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Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year == The document seems to use 'NOT RECOMMENDED' as an RFC 2119 keyword, but does not include the phrase in its RFC 2119 key words list. -- The document date (September 7, 2010) is 4274 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- ** Obsolete normative reference: RFC 2617 (Obsoleted by RFC 7235, RFC 7615, RFC 7616, RFC 7617) Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group L. Morand 3 Internet-Draft France Telecom - Orange 4 Intended status: Informational September 7, 2010 5 Expires: March 11, 2011 7 Hypertext Transfer Protocol (HTTP) Digest Authentication Using GSM 2G 8 Authentication and Key Agreement (AKA) 9 draft-morand-http-digest-2g-aka-01 11 Abstract 13 This memo specifies a one-time password generation mechanism for 14 Hypertext Transfer Protocol (HTTP) Digest access authentication based 15 on Global System for Mobile Communications (GSM) authentication and 16 key generation functions A3 and A8, also known as GSM AKA or 2G AKA. 17 The HTTP Authentication Framework includes two authentication 18 schemes: Basic and Digest. Both schemes employ a shared secret based 19 mechanism for access authentication. The GSM AKA mechanism performs 20 user authentication and session key distribution in GSM and Universal 21 Mobile Telecommunications System (UMTS) networks. GSM AKA is a 22 challenge-response based mechanism that uses symmetric cryptography. 24 Requirements Language 26 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 27 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 28 document are to be interpreted as described in RFC 2119 [RFC2119]. 30 Status of this Memo 32 This Internet-Draft is submitted in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at http://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on March 11, 2011. 47 Copyright Notice 48 Copyright (c) 2010 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (http://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction and Motivation . . . . . . . . . . . . . . . . . 3 64 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3 65 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 66 4. GSM 2G AKA Mechanism Overview . . . . . . . . . . . . . . . . 4 67 5. Specification of Digest 2G AKA . . . . . . . . . . . . . . . . 6 68 5.1. Algorithm Directive . . . . . . . . . . . . . . . . . . . 6 69 5.2. Creating a Challenge . . . . . . . . . . . . . . . . . . . 7 70 5.3. Client Authentication . . . . . . . . . . . . . . . . . . 7 71 5.4. Server Authentication . . . . . . . . . . . . . . . . . . 8 72 6. Example of Digest 2G AKA operations . . . . . . . . . . . . . 8 73 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 74 8. Security Considerations . . . . . . . . . . . . . . . . . . . 11 75 8.1. Authentication of Clients using Digest 2G AKA . . . . . . 11 76 8.2. Limited Use of Nonce Values . . . . . . . . . . . . . . . 11 77 8.3. Multiple Authentication Schemes and Algorithms . . . . . . 12 78 8.4. Online Dictionary Attacks . . . . . . . . . . . . . . . . 12 79 8.5. Session Protection . . . . . . . . . . . . . . . . . . . . 12 80 8.6. Replay Protection . . . . . . . . . . . . . . . . . . . . 13 81 8.7. Mutual Authentication . . . . . . . . . . . . . . . . . . 13 82 8.8. Flooding the Authentication Centre . . . . . . . . . . . . 13 83 8.9. AKA Security . . . . . . . . . . . . . . . . . . . . . . . 13 84 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14 85 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14 86 10.1. Normative References . . . . . . . . . . . . . . . . . . . 14 87 10.2. Informative References . . . . . . . . . . . . . . . . . . 14 88 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14 90 1. Introduction and Motivation 92 The Hypertext Transfer Protocol (HTTP) Authentication Framework, 93 described in [RFC2617], includes two authentication schemes: Basic 94 and Digest. Both schemes employ a shared secret based mechanism for 95 access authentication. The Basic scheme is inherently insecure in 96 that it transmits user credentials in plain text. The Digest scheme 97 improves security by hiding user credentials with cryptographic 98 hashes, and additionally by providing limited message integrity. 100 The 2G AKA functions [TS55.205] perform authentication and session 101 key distribution in Global System for Mobile Communication (GSM) and 102 Universal Mobile Telecommunications System (UMTS) networks. 