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'HTTP-P1' -- Possible downref: Non-RFC (?) normative reference: ref. 'HTTP-P6' -- Possible downref: Non-RFC (?) normative reference: ref. 'HTTP-P7' Summary: 5 errors (**), 0 flaws (~~), 9 warnings (==), 5 comments (--). 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-03 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 . . . . . . . . . . . . . . . 16 75 3.8 Proxy-Authenticate and Proxy-Authorization . . . . . . . . . 16 76 3.9 Example . . . . . . . . . . . . . . . . . . . . . . . . . . 17 77 4 Internationalization . . . . . . . . . . . . . . . . . . . . . . 19 78 5 Security Considerations . . . . . . . . . . . . . . . . . . . . 19 79 5.1 Limitations . . . . . . . . . . . . . . . . . . . . . . . . 19 80 5.2 Authentication of Clients using Digest Authentication . . . 19 81 5.3 Limited Use Nonce Values . . . . . . . . . . . . . . . . . . 20 82 5.4 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . . 21 83 5.5 Weakness Created by Multiple Authentication Schemes . . . . 21 84 5.6 Online dictionary attacks . . . . . . . . . . . . . . . . . 22 85 5.7 Man in the Middle . . . . . . . . . . . . . . . . . . . . . 22 86 5.8 Chosen plaintext attacks . . . . . . . . . . . . . . . . . . 23 87 5.9 Precomputed dictionary attacks . . . . . . . . . . . . . . . 23 88 5.10 Batch brute force attacks . . . . . . . . . . . . . . . . . 24 89 5.11 Spoofing by Counterfeit Servers . . . . . . . . . . . . . . 24 90 5.12 Storing passwords . . . . . . . . . . . . . . . . . . . . . 24 91 5.13 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 25 93 6 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 26 94 6.1 HTTP Digest Hash Algorithms Registry . . . . . . . . . . . 26 95 6.2 Digest . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 96 7 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 27 97 8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 98 8.1 Normative References . . . . . . . . . . . . . . . . . . . . 27 99 8.2 Informative References . . . . . . . . . . . . . . . . . . . 28 100 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28 102 1 Introduction 104 HTTP provides a simple challenge-response authentication mechanism 105 that may be used by a server to challenge a client request and by a 106 client to provide authentication information. This document defines 107 the HTTP Digest Authentication scheme that may be used with the 108 authentication mechanism. 110 The details of the challenge-response authentication mechanism are 111 specified in the [HTTP-P7] document. 113 1.1 Terminology 115 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 116 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 117 document are to be interpreted as described in RFC 2119 [RFC2119]. 119 2 Syntax Convention 121 In the interest of clarity and readability, the extended parameters 122 or the headers and parameters in the examples in this document might 123 be broken into multiple lines. Any line that is indented in this 124 document is a continuation of the preceding line. 126 3 Digest Access Authentication Scheme 128 3.1 Overall Operation 130 The Digest scheme is based on a simple challenge-response paradigm. 131 The Digest scheme challenges using a nonce value. A valid response 132 contains a checksum of the username, the password, the given nonce 133 value, the HTTP method, and the requested URI. In this way, the 134 password is never sent in the clear. The username and password must 135 be prearranged in some fashion not addressed by this document. 137 3.2 Representation of Digest Values 139 An optional header allows the server to specify the algorithm used to 140 create the checksum or digest. This documents adds SHA2-256 and SHA2- 141 512/256 algorithms. To maintain backwards compatibility, the MD5 142 algorithm is still supported but not recommended. 144 The size of the digest depends on the algorithm used. The bits in 145 the digest are converted from the most significant to the least 146 significant bit, four bits at a time to the ASCII representation as 147 follows. Each four bits is represented by its familiar hexadecimal 148 notation from the characters 0123456789abcdef, that is binary 0000 is 149 represented by the character '0', 0001 by '1' and so on up to the 150 representation of 1111 as 'f'. If the MD5 algorithm is used to 151 calculate the digest, then the digest will be represented as 32 152 hexadecimal characters, SHA2-256 and SHA2-512/256 by 64 hexadecimal 153 characters. 155 3.3 The WWW-Authenticate Response Header 157 If a server receives a request for an access-protected object, and an 158 acceptable Authorization header is not sent, the server responds with 159 a "401 Unauthorized" status code, and a WWW-Authenticate header as 160 per the framework defined above, and include some or all of the 161 following parameters: 163 realm 164 A string to be displayed to users so they know which username and 165 password to use. This string should contain at least the name of 166 the host performing the authentication and might additionally 167 indicate the collection of users who might have access. An example 168 might be "registered_users@gotham.news.com". 170 domain 171 A quoted, space-separated list of URIs, as specified in RFC XURI 172 [RFC2396], that define the protection space. If a URI is an 173 abs_path, it is relative to the canonical root URL of the server 174 being accessed. An absolute-URI in this list may refer to a 175 different server than the one being accessed. The client can use 176 this list to determine the set of URIs for which the same 177 authentication information may be sent: any URI that has a URI in 178 this list as a prefix (after both have been made absolute) may be 179 assumed to be in the same protection space. If this directive is 180 omitted or its value is empty, the client should assume that the 181 protection space consists of all URIs on the responding server. 183 This directive is not meaningful in Proxy-Authenticate headers, 184 for which the protection space is always the entire proxy; if 185 present it should be ignored. 187 nonce 188 A server-specified data string which should be uniquely generated 189 each time a 401 response is made. It is recommended that this 190 string be base64 or hexadecimal data. Specifically, since the 191 string is passed in the header lines as a quoted string, the 192 double-quote character is not allowed. 