An Extension to HTTP : Digest Access Authentication
Author(s): P. Hallam-Baker, L. Stewart, A. Luotonen, J. Hostetler, E. Sink, P. Leach, J. Franks
replaced by: RFC 2617
The protocol referred to as 'HTTP/1.0' includes the specification for a Basic Access Authentication scheme. This scheme is not considered to be a secure method of user authentication, as the user name and password are passed over the network...
Network Working Group J. Franks Request for Comments: 2069 Northwestern University Category: Standards Track P. Hallam-Baker CERN J. Hostetler Spyglass, Inc. P. Leach Microsoft Corporation A. Luotonen Netscape Communications Corporation E. Sink Spyglass, Inc. L. Stewart Open Market, Inc. January 1997 An Extension to HTTP : Digest Access Authentication Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Abstract The protocol referred to as "HTTP/1.0" includes the specification for a Basic Access Authentication scheme. This scheme is not considered to be a secure method of user authentication, as the user name and password are passed over the network as clear text. A specification for a different authentication scheme is needed to address this severe limitation. This document provides specification for such a scheme, referred to as "Digest Access Authentication". Like Basic, Digest access authentication verifies that both parties to a communication know a shared secret (a password); unlike Basic, this verification can be done without sending the password in the clear, which is Basic's biggest weakness. As with most other authentication protocols, the greatest sources of risks are usually found not in the core protocol itself but in policies and procedures surrounding its use. Franks, et. al. Standards Track [Page 1] RFC 2069 Digest Access Authentication January 1997 Table of Contents INTRODUCTION...................................................... 2 1.1 PURPOSE .................................................... 2 1.2 OVERALL OPERATION .......................................... 3 1.3 REPRESENTATION OF DIGEST VALUES ............................ 3 1.4 LIMITATIONS ................................................ 3 2. DIGEST ACCESS AUTHENTICATION SCHEME............................ 3 2.1 SPECIFICATION OF DIGEST HEADERS ............................. 3 2.1.1 THE WWW-AUTHENTICATE RESPONSE HEADER ..................... 4 2.1.2 THE AUTHORIZATION REQUEST HEADER ......................... 6 2.1.3 THE AUTHENTICATION-INFO HEADER ........................... 9 2.2 DIGEST OPERATION ............................................ 10 2.3 SECURITY PROTOCOL NEGOTIATION ............................... 10 2.4 EXAMPLE ..................................................... 11 2.5 PROXY-AUTHENTICATION AND PROXY-AUTHORIZATION ................ 11 3. SECURITY CONSIDERATIONS........................................ 12 3.1 COMPARISON WITH BASIC AUTHENTICATION ........................ 13 3.2 REPLAY ATTACKS .............................................. 13 3.3 MAN IN THE MIDDLE ........................................... 14 3.4 SPOOFING BY COUNTERFEIT SERVERS ............................. 15 3.5 STORