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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 HTTPAuth Working Group R. Shekh-Yusef, Ed. 3 Internet-Draft D. Ahrens 4 Obsoletes: 2617 (if approved) Avaya 5 Intended Status: Standards Track S. Bremer 6 Expires: July 23, 2014 Netzkonform 7 January 19, 2014 9 HTTP Digest Access Authentication 10 draft-ietf-httpauth-digest-04 12 Abstract 14 HTTP provides a simple challenge-response authentication mechanism 15 that may be used by a server to challenge a client request and by a 16 client to provide authentication information. This document defines 17 the HTTP Digest Authentication scheme that may be used with the 18 authentication mechanism. 20 Status of this Memo 22 This Internet-Draft is submitted to IETF in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF), its areas, and its working groups. Note that 27 other groups may also distribute working documents as 28 Internet-Drafts. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 The list of current Internet-Drafts can be accessed at 36 http://www.ietf.org/1id-abstracts.html 38 The list of Internet-Draft Shadow Directories can be accessed at 39 http://www.ietf.org/shadow.html 41 Copyright and License Notice 43 Copyright (c) 2014 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (http://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 4 59 1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4 60 2 Syntax Convention . . . . . . . . . . . . . . . . . . . . . . . 4 61 3 Digest Access Authentication Scheme . . . . . . . . . . . . . . 4 62 3.1 Overall Operation . . . . . . . . . . . . . . . . . . . . . 4 63 3.2 Representation of Digest Values . . . . . . . . . . . . . . 4 64 3.3 The WWW-Authenticate Response Header . . . . . . . . . . . . 5 65 3.4 The Authorization Request Header . . . . . . . . . . . . . . 8 66 3.4.1 Request-Digest . . . . . . . . . . . . . . . . . . . . . 10 67 3.4.2 A1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 68 3.4.3 A2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 69 3.4.4 Username Hashing . . . . . . . . . . . . . . . . . . . . 11 70 3.4.5 Directive Values and Quoted-String . . . . . . . . . . . 11 71 3.4.6 Various Considerations . . . . . . . . . . . . . . . . . 12 72 3.5 The Authentication-Info Header . . . . . . . . . . . . . . . 13 73 3.6 Digest Operation . . . . . . . . . . . . . . . . . . . . . . 14 74 3.7 Security Protocol Negotiation . . . . . . . . . . . . . . . 15 75 3.8 Proxy-Authenticate and Proxy-Authorization . . . . . . . . . 16 76 3.9 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 17 77 3.9.1 Example with SHA2-256 and MD5 . . . . . . . . . . . . . 17 78 3.9.2 Example with SHA-512-256, Charset, and Userhash . . . . 18 79 4 Internationalization . . . . . . . . . . . . . . . . . . . . . . 19 80 5 Security Considerations . . . . . . . . . . . . . . . . . . . . 20 81 5.1 Limitations . . . . . . . . . . . . . . . . . . . . . . . . 20 82 5.2 Authentication of Clients using Digest Authentication . . . 20 83 5.3 Limited Use Nonce Values . . . . . . . . . . . . . . . . . . 21 84 5.4 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . . 21 85 5.5 Weakness Created by Multiple Authentication Schemes . . . . 22 86 5.6 Online dictionary attacks . . . . . . . . . . . . . . . . . 23 87 5.7 Man in the Middle . . . . . . . . . . . . . . . . . . . . . 23 88 5.8 Chosen plaintext attacks . . . . . . . . . . . . . . . . . . 24 89 5.9 Precomputed dictionary attacks . . . . . . . . . . . . . . . 24 90 5.10 Batch brute force attacks . . . . . . . . . . . . . . . . . 24 91 5.11 Spoofing by Counterfeit Servers . . . . . . . . . . . . . . 25 92 5.12 Storing passwords . . . . . . . . . . . . . . . . . . . . . 25 93 5.13 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 26 94 6 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 27 95 6.1 HTTP Digest Hash Algorithms Registry . . . . . . . . . . . 27 96 6.2 Digest . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 97 7 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 28 98 8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 99 8.1 Normative References . . . . . . . . . . . . . . . . . . . . 28 100 8.2 Informative References . . . . . . . . . . . . . . . . . . . 29 101 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29 103 1 Introduction 105 HTTP provides a simple challenge-response authentication mechanism 106 that may be used by a server to challenge a client request and by a 107 client to provide authentication information. This document defines 108 the HTTP Digest Authentication scheme that may be used with the 109 authentication mechanism. 111 The details of the challenge-response authentication mechanism are 112 specified in the [HTTP-P7] document. 114 1.1 Terminology 116 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 117 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 118 document are to be interpreted as described in RFC 2119 [RFC2119]. 120 2 Syntax Convention 122 In the interest of clarity and readability, the extended parameters 123 or the headers and parameters in the examples in this document might 124 be broken into multiple lines. Any line that is indented in this 125 document is a continuation of the preceding line. 127 3 Digest Access Authentication Scheme 129 3.1 Overall Operation 131 The Digest scheme is based on a simple challenge-response paradigm. 132 The Digest scheme challenges using a nonce value. A valid response 133 contains a checksum of the username, the password, the given nonce 134 value, the HTTP method, and the requested URI. In this way, the 135 password is never sent in the clear. The username and password must 136 be prearranged in some fashion not addressed by this document. 138 3.2 Representation of Digest Values 140 An optional header allows the server to specify the algorithm used to 141 create the checksum or digest. This documents adds SHA2-256 and SHA2- 142 512/256 algorithms. To maintain backwards compatibility, the MD5 143 algorithm is still supported but not recommended. 145 The size of the digest depends on the algorithm used. The bits in 146 the digest are converted from the most significant to the least 147 significant bit, four bits at a time to the ASCII representation as 148 follows. Each four bits is represented by its familiar hexadecimal 149 notation from the characters 0123456789abcdef, that is binary 0000 is 150 represented by the character '0', 0001 by '1' and so on up to the 151 representation of 1111 as 'f'. If the MD5 algorithm is used to 152 calculate the digest, then the digest will be represented as 32 153 hexadecimal characters, SHA2-256 and SHA2-512/256 by 64 hexadecimal 154 characters. 156 3.3 The WWW-Authenticate Response Header 158 If a server receives a request for an access-protected object, and an 159 acceptable Authorization header is not sent, the server responds with 160 a "401 Unauthorized" status code, and a WWW-Authenticate header with 161 Digest scheme as per the framework defined above, and include some or 162 all of the following parameters: 164 realm 165 A string to be displayed to users so they know which username and 166 password to use. This string should contain at least the name of 167 the host performing the authentication and might additionally 168 indicate the collection of users who might have access. An example 169 might be "registered_users@gotham.news.com". (See section 2.2 of 170 [HTTP-P7] for more details). 172 domain 173 A quoted, space-separated list of URIs, as specified in RFC 3986 174 [RFC3986], that define the protection space. If a URI is an 175 abs_path, it is relative to the canonical root URL of the server 176 being accessed. An absolute-URI in this list may refer to a 177 different server than the one being accessed. The client can use 178 this list to determine the set of URIs for which the same 179 authentication information may be sent: any URI that has a URI in 180 this list as a prefix (after both have been made absolute) may be 181 assumed to be in the same protection space. If this directive is 182 omitted or its value is empty, the client should assume that the 183 protection space consists of all URIs on the responding server. 185 This directive is not meaningful in Proxy-Authenticate headers, 186 for which the protection space is always the entire proxy; if 187 present it should be ignored. 189 nonce 190 A server-specified data string which should be uniquely generated 191 each time a 401 response is made. It is recommended that this 192 string be base64 or hexadecimal data. Specifically, since the 193 string is passed in the header lines as a quoted string, the 194 double-quote character is not allowed. 196 The contents of the nonce are implementation dependent. The 197 quality of the implementation depends on a good choice. A nonce 198 might, for example, be constructed as the base 64 encoding of 200 time-stamp H(time-stamp ":" ETag ":" private-key) 202 where time-stamp is a server-generated time or other non-repeating 203 value, ETag is the value of the HTTP ETag header associated with 204 the requested entity, and private-key is data known only to the 205 server. With a nonce of this form a server would recalculate the 206 hash portion after receiving the client authentication header and 207 reject the request if it did not match the nonce from that header 208 or if the time-stamp value is not recent enough. In this way the 209 server can limit the time of the nonce's validity. The inclusion 210 of the ETag prevents a replay request for an updated version of 211 the resource. (Note: including the IP address of the client in the 212 nonce would appear to offer the server the ability to limit the 213 reuse of the nonce to the same client that originally got it. 214 However, that would break proxy farms, where requests from a 215 single user often go through different proxies in the farm. Also, 216 IP address spoofing is not that hard.) 218 An implementation might choose not to accept a previously used 219 nonce or a previously used digest, in order to protect against a 220 replay attack. Or, an implementation might choose to use one-time 221 nonces or digests for POST or PUT requests and a time-stamp for 222 GET requests. For more details on the issues involved see section 223 5 of this document. 225 The nonce is opaque to the client. 227 opaque 228 A string of data, specified by the server, which should be 229 returned by the client unchanged in the Authorization header of 230 subsequent requests with URIs in the same protection space. It is 231 recommended that this string be base64 or hexadecimal data. 233 stale 234 A case-insensitive flag, indicating that the previous request from 235 the client was rejected because the nonce value was stale. If 236 stale is TRUE, the client may wish to simply retry the request 237 with a new encrypted response, without reprompting the user for a 238 new username and password. The server should only set stale to 239 TRUE if it receives a request for which the nonce is invalid but 240 with a valid digest for that nonce (indicating that the client 241 knows the correct username/password). If stale is FALSE, or 242 anything other than TRUE, or the stale directive is not present, 243 the username and/or password are invalid, and new values must be 244 obtained. 246 algorithm 247 A string indicating a pair of algorithms used to produce the 248 digest and a checksum. If this is not present it is assumed to be 249 "MD5". If the algorithm is not understood, the challenge should be 250 ignored (and a different one used, if there is more than one). 252 In this document the string obtained by applying the digest 253 algorithm to the data "data" with secret "secret" will be denoted 254 by KD(secret, data), and the string obtained by applying the 255 checksum algorithm to the data "data" will be denoted H(data). The 256 notation unq(X) means the value of the quoted-string X without the 257 surrounding quotes. 259 For the "MD5" and "MD5-sess" algorithms 261 H(data) = MD5(data) 263 For the "SHA2-256" and "SHA2-256-sess" algorithms 265 H(data) = SHA2-256(data) 267 For the "SHA2-512-256" and "SHA2-512-256-sess" algorithms 269 H(data) = SHA2-512-256(data) 271 and 273 KD(secret, data) = H(concat(secret, ":", data)) 275 i.e., the digest is the MD5 of the secret concatenated with a 276 colon concatenated with the data. The "MD5-sess" algorithm is 277 intended to allow efficient 3rd party authentication servers; 278 for the difference in usage, see the description in section 279 3.4.2. 281 qop-options 282 This directive is optional, but is made so only for backward 283 compatibility with RFC 2069 [RFC2069]; it SHOULD be used by all 284 implementations compliant with this version of the Digest scheme. 285 If present, it is a quoted string of one or more tokens indicating 286 the "quality of protection" values supported by the server. The 287 value "auth" indicates authentication; the value "auth-int" 288 indicates authentication with integrity protection; see the 289 descriptions below for calculating the response directive value 290 for the application of this choice. Unrecognized options MUST be 291 ignored. 293 charset 294 This is an OPTIONAL parameter that is used by the server to 295 indicate the encoding scheme it supports. 297 userhash 298 This is an OPTIONAL parameter that is used by the server to 299 indicate that it supports username hashing. Valid value are: 300 "true" or "false". 302 3.4 The Authorization Request Header 304 The client is expected to retry the request, passing an 305 Authorization header line with Digest scheme, which is defined 306 according to the framework above. The values of the opaque and 307 algorithm fields must be those supplied in the WWW-Authenticate 308 response header for the entity being requested. 310 The request includes some or all of the following parameters: 312 response 313 A string of the hex digits computed as a request-digest defined 314 below, which proves that the user knows a password 316 username 317 The user's name in the specified realm. 