194 The contents of the nonce are implementation dependent. The 195 quality of the implementation depends on a good choice. A nonce 196 might, for example, be constructed as the base 64 encoding of 198 time-stamp H(time-stamp ":" ETag ":" private-key) 200 where time-stamp is a server-generated time or other non-repeating 201 value, ETag is the value of the HTTP ETag header associated with 202 the requested entity, and private-key is data known only to the 203 server. With a nonce of this form a server would recalculate the 204 hash portion after receiving the client authentication header and 205 reject the request if it did not match the nonce from that header 206 or if the time-stamp value is not recent enough. In this way the 207 server can limit the time of the nonce's validity. The inclusion 208 of the ETag prevents a replay request for an updated version of 209 the resource. (Note: including the IP address of the client in the 210 nonce would appear to offer the server the ability to limit the 211 reuse of the nonce to the same client that originally got it. 212 However, that would break proxy farms, where requests from a 213 single user often go through different proxies in the farm. Also, 214 IP address spoofing is not that hard.) 216 An implementation might choose not to accept a previously used 217 nonce or a previously used digest, in order to protect against a 218 replay attack. Or, an implementation might choose to use one-time 219 nonces or digests for POST or PUT requests and a time-stamp for 220 GET requests. For more details on the issues involved see section 221 5 of this document. 223 The nonce is opaque to the client. 225 opaque 226 A string of data, specified by the server, which should be 227 returned by the client unchanged in the Authorization header of 228 subsequent requests with URIs in the same protection space. It is 229 recommended that this string be base64 or hexadecimal data. 231 stale 232 A case-insensitive flag, indicating that the previous request from 233 the client was rejected because the nonce value was stale. If 234 stale is TRUE, the client may wish to simply retry the request 235 with a new encrypted response, without reprompting the user for a 236 new username and password. The server should only set stale to 237 TRUE if it receives a request for which the nonce is invalid but 238 with a valid digest for that nonce (indicating that the client 239 knows the correct username/password). If stale is FALSE, or 240 anything other than TRUE, or the stale directive is not present, 241 the username and/or password are invalid, and new values must be 242 obtained. 244 algorithm 245 A string indicating a pair of algorithms used to produce the 246 digest and a checksum. If this is not present it is assumed to be 247 "MD5". If the algorithm is not understood, the challenge should be 248 ignored (and a different one used, if there is more than one). 250 In this document the string obtained by applying the digest 251 algorithm to the data "data" with secret "secret" will be denoted 252 by KD(secret, data), and the string obtained by applying the 253 checksum algorithm to the data "data" will be denoted H(data). The 254 notation unq(X) means the value of the quoted-string X without the 255 surrounding quotes. 257 For the "MD5" and "MD5-sess" algorithms 259 H(data) = MD5(data) 261 For the "SHA2-256" and "SHA2-256-sess" algorithms 263 H(data) = SHA2-256(data) 265 For the "SHA2-512-256" and "SHA2-512-256-sess" algorithms 267 H(data) = SHA2-512-256(data) 269 and 271 KD(secret, data) = H(concat(secret, ":", data)) 273 i.e., the digest is the MD5 of the secret concatenated with a 274 colon concatenated with the data. The "MD5-sess" algorithm is 275 intended to allow efficient 3rd party authentication servers; 276 for the difference in usage, see the description in section 277 3.4.2. 279 qop-options 280 This directive is optional, but is made so only for backward 281 compatibility with RFC 2069 [RFC2069]; it SHOULD be used by all 282 implementations compliant with this version of the Digest scheme. 283 If present, it is a quoted string of one or more tokens indicating 284 the "quality of protection" values supported by the server. The 285 value "auth" indicates authentication; the value "auth-int" 286 indicates authentication with integrity protection; see the 287 descriptions below for calculating the response directive value 288 for the application of this choice. Unrecognized options MUST be 289 ignored. 291 charset 292 This is an OPTIONAL parameter that is used by the server to 293 indicate the encoding scheme it supports. 295 userhash 296 This is an OPTIONAL parameter that is used by the server to 297 indicate that it supports username hashing. 299 3.4 The Authorization Request Header 301 The client is expected to retry the request, passing an 302 Authorization header line, which is defined according to the 303 framework above. The values of the opaque and algorithm fields 304 must be those supplied in the WWW-Authenticate response header for 305 the entity being requested. 307 The request includes some or all of the following parameters: 309 response 310 A string of the hex digits computed as defined below, which proves 311 that the user knows a password 313 username 314 The user's name in the specified realm. 316 digest-uri 317 The URI from request-target of the Request-Line; duplicated here 318 because proxies are allowed to change the Request-Line in transit. 320 qop 321 Indicates what "quality of protection" the client has applied to 322 the message. If present, its value MUST be one of the alternatives 323 the server indicated it supports in the WWW-Authenticate header. 324 These values affect the computation of the request-digest. Note 325 that this is a single token, not a quoted list of alternatives as 326 in WWW-Authenticate. This directive is optional in order to 327 preserve backward compatibility with a minimal implementation of 328 RFC 2069 [RFC2069], but SHOULD be used if the server indicated 329 that qop is supported by providing a qop directive in the WWW- 330 Authenticate header field. 332 cnonce 333 This MUST be specified if a qop directive is sent (see above), and 334 MUST NOT be specified if the server did not send a qop directive 335 in the WWW-Authenticate header field. The cnonce-value is an 336 opaque quoted string value provided by the client and used by both 337 client and server to avoid chosen plaintext attacks, to provide 338 mutual authentication, and to provide some message integrity 339 protection. See the descriptions below of the calculation of the 340 response- digest and request-digest values. 342 nonce-count 343 This MUST be specified if a qop directive is sent (see above), and 344 MUST NOT be specified if the server did not send a qop directive 345 in the WWW-Authenticate header field. The nc-value is the 346 hexadecimal count of the number of requests (including the current 347 request) that the client has sent with the nonce value in this 348 request. For example, in the first request sent in response to a 349 given nonce value, the client sends "nc=00000001". The purpose of 350 this directive is to allow the server to detect request replays by 351 maintaining its own copy of this count - if the same nc-value is 352 seen twice, then the request is a replay. See the description 353 below of the construction of the request-digest value. 355 charset 356 This OPTIONAL parameter is used by the client to indicate the 357 character encoding used for the username and password. 359 userhash 360 This OPTIONAL parameter is used by the client to indicate that the 361 username has been hashed. 363 If a directive or its value is improper, or required directives are 364 missing, the proper response is 400 Bad Request. If the request- 365 digest is invalid, then a login failure should be logged, since 366 repeated login failures from a single client may indicate an attacker 367 attempting to guess passwords. 369 The definition of request-digest above indicates the encoding for its 370 value. The following definitions show how the value is computed. 372 3.4.1 Request-Digest 374 If the "qop" value is "auth" or "auth-int": 376 request-digest = <"> < KD ( H(A1), unq(nonce-value) 377 ":" nc-value 378 ":" unq(cnonce-value) 379 ":" unq(qop-value) 380 ":" H(A2) 381 ) <"> 383 If the "qop" directive is not present (this construction is for 384 compatibility with RFC 2069): 386 request-digest = 387 <"> < KD ( H(A1), unq(nonce-value) ":" H(A2) ) > <"> 389 See below for the definitions for A1 and A2. 391 3.4.2 A1 393 If the "algorithm" directive's value is "MD5", "SHA2-256", or "SHA2- 394 512-256", then A1 is: 396 A1 = unq(username-value) ":" unq(realm-value) ":" passwd 398 where 400 passwd = < user's password > 402 If the "algorithm" directive's value is "MD5-sess", "SHA2-256-sess", 403 or "SHA2-512-256-sess", then A1 is calculated only once - on the 404 first request by the client following receipt of a WWW-Authenticate 405 challenge from the server. It uses the server nonce from that 406 challenge, and the first client nonce value to construct A1 as 407 follows: 409 A1 = H( unq(username-value) ":" unq(realm-value) 410 ":" passwd ) 411 ":" unq(nonce-value) ":" unq(cnonce-value) 413 This creates a 'session key' for the authentication of subsequent 414 requests and responses which is different for each "authentication 415 session", thus limiting the amount of material hashed with any one 416 key. (Note: see further discussion of the authentication session in 417 section 3.6.) Because the server need only use the hash of the user 418 credentials in order to create the A1 value, this construction could 419 be used in conjunction with a third party authentication service so 420 that the web server would not need the actual password value. The 421 specification of such a protocol is beyond the scope of this 422 specification. 424 3.4.3 A2 426 If the "qop" directive's value is "auth" or is unspecified, then A2 427 is: 429 A2 = Method ":" digest-uri-value 431 If the "qop" value is "auth-int", then A2 is: 433 A2 = Method ":" digest-uri-value ":" H(entity-body) 435 3.4.4 Username Hashing 437 To protect the transport of the username from the client to the 438 server, the server SHOULD set the "userhash" parameter with the value 439 of "true" in the WWW-Authentication header. 441 If the client supports the "userhash" parameter, and the "userhash" 442 parameter value in the WWW-Authentication header is set to "true", 443 then the client MUST calculate a hash of the username after any other 444 hash calculation and include the "userhash" parameter with the value 445 of "true" in the Authorization Request Header. If the client does not 446 provide the "username" as a hash value or the "userhash" parameter 447 with the value of "true", the server MAY reject the request. 449 The server may change the nonce value, so the client should be ready 450 to recalculate the hashed username. 452 The following is the operation that the client will take to hash the 453 username: 455 username = H( H( username ":" realm ) ":" nonce) 457 3.4.5 Directive Values and Quoted-String 459 Note that the value of many of the directives, such as "username- 460 value", are defined as a "quoted-string". However, the "unq" notation 461 indicates that surrounding quotation marks are removed in forming the 462 string A1. Thus if the Authorization header includes the fields 464 username="Mufasa", realm=myhost@testrealm.com 466 and the user Mufasa has password "Circle Of Life" then H(A1) would be 467 H(Mufasa:myhost@testrealm.com:Circle Of Life) with no quotation marks 468 in the digested string. 470 No white space is allowed in any of the strings to which the digest 471 function H() is applied unless that white space exists in the quoted 472 strings or entity body whose contents make up the string to be 473 digested. For example, the string A1 illustrated above must be 475 Mufasa:myhost@testrealm.com:Circle Of Life 477 with no white space on either side of the colons, but with the white 478 space between the words used in the password value. Likewise, the 479 other strings digested by H() must not have white space on either 480 side of the colons which delimit their fields unless that white space 481 was in the quoted strings or entity body being digested. 483 Also note that if integrity protection is applied (qop=auth-int), the 484 H(entity-body) is the hash of the entity body, not the message body - 485 it is computed before any transfer encoding is applied by the sender 486 and after it has been removed by the recipient. Note that this 487 includes multipart boundaries and embedded headers in each part of 488 any multipart content-type. 490 3.4.6 Various Considerations 492 The "Method" value is the HTTP request method as specified in section 493 3.1.1 of [HTTP-P1]. The "request-target" value is the request-target 494 from the request line as specified in section 3.1.1 of [HTTP-P1]. 495 This may be "*", an "absolute-URI" or an "absolute-path" as specified 496 in section 2.7 of [HTTP-P1], but it MUST agree with the request- 497 target. In particular, it MUST be an "absolute-URI" if the request- 498 target is an "absolute-URI". The "cnonce-value" is an optional 499 client-chosen value whose purpose is to foil chosen plaintext 500 attacks. 502 The authenticating server must assure that the resource designated by 503 the "uri" directive is the same as the resource specified in the 504 Request-Line; if they are not, the server SHOULD return a 400 Bad 505 Request error. (Since this may be a symptom of an attack, server 506 implementers may want to consider logging such errors.) The purpose 507 of duplicating information from the request URL in this field is to 508 deal with the possibility that an intermediate proxy may alter the 509 client's Request-Line. This altered (but presumably semantically 510 equivalent) request would not result in the same digest as that 511 calculated by the client. 513 Implementers should be aware of how authenticated transactions 514 interact with shared caches. The HTTP/1.1 protocol specifies that 515 when a shared cache (see [HTTP-P6]) has received a request containing 516 an Authorization header and a response from relaying that request, it 517 MUST NOT return that response as a reply to any other request, unless 518 one of two Cache-Control (see section 3.2 of [HTTP-P6]) directives 519 was present in the response. If the original response included the 520 "must-revalidate" Cache-Control directive, the cache MAY use the 521 entity of that response in replying to a subsequent request, but MUST 522 first revalidate it with the origin server, using the request headers 523 from the new request to allow the origin server to authenticate the 524 new request. Alternatively, if the original response included the 525 "public" Cache-Control directive, the response entity MAY be returned 526 in reply to any subsequent request. 528 3.5 The Authentication-Info Header 530 The Authentication-Info header is used by the server to communicate 531 some information regarding the successful authentication in the 532 response. 534 AuthenticationInfo = "Authentication-Info" ":" auth-info 535 auth-info = 1#(nextnonce | [ message-qop ] 536 | [ response-auth ] | [ cnonce ] 537 | [nonce-count] ) 538 nextnonce = "nextnonce" "=" nonce-value 539 response-auth = "rspauth" "=" response-digest 540 response-digest = <"> *LHEX <"> 542 The value of the nextnonce directive is the nonce the server wishes 543 the client to use for a future authentication response. The server 544 may send the Authentication-Info header with a nextnonce field as a 545 means of implementing one-time or otherwise changing nonces. If the 546 nextnonce field is present the client SHOULD use it when constructing 547 the Authorization header for its next request. Failure of the client 548 to do so may result in a request to re-authenticate from the server 549 with the "stale=TRUE". 551 Server implementations should carefully consider the performance 552 implications of the use of this mechanism; pipelined requests will 553 not be possible if every response includes a nextnonce directive 554 that must be used on the next request received by the server. 555 Consideration should be given to the performance vs. security 556 tradeoffs of allowing an old nonce value to be used for a limited 557 time to permit request pipelining. Use of the nonce-count can 558 retain most of the security advantages of a new server nonce 559 without the deleterious affects on pipelining. 561 message-qop 562 Indicates the "quality of protection" options applied to the 563 response by the server. The value "auth" indicates 564 authentication; the value "auth-int" indicates authentication with 565 integrity protection. The server SHOULD use the same value for the 566 message- qop directive in the response as was sent by the client 567 in the corresponding request. 569 The optional response digest in the "response-auth" directive 570 supports mutual authentication -- the server proves that it knows the 571 user's secret, and with qop=auth-int also provides limited integrity 572 protection of the response. The "response-digest" value is calculated 573 as for the "request-digest" in the Authorization header, except that 574 if "qop=auth" or is not specified in the Authorization header for the 575 request, A2 is 577 A2 = ":" digest-uri-value 579 and if "qop=auth-int", then A2 is 581 A2 = ":" digest-uri-value ":" H(entity-body) 583 where "digest-uri-value" is the value of the "uri" directive on the 584 Authorization header in the request. The "cnonce-value" and "nc- 585 value" MUST be the ones for the client request to which this message 586 is the response. The "response-auth", "cnonce", and "nonce-count" 587 directives MUST BE present if "qop=auth" or "qop=auth-int" is 588 specified. 590 The Authentication-Info header is allowed in the trailer of an HTTP 591 message transferred via chunked transfer-coding. 593 3.6 Digest Operation 595 Upon receiving the Authorization header, the server may check its 596 validity by looking up the password that corresponds to the submitted 597 username. Then, the server must perform the same digest operation 598 (e.g., MD5) performed by the client, and compare the result to the 599 given request-digest value. 601 Note that the HTTP server does not actually need to know the user's 602 cleartext password. As long as H(A1) is available to the server, the 603 validity of an Authorization header may be verified. 605 The client response to a WWW-Authenticate challenge for a protection 606 space starts an authentication session with that protection space. 607 The authentication session lasts until the client receives another 608 WWW-Authenticate challenge from any server in the protection space. A 609 client should remember the username, password, nonce, nonce count and 610 opaque values associated with an authentication session to use to 611 construct the Authorization header in future requests within that 612 protection space. The Authorization header may be included 613 preemptively; doing so improves server efficiency and avoids extra 614 round trips for authentication challenges. The server may choose to 615 accept the old Authorization header information, even though the 616 nonce value included might not be fresh. Alternatively, the server 617 may return a 401 response with a new nonce value, causing the client 618 to retry the request; by specifying stale=TRUE with this response, 619 the server tells the client to retry with the new nonce, but without 620 prompting for a new username and password. 