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Internet Engineering Task Force (IETF)                       A. MelnikovRequest for Comments: 7804                                     Isode LtdCategory: Experimental                                        March 2016ISSN: 2070-1721Salted Challenge Response HTTP Authentication MechanismAbstract   This specification describes a family of HTTP authentication   mechanisms called the Salted Challenge Response Authentication   Mechanism (SCRAM), which provides a more robust authentication   mechanism than a plaintext password protected by Transport Layer   Security (TLS) and avoids the deployment obstacles presented by   earlier TLS-protected challenge response authentication mechanisms.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for examination, experimental implementation, and   evaluation.   This document defines an Experimental Protocol for the Internet   community.  This document is a product of the Internet Engineering   Task Force (IETF).  It represents the consensus of the IETF   community.  It has received public review and has been approved for   publication by the Internet Engineering Steering Group (IESG).  Not   all documents approved by the IESG are a candidate for any level of   Internet Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7804.Melnikov                      Experimental                      [Page 1]

RFC 7804                       HTTP SCRAM                     March 2016Copyright Notice   Copyright (c) 2016 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Conventions Used in This Document . . . . . . . . . . . . . .32.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .42.2.  Notation  . . . . . . . . . . . . . . . . . . . . . . . .43.  SCRAM Algorithm Overview  . . . . . . . . . . . . . . . . . .64.  SCRAM Mechanism Names . . . . . . . . . . . . . . . . . . . .75.  SCRAM Authentication Exchange . . . . . . . . . . . . . . . .75.1.  One Round-Trip Reauthentication . . . . . . . . . . . . .106.  Use of the Authentication-Info Header Field with SCRAM  . . .127.  Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . .138.  Security Considerations . . . . . . . . . . . . . . . . . . .149.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .1510. Design Motivations  . . . . . . . . . . . . . . . . . . . . .1511. References  . . . . . . . . . . . . . . . . . . . . . . . . .1611.1.  Normative References . . . . . . . . . . . . . . . . . .1611.2.  Informative References . . . . . . . . . . . . . . . . .17   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .18   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .181.  Introduction   The authentication mechanism most widely deployed and used by   Internet application protocols is the transmission of clear-text   passwords over a channel protected by Transport Layer Security (TLS).   There are some significant security concerns with that mechanism,   which could be addressed by the use of a challenge response   authentication mechanism protected by TLS.  Unfortunately, the HTTP   Digest challenge response mechanism presently on the Standards Track   failed widespread deployment and has had only limited success.Melnikov                      Experimental                      [Page 2]

RFC 7804                       HTTP SCRAM                     March 2016   This specification describes a family of authentication mechanisms   called the Salted Challenge Response Authentication Mechanism   (SCRAM), which addresses the requirements necessary to deploy a   challenge response mechanism more widely than past attempts (see   [RFC5802]).  In particular, it addresses some of the issues   identified with HTTP Digest, as described in [RFC6331], such as the   complexity of implementation and protection of the whole   authentication exchange in order to protect against certain man-in-   the-middle attacks.   HTTP SCRAM is an adaptation of [RFC5802] for use in HTTP.  The SCRAM   data exchanged is identical to what is defined in [RFC5802].  This   document also adds a 1 round-trip reauthentication mode.   HTTP SCRAM provides the following protocol features:   o  The authentication information stored in the authentication      database is not sufficient by itself (without a dictionary attack)      to impersonate the client.  The information is salted to make it      harder to do a pre-stored dictionary attack if the database is      stolen.   o  The server does not gain the ability to impersonate the client to      other servers (with an exception for server-authorized proxies),      unless it performs a dictionary attack.   o  The mechanism permits the use of a server-authorized proxy without      requiring that proxy to have super-user rights with the back-end      server.   o  Mutual authentication is supported, but only the client is named      (i.e., the server has no name).2.  Conventions Used in This Document   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].   Formal syntax is defined by [RFC5234] including the core rules   defined inAppendix B of [RFC5234].   Example lines prefaced by "C:" are sent by the client and ones   prefaced by "S:" by the server.  If a single "C:" or "S:" label   applies to multiple lines, then the line breaks between those lines   are for editorial clarity only and are not part of the actual   protocol exchange.Melnikov                      Experimental                      [Page 3]