319 digest-uri 320 The URI from request-target of the Request-Line; duplicated here 321 because proxies are allowed to change the Request-Line in transit. 323 qop 324 Indicates what "quality of protection" the client has applied to 325 the message. If present, its value MUST be one of the alternatives 326 the server indicated it supports in the WWW-Authenticate header. 327 These values affect the computation of the request-digest. Note 328 that this is a single token, not a quoted list of alternatives as 329 in WWW-Authenticate. This directive is optional in order to 330 preserve backward compatibility with a minimal implementation of 331 RFC 2069 [RFC2069], but SHOULD be used if the server indicated 332 that qop is supported by providing a qop directive in the WWW- 333 Authenticate header field. 335 cnonce 336 This MUST be specified if a qop directive is sent (see above), and 337 MUST NOT be specified if the server did not send a qop directive 338 in the WWW-Authenticate header field. The cnonce-value is an 339 opaque quoted string value provided by the client and used by both 340 client and server to avoid chosen plaintext attacks, to provide 341 mutual authentication, and to provide some message integrity 342 protection. See the descriptions below of the calculation of the 343 response- digest and request-digest values. 345 nonce-count 346 This MUST be specified if a qop directive is sent (see above), and 347 MUST NOT be specified if the server did not send a qop directive 348 in the WWW-Authenticate header field. The nc-value is the 349 hexadecimal count of the number of requests (including the current 350 request) that the client has sent with the nonce value in this 351 request. For example, in the first request sent in response to a 352 given nonce value, the client sends "nc=00000001". The purpose of 353 this directive is to allow the server to detect request replays by 354 maintaining its own copy of this count - if the same nc-value is 355 seen twice, then the request is a replay. See the description 356 below of the construction of the request-digest value. 358 charset 359 This OPTIONAL parameter is used by the client to indicate the 360 character encoding used for the username and password. 362 userhash 363 This OPTIONAL parameter is used by the client to indicate that the 364 username has been hashed. Valid value are: "true" or "false". 366 If a directive or its value is improper, or required directives are 367 missing, the proper response is 400 Bad Request. If the request- 368 digest is invalid, then a login failure should be logged, since 369 repeated login failures from a single client may indicate an attacker 370 attempting to guess passwords. 372 The definition of request-digest above indicates the encoding for its 373 value. The following definitions show how the value is computed. 375 3.4.1 Request-Digest 377 If the "qop" value is "auth" or "auth-int": 379 request-digest = <"> < KD ( H(A1), unq(nonce-value) 380 ":" nc-value 381 ":" unq(cnonce-value) 382 ":" unq(qop-value) 383 ":" H(A2) 384 ) <"> 386 If the "qop" directive is not present (this construction is for 387 compatibility with RFC 2069): 389 request-digest = 390 <"> < KD ( H(A1), unq(nonce-value) ":" H(A2) ) > <"> 392 See below for the definitions for A1 and A2. 394 3.4.2 A1 396 If the "algorithm" directive's value is "MD5", "SHA2-256", or "SHA2- 397 512-256", then A1 is: 399 A1 = unq(username-value) ":" unq(realm-value) ":" passwd 401 where 403 passwd = < user's password > 405 If the "algorithm" directive's value is "MD5-sess", "SHA2-256-sess", 406 or "SHA2-512-256-sess", then A1 is calculated only once - on the 407 first request by the client following receipt of a WWW-Authenticate 408 challenge from the server. It uses the server nonce from that 409 challenge, and the first client nonce value to construct A1 as 410 follows: 412 A1 = H( unq(username-value) ":" unq(realm-value) 413 ":" passwd ) 414 ":" unq(nonce-value) ":" unq(cnonce-value) 416 This creates a 'session key' for the authentication of subsequent 417 requests and responses which is different for each "authentication 418 session", thus limiting the amount of material hashed with any one 419 key. (Note: see further discussion of the authentication session in 420 section 3.6.) Because the server need only use the hash of the user 421 credentials in order to create the A1 value, this construction could 422 be used in conjunction with a third party authentication service so 423 that the web server would not need the actual password value. The 424 specification of such a protocol is beyond the scope of this 425 specification. 427 3.4.3 A2 429 If the "qop" directive's value is "auth" or is unspecified, then A2 430 is: 432 A2 = Method ":" digest-uri-value 434 If the "qop" value is "auth-int", then A2 is: 436 A2 = Method ":" digest-uri-value ":" H(entity-body) 438 3.4.4 Username Hashing 440 To protect the transport of the username from the client to the 441 server, the server SHOULD set the "userhash" parameter with the value 442 of "true" in the WWW-Authentication header. 444 If the client supports the "userhash" parameter, and the "userhash" 445 parameter value in the WWW-Authentication header is set to "true", 446 then the client MUST calculate a hash of the username after any other 447 hash calculation and include the "userhash" parameter with the value 448 of "true" in the Authorization Request Header. If the client does not 449 provide the "username" as a hash value or the "userhash" parameter 450 with the value of "true", the server MAY reject the request. 452 The server may change the nonce value, so the client should be ready 453 to recalculate the hashed username. 455 The following is the operation that the client will take to hash the 456 username: 458 username = H( H( username ":" realm ) ":" nonce) 460 3.4.5 Directive Values and Quoted-String 462 Note that the value of many of the directives, such as "username- 463 value", are defined as a "quoted-string". However, the "unq" notation 464 indicates that surrounding quotation marks are removed in forming the 465 string A1. Thus if the Authorization header includes the fields 467 username="Mufasa", realm=myhost@testrealm.com 469 and the user Mufasa has password "Circle Of Life" then H(A1) would be 470 H(Mufasa:myhost@testrealm.com:Circle Of Life) with no quotation marks 471 in the digested string. 473 No white space is allowed in any of the strings to which the digest 474 function H() is applied unless that white space exists in the quoted 475 strings or entity body whose contents make up the string to be 476 digested. For example, the string A1 illustrated above must be 478 Mufasa:myhost@testrealm.com:Circle Of Life 480 with no white space on either side of the colons, but with the white 481 space between the words used in the password value. Likewise, the 482 other strings digested by H() must not have white space on either 483 side of the colons which delimit their fields unless that white space 484 was in the quoted strings or entity body being digested. 486 Also note that if integrity protection is applied (qop=auth-int), the 487 H(entity-body) is the hash of the entity body, not the message body - 488 it is computed before any transfer encoding is applied by the sender 489 and after it has been removed by the recipient. Note that this 490 includes multipart boundaries and embedded headers in each part of 491 any multipart content-type. 493 3.4.6 Various Considerations 495 The "Method" value is the HTTP request method as specified in section 496 3.1.1 of [HTTP-P1]. The "request-target" value is the request-target 497 from the request line as specified in section 3.1.1 of [HTTP-P1]. 498 This may be "*", an "absolute-URI" or an "absolute-path" as specified 499 in section 2.7 of [HTTP-P1], but it MUST agree with the request- 500 target. In particular, it MUST be an "absolute-URI" if the request- 501 target is an "absolute-URI". The "cnonce-value" is an optional 502 client-chosen value whose purpose is to foil chosen plaintext 503 attacks. 505 The authenticating server must assure that the resource designated by 506 the "uri" directive is the same as the resource specified in the 507 Request-Line; if they are not, the server SHOULD return a 400 Bad 508 Request error. (Since this may be a symptom of an attack, server 509 implementers may want to consider logging such errors.) The purpose 510 of duplicating information from the request URL in this field is to 511 deal with the possibility that an intermediate proxy may alter the 512 client's Request-Line. This altered (but presumably semantically 513 equivalent) request would not result in the same digest as that 514 calculated by the client. 516 Implementers should be aware of how authenticated transactions 517 interact with shared caches. The HTTP/1.1 protocol specifies that 518 when a shared cache (see [HTTP-P6]) has received a request containing 519 an Authorization header and a response from relaying that request, it 520 MUST NOT return that response as a reply to any other request, unless 521 one of two Cache-Control (see section 3.2 of [HTTP-P6]) directives 522 was present in the response. If the original response included the 523 "must-revalidate" Cache-Control directive, the cache MAY use the 524 entity of that response in replying to a subsequent request, but MUST 525 first revalidate it with the origin server, using the request headers 526 from the new request to allow the origin server to authenticate the 527 new request. Alternatively, if the original response included the 528 "public" Cache-Control directive, the response entity MAY be returned 529 in reply to any subsequent request. 531 3.5 The Authentication-Info Header 533 The Authentication-Info header is used by the server to communicate 534 some information regarding the successful authentication in the 535 response. 537 AuthenticationInfo = "Authentication-Info" ":" auth-info 538 auth-info = 1#(nextnonce | [ message-qop ] 539 | [ response-auth ] | [ cnonce ] 540 | [nonce-count] ) 541 nextnonce = "nextnonce" "=" nonce-value 542 response-auth = "rspauth" "=" response-digest 543 response-digest = <"> *LHEX <"> 545 The value of the nextnonce directive is the nonce the server wishes 546 the client to use for a future authentication response. The server 547 may send the Authentication-Info header with a nextnonce field as a 548 means of implementing one-time or otherwise changing nonces. If the 549 nextnonce field is present the client SHOULD use it when constructing 550 the Authorization header for its next request. Failure of the client 551 to do so may result in a request to re-authenticate from the server 552 with the "stale=TRUE". 554 Server implementations should carefully consider the performance 555 implications of the use of this mechanism; pipelined requests will 556 not be possible if every response includes a nextnonce directive 557 that must be used on the next request received by the server. 558 Consideration should be given to the performance vs. security 559 tradeoffs of allowing an old nonce value to be used for a limited 560 time to permit request pipelining. Use of the nonce-count can 561 retain most of the security advantages of a new server nonce 562 without the deleterious affects on pipelining. 564 message-qop 565 Indicates the "quality of protection" options applied to the 566 response by the server. The value "auth" indicates 567 authentication; the value "auth-int" indicates authentication with 568 integrity protection. The server SHOULD use the same value for the 569 message- qop directive in the response as was sent by the client 570 in the corresponding request. 572 The optional response digest in the "response-auth" directive 573 supports mutual authentication -- the server proves that it knows the 574 user's secret, and with qop=auth-int also provides limited integrity 575 protection of the response. The "response-digest" value is calculated 576 as for the "request-digest" in the Authorization header, except that 577 if "qop=auth" or is not specified in the Authorization header for the 578 request, A2 is 580 A2 = ":" digest-uri-value 582 and if "qop=auth-int", then A2 is 584 A2 = ":" digest-uri-value ":" H(entity-body) 586 where "digest-uri-value" is the value of the "uri" directive on the 587 Authorization header in the request. The "cnonce-value" and "nc- 588 value" MUST be the ones for the client request to which this message 589 is the response. The "response-auth", "cnonce", and "nonce-count" 590 directives MUST BE present if "qop=auth" or "qop=auth-int" is 591 specified. 593 The Authentication-Info header is allowed in the trailer of an HTTP 594 message transferred via chunked transfer-coding. 596 3.6 Digest Operation 598 Upon receiving the Authorization header, the server may check its 599 validity by looking up the password that corresponds to the submitted 600 username. Then, the server must perform the same digest operation 601 (e.g., MD5) performed by the client, and compare the result to the 602 given request-digest value. 604 Note that the HTTP server does not actually need to know the user's 605 cleartext password. As long as H(A1) is available to the server, the 606 validity of an Authorization header may be verified. 608 The client response to a WWW-Authenticate challenge for a protection 609 space starts an authentication session with that protection space. 610 The authentication session lasts until the client receives another 611 WWW-Authenticate challenge from any server in the protection space. A 612 client should remember the username, password, nonce, nonce count and 613 opaque values associated with an authentication session to use to 614 construct the Authorization header in future requests within that 615 protection space. The Authorization header may be included 616 preemptively; doing so improves server efficiency and avoids extra 617 round trips for authentication challenges. The server may choose to 618 accept the old Authorization header information, even though the 619 nonce value included might not be fresh. Alternatively, the server 620 may return a 401 response with a new nonce value, causing the client 621 to retry the request; by specifying stale=TRUE with this response, 622 the server tells the client to retry with the new nonce, but without 623 prompting for a new username and password. 