2G AKA is 103 a challenge-response based mechanism that uses symmetric 104 cryptography. GSM AKA is typically run in a GSM Subscriber Identity 105 Module (SIM), which resides in a smart card like device that also 106 provides tamper resistant storage of shared secrets. The 3G 107 Authentication and Key Agreement (AKA) mechanism, also known as UMTS 108 AKA, relying on the use of the UMTS Subscriber Identity Module (USIM) 109 instead of the GSM SIM, is most closely associated with UMTS; 110 however, mobile operators commonly distribute GSM SIMs with UMTS 111 mobile phones, resulting in the use of GSM 2G AKA in place of UMTS 112 AKA. 114 This document specifies a mapping of GSM AKA parameters onto HTTP 115 Digest authentication. In essence, this mapping enables the usage of 116 GSM 2G AKA as a one-time password generation mechanism for Digest 117 authentication. 119 This document is based heavily on [RFC3310] which specified a mapping 120 of Authentication and Key Agreement (AKA) onto HTTP Digest 121 authentication. While Digest AKA can be generally used when the 122 mobile phones are equipped with a UMTS SIM card, it may be useful for 123 mobile operators who have not yet fully deployed USIMs and have still 124 millions of SIMs deployed in the network. Digest 2G AKA allows 125 access to applications in a more secure way than would be possible 126 with the use of passwords or with GSM without enhancements. 128 Moreover, as the Session Initiation Protocol (SIP) [RFC3261] 129 Authentication Framework closely follows the HTTP Authentication 130 Framework, Digest 2G AKA is directly applicable to SIP as well as to 131 any other embodiment of HTTP Digest. 133 2. Requirements Language 135 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 136 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 137 document are to be interpreted as described in [RFC2119]. 139 3. Terminology 141 This section explains the terminology used in this document. 143 AuC Authentication Center. 145 AKA Authentication and Key Agreement. 147 GSM Global System for Mobile Communication. 149 IMS IP Multimedia Subsystem. 151 ISIM IMS Subscriber Identity Module. 153 Kc Cypher Key. Ki Subscriber Key. 155 RAND Random Challenge. 157 SIM Subscriber Identity Module. 159 SRES Signed Authentication Response. 161 UMTS Universal Mobile Telecommunications System. 163 USIM UMTS Subscriber Identity Module. 165 4. GSM 2G AKA Mechanism Overview 167 This chapter describes the GSM AKA mechanism based on a shared secret 168 key (Ki) and the use of A3/A8 algorithms: 170 1. A shared secret Ki is established beforehand between the SIM and 171 the Authentication Center (AuC). The secret is stored in the 172 SIM, which resides on a smart card like, tamper resistant device. 174 2. The AuC of the home network produces an authentication vector 175 (AV), based on the shared secret Ki and a generated random value 176 (RAND). Using Ki and RAND, the A3 algorihm is used for 177 calculating the expected response (SRES) and the A8 algorithm is 178 used for calculating the ciffering key Kc. 180 3. The authentication vector (RAND, SRES, Kc) is downloaded to a 181 server. Optionally, the server can also download a batch of AVs, 182 containing more than one authentication vector generated by 183 adding different RAND values. 185 4. The server creates an authentication request, which contains the 186 random challenge RAND. 188 5. The authentication request is delivered to the client. 190 6. The client produces a signed authentication response RES, using 191 the shared secret Ki and the random challenge RAND. 193 7. The authentication response RES is delivered to the server. 195 8. The server compares the authentication response RES with the 196 expected response SRES. If the two match, the user has been 197 successfully authenticated, and the session key Kc can be used 198 for protecting further communication between the client and the 199 server 200 SIM Server AuC 201 (Ki) (Ki) 203 | | | 204 |--- Request (IMSI) ------>| | 205 | | Authen. Data Request (IMSI) | 206 | |---------------------------->| 207 | | | 208 | | +-----------------+----+ 209 | | | IMSI --> Ki | 210 | | | A3(RAND,Ki) = SRES | 211 | | | A8(RAND,Ki) = Kc | 212 | | +----------------+-----+ 213 | | | 214 | |<--- AV (RAND, SRES, Kc) ---| 215 |<-- Auth. Request (RAND) -| | 216 | | | 217 +-----+---------------+ | | 218 | A3(RAND,Ki) = SRES* | | | 219 | A8(RAND,Ki) = Kc | | | 220 +-----+---------------+ | | 221 | | | 222 |- Auth. Response (SRES*)->| | 223 | | | 224 | +-------+------+ | 225 | | SRES*= SRES? | | 226 | +-------+------+ | 227 | | | 229 Figure 1. GSM 2G AKA overview 231 5. Specification of Digest 2G AKA 233 In general, the Digest GSM AKA operation is identical to the Digest 234 operation in [RFC2617]. This chapter specifies the parts in which 235 Digest 2G AKA extends the Digest operation. The notation used in the 236 Augmented BNF definitions for the new and modified syntax elements in 237 this section is as used in SIP [RFC3261], and any elements not 238 defined in this section are as defined in SIP and the documents to 239 which it refers. 