622 Because the client is required to return the value of the opaque 623 directive given to it by the server for the duration of a session, 624 the opaque data may be used to transport authentication session state 625 information. (Note that any such use can also be accomplished more 626 easily and safely by including the state in the nonce.) For example, 627 a server could be responsible for authenticating content that 628 actually sits on another server. It would achieve this by having the 629 first 401 response include a domain directive whose value includes a 630 URI on the second server, and an opaque directive whose value 631 contains the state information. The client will retry the request, at 632 which time the server might respond with a 301/302 redirection, 633 pointing to the URI on the second server. The client will follow the 634 redirection, and pass an Authorization header , including the 635 data. 637 As with the basic scheme, proxies must be completely transparent in 638 the Digest access authentication scheme. That is, they must forward 639 the WWW-Authenticate, Authentication-Info and Authorization headers 640 untouched. If a proxy wants to authenticate a client before a request 641 is forwarded to the server, it can be done using the Proxy- 642 Authenticate and Proxy-Authorization headers described in section 3.6 643 below. 645 3.7 Security Protocol Negotiation 647 It is useful for a server to be able to know which security schemes a 648 client is capable of handling. 650 It is possible that a server may want to require Digest as its 651 authentication method, even if the server does not know that the 652 client supports it. A client is encouraged to fail gracefully if the 653 server specifies only authentication schemes it cannot handle. 655 When a server receives a request to access a resource, the server 656 might challenge the client by responding with "401 Unauthorized" 657 status code, and include one or more WWW-Authenticate headers. If the 658 server challenges with multiple Digest headers, then each one of 659 these headers MUST use a different digest algorithm. The server MUST 660 add these Digest headers to the response in order of preference, 661 starting with the most preferred header, followed by the less 662 preferred headers. 664 This specification defines the following preference list, starting 665 with the most preferred algorithm: 667 * SHA2-256. 668 * SHA2-512/256. 669 * MD5 (for backward compatibility). 671 A future version of this document might add SHA3 [SHA3] as a backup 672 algorithm, once its definition has been finalized and published. 674 When the client receives the response it SHOULD use the topmost 675 header that it supports, unless a local policy dictates otherwise. 676 The client should ignore any challenge it does not understand. 678 3.8 Proxy-Authenticate and Proxy-Authorization 680 The digest authentication scheme may also be used for authenticating 681 users to proxies, proxies to proxies, or proxies to origin servers by 682 use of the Proxy-Authenticate and Proxy-Authorization headers. These 683 headers are instances of the Proxy-Authenticate and Proxy- 684 Authorization headers specified in sections 4.2 and 4.3 of the 685 HTTP/1.1 specification [HTTP-P7] and their behavior is subject to 686 restrictions described there. The transactions for proxy 687 authentication are very similar to those already described. Upon 688 receiving a request which requires authentication, the proxy/server 689 must issue the "407 Proxy Authentication Required" response with a 690 "Proxy-Authenticate" header. The digest-challenge used in the Proxy- 691 Authenticate header is the same as that for the WWW- Authenticate 692 header as defined above in section 3.2.1. 694 The client/proxy must then re-issue the request with a Proxy- 695 Authorization header, with directives as specified for the 696 Authorization header in section 3.4 above. 698 On subsequent responses, the server sends Proxy-Authenticate-Info 699 with directives the same as those for the Authentication-Info header 700 field. 702 Note that in principle a client could be asked to authenticate itself 703 to both a proxy and an end-server, but never in the same response. 705 3.9 Example 707 The following example assumes that an access protected document is 708 being requested from the server via a GET request. The URI of the 709 document is http://www.nowhere.org/dir/index.html". Both client and 710 server know that the username for this document is "Mufasa" and the 711 password is "Circle of Life" ( with one space between each of the 712 three words). 714 The first time the client requests the document, no Authorization 715 header is sent, so the server responds with: 717 HTTP/1.1 401 Unauthorized 718 WWW-Authenticate: Digest 719 realm = "testrealm@host.com", 720 qop="auth, auth-int", 721 algorithm="SHA2-256", 722 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093", 723 opaque="5ccc069c403ebaf9f0171e9517f40e41" 724 WWW-Authenticate: Digest 725 realm="testrealm@host.com", 726 qop="auth, auth-int", 727 algorithm="MD5", 728 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093", 729 opaque="5ccc069c403ebaf9f0171e9517f40ef41" 731 The client may prompt the user for their username and password, after 732 which it will respond with a new request, including the following 733 Authorization header if the client chooses MD5 digest: 735 Authorization:Digest username="Mufasa", 736 realm="testrealm@host.com", 737 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093", 738 uri="/dir/index.html", 739 qop="auth", 740 algorithm="MD5", 741 nc=00000001, 742 cnonce="0a4f113b", 743 response="6629fae49393a05397450978507c4ef1", 744 opaque="5ccc069c403ebaf9f0171e9517f40e41" 746 If the client chooses to use the SHA2-256 algorithm for calculating 747 the response, the client responds with a new request including the 748 following Authorization header: 750 Authorization:Digest username="Mufasa", 751 realm="testrealm@host.com", 752 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093", 753 uri="/dir/index.html", 754 qop="auth", 755 algorithm="SHA2-256", 756 nc=00000001, 757 cnonce="0a4f113b", 758 response="5abdd07184ba512a22c53f41470e5eea7dcaa3a93 759 a59b630c13dfe0a5dc6e38b", 760 opaque="5ccc069c403ebaf9f0171e9517f40e41" 762 4 Internationalization 764 In challenges, servers SHOULD use the "charset" authentication 765 parameter (case-insensitive) to express the character encoding they 766 expect the user agent to use. 768 The only allowed value is "UTF-8", to be matched case-insensitively, 769 indicating that the server expects the UTF-8 character encoding to be 770 used ([RFC3629]). 