625 Because the client is required to return the value of the opaque 626 directive given to it by the server for the duration of a session, 627 the opaque data may be used to transport authentication session state 628 information. (Note that any such use can also be accomplished more 629 easily and safely by including the state in the nonce.) For example, 630 a server could be responsible for authenticating content that 631 actually sits on another server. It would achieve this by having the 632 first 401 response include a domain directive whose value includes a 633 URI on the second server, and an opaque directive whose value 634 contains the state information. The client will retry the request, at 635 which time the server might respond with a 301/302 redirection, 636 pointing to the URI on the second server. The client will follow the 637 redirection, and pass an Authorization header , including the 638 data. 640 As with the basic scheme, proxies must be completely transparent in 641 the Digest access authentication scheme. That is, they must forward 642 the WWW-Authenticate, Authentication-Info and Authorization headers 643 untouched. If a proxy wants to authenticate a client before a request 644 is forwarded to the server, it can be done using the Proxy- 645 Authenticate and Proxy-Authorization headers described in section 3.6 646 below. 648 3.7 Security Protocol Negotiation 650 It is useful for a server to be able to know which security schemes a 651 client is capable of handling. 653 It is possible that a server may want to require Digest as its 654 authentication method, even if the server does not know that the 655 client supports it. A client is encouraged to fail gracefully if the 656 server specifies only authentication schemes it cannot handle. 658 When a server receives a request to access a resource, the server 659 might challenge the client by responding with "401 Unauthorized" 660 status code, and include one or more WWW-Authenticate headers. If the 661 server challenges with multiple Digest headers, then each one of 662 these headers MUST use a different digest algorithm. The server MUST 663 add these Digest headers to the response in order of preference, 664 starting with the most preferred header, followed by the less 665 preferred headers. 667 This specification defines the following preference list, starting 668 with the most preferred algorithm: 670 * SHA2-256. 671 * SHA2-512/256. 672 * MD5 (for backward compatibility). 674 A future version of this document might add SHA3 [SHA3] as a backup 675 algorithm, once its definition has been finalized and published. 677 When the client receives the response it SHOULD use the topmost 678 header that it supports, unless a local policy dictates otherwise. 679 The client should ignore any challenge it does not understand. 681 3.8 Proxy-Authenticate and Proxy-Authorization 683 The digest authentication scheme may also be used for authenticating 684 users to proxies, proxies to proxies, or proxies to origin servers by 685 use of the Proxy-Authenticate and Proxy-Authorization headers. These 686 headers are instances of the Proxy-Authenticate and Proxy- 687 Authorization headers specified in sections 4.2 and 4.3 of the 688 HTTP/1.1 specification [HTTP-P7] and their behavior is subject to 689 restrictions described there. The transactions for proxy 690 authentication are very similar to those already described. Upon 691 receiving a request which requires authentication, the proxy/server 692 must issue the "407 Proxy Authentication Required" response with a 693 "Proxy-Authenticate" header. The digest-challenge used in the Proxy- 694 Authenticate header is the same as that for the WWW- Authenticate 695 header as defined above in section 3.2.1. 697 The client/proxy must then re-issue the request with a Proxy- 698 Authorization header, with directives as specified for the 699 Authorization header in section 3.4 above. 701 On subsequent responses, the server sends Proxy-Authenticate-Info 702 with directives the same as those for the Authentication-Info header 703 field. 705 Note that in principle a client could be asked to authenticate itself 706 to both a proxy and an end-server, but never in the same response. 708 3.9 Examples 710 3.9.1 Example with SHA2-256 and MD5 712 The following example assumes that an access protected document is 713 being requested from the server via a GET request. The URI of the 714 document is http://www.nowhere.org/dir/index.html". Both client and 715 server know that the username for this document is "Mufasa" and the 716 password is "Circle of Life" ( with one space between each of the 717 three words). 719 The first time the client requests the document, no Authorization 720 header is sent, so the server responds with: 722 HTTP/1.1 401 Unauthorized 723 WWW-Authenticate: Digest 724 realm = "testrealm@host.com", 725 qop="auth, auth-int", 726 algorithm="SHA2-256", 727 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093", 728 opaque="5ccc069c403ebaf9f0171e9517f40e41" 729 WWW-Authenticate: Digest 730 realm="testrealm@host.com", 731 qop="auth, auth-int", 732 algorithm="MD5", 733 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093", 734 opaque="5ccc069c403ebaf9f0171e9517f40ef41" 736 The client may prompt the user for their username and password, after 737 which it will respond with a new request, including the following 738 Authorization header if the client chooses MD5 digest: 740 Authorization:Digest username="Mufasa", 741 realm="testrealm@host.com", 742 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093", 743 uri="/dir/index.html", 744 qop="auth", 745 algorithm="MD5", 746 nc=00000001, 747 cnonce="0a4f113b", 748 response="6629fae49393a05397450978507c4ef1", 749 opaque="5ccc069c403ebaf9f0171e9517f40e41" 751 If the client chooses to use the SHA2-256 algorithm for calculating 752 the response, the client responds with a new request including the 753 following Authorization header: 755 Authorization:Digest username="Mufasa", 756 realm="testrealm@host.com", 757 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093", 758 uri="/dir/index.html", 759 qop="auth", 760 algorithm="SHA2-256", 761 nc=00000001, 762 cnonce="0a4f113b", 763 response="5abdd07184ba512a22c53f41470e5eea7dcaa3a93 764 a59b630c13dfe0a5dc6e38b", 765 opaque="5ccc069c403ebaf9f0171e9517f40e41" 767 3.9.2 Example with SHA-512-256, Charset, and Userhash 769 The following example assumes that an access protected document is 770 being requested from the server via a GET request. The URI for the 771 request is "http://api.example.org/doe.json". Both client and server 772 know the userhash of the username, support the UTF-8 charset, and use 773 the SHA-512-256 algorithm. The username for the request is "Jason 774 Doe" and the password is "Secret, or not?". 776 The first time the client requests the document, no Authorization 777 header is sent, so the server responds with: 779 HTTP/2.0 401 Unauthorized 780 WWW-Authenticate: Digest 781 realm="api@example.org", 782 qop=auth, 783 algorithm=SHA-512-256, 784 nonce="e145a96d70d40739596e60c6340f13be03290bd73c676d 785 3f25c01271af522eb2", 786 opaque="192cbcf2a2576846522c1a367c1dfdf0359a87719c5cc1 787 839e4f3d2ffeb82517", 788 charset=UTF-8, 789 userhash=true 791 The client may prompt the user for the required credentials and send 792 a new request with following Authorization header: 794 Authorization: Digest 795 username="298bc3decec198ec5e7ecc1d69f059ca33044dd15baf45 796 a1f87bbd7adb3784fd", 797 realm="api@example.org", 798 uri="/doe.json", 799 algorithm=SHA-512-256, 800 nonce="e145a96d70d40739596e60c6340f13be03290bd73c676d 801 3f25c01271af522eb2", 802 nc=00000001, 803 cnonce="cde966df34a49d5d842a263604159141c81db8d468e1bf 804 657230429424fc337a", 805 qop=auth, 806 response="ec180fc03b7a0dcd43c414f66f2335399bbe5f4d4ad469 807 f8233106ba453213c8", 808 opaque="192cbcf2a2576846522c1a367c1dfdf0359a87719c5cc1 809 839e4f3d2ffeb82517", 810 charset=UTF-8, 811 userhash=true 813 If the client can not provide a hashed username for any reason, the 814 client may try a request with this Authorization header: 816 Authorization: Digest 817 username="Jason Doe", 818 realm="api@example.org", 819 uri="/doe.json", 820 algorithm=SHA-512-256, 821 nonce="e145a96d70d40739596e60c6340f13be03290bd73c676d 822 3f25c01271af522eb2", 823 nc=00000001, 824 cnonce="cde966df34a49d5d842a263604159141c81db8d468e1bf 825 657230429424fc337a", 826 qop=auth, 827 response="ec180fc03b7a0dcd43c414f66f2335399bbe5f4d4ad469 828 f8233106ba453213c8", 829 opaque="192cbcf2a2576846522c1a367c1dfdf0359a87719c5cc1 830 839e4f3d2ffeb82517", 831 charset=UTF-8, 832 userhash=false 834 4 Internationalization 836 In challenges, servers SHOULD use the "charset" authentication 837 parameter (case-insensitive) to express the character encoding they 838 expect the user agent to use. 840 The only allowed value is "UTF-8", to be matched case-insensitively, 841 indicating that the server expects the UTF-8 character encoding to be 842 used ([RFC3629]). 844 If the user agent supports the encoding indicated by the server, it 845 MAY add the "charset" parameter, with the value it received from the 846 server, to the Proxy-Authenticate or WWW-Authenticate header fields 847 it sends back to the server. 849 If the user agent does not support the encoding indicated by the 850 server, it MUST fail the request. 852 5 Security Considerations 854 5.1 Limitations 856 HTTP Digest authentication, when used with human-memorable passwords, 857 is vulnerable to dictionary attacks. Such attacks are much easier 858 than cryptographic attacks on any widely used algorithm, including 859 those that are no longer considered secure. In other words, algorithm 860 agility does not make this usage any more secure. 862 As a result, Digest authentication SHOULD be used only with passwords 863 that have a reasonable amount of entropy, e.g. 128-bit or more. Such 864 passwords typically cannot be memorized by humans but can be used for 865 automated web services. 867 Digest authentication SHOULD be used over a secure channel like HTTPS 868 [RFC2818]. 870 5.2 Authentication of Clients using Digest Authentication 872 Digest Authentication does not provide a strong authentication 873 mechanism, when compared to public key based mechanisms, for example. 875 However, it is significantly stronger than (e.g.) CRAM-MD5, which has 876 been proposed for use with LDAP [RFC4513], POP and IMAP (see 877 [RFC2195]). It is intended to replace the much weaker and even more 878 dangerous Basic mechanism. 880 Digest Authentication offers no confidentiality protection beyond 881 protecting the actual username and password. All of the rest of the 882 request and response are available to an eavesdropper. 884 Digest Authentication offers only limited integrity protection for 885 the messages in either direction. If qop=auth-int mechanism is used, 886 those parts of the message used in the calculation of the WWW- 887 Authenticate and Authorization header field response directive values 888 (see section 3.2 above) are protected. Most header fields and their 889 values could be modified as a part of a man-in-the-middle attack. 891 Many needs for secure HTTP transactions cannot be met by Digest 892 Authentication. For those needs TLS or SHTTP are more appropriate 893 protocols. In particular Digest authentication cannot be used for any 894 transaction requiring confidentiality protection. Nevertheless many 895 functions remain for which Digest authentication is both useful and 896 appropriate. 898 5.3 Limited Use Nonce Values 900 The Digest scheme uses a server-specified nonce to seed the 901 generation of the request-digest value (as specified in section 902 3.2.2.1 above). As shown in the example nonce in section 3.2.1, the 903 server is free to construct the nonce such that it may only be used 904 from a particular client, for a particular resource, for a limited 905 period of time or number of uses, or any other restrictions. Doing 906 so strengthens the protection provided against, for example, replay 907 attacks (see 4.5). However, it should be noted that the method 908 chosen for generating and checking the nonce also has performance and 909 resource implications. For example, a server may choose to allow 910 each nonce value to be used only once by maintaining a record of 911 whether or not each recently issued nonce has been returned and 912 sending a next-nonce directive in the Authentication-Info header 913 field of every response. This protects against even an immediate 914 replay attack, but has a high cost checking nonce values, and perhaps 915 more important will cause authentication failures for any pipelined 916 requests (presumably returning a stale nonce indication). Similarly, 917 incorporating a request-specific element such as the Etag value for a 918 resource limits the use of the nonce to that version of the resource 919 and also defeats pipelining. Thus it may be useful to do so for 920 methods with side effects but have unacceptable performance for those 921 that do not. 923 5.4 Replay Attacks 925 A replay attack against Digest authentication would usually be 926 pointless for a simple GET request since an eavesdropper would 927 already have seen the only document he could obtain with a replay. 928 This is because the URI of the requested document is digested in the 929 client request and the server will only deliver that document. By 930 contrast under Basic Authentication once the eavesdropper has the 931 user's password, any document protected by that password is open to 932 him. 934 Thus, for some purposes, it is necessary to protect against replay 935 attacks. A good Digest implementation can do this in various ways. 936 The server created "nonce" value is implementation dependent, but if 937 it contains a digest of the client IP, a time-stamp, the resource 938 ETag, and a private server key (as recommended above) then a replay 939 attack is not simple. An attacker must convince the server that the 940 request is coming from a false IP address and must cause the server 941 to deliver the document to an IP address different from the address 942 to which it believes it is sending the document. An attack can only 943 succeed in the period before the time-stamp expires. Digesting the 944 client IP and time-stamp in the nonce permits an implementation which 945 does not maintain state between transactions. 