241 5.1. Algorithm Directive 243 In order to direct the client into using 2G AKA for authentication 244 instead of the standard password system, the RFC 2617 defined 245 algorithm directive is overloaded in Digest 2G AKA: 247 algorithm = "algorithm" EQUAL ( 2GAKA-namespace / algorithm-value 248 ) 250 2GAKA-namespace = "2GAKA" "-" algorithm-value 252 algorithm-value = ( "MD5" / "MD5-sess" / token ) 254 algorithm 256 A string indicating the algorithm used in producing the digest and 257 the checksum. If the directive is not understood, the nonce 258 SHOULD be ignored, and another challenge (if one is present) 259 should be used instead. Reuse of the same SRES value in 260 authenticating subsequent requests and responses is NOT 261 RECOMMENDED. An SRES value SHOULD only be used as a one-time 262 password, and algorithms such as "MD5-sess", which limit the 263 amount of material hashed with a single key, by producing a 264 session key for authentication, SHOULD NOT be used. 266 5.2. Creating a Challenge 268 In order to deliver the GSM AKA authentication challenge to the 269 client in Digest 2G AKA, the nonce directive defined in [RFC2617] is 270 extended: 272 nonce = "nonce" EQUAL ( 2GAKA-nonce / nonce-value ) 274 2GAKA-nonce = LDQUOT 2GAKA-nonce-value RDQUOT 276 2GAKA-nonce-value = 278 nonce 280 A parameter which is populated with the Base64 [RFC2045] encoding 281 of the 2G AKA authentication random challenge RAND. 283 5.3. Client Authentication 285 When a client receives a Digest 2G AKA authentication challenge, it 286 extracts the RAND from the "nonce" parameter and runs the A3-A8 287 algorithms with the RAND challenge and shared secret Ki. 289 The resulting A3-A8 SRES parameter is treated as a "password" when 290 calculating the response directive of [RFC2617]. Due to the fact 291 that the SRES parameter is 32 bits and the response directive of 292 [RFC2617] is defined as 32 hex digits, SRES is encoded in the low 293 order (i.e. rightmost) 32 bits of "response", padded with leading 294 zeroes. 296 Example: 298 SRES="0000000000000000000000007018d8a1" 300 5.4. Server Authentication 302 With Digest 2G AKA, the server MUST use the expected response SRES 303 received in the authentication vector as "password" when calculating 304 the "response-auth" of the "Authentication-Info" header defined in 305 [RFC2617]. 307 6. Example of Digest 2G AKA operations 309 Figure 2 below describes a message flow describing a Digest 2G AKA 310 process of authenticating a SIP request, namely the SIP REGISTER 311 request. 313 SIM HTTP Server AuC 314 (Ki) (Ki) 316 | 1) GET (IMSI) | | 317 |--------------------------->| | 318 | | 2) Authen. Data Request (IMSI)| 319 | |------------------------------>| 320 | | | 321 | | +-----------------+--+ 322 | | | IMSI --> Ki | 323 | | | A3(RAND,Ki) = SRES | 324 | | | A8(RAND,Ki) = Kc | 325 | | +-----------------+--+ 326 | | | 327 | | 3) AV (RAND, SRES, Kc) | 328 | |<------------------------------| 329 | 4) 401 Unauthorized (RAND) | | 330 |<---------------------------| | 331 | | | 332 +-----+---------------+ | | 333 | A3(RAND,Ki) = SRES* | | | 334 | A8(RAND,Ki) = Kc | | | 335 +-----+---------------+ | | 336 | | | 337 | 5) GET (IMSI, RAND, SRES) | | 338 |--------------------------->| | 339 | | | 340 | +-------+------+ | 341 | | SRES*= SRES? | | 342 | +-------+------+ | 343 | | | 344 | 6) 200 OK | | 345 |<---------------------------| | 346 | | | 348 Figure 2: Message flow representing a successful authentication 350 1) Initial request 352 GET / HTTP/1.1 353 Authorization: Digest 354 username="user1_private@home1.net", 355 realm="service1.home1.net", 356 nonce="", 357 uri="/", 358 response="" 360 2) Request to the AuC for 2G AKA authentication vector (AV) for the 361 given IMSI 363 3) Response from the AuC providing 2G AKA AV (RAND, SRES, Kc) 364 associated with the IMSI 366 4) Response containing a challenge 368 HTTP/1.1 401 Unauthorized 369 WWW-Authenticate: Digest 370 realm="service1.home1.net", 371 nonce="base64(RAND)", 372 qop="auth,auth-int", 373 opaque="6dae728da9089dab9112373c9f0a9731", 374 algorithm=2GAKA-MD5 376 5) Request containing credentials 378 GET / HTTP/1.1 379 Authorization: Digest 380 username="user1_private@home1.net", 381 realm="service1.home1.net", 382 nonce="base64(RAND)", 383 uri="/", 384 nc=00000001, 385 cnonce="0b8f29d6", 386 response="6629fae49393a05397450978507c4ef1", 387 opaque="6dae728da9089dab9112373c9f0a9731", 388 algorithm=2GAKA-MD5 390 6) Successful response 392 HTTP/1.1 200 OK 393 Authentication-Info: 394 qop=auth-int, 395 rspauth="6629fae49394a05397450978507c4ef1", 396 cnonce="6629fae49393a05397450978507c4ef1", 397 nc=00000001, 398 opaque="6dae728da9089dab9112373c9f0a9731", 399 nonce="base64(RAND)" 401 7. IANA Considerations 403 This document has no actions for IANA. 405 Note to RFC Editor: this section may be removed on publication as an 406 RFC. 408 8. Security Considerations 410 In general, Digest 2G AKA is vulnerable to the same security threats 411 as HTTP authentication [RFC2617]. This chapter discusses the 412 relevant exceptions. 