RFC 7804                       HTTP SCRAM                     March 20162.1.  Terminology   This document uses several terms defined in the "Internet Security   Glossary" [RFC4949], including the following: authentication,   authentication exchange, authentication information, brute force,   challenge-response, cryptographic hash function, dictionary attack,   eavesdropping, hash result, keyed hash, man-in-the-middle, nonce,   one-way encryption function, password, replay attack, and salt.   Readers not familiar with these terms should use that glossary as a   reference.   Some clarifications and additional definitions follow:   o  Authentication information: Information used to verify an identity      claimed by a SCRAM client.  The authentication information for a      SCRAM identity consists of salt, iteration count, the StoredKey,      and the ServerKey (as defined in the algorithm overview) for each      supported cryptographic hash function.   o  Authentication database: The database used to look up the      authentication information associated with a particular identity.      For application protocols, LDAPv3 (see [RFC4510]) is frequently      used as the authentication database.  For lower-layer protocols      such as PPP or 802.11x, the use of RADIUS [RFC2865] is more      common.   o  Base64: An encoding mechanism defined inSection 4 of [RFC4648]      that converts an octet string input to a textual output string      that can be easily displayed to a human.  The use of base64 in      SCRAM is restricted to the canonical form with no whitespace.   o  Octet: An 8-bit byte.   o  Octet string: A sequence of 8-bit bytes.   o  Salt: A random octet string that is combined with a password      before applying a one-way encryption function.  This value is used      to protect passwords that are stored in an authentication      database.2.2.  Notation   The pseudocode description of the algorithm uses the following   notation:   o  ":=": The variable on the left-hand side represents the octet      string resulting from the expression on the right-hand side.Melnikov                      Experimental                      [Page 4]

RFC 7804                       HTTP SCRAM                     March 2016   o  "+": Octet string concatenation.   o  "[ ]": A portion of an expression enclosed in "[" and "]" is      optional in the result under some circumstances.  See the      associated text for a description of those circumstances.   o  Normalize(str): Apply the Preparation and Enforcement steps      according to the OpaqueString profile (see [RFC7613]) to a UTF-8      [RFC3629] encoded "str".  The resulting string is also in UTF-8.      Note that implementations MUST either implement OpaqueString      profile operations from [RFC7613] or disallow the use of non      US-ASCII Unicode codepoints in "str".  The latter is a particular      case of compliance with [RFC7613].   o  HMAC(key, str): Apply the HMAC-keyed hash algorithm (defined in      [RFC2104]) using the octet string represented by "key" as the key      and the octet string "str" as the input string.  The size of the      result is the hash result size for the hash function in use.  For      example, it is 32 octets for SHA-256 and 20 octets for SHA-1 (see      [RFC6234]).   o  H(str): Apply the cryptographic hash function to the octet string      "str", producing an octet string as a result.  The size of the      result depends on the hash result size for the hash function in      use.   o  XOR: Apply the exclusive-or operation to combine the octet string      on the left of this operator with the octet string on the right of      this operator.  The length of the output and each of the two      inputs will be the same for this use.   o  Hi(str, salt, i):      U1   := HMAC(str, salt + INT(1))      U2   := HMAC(str, U1)      ...      Ui-1 := HMAC(str, Ui-2)      Ui   := HMAC(str, Ui-1)      Hi := U1 XOR U2 XOR ... XOR Ui      where "i" is the iteration count, "+" is the string concatenation      operator, and INT(g) is a four-octet encoding of the integer g,      most significant octet first.      Hi() is, essentially, PBKDF2 [RFC2898] with HMAC() as the      Pseudorandom Function (PRF) and with dkLen == output length of      HMAC() == output length of H().Melnikov                      Experimental                      [Page 5]

RFC 7804                       HTTP SCRAM                     March 20163.  SCRAM Algorithm Overview   The following is a description of a full HTTP SCRAM authentication   exchange.  Note that this section omits some details, such as client   and server nonces.  SeeSection 5 for more details.   To begin with, the SCRAM client is in possession of a username and   password, both encoded in UTF-8 [RFC3629] (or a ClientKey/ServerKey,   or SaltedPassword).  It sends the username to the server, which   retrieves the corresponding authentication information: a salt, a   StoredKey, a ServerKey, and an iteration count ("i").  (Note that a   server implementation may choose to use the same iteration count for   all accounts.)  The server sends the salt and the iteration count to   the client, which then computes the following values and sends a   ClientProof to the server:   Informative Note: Implementors are encouraged to create test cases   that use both usernames and passwords with non-ASCII codepoints.  In   particular, it is useful to test codepoints whose Unicode   Normalization Canonical Composition (NFC) and Unicode Normalization   Form Compatibility Composition (NFKC) are different (see   [Unicode-UAX15]).  Some examples of such codepoints include Vulgar   Fraction One Half (U+00BD) and Acute Accent (U+00B4).      SaltedPassword  := Hi(Normalize(password), salt, i)      ClientKey       := HMAC(SaltedPassword, "Client Key")      StoredKey       := H(ClientKey)      AuthMessage     := client-first-message-bare + "," +                         server-first-message + "," +                         client-final-message-without-proof      ClientSignature := HMAC(StoredKey, AuthMessage)      ClientProof     := ClientKey XOR ClientSignature      ServerKey       := HMAC(SaltedPassword, "Server Key")      ServerSignature := HMAC(ServerKey, AuthMessage)   The server authenticates the client by computing the ClientSignature,   exclusive-ORing that with the ClientProof to recover the ClientKey,   and verifying the correctness of the ClientKey by applying the hash   function and comparing the result to the StoredKey.  If the ClientKey   is correct, this proves that the client has access to the user's   password.   Similarly, the client authenticates the server by computing the   ServerSignature and comparing it to the value sent by the server.  If   the two are equal, this proves that the server had access to the   user's ServerKey.Melnikov                      Experimental                      [Page 6]

RFC 7804                       HTTP SCRAM                     March 2016   For initial authentication, the AuthMessage is computed by   concatenating decoded "data" attribute values from the authentication   exchange.  The format of each of these 3 decoded "data" attributes is   defined in [RFC5802].4.  SCRAM Mechanism Names   A SCRAM mechanism name (authentication scheme) is a string "SCRAM-"   followed by the uppercased name of the underlying hash function taken   from the IANA "Hash Function Textual Names" registry (see   <http://www.iana.org/assignments/hash-function-text-names>).   For interoperability, all HTTP clients and servers supporting SCRAM   MUST implement the SCRAM-SHA-256 authentication mechanism, i.e., an   authentication mechanism from the SCRAM family that uses the SHA-256   hash function as defined in [RFC7677].5.  SCRAM Authentication Exchange   HTTP SCRAM is an HTTP Authentication mechanism whose client response   (<credentials-scram>) and server challenge (<challenge-scram>)   messages are text-based messages containing one or more attribute-   value pairs separated by commas.  The messages and their attributes   are described below and defined inSection 7.    challenge-scram   = scram-name [1*SP 1#auth-param]          ; Complies with <challenge> ABNF fromRFC 7235.          ; Included in the WWW-Authenticate header field.    credentials-scram = scram-name [1*SP 1#auth-param]          ; Complies with <credentials> fromRFC 7235.          ; Included in the Authorization header field.    scram-name = "SCRAM-SHA-256" / "SCRAM-SHA-1" / other-scram-name          ; SCRAM-SHA-256 and SCRAM-SHA-1 are registered by this RFC.          ;          ; SCRAM-SHA-1 is registered for database compatibility          ; with implementations ofRFC 5802 (such as IMAP or Extensible            Messaging and Presence Protocol (XMPP)          ; servers), but it is not recommended for new deployments.    other-scram-name = "SCRAM-" hash-name          ; hash-name is a capitalized form of names from IANA.          ; "Hash Function Textual Names" registry.          ; Additional SCRAM names must be registered in both          ; the IANA "SASL Mechanisms" registry          ; and the IANA "HTTP Authentication Schemes" registry.Melnikov                      Experimental                      [Page 7]

