET Response . . . . . . . . . . . . . . . . . 11 + 5.1. Successful GET Response . . . . . . . . . . . . . . . . . 12 5.2. Encryption and Compression . . . . . . . . . . . . . . . 12 5.3. Encryption with More Than One Key . . . . . . . . . . . . 12 - 5.4. Encryption with Explicit Key . . . . . . . . . . . . . . 12 + 5.4. Encryption with Explicit Key . . . . . . . . . . . . . . 13 5.5. Encryption with Multiple Records . . . . . . . . . . . . 13 - 5.6. Diffie-Hellman Encryption . . . . . . . . . . . . . . . . 13 + 5.6. Diffie-Hellman Encryption . . . . . . . . . . . . . . . . 14 5.7. Diffie-Hellman with Authentication Secret . . . . . . . . 14 - 6. Security Considerations . . . . . . . . . . . . . . . . . . . 14 + 6. Security Considerations . . . . . . . . . . . . . . . . . . . 15 6.1. Key and Nonce Reuse . . . . . . . . . . . . . . . . . . . 15 - 6.2. Content Integrity . . . . . . . . . . . . . . . . . . . . 15 - 6.3. Leaking Information in Headers . . . . . . . . . . . . . 15 - 6.4. Poisoning Storage . . . . . . . . . . . . . . . . . . . . 16 - 6.5. Sizing and Timing Attacks . . . . . . . . . . . . . . . . 16 - 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 - 7.1. The "aesgcm" HTTP Content Encoding . . . . . . . . . . . 16 - 7.2. Encryption Header Fields . . . . . . . . . . . . . . . . 17 - 7.3. The HTTP Encryption Parameter Registry . . . . . . . . . 17 - 7.3.1. keyid . . . . . . . . . . . . . . . . . . . . . . . . 18 - 7.3.2. salt . . . . . . . . . . . . . . . . . . . . . . . . 18 - 7.3.3. rs . . . . . . . . . . . . . . . . . . . . . . . . . 18 - 7.4. The HTTP Crypto-Key Parameter Registry . . . . . . . . . 18 - 7.4.1. keyid . . . . . . . . . . . . . . . . . . . . . . . . 19 - 7.4.2. aesgcm . . . . . . . . . . . . . . . . . . . . . . . 19 - 7.4.3. dh . . . . . . . . . . . . . . . . . . . . . . . . . 19 - 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 - 8.1. Normative References . . . . . . . . . . . . . . . . . . 19 - 8.2. Informative References . . . . . . . . . . . . . . . . . 20 - Appendix A. JWE Mapping . . . . . . . . . . . . . . . . . . . . 21 - Appendix B. Intermediate Values for Encryption . . . . . . . . . 22 - Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 23 - Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 23 + 6.2. Data Encryption Limits . . . . . . . . . . . . . . . . . 16 + 6.3. Content Integrity . . . . . . . . . . . . . . . . . . . . 16 + 6.4. Leaking Information in Headers . . . . . . . . . . . . . 16 + 6.5. Poisoning Storage . . . . . . . . . . . . . . . . . . . . 17 + 6.6. Sizing and Timing Attacks . . . . . . . . . . . . . . . . 17 + 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 + 7.1. The "aesgcm" HTTP Content Encoding . . . . . . . . . . . 17 + 7.2. Encryption Header Fields . . . . . . . . . . . . . . . . 18 + 7.3. The HTTP Encryption Parameter Registry . . . . . . . . . 18 + 7.3.1. keyid . . . . . . . . . . . . . . . . . . . . . . . . 19 + 7.3.2. salt . . . . . . . . . . . . . . . . . . . . . . . . 19 + 7.3.3. rs . . . . . . . . . . . . . . . . . . . . . . . . . 19 + 7.4. The HTTP Crypto-Key Parameter Registry . . . . . . . . . 19 + 7.4.1. keyid . . . . . . . . . . . . . . . . . . . . . . . . 20 + 7.4.2. aesgcm . . . . . . . . . . . . . . . . . . . . . . . 20 + 7.4.3. dh . . . . . . . . . . . . . . . . . . . . . . . . . 20 + 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 + 8.1. Normative References . . . . . . . . . . . . . . . . . . 20 + 8.2. Informative References . . . . . . . . . . . . . . . . . 21 + Appendix A. JWE Mapping . . . . . . . . . . . . . . . . . . . . 22 + Appendix B. Intermediate Values for Encryption . . . . . . . . . 23 + Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 24 + Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 24 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 @@ -158,49 +159,49 @@ Key header field (Section 4) can be included to describe how the content encryption key is derived or retrieved. The "aesgcm" 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 a series of fixed-size records, with a final record that - is one or more octets shorter than a fixed sized record. + 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. - +------+ input of between rs-65537 - | data | and rs-2 octets - +------+ (one fewer for the last record) + +-----------+ content is rs octets minus padding + | data | of between 2 and 65537 octets; + +-----------+ the last record is smaller | v - +-----+-----------+ - | pad | data | add padding to form plaintext - +-----+-----------+ + +-----+-----------+ add padding to get rs octets; + | pad | data | the last record contains + +-----+-----------+ up to rs minus 1 octets | v - +--------------------+ - | ciphertext | encrypt with AEAD_AES_128_GCM - +--------------------+ expands by 16 octets - + +--------------------+ 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. - AEAD_AES_128_GCM expands ciphertext to be 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 16 octets or less in size. Valid records always - contain at least one byte of padding and a 16 octet authentication + 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 + 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 @@ -215,108 +216,120 @@ 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 - records start and end on multiples of the record size, plus the 16 - octet authentication tag size. + range requests start and end on multiples of the record size, plus + the 16 octet authentication tag size. + + 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 encoding(s) that have been applied to a payload body, and therefore how those content encoding(s) can be removed. The "Encryption" header field uses the extended ABNF syntax defined - in Section 1.2 of [RFC7230] and the "parameter" rule from [RFC7231] + in Section 1.2 of [RFC7230] and the "parameter" and "OWS" rules from + [RFC7231]. Encryption = #encryption_params - encryption_params = [ parameter *( ";" parameter ) ] + 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 encoding is reflected in the Encryption header field, in - the order in which they were applied. + the content encoding 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. - The Encryption header MAY be omitted if the sender does not intend - for the immediate recipient to be able to decrypt the payload body. - Alternatively, the Encryption header field MAY be omitted if the - sender intends for the recipient to acquire the header field by other - means. - Servers processing PUT requests MUST persist the value of the Encryption header field, unless they remove the content-coding by decrypting the payload. 3.1. Encryption Header Field Parameters The following parameters are used in determining the content encryption key that is used for encryption: - keyid: The "keyid" parameter contains a string that identifies the - keying material that is used. The "keyid" parameter SHOULD be - included, unless key identification is guaranteed by other means. - The "keyid" parameter MUST be used if keying material included in - an Crypto-Key header field is needed to derive the content - encryption key. + 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 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 encoding ensures that content encryption key reuse is highly unlikely. rs: The "rs" parameter contains a positive decimal integer that describes the record size in octets. This value MUST be greater - than 1. If the "rs" parameter is absent, the record size defaults - to 4096 octets. + than 1. For the "aesgcm" content encoding, 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. 3.2. 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 first step of HKDF is therefore: + header field (Section 4). 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: cek_info = "Content-Encoding: aesgcm" || 0x00 || context Unless otherwise specified, the context is a zero length octet sequence. Specifications that use this content encoding MAY specify the use of an expanded context to cover additional inputs in the key derivation. - AEAD_AES_128_GCM requires a 16 octet (128 bit) content encryption - key, so the length (L) parameter to HKDF is 16. The second step of + 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 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 @@ -336,36 +349,42 @@ 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 - An Crypto-Key header field can be used to describe the input keying + A Crypto-Key header field can be used to describe the input keying material used in the Encryption header field. The Crypto-Key header field uses the extended ABNF syntax defined in - Section 1.2 of [RFC7230] and the "parameter" rule from [RFC7231]. + Section 1.2 of [RFC7230] and the "parameter" and "OWS" rules from + [RFC7231]. Crypto-Key = #crypto_key_params - crypto_key_params = [ parameter *( ";" parameter ) ] + crypto_key_params = [ parameter *( OWS ";" OWS parameter ) ] keyid: The "keyid" parameter corresponds to the "keyid" parameter in the Encryption header field. aesgcm: The "aesgcm" parameter contains the base64url-encoded octets - [RFC7515] of the input keying material. + [RFC7515] of the input keying material for the "aesgcm" content + encoding. dh: The "dh" parameter contains an ephemeral Diffie-Hellman share. This form of the header field can be used to encrypt content for a specific recipient. Crypto-Key header field values with multiple instances of the same parameter name are invalid. The input keying material used by the key derivation (see Section 3.2) can be determined based on the information in the @@ -490,60 +509,60 @@ and filename safe variant of base64 [RFC4648]. This includes the bodies of requests. Whitespace in these values is added to fit formatting constraints. 5.1. Successful GET Response HTTP/1.1 200 OK Content-Type: application/octet-stream Content-Encoding: aesgcm Connection: close - Encryption: keyid="http://example.org/bob/keys/123"; + Encryption: keyid="bob/keys/123"; salt="XZwpw6o37R-6qoZjw6KwAw" [encrypted payload] Here, a successful HTTP GET response has been encrypted using input keying material that is identified by a URI. Note that the media type has been changed to "application/octet- stream" to avoid exposing information about the content. 5.2. Encryption and Compression In this example, a response is first compressed, then encrypted. Note that this particular encoding might compromise confidentiality if the contents of the response could be influenced by an attacker. HTTP/1.1 200 OK Content-Type: text/html Content-Encoding: gzip, aesgcm Transfer-Encoding: chunked - Encryption: keyid="mailto:me@example.com"; + Encryption: keyid="me@example.com"; salt="m2hJ_NttRtFyUiMRPwfpHA" [encrypted payload] 5.3. 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="http://example.org/bob/keys/123"; + keyid="bob/keys/123"; salt="bDMSGoc2uobK_IhavSHsHA"; rs=1200 [encrypted payload] 5.4. Encryption with Explicit Key This example shows the UTF-8 encoded string "I am the walrus" encrypted using an directly provided value for the input keying material. The content body contains a single record only and is shown here using base64url encoding for presentation reasons. @@ -649,37 +668,52 @@ 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. Content Integrity +6.2. 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]. + + 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. + +6.3. 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 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. -6.3. Leaking Information in Headers +6.4. 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' @@ -695,50 +729,51 @@ 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. -6.4. Poisoning Storage +6.5. 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. -6.5. Sizing and Timing Attacks +6.6. 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 7.1. The "aesgcm" HTTP Content Encoding - This memo registers the "encrypted" HTTP content-coding in the HTTP + This memo registers the "aesgcm" HTTP content-coding in the HTTP Content Codings Registry, as detailed in Section 2. o Name: aesgcm + 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. @@ -899,24 +934,33 @@ 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 + [AEBounds] + Luykx, A. and K. Paterson, "Limits on Authenticated + Encryption Use in TLS", March 2016, + . + [FIPS186] National Institute of Standards and Technology (NIST), "Digital Signature Standard (DSS)", NIST PUB 186-4 , July 2013. + [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data + Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, + . + [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, "OpenPGP Message Format", RFC 4880, DOI 10.17487/RFC4880, November 2007, . [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, .