772 If the user agent supports the encoding indicated by the server, it 773 MAY add the "charset" parameter, with the value it received from the 774 server, to the Proxy-Authenticate or WWW-Authenticate header fields 775 it sends back to the server. 777 If the user agent does not support the encoding indicated by the 778 server, it MUST fail the request. 780 5 Security Considerations 782 5.1 Limitations 784 HTTP Digest authentication, when used with human-memorable passwords, 785 is vulnerable to dictionary attacks. Such attacks are much easier 786 than cryptographic attacks on any widely used algorithm, including 787 those that are no longer considered secure. In other words, algorithm 788 agility does not make this usage any more secure. 790 As a result, Digest authentication SHOULD be used only with passwords 791 that have a reasonable amount of entropy, e.g. 128-bit or more. Such 792 passwords typically cannot be memorized by humans but can be used for 793 automated web services. 795 Digest authentication SHOULD be used over a secure channel like HTTPS 796 [RFC2818]. 798 5.2 Authentication of Clients using Digest Authentication 800 Digest Authentication does not provide a strong authentication 801 mechanism, when compared to public key based mechanisms, for example. 803 However, it is significantly stronger than (e.g.) CRAM-MD5, which has 804 been proposed for use with LDAP [RFC2829], POP and IMAP (see RFC 805 2195). It is intended to replace the much weaker and even more 806 dangerous Basic mechanism. 808 Digest Authentication offers no confidentiality protection beyond 809 protecting the actual username and password. All of the rest of the 810 request and response are available to an eavesdropper. 812 Digest Authentication offers only limited integrity protection for 813 the messages in either direction. If qop=auth-int mechanism is used, 814 those parts of the message used in the calculation of the WWW- 815 Authenticate and Authorization header field response directive values 816 (see section 3.2 above) are protected. Most header fields and their 817 values could be modified as a part of a man-in-the-middle attack. 819 Many needs for secure HTTP transactions cannot be met by Digest 820 Authentication. For those needs TLS or SHTTP are more appropriate 821 protocols. In particular Digest authentication cannot be used for any 822 transaction requiring confidentiality protection. Nevertheless many 823 functions remain for which Digest authentication is both useful and 824 appropriate. 826 5.3 Limited Use Nonce Values 828 The Digest scheme uses a server-specified nonce to seed the 829 generation of the request-digest value (as specified in section 830 3.2.2.1 above). As shown in the example nonce in section 3.2.1, the 831 server is free to construct the nonce such that it may only be used 832 from a particular client, for a particular resource, for a limited 833 period of time or number of uses, or any other restrictions. Doing 834 so strengthens the protection provided against, for example, replay 835 attacks (see 4.5). However, it should be noted that the method 836 chosen for generating and checking the nonce also has performance and 837 resource implications. For example, a server may choose to allow 838 each nonce value to be used only once by maintaining a record of 839 whether or not each recently issued nonce has been returned and 840 sending a next-nonce directive in the Authentication-Info header 841 field of every response. This protects against even an immediate 842 replay attack, but has a high cost checking nonce values, and perhaps 843 more important will cause authentication failures for any pipelined 844 requests (presumably returning a stale nonce indication). Similarly, 845 incorporating a request-specific element such as the Etag value for a 846 resource limits the use of the nonce to that version of the resource 847 and also defeats pipelining. Thus it may be useful to do so for 848 methods with side effects but have unacceptable performance for those 849 that do not. 851 5.4 Replay Attacks 853 A replay attack against Digest authentication would usually be 854 pointless for a simple GET request since an eavesdropper would 855 already have seen the only document he could obtain with a replay. 856 This is because the URI of the requested document is digested in the 857 client request and the server will only deliver that document. By 858 contrast under Basic Authentication once the eavesdropper has the 859 user's password, any document protected by that password is open to 860 him. 862 Thus, for some purposes, it is necessary to protect against replay 863 attacks. A good Digest implementation can do this in various ways. 864 The server created "nonce" value is implementation dependent, but if 865 it contains a digest of the client IP, a time-stamp, the resource 866 ETag, and a private server key (as recommended above) then a replay 867 attack is not simple. An attacker must convince the server that the 868 request is coming from a false IP address and must cause the server 869 to deliver the document to an IP address different from the address 870 to which it believes it is sending the document. An attack can only 871 succeed in the period before the time-stamp expires. Digesting the 872 client IP and time-stamp in the nonce permits an implementation which 873 does not maintain state between transactions. 875 For applications where no possibility of replay attack can be 876 tolerated the server can use one-time nonce values which will not be 877 honored for a second use. This requires the overhead of the server 879 remembering which nonce values have been used until the nonce time- 880 stamp (and hence the digest built with it) has expired, but it 881 effectively protects against replay attacks. 