947 For applications where no possibility of replay attack can be 948 tolerated the server can use one-time nonce values which will not be 949 honored for a second use. This requires the overhead of the server 951 remembering which nonce values have been used until the nonce time- 952 stamp (and hence the digest built with it) has expired, but it 953 effectively protects against replay attacks. 955 An implementation must give special attention to the possibility of 956 replay attacks with POST and PUT requests. Unless the server employs 957 one-time or otherwise limited-use nonces and/or insists on the use of 958 the integrity protection of qop=auth-int, an attacker could replay 959 valid credentials from a successful request with counterfeit form 960 data or other message body. Even with the use of integrity protection 961 most metadata in header fields is not protected. Proper nonce 962 generation and checking provides some protection against replay of 963 previously used valid credentials, but see 4.8. 965 5.5 Weakness Created by Multiple Authentication Schemes 967 An HTTP/1.1 server may return multiple challenges with a 401 968 (Authenticate) response, and each challenge may use a different auth- 969 scheme. A user agent MUST choose to use the strongest auth- scheme it 970 understands and request credentials from the user based upon that 971 challenge. 973 Note that many browsers will only recognize Basic and will require 974 that it be the first auth-scheme presented. Servers should only 975 include Basic if it is minimally acceptable. 977 When the server offers choices of authentication schemes using the 978 WWW-Authenticate header, the strength of the resulting authentication 979 is only as good as that of the of the weakest of the authentication 980 schemes. See section 5.7 below for discussion of particular attack 981 scenarios that exploit multiple authentication schemes. 983 5.6 Online dictionary attacks 985 If the attacker can eavesdrop, then it can test any overheard 986 nonce/response pairs against a list of common words. Such a list is 987 usually much smaller than the total number of possible passwords. The 988 cost of computing the response for each password on the list is paid 989 once for each challenge. 991 The server can mitigate this attack by not allowing users to select 992 passwords that are in a dictionary. 994 5.7 Man in the Middle 996 Both Basic and Digest authentication are vulnerable to "man in the 997 middle" (MITM) attacks, for example, from a hostile or compromised 998 proxy. Clearly, this would present all the problems of eavesdropping. 999 But it also offers some additional opportunities to the attacker. 1001 A possible man-in-the-middle attack would be to add a weak 1002 authentication scheme to the set of choices, hoping that the client 1003 will use one that exposes the user's credentials (e.g. password). For 1004 this reason, the client should always use the strongest scheme that 1005 it understands from the choices offered. 1007 An even better MITM attack would be to remove all offered choices, 1008 replacing them with a challenge that requests only Basic 1009 authentication, then uses the cleartext credentials from the Basic 1010 authentication to authenticate to the origin server using the 1011 stronger scheme it requested. A particularly insidious way to mount 1012 such a MITM attack would be to offer a "free" proxy caching service 1013 to gullible users. 1015 User agents should consider measures such as presenting a visual 1016 indication at the time of the credentials request of what 1017 authentication scheme is to be used, or remembering the strongest 1018 authentication scheme ever requested by a server and produce a 1019 warning message before using a weaker one. It might also be a good 1020 idea for the user agent to be configured to demand Digest 1021 authentication in general, or from specific sites. 1023 Or, a hostile proxy might spoof the client into making a request the 1024 attacker wanted rather than one the client wanted. Of course, this is 1025 still much harder than a comparable attack against Basic 1026 Authentication. 1028 5.8 Chosen plaintext attacks 1030 With Digest authentication, a MITM or a malicious server can 1031 arbitrarily choose the nonce that the client will use to compute the 1032 response. This is called a "chosen plaintext" attack. The ability to 1033 choose the nonce is known to make cryptanalysis much easier. 1035 However, no way to analyze the MD5 one-way function used by Digest 1036 using chosen plaintext is currently known. 1038 The countermeasure against this attack is for clients to be 1039 configured to require the use of the optional "cnonce" directive; 1040 this allows the client to vary the input to the hash in a way not 1041 chosen by the attacker. 1043 5.9 Precomputed dictionary attacks 1045 With Digest authentication, if the attacker can execute a chosen 1046 plaintext attack, the attacker can precompute the response for many 1047 common words to a nonce of its choice, and store a dictionary of 1048 (response, password) pairs. Such precomputation can often be done in 1049 parallel on many machines. It can then use the chosen plaintext 1050 attack to acquire a response corresponding to that challenge, and 1051 just look up the password in the dictionary. Even if most passwords 1052 are not in the dictionary, some might be. Since the attacker gets to 1053 pick the challenge, the cost of computing the response for each 1054 password on the list can be amortized over finding many passwords. A 1055 dictionary with 100 million password/response pairs would take about 1056 3.2 gigabytes of disk storage. 1058 The countermeasure against this attack is to for clients to be 1059 configured to require the use of the optional "cnonce" directive. 1061 5.10 Batch brute force attacks 1063 With Digest authentication, a MITM can execute a chosen plaintext 1064 attack, and can gather responses from many users to the same nonce. 1065 It can then find all the passwords within any subset of password 1066 space that would generate one of the nonce/response pairs in a single 1067 pass over that space. It also reduces the time to find the first 1068 password by a factor equal to the number of nonce/response pairs 1069 gathered. This search of the password space can often be done in 1070 parallel on many machines, and even a single machine can search large 1071 subsets of the password space very quickly -- reports exist of 1072 searching all passwords with six or fewer letters in a few hours. 