414 8.1. Authentication of Clients using Digest 2G AKA 416 2G AKA is typically -- though this isn't a theoretical limitation -- 417 run on a SIM application that usually resides in a tamper resistant 418 smart card. Interfaces to the SIM exist, which enable the host 419 device to request authentication to be performed on the card. 420 However, these interfaces do not allow access to the long-term secret 421 outside the SIM, and the authentication can only be performed if the 422 device accessing the SIM has knowledge of a PIN code, shared between 423 the user and the SIM. Such PIN codes are typically obtained from 424 user input, and are usually required when the device is powered on. 426 The use of tamper resistant cards with secure interfaces implies that 427 Digest 2G AKA is typically more secure than regular Digest 428 implementations, as neither possession of the host device nor Trojan 429 Horses in the software give access to the long-term secret. Where a 430 PIN scheme is used, the user is also authenticated when the device is 431 powered on. However, there may be a difference in the resulting 432 security of Digest 2G AKA, compared to traditional Digest 433 implementations, depending on whether those implementations cache/ 434 store passwords that are received from the user. 436 8.2. Limited Use of Nonce Values 438 The Digest scheme uses server-specified nonce values to seed the 439 generation of the request-digest value. The server is free to 440 construct the nonce in such a way that it may only be used from a 441 particular client, for a particular resource, for a limited period of 442 time or number or uses, or any other restrictions. Doing so 443 strengthens the protection provided against, for example, replay 444 attacks. 446 Digest 2G AKA limits the applicability of a nonce value to a 447 particular SIM. Typically, the SIM is accessible only to one client 448 device at a time. However, the nonce values are strong and secure 449 even though limited to a particular SIM. Additionally, this requires 450 that the server is provided with the client identity before an 451 authentication challenge can be generated. If a client identity is 452 not available, an additional round trip is needed to acquire it. 454 8.3. Multiple Authentication Schemes and Algorithms 456 In HTTP authentication, a user agent MUST choose the strongest 457 authentication scheme it understands and request credentials from the 458 user, based upon that challenge. 460 In general, using passwords generated by Digest 2G AKA with other 461 HTTP authentication schemes is not recommended even though the realm 462 values or protection domains would coincide. In these cases, a 463 password should be requested from the end-user instead. Digest 2G 464 AKA passwords MUST NOT be re-used with such HTTP authentication 465 schemes, which send the password in the clear. In particular, 2G AKA 466 passwords must not be re-used with HTTP Basic. 468 The same principle must be applied within a scheme if several 469 algorithms are supported. A client receiving an HTTP Digest 470 challenge with several available algorithms MUST choose the strongest 471 algorithm it understands. For example, Digest with "2GAKA-MD5" would 472 be stronger than Digest with "MD5". 474 8.4. Online Dictionary Attacks 476 Since user-selected passwords are typically quite simple, it has been 477 proposed that servers should not accept passwords for HTTP Digest 478 which are in the dictionary [RFC2617]. This potential threat does 479 not exist in HTTP Digest 2G AKA because the algorithm will use SIM 480 originated passwords. However, the end-user must still be careful 481 with PIN codes. Even though HTTP Digest 2G AKA password requests are 482 never displayed to the end-user, the end-user will be authenticated 483 to the SIM via a PIN code. Commonly known initial PIN codes are 484 typically installed to the SIM during manufacturing and if the end- 485 users do not change them, there is a danger than an unauthorized user 486 may be able to use the device. Naturally this requires that the 487 unauthorized user has access to the physical device, and that the 488 end-user has not changed the initial PIN code. For this reason, end- 489 users are strongly encouraged to change their PIN codes when they 490 receive a SIM. 492 8.5. Session Protection 494 Digest 2G AKA is able to generate an additional session key for 495 integrity (Kc) protection. Even though this document does not 496 specify the use of these additional keys, they may be used for 497 creating additional security within HTTP authentication or some other 498 security mechanisms. 