883 An implementation must give special attention to the possibility of 884 replay attacks with POST and PUT requests. Unless the server employs 885 one-time or otherwise limited-use nonces and/or insists on the use of 886 the integrity protection of qop=auth-int, an attacker could replay 887 valid credentials from a successful request with counterfeit form 888 data or other message body. Even with the use of integrity protection 889 most metadata in header fields is not protected. Proper nonce 890 generation and checking provides some protection against replay of 891 previously used valid credentials, but see 4.8. 893 5.5 Weakness Created by Multiple Authentication Schemes 895 An HTTP/1.1 server may return multiple challenges with a 401 896 (Authenticate) response, and each challenge may use a different auth- 897 scheme. A user agent MUST choose to use the strongest auth- scheme it 898 understands and request credentials from the user based upon that 899 challenge. 901 Note that many browsers will only recognize Basic and will require 902 that it be the first auth-scheme presented. Servers should only 903 include Basic if it is minimally acceptable. 905 When the server offers choices of authentication schemes using the 906 WWW-Authenticate header, the strength of the resulting authentication 907 is only as good as that of the of the weakest of the authentication 908 schemes. See section 5.7 below for discussion of particular attack 909 scenarios that exploit multiple authentication schemes. 911 5.6 Online dictionary attacks 913 If the attacker can eavesdrop, then it can test any overheard 914 nonce/response pairs against a list of common words. Such a list is 915 usually much smaller than the total number of possible passwords. The 916 cost of computing the response for each password on the list is paid 917 once for each challenge. 919 The server can mitigate this attack by not allowing users to select 920 passwords that are in a dictionary. 922 5.7 Man in the Middle 924 Both Basic and Digest authentication are vulnerable to "man in the 925 middle" (MITM) attacks, for example, from a hostile or compromised 926 proxy. Clearly, this would present all the problems of eavesdropping. 927 But it also offers some additional opportunities to the attacker. 929 A possible man-in-the-middle attack would be to add a weak 930 authentication scheme to the set of choices, hoping that the client 931 will use one that exposes the user's credentials (e.g. password). For 932 this reason, the client should always use the strongest scheme that 933 it understands from the choices offered. 935 An even better MITM attack would be to remove all offered choices, 936 replacing them with a challenge that requests only Basic 937 authentication, then uses the cleartext credentials from the Basic 938 authentication to authenticate to the origin server using the 939 stronger scheme it requested. A particularly insidious way to mount 940 such a MITM attack would be to offer a "free" proxy caching service 941 to gullible users. 943 User agents should consider measures such as presenting a visual 944 indication at the time of the credentials request of what 945 authentication scheme is to be used, or remembering the strongest 946 authentication scheme ever requested by a server and produce a 947 warning message before using a weaker one. It might also be a good 948 idea for the user agent to be configured to demand Digest 949 authentication in general, or from specific sites. 951 Or, a hostile proxy might spoof the client into making a request the 952 attacker wanted rather than one the client wanted. Of course, this is 953 still much harder than a comparable attack against Basic 954 Authentication. 956 5.8 Chosen plaintext attacks 958 With Digest authentication, a MITM or a malicious server can 959 arbitrarily choose the nonce that the client will use to compute the 960 response. This is called a "chosen plaintext" attack. The ability to 961 choose the nonce is known to make cryptanalysis much easier. 963 However, no way to analyze the MD5 one-way function used by Digest 964 using chosen plaintext is currently known. 966 The countermeasure against this attack is for clients to be 967 configured to require the use of the optional "cnonce" directive; 968 this allows the client to vary the input to the hash in a way not 969 chosen by the attacker. 971 5.9 Precomputed dictionary attacks 973 With Digest authentication, if the attacker can execute a chosen 974 plaintext attack, the attacker can precompute the response for many 975 common words to a nonce of its choice, and store a dictionary of 976 (response, password) pairs. Such precomputation can often be done in 977 parallel on many machines. It can then use the chosen plaintext 978 attack to acquire a response corresponding to that challenge, and 979 just look up the password in the dictionary. Even if most passwords 980 are not in the dictionary, some might be. Since the attacker gets to 981 pick the challenge, the cost of computing the response for each 982 password on the list can be amortized over finding many passwords. A 983 dictionary with 100 million password/response pairs would take about 984 3.2 gigabytes of disk storage. 986 The countermeasure against this attack is to for clients to be 987 configured to require the use of the optional "cnonce" directive. 989 5.10 Batch brute force attacks 991 With Digest authentication, a MITM can execute a chosen plaintext 992 attack, and can gather responses from many users to the same nonce. 993 It can then find all the passwords within any subset of password 994 space that would generate one of the nonce/response pairs in a single 995 pass over that space. It also reduces the time to find the first 996 password by a factor equal to the number of nonce/response pairs 997 gathered. This search of the password space can often be done in 998 parallel on many machines, and even a single machine can search large 999 subsets of the password space very quickly -- reports exist of 1000 searching all passwords with six or fewer letters in a few hours. 1002 The countermeasure against this attack is to for clients to be 1003 configured to require the use of the optional "cnonce" directive. 1005 5.11 Spoofing by Counterfeit Servers 1007 Basic Authentication is vulnerable to spoofing by counterfeit 1008 servers. If a user can be led to believe that she is connecting to a 1009 host containing information protected by a password she knows, when 1010 in fact she is connecting to a hostile server, then the hostile 1011 server can request a password, store it away for later use, and feign 1012 an error. This type of attack is more difficult with Digest 1013 Authentication -- but the client must know to demand that Digest 1014 authentication be used, perhaps using some of the techniques 1015 described above to counter "man-in-the-middle" attacks. Again, the 1016 user can be helped in detecting this attack by a visual indication of 1017 the authentication mechanism in use with appropriate guidance in 1018 interpreting the implications of each scheme. 