RFC 7804                       HTTP SCRAM                     March 2016   This is a simple example of a SCRAM-SHA-256 authentication exchange   (no support for channel bindings, as this feature is not currently   supported by HTTP).  Username 'user' and password 'pencil' are used.   Note that long lines are folded for readability.   C: GET /resource HTTP/1.1   C: Host: server.example.com   C: [...]   S: HTTP/1.1 401 Unauthorized   S: WWW-Authenticate: Digest realm="realm1@example.com",          Digest realm="realm2@example.com",          Digest realm="realm3@example.com",          SCRAM-SHA-256 realm="realm3@example.com",          SCRAM-SHA-256 realm="testrealm@example.com"   S: [...]   C: GET /resource HTTP/1.1   C: Host: server.example.com   C: Authorization: SCRAM-SHA-256 realm="testrealm@example.com",          data=biwsbj11c2VyLHI9ck9wck5HZndFYmVSV2diTkVrcU8K   C: [...]   S: HTTP/1.1 401 Unauthorized   S: WWW-Authenticate: SCRAM-SHA-256           sid=AAAABBBBCCCCDDDD,           data=cj1yT3ByTkdmd0ViZVJXZ2JORWtxTyVodllEcFdVYTJSYVRDQWZ1eEZJ              bGopaE5sRixzPVcyMlphSjBTTlk3c29Fc1VFamI2Z1E9PSxpPTQwOTYK   S: [...]   C: GET /resource HTTP/1.1   C: Host: server.example.com   C: Authorization: SCRAM-SHA-256 sid=AAAABBBBCCCCDDDD,          data=Yz1iaXdzLHI9ck9wck5HZndFYmVSV2diTkVrcU8laHZZRHBXVWEyUmFUQ             0FmdXhGSWxqKWhObEYscD1kSHpiWmFwV0lrNGpVaE4rVXRlOXl0YWc5empm             TUhnc3FtbWl6N0FuZFZRPQo=   C: [...]   S: HTTP/1.1 200 Ok   S: Authentication-Info: sid=AAAABBBBCCCCDDDD,          data=dj02cnJpVFJCaTIzV3BSUi93dHVwK21NaFVaVW4vZEI1bkxUSlJzamw5N             Uc0PQo=   S: [...Other header fields and resource body...]Melnikov                      Experimental                      [Page 8]

RFC 7804                       HTTP SCRAM                     March 2016   In the above example, the first client request contains a "data"   attribute that base64 decodes as follows:      n,,n=user,r=rOprNGfwEbeRWgbNEkqO   The server then responds with a "data" attribute that base64 decodes   as follows:      r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF,s=W22ZaJ0SNY7soE      sUEjb6gQ==,i=4096   The next client request contains a "data" attribute that base64   decodes as follows:      c=biws,r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF,p=dHzbZap      WIk4jUhN+Ute9ytag9zjfMHgsqmmiz7AndVQ=   The final server response contains a "data" attribute that base64   decodes as follows:      v=6rriTRBi23WpRR/wtup+mMhUZUn/dB5nLTJRsjl95G4=   Note that in the example above, the client can also initiate SCRAM   authentication without first being prompted by the server.   Initial "SCRAM-SHA-256" authentication starts with sending the   Authorization request header field (defined by HTTP/1.1, Part 7   [RFC7235]) containing the "SCRAM-SHA-256" authentication scheme and   the following attributes:   o  A "realm" attribute MAY be included to indicate the scope of      protection in the manner described in HTTP/1.1, Part 7 [RFC7235].      As specified in [RFC7235], the "realm" attribute MUST NOT appear      more than once.  The "realm" attribute only appears in the first      SCRAM message to the server and in the first SCRAM response from      the server.   o  The client also includes the "data" attribute that contains the      base64-encoded "client-first-message" [RFC5802] containing:      *  a header consisting of a flag indicating whether channel         binding is supported-but-not-used, not supported, or used.         Note that this version of SCRAM doesn't support HTTP channel         bindings, so this header always starts with "n"; otherwise, the         message is invalid and authentication MUST fail.      *  SCRAM username and a random, unique "nonce" attribute.Melnikov                      Experimental                      [Page 9]