500 8.6. Replay Protection 502 The generation of RAND used as one-time or very limited-use nonces 503 and the use of the integrity protection of qop=auth-int will limit 504 the possibility of replay attacks. 506 In GSM, the network is allowed to re-use the RAND challenge in 507 consecutive authentication exchanges. This is not allowed in Digest 508 2G AKA. The server is mandated to use fresh triplets (RAND 509 challenges) in consecutive authentication exchanges. Digest 2G AKA 510 does not mandate any means for the client to check if the RANDs are 511 fresh, so the security of the scheme leans on the secrecy of the 512 triplets. However, the peer MAY employ implementation-specific 513 mechanisms to remember some of the previously used RANDs, and the 514 client MAY check the freshness of the server's RANDs. 516 8.7. Mutual Authentication 518 With Digest 2G AKA, network authentication is performed only after UE 519 authentication, in contrary to Digest AKA [RFC3310] in which the UE 520 authenticates the network before responding to the challenge. To 521 prevent an impersonation attack of the server to the UE, the 522 authentication of the server to the UE SHOULD be improved by 523 protecting the communication with TLS. An attacker succeeds only if 524 he can break both, the certificate-based TLS authentication to the UE 525 and mutual authentication provided by HTTP Digest using a password 526 derived from GSM procedures. One way to break TLS is to compromise 527 the certificate. 529 8.8. Flooding the Authentication Centre 531 The server typically obtains authentication vectors from the 532 Authentication Centre (AuC). Digest 2G AKA introduces a new usage 533 for the AuC. The protocols between the server and the AuC are out of 534 the scope of this document. However, it should be noted that a 535 malicious client may generate a lot of protocol requests to mount a 536 denial of service attack. The server implementation SHOULD take this 537 into account and SHOULD take steps to limit the traffic that it 538 generates towards the AuC, preventing the attacker from flooding the 539 AuC and from extending the denial of service attack from Digest 2G 540 AKA to other users of the AuC. 542 8.9. AKA Security 544 Evolutions of GSM networks, specifically Universal Mobile 545 Telecommunications System (UMTS) and IP Multimedia System (IMS) 546 networks, use an enhanced shared secret based mechanism for 547 authentication known as Authentication and Key Agreement (AKA). In 548 these networks, AKA is typically run in a UMTS Services Identity 549 Module (USIM) or IP Multimedia Services Identity Module (ISIM). GSM 550 phones can also be equipped with a USIM or ISIM. In that case, 551 Digest AKA as described in [RFC3310] is used for authentication as 552 opposed to Digest 2G AKA. 554 9. Acknowledgements 556 This memo is based on an initial draft written by Brett Wallis 557 (draft-ietf-http-digest-auth-a3a8-01). 559 10. References 561 10.1. Normative References 563 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 564 Extensions (MIME) Part One: Format of Internet Message 565 Bodies", RFC 2045, November 1996. 567 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 568 Requirement Levels", BCP 14, RFC 2119, March 1997. 570 [RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., 571 Leach, P., Luotonen, A., and L. Stewart, "HTTP 572 Authentication: Basic and Digest Access Authentication", 573 RFC 2617, June 1999. 575 10.2. Informative References 577 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 578 A., Peterson, J., Sparks, R., Handley, M., and E. 579 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 580 June 2002. 582 [RFC3310] Niemi, A., Arkko, J., and V. Torvinen, "Hypertext Transfer 583 Protocol (HTTP) Digest Authentication Using Authentication 584 and Key Agreement (AKA)", RFC 3310, September 2002. 586 [TS55.205] 587 "Specification of the GSM- MILENAGE algorithms (Release 588 9)", December 2009. 590 Author's Address 592 Lionel Morand 593 France Telecom - Orange 594 38-40 rue du general Leclerc 595 Issy-Les-Moulineaux Cedex 9 92794 596 France 598 Phone: +33 1 4529 6257 599 Email: lionel.morand@orange-ftgroup.com