1074 The countermeasure against this attack is to for clients to be 1075 configured to require the use of the optional "cnonce" directive. 1077 5.11 Spoofing by Counterfeit Servers 1079 Basic Authentication is vulnerable to spoofing by counterfeit 1080 servers. If a user can be led to believe that she is connecting to a 1081 host containing information protected by a password she knows, when 1082 in fact she is connecting to a hostile server, then the hostile 1083 server can request a password, store it away for later use, and feign 1084 an error. This type of attack is more difficult with Digest 1085 Authentication -- but the client must know to demand that Digest 1086 authentication be used, perhaps using some of the techniques 1087 described above to counter "man-in-the-middle" attacks. Again, the 1088 user can be helped in detecting this attack by a visual indication of 1089 the authentication mechanism in use with appropriate guidance in 1090 interpreting the implications of each scheme. 1092 5.12 Storing passwords 1094 Digest authentication requires that the authenticating agent (usually 1095 the server) store some data derived from the user's name and password 1096 in a "password file" associated with a given realm. Normally this 1097 might contain pairs consisting of username and H(A1), where H(A1) is 1098 the digested value of the username, realm, and password as described 1099 above. 1101 The security implications of this are that if this password file is 1102 compromised, then an attacker gains immediate access to documents on 1103 the server using this realm. Unlike, say a standard UNIX password 1104 file, this information need not be decrypted in order to access 1105 documents in the server realm associated with this file. On the other 1106 hand, decryption, or more likely a brute force attack, would be 1107 necessary to obtain the user's password. This is the reason that the 1108 realm is part of the digested data stored in the password file. It 1109 means that if one Digest authentication password file is compromised, 1110 it does not automatically compromise others with the same username 1111 and password (though it does expose them to brute force attack). 1113 There are two important security consequences of this. First the 1114 password file must be protected as if it contained unencrypted 1115 passwords, because for the purpose of accessing documents in its 1116 realm, it effectively does. 1118 A second consequence of this is that the realm string should be 1119 unique among all realms which any single user is likely to use. In 1120 particular a realm string should include the name of the host doing 1121 the authentication. The inability of the client to authenticate the 1122 server is a weakness of Digest Authentication. 1124 5.13 Summary 1126 By modern cryptographic standards Digest Authentication is weak. But 1127 for a large range of purposes it is valuable as a replacement for 1128 Basic Authentication. It remedies some, but not all, weaknesses of 1129 Basic Authentication. Its strength may vary depending on the 1130 implementation. In particular the structure of the nonce (which is 1131 dependent on the server implementation) may affect the ease of 1132 mounting a replay attack. A range of server options is appropriate 1133 since, for example, some implementations may be willing to accept the 1134 server overhead of one-time nonces or digests to eliminate the 1135 possibility of replay. Others may satisfied with a nonce like the one 1136 recommended above restricted to a single IP address and a single ETag 1137 or with a limited lifetime. 1139 The bottom line is that *any* compliant implementation will be 1140 relatively weak by cryptographic standards, but *any* compliant 1141 implementation will be far superior to Basic Authentication. 1143 6 IANA Considerations 1145 6.1 HTTP Digest Hash Algorithms Registry 1147 This specification creates a new IANA registry named "HTTP Digest 1148 Hash Algorithms". When registering a new hash algorithm, the 1149 following information MUST be provided: 1151 o Hash Algorithm 1152 The textual name of the hash algorithm. 1154 o Digest Size 1155 The size of the algorithm's output in hexadecimal characters. 1157 o Preference 1158 The preference of the algorithm, with zero being the least 1159 preferred. 1161 o Reference 1162 A reference to the specification that describes the new algorithm. 1164 The update policy for this registry shall be Specification Required. 1166 The initial registry will contain the following entries: 1168 Hash Algorithm Digest Size Preference Reference 1169 -------------- ----------- ---------- --------- 1170 "MD5" 32 0 RFC XXXX 1171 "SHA2-512-256" 64 1 RFC XXXX 1172 "SHA2-256" 64 2 RFC XXXX 1174 Each one of the algorithms defined in the registry might have a -sess 1175 variant, e.g. MD5-sess, SHA2-256-sess, etc. 1177 6.2 Digest 1179 This specification registers the Digest scheme with the 1180 Authentication Scheme Registry. 1182 Authentication Scheme Name: Digest 1184 Pointer to specification text: RFCXXX 1186 7 Acknowledgments 1188 TODO 1190 8 References 1192 8.1 Normative References 1194 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1195 Requirement Levels", BCP 14, RFC 2119, March 1997. 1197 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 1198 10646", STD 63, RFC 3629, November 2003. 1200 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 1201 Resource Identifier (URI): Generic Syntax", STD 66, 1202 RFC 3986, January 2005. 1204 [RFC4513] Harrison, R., Ed., "Lightweight Directory Access Protocol 1205 (LDAP): Authentication Methods and Security Mechanisms", 1206 RFC 4513, June 2006. 1208 [HTTP-P1] Fielding, R., Reschke, J., "Hypertext Transfer Protocol 1209 (HTTP/1.1): Message Syntax and Routing", Work in Progress, 1210 November 2013. 1212 [HTTP-P6] Fielding, R., Nottingham, M., Reschke, J., "Hypertext 1213 Transfer Protocol (HTTP/1.1): Caching", Work in Progress, 1214 November 2013. 1216 [HTTP-P7] Fielding, R., Reschke, J., "Hypertext Transfer Protocol 1217 (HTTP/1.1): Authentication", Work in Progress, November 1218 2013. 1220 8.2 Informative References 1222 [RFC2069] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P., 1223 Luotonen, A., Sink, E., and L. Stewart, "An Extension to 1224 HTTP : Digest Access Authentication", RFC 2069, January 1225 1997. 1227 [RFC2195] Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP 1228 AUTHorize Extension for Simple Challenge/Response", 1229 RFC 2195, September 1997. 1231 [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. 1233 [SHA3] "SHA-3 STANDARDIZATION" 1234 http://csrc.nist.gov/groups/ST/hash/sha-3/sha-3_standardization.html 1236 Authors' Addresses 1238 Rifaat Shekh-Yusef (Editor) 1239 Avaya 1240 250 Sydney Street 1241 Belleville, Ontario 1242 Canada 1244 Phone: +1-613-967-5267 1245 Email: rifaat.ietf@gmail.com 1247 David Ahrens 1248 Avaya 1249 California 1250 USA 1252 EMail: ahrensdc@gmail.com 1254 Sophie Bremer 1255 Netzkonform 1256 Germany 1258 Email: sophie.bremer@netzkonform.de