RFC 7804                       HTTP SCRAM                     March 2016   In an HTTP response, the server sends the WWW-Authenticate header   field containing a unique session identifier (the "sid" attribute)   plus the "data" attribute containing the base64-encoded "server-   first-message" [RFC5802].  The "server-first-message" contains the   user's iteration count i, the user's salt, and the nonce with a   concatenation of the client-specified one (taken from the "client-   first-message") with a freshly generated server nonce.   The client then responds with another HTTP request with the   Authorization header field, which includes the "sid" attribute   received in the previous server response, together with the "data"   attribute containing base64-encoded "client-final-message" data.  The   latter has the same nonce as in "server-first-message" and a   ClientProof computed using the selected hash function (e.g., SHA-256)   as explained earlier.   The server verifies the nonce and the proof, and, finally, it   responds with a 200 HTTP response with the Authentication-Info header   field [RFC7615] containing the "sid" attribute (as received from the   client) and the "data" attribute containing the base64-encoded   "server-final-message", concluding the authentication exchange.   The client then authenticates the server by computing the   ServerSignature and comparing it to the value sent by the server.  If   the two are different, the client MUST consider the authentication   exchange to be unsuccessful, and it might have to drop the   connection.5.1.  One Round-Trip Reauthentication   If the server supports SCRAM reauthentication, the server sends in   its initial HTTP response a WWW-Authenticate header field containing   the "realm" attribute (as defined earlier), the "sr" attribute that   contains the server part of the "r" attribute (see s-nonce in   [RFC5802]), and an optional "ttl" attribute (which contains the "sr"   value validity in seconds).   If the client has authenticated to the same realm before (i.e., it   remembers "i" and "s" attributes for the user from earlier   authentication exchanges with the server), it can respond to that   with "client-final-message".  When constructing the "client-final-   message", the client constructs the c-nonce part of the "r" attribute   as on initial authentication and the s-nonce part as follows: s-nonce   is a concatenation of nonce-count and the "sr" attribute (in that   order).  The nonce-count is a positive integer that is equal to the   user's "i" attribute on first reauthentication and is incremented by   1 on each successful reauthentication.Melnikov                      Experimental                     [Page 10]

RFC 7804                       HTTP SCRAM                     March 2016      The purpose of the nonce-count is to allow the server to detect      request replays by maintaining its own copy of this count -- if      the same nonce-count value is seen twice, then the request is a      replay.   If the server considers the s-nonce part of the "nonce" attribute   (the "r" attribute) to still be valid (i.e., the nonce-count part is   as expected (see above) and the "sr" part is still fresh), it will   provide access to the requested resource (assuming the client hash   verifies correctly, of course).  However, if the server considers   that the server part of the nonce is stale (for example, if the "sr"   value is used after the "ttl" seconds), the server returns "401   Unauthorized" containing the SCRAM mechanism name with the following   attributes: a new "sr", "stale=true", and an optional "ttl".  The   "stale" attribute signals to the client that there is no need to ask   the user for the password.      Formally, the "stale" attribute is defined as a flag, indicating      that the previous request from the client was rejected because the      nonce value was stale.  If stale is TRUE (case-insensitive), the      client may wish to simply retry the request with a new encrypted      response without reprompting the user for a new username and      password.  The server should only set stale to TRUE if it receives      a request for which the nonce is invalid but with a valid digest      for that nonce (indicating that the client knows the correct      username/password).  If stale is FALSE or anything other than      TRUE, or the stale directive is not present, the username and/or      password are invalid, and new values must be obtained.   When constructing AuthMessage (seeSection 3) to be used for   calculating client and server proofs, "client-first-message-bare" and   "server-first-message" are reconstructed from data known to the   client and the server.Melnikov                      Experimental                     [Page 11]