1020 5.12 Storing passwords 1022 Digest authentication requires that the authenticating agent (usually 1023 the server) store some data derived from the user's name and password 1024 in a "password file" associated with a given realm. Normally this 1025 might contain pairs consisting of username and H(A1), where H(A1) is 1026 the digested value of the username, realm, and password as described 1027 above. 1029 The security implications of this are that if this password file is 1030 compromised, then an attacker gains immediate access to documents on 1031 the server using this realm. Unlike, say a standard UNIX password 1032 file, this information need not be decrypted in order to access 1033 documents in the server realm associated with this file. On the other 1034 hand, decryption, or more likely a brute force attack, would be 1035 necessary to obtain the user's password. This is the reason that the 1036 realm is part of the digested data stored in the password file. It 1037 means that if one Digest authentication password file is compromised, 1038 it does not automatically compromise others with the same username 1039 and password (though it does expose them to brute force attack). 1041 There are two important security consequences of this. First the 1042 password file must be protected as if it contained unencrypted 1043 passwords, because for the purpose of accessing documents in its 1044 realm, it effectively does. 1046 A second consequence of this is that the realm string should be 1047 unique among all realms which any single user is likely to use. In 1048 particular a realm string should include the name of the host doing 1049 the authentication. The inability of the client to authenticate the 1050 server is a weakness of Digest Authentication. 1052 5.13 Summary 1054 By modern cryptographic standards Digest Authentication is weak. But 1055 for a large range of purposes it is valuable as a replacement for 1056 Basic Authentication. It remedies some, but not all, weaknesses of 1057 Basic Authentication. Its strength may vary depending on the 1058 implementation. In particular the structure of the nonce (which is 1059 dependent on the server implementation) may affect the ease of 1060 mounting a replay attack. A range of server options is appropriate 1061 since, for example, some implementations may be willing to accept the 1062 server overhead of one-time nonces or digests to eliminate the 1063 possibility of replay. Others may satisfied with a nonce like the one 1064 recommended above restricted to a single IP address and a single ETag 1065 or with a limited lifetime. 1067 The bottom line is that *any* compliant implementation will be 1068 relatively weak by cryptographic standards, but *any* compliant 1069 implementation will be far superior to Basic Authentication. 1071 6 IANA Considerations 1073 6.1 HTTP Digest Hash Algorithms Registry 1075 This specification creates a new IANA registry named "HTTP Digest 1076 Hash Algorithms". When registering a new hash algorithm, the 1077 following information MUST be provided: 1079 o Hash Algorithm 1080 The textual name of the hash algorithm. 1082 o Digest Size 1083 The size of the algorithm's output in hexadecimal characters. 1085 o Preference 1086 The preference of the algorithm, with zero being the least 1087 preferred. 1089 o Reference 1090 A reference to the specification that describes the new algorithm. 1092 The update policy for this registry shall be Specification Required. 1094 The initial registry will contain the following entries: 1096 Hash Algorithm Digest Size Preference Reference 1097 -------------- ----------- ---------- --------- 1098 "MD5" 32 0 RFC XXXX 1099 "SHA2-512-256" 64 1 RFC XXXX 1100 "SHA2-256" 64 2 RFC XXXX 1102 Each one of the algorithms defined in the registry might have a -sess 1103 variant, e.g. MD5-sess, SHA2-256-sess, etc. 1105 6.2 Digest 1107 This specification registers the Digest scheme with the 1108 Authentication Scheme Registry. 1110 Authentication Scheme Name: Digest 1112 Pointer to specification text: RFCXXX 1114 7 Acknowledgments 1116 TODO 1118 8 References 1120 8.1 Normative References 1122 [RFC2069] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P., 1123 Luotonen, A., Sink, E., and L. Stewart, "An Extension to 1124 HTTP : Digest Access Authentication", RFC 2069, January 1125 1997. 1127 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1128 Requirement Levels", BCP 14, RFC 2119, March 1997. 1130 [RFC2396] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 1131 Resource Identifiers (URI): Generic Syntax", RFC 2396, 1132 August 1998. 1134 [RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., 1135 Leach, P., Luotonen, A., and L. Stewart, "HTTP 1136 Authentication: Basic and Digest Access Authentication", 1137 RFC 2617, June 1999. 1139 [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. 1141 [RFC2829] Wahl, M., Alvestrand, H., Hodges, J., and R. Morgan, 1142 "Authentication Methods for LDAP", RFC 2829, May 2000. 1144 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 1145 10646", STD 63, RFC 3629, November 2003. 1147 [HTTP-P1] Fielding, R., Reschke, J., "Hypertext Transfer Protocol 1148 (HTTP/1.1): Message Syntax and Routing", Work in Progress, 1149 November 2013. 1151 [HTTP-P6] Fielding, R., Nottingham, M., Reschke, J., "Hypertext 1152 Transfer Protocol (HTTP/1.1): Caching", Work in Progress, 1153 November 2013. 1155 [HTTP-P7] Fielding, R., Reschke, J., "Hypertext Transfer Protocol 1156 (HTTP/1.1): Authentication", Work in Progress, November 1157 2013. 1159 8.2 Informative References 1161 [RFC2195] Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP 1162 AUTHorize Extension for Simple Challenge/Response", 1163 RFC 2195, September 1997. 1165 Authors' Addresses 1167 Rifaat Shekh-Yusef (Editor) 1168 Avaya 1169 250 Sydney Street 1170 Belleville, Ontario 1171 Canada 1173 Phone: +1-613-967-5267 1174 Email: rifaat.ietf@gmail.com 1176 David Ahrens 1177 Avaya 1178 California 1179 USA 1181 EMail: ahrensdc@gmail.com 1183 Sophie Bremer 1184 Netzkonform 1185 Germany 1187 Email: sophie.bremer@netzkonform.de