RFC 7804                       HTTP SCRAM                     March 2016   Reauthentication can look like this:   C: GET /resource HTTP/1.1   C: Host: server.example.com   C: [...]   S: HTTP/1.1 401 Unauthorized   S: WWW-Authenticate: Digest realm="realm1@example.com",          Digest realm="realm2@example.com",          Digest realm="realm3@example.com",          SCRAM-SHA-256 realm="realm3@example.com",          SCRAM-SHA-256 realm="testrealm@example.com", sr=%hvYDpWUa2RaTC           AfuxFIlj)hNlF          SCRAM-SHA-256 realm="testrealm2@example.com", sr=AAABBBCCCDDD,           ttl=120   S: [...]   [The client authenticates as usual to realm "testrealm@example.com"]   [Some time later, client decides to reauthenticate.   It will use the cached "i" (4096) and "s" (W22ZaJ0SNY7soEsUEjb6gQ==)   from earlier exchanges.  It will use the nonce-value of 4096 together   with the server advertised "sr" value as the server part of the "r".]   C: GET /resource HTTP/1.1   C: Host: server.example.com   C: Authorization: SCRAM-SHA-256 realm="testrealm@example.com",          data=Yz1iaXdzLHI9ck9wck5HZndFYmVSV2diTkVrcU80MDk2JWh2WURwV1VhM           lJhVENBZnV4RklsailoTmxGLHA9ZEh6YlphcFdJazRqVWhOK1V0ZTl5dGFnOX           pqZk1IZ3NxbW1pejdBbmRWUT0K   C: [...]   S: HTTP/1.1 200 Ok   S: Authentication-Info: sid=AAAABBBBCCCCDDDD,          data=dj02cnJpVFJCaTIzV3BSUi93dHVwK21NaFVaVW4vZEI1bkxUSlJzamw5N           Uc0PQo=   S: [...Other header fields and resource body...]6.  Use of the Authentication-Info Header Field with SCRAM   When used with SCRAM, the Authentication-Info header field is allowed   in the trailer of an HTTP message transferred via chunked transfer-   coding.Melnikov                      Experimental                     [Page 12]

RFC 7804                       HTTP SCRAM                     March 20167.  Formal Syntax   The following syntax specification uses the Augmented Backus-Naur   Form (ABNF) notation as specified in [RFC5234].      ALPHA = <as defined inRFC 5234 Appendix B.1>      DIGIT = <as defined inRFC 5234 Appendix B.1>      base64-char     = ALPHA / DIGIT / "/" / "+"      base64-4        = 4base64-char      base64-3        = 3base64-char "="      base64-2        = 2base64-char "=="      base64          = *base64-4 [base64-3 / base64-2]      sr              = "sr=" s-nonce                        ;; s-nonce is defined inRFC 5802.      data            = "data=" base64                        ;; The "data" attribute value is base64-encoded                        ;; SCRAM challenge or response defined in                        ;;RFC 5802.      ttl             = "ttl=" 1*DIGIT                        ;; "sr" value validity in seconds.                        ;; No leading 0s.      reauth-s-nonce  = nonce-count s-nonce      nonce-count     = posit-number                        ;; posit-number is defined inRFC 5802.                        ;; The initial value is taken from the "i"                        ;; attribute for the user and is incremented                        ;; by 1 on each successful reauthentication.      sid             = "sid=" token                        ;; See token definition inRFC 7235.      stale           = "stale=" ( "true" / "false" )      realm           = "realm=" <as defined inRFC 7235>Melnikov                      Experimental                     [Page 13]

RFC 7804                       HTTP SCRAM                     March 20168.  Security Considerations   If the authentication exchange is performed without a strong session   encryption (such as TLS with data confidentiality), then a passive   eavesdropper can gain sufficient information to mount an offline   dictionary or brute-force attack that can be used to recover the   user's password.  The amount of time necessary for this attack   depends on the cryptographic hash function selected, the strength of   the password, and the iteration count supplied by the server.  SCRAM   allows the server/server administrator to increase the iteration   count over time in order to slow down the above attacks.  (Note that   a server that is only in possession of StoredKey and ServerKey can't   automatically increase the iteration count upon successful   authentication.  Such an increase would require resetting the user's   password.)  An external security layer with strong encryption will   prevent these attacks.   If the authentication information is stolen from the authentication   database, then an offline dictionary or brute-force attack can be   used to recover the user's password.  The use of salt mitigates this   attack somewhat by requiring a separate attack on each password.   Authentication mechanisms that protect against this attack are   available (e.g., the Encrypted Key Exchange (EKE) class of   mechanisms).RFC 2945 [RFC2945] is an example of such technology.   If an attacker obtains the authentication information from the   authentication repository and either eavesdrops on one authentication   exchange or impersonates a server, the attacker gains the ability to   impersonate that user to all servers providing SCRAM access using the   same hash function, password, iteration count, and salt.  For this   reason, it is important to use randomly generated salt values.   SCRAM does not negotiate which hash function to use.  Hash function   negotiation is left to the HTTP authentication mechanism negotiation.   It is important that clients be able to sort a locally available list   of mechanisms by preference so that the client may pick the most   preferred of a server's advertised mechanism list.  This preference   order is not specified here as it is a local matter.  The preference   order should include objective and subjective notions of mechanism   cryptographic strength (e.g., SCRAM with SHA-256 should be preferred   over SCRAM with SHA-1).   This document recommends use of SCRAM with SHA-256 hash.  SCRAM-SHA-1   is registered for database compatibility with implementations ofRFC5802 (such as IMAP or XMPP servers) that want to also expose HTTP   access to a related service, but it is not recommended for new   deployments.Melnikov                      Experimental                     [Page 14]

RFC 7804                       HTTP SCRAM                     March 2016   A hostile server can perform a computational denial-of-service attack   on clients by sending a big iteration count value.  In order to   defend against that, a client implementation can pick a maximum   iteration count that it is willing to use and reject any values that   exceed that threshold (in such cases, the client, of course, has to   fail the authentication).   See [RFC4086] for more information about generating randomness.9.  IANA Considerations   New mechanisms in the SCRAM family are registered according to the   IANA procedure specified in [RFC5802].   Note to future "SCRAM-" mechanism designers: Each new "SCRAM-" HTTP   authentication mechanism MUST be explicitly registered with IANA and   MUST comply with "SCRAM-" mechanism naming convention defined inSection 4 of this document.   IANA has added the following entries to the "HTTP Authentication   Schemes" registry defined in HTTP/1.1, Part 7 [RFC7235]:      Authentication Scheme Name: SCRAM-SHA-256      Pointer to specification text:RFC 7804      Notes (optional): (none)      Authentication Scheme Name: SCRAM-SHA-1      Pointer to specification text:RFC 7804      Notes (optional): (none)10.  Design Motivations   The following design goals shaped this document.  Note that some of   the goals have changed since the initial draft version of the   document.   o  The HTTP authentication mechanism has all modern features: support      for internationalized usernames and passwords.   o  The protocol supports mutual authentication.   o  The authentication information stored in the authentication      database is not sufficient by itself to impersonate the client.   o  The server does not gain the ability to impersonate the client to      other servers (with an exception for server-authorized proxies),      unless such other servers allow SCRAM authentication and use the      same salt and iteration count for the user.Melnikov                      Experimental                     [Page 15]

RFC 7804                       HTTP SCRAM                     March 2016   o  The mechanism is extensible, but (hopefully) not over-engineered      in this respect.   o  The mechanism is easier to implement than HTTP Digest in both      clients and servers.   o  The protocol supports 1 round-trip reauthentication.11.  References11.1.  Normative References   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-              Hashing for Message Authentication",RFC 2104,              DOI 10.17487/RFC2104, February 1997,              <http://www.rfc-editor.org/info/rfc2104>.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <http://www.rfc-editor.org/info/rfc2119>.   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO              10646", STD 63,RFC 3629, DOI 10.17487/RFC3629, November              2003, <http://www.rfc-editor.org/info/rfc3629>.   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data              Encodings",RFC 4648, DOI 10.17487/RFC4648, October 2006,              <http://www.rfc-editor.org/info/rfc4648>.   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax              Specifications: ABNF", STD 68,RFC 5234,              DOI 10.17487/RFC5234, January 2008,              <http://www.rfc-editor.org/info/rfc5234>.   [RFC5802]  Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,              "Salted Challenge Response Authentication Mechanism              (SCRAM) SASL and GSS-API Mechanisms",RFC 5802,              DOI 10.17487/RFC5802, July 2010,              <http://www.rfc-editor.org/info/rfc5802>.   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms              (SHA and SHA-based HMAC and HKDF)",RFC 6234,              DOI 10.17487/RFC6234, May 2011,              <http://www.rfc-editor.org/info/rfc6234>.Melnikov                      Experimental                     [Page 16]

RFC 7804                       HTTP SCRAM                     March 2016   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer              Protocol (HTTP/1.1): Authentication",RFC 7235,              DOI 10.17487/RFC7235, June 2014,              <http://www.rfc-editor.org/info/rfc7235>.   [RFC7613]  Saint-Andre, P. and A. Melnikov, "Preparation,              Enforcement, and Comparison of Internationalized Strings              Representing Usernames and Passwords",RFC 7613,              DOI 10.17487/RFC7613, August 2015,              <http://www.rfc-editor.org/info/rfc7613>.   [RFC7615]  Reschke, J., "HTTP Authentication-Info and Proxy-              Authentication-Info Response Header Fields",RFC 7615,              DOI 10.17487/RFC7615, September 2015,              <http://www.rfc-editor.org/info/rfc7615>.   [RFC7677]  Hansen, T., "SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple              Authentication and Security Layer (SASL) Mechanisms",RFC 7677, DOI 10.17487/RFC7677, November 2015,              <http://www.rfc-editor.org/info/rfc7677>.11.2.  Informative References   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,              "Remote Authentication Dial In User Service (RADIUS)",RFC 2865, DOI 10.17487/RFC2865, June 2000,              <http://www.rfc-editor.org/info/rfc2865>.   [RFC2898]  Kaliski, B., "PKCS #5: Password-Based Cryptography              Specification Version 2.0",RFC 2898,              DOI 10.17487/RFC2898, September 2000,              <http://www.rfc-editor.org/info/rfc2898>.   [RFC2945]  Wu, T., "The SRP Authentication and Key Exchange System",RFC 2945, DOI 10.17487/RFC2945, September 2000,              <http://www.rfc-editor.org/info/rfc2945>.   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,              "Randomness Requirements for Security",BCP 106,RFC 4086,              DOI 10.17487/RFC4086, June 2005,              <http://www.rfc-editor.org/info/rfc4086>.   [RFC4510]  Zeilenga, K., Ed., "Lightweight Directory Access Protocol              (LDAP): Technical Specification Road Map",RFC 4510,              DOI 10.17487/RFC4510, June 2006,              <http://www.rfc-editor.org/info/rfc4510>.Melnikov                      Experimental                     [Page 17]

RFC 7804                       HTTP SCRAM                     March 2016   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",              FYI 36,RFC 4949, DOI 10.17487/RFC4949, August 2007,              <http://www.rfc-editor.org/info/rfc4949>.   [RFC6331]  Melnikov, A., "Moving DIGEST-MD5 to Historic",RFC 6331,              DOI 10.17487/RFC6331, July 2011,              <http://www.rfc-editor.org/info/rfc6331>.   [Unicode-UAX15]              The Unicode Consortium, "Unicode Standard Annex #15:              Unicode Normalization Forms", June 2015,              <http://www.unicode.org/reports/tr15/>.Acknowledgements   This document benefited from discussions on the mailing lists for the   HTTPAuth, SASL, and Kitten working groups.  The author would like to   specially thank the co-authors of [RFC5802] from which lots of text   was copied.   Thank you to Martin Thomson for the idea of adding the "ttl"   attribute.   Thank you to Julian F. Reschke for corrections regarding use of the   Authentication-Info header field.   A special thank you to Tony Hansen for doing an early implementation   and providing extensive comments on the document.   Thank you to Russ Housley, Stephen Farrell, Barry Leiba, and Tim   Chown for doing detailed reviews of the document.Author's Address   Alexey Melnikov   Isode Ltd   Email: Alexey.Melnikov@isode.comMelnikov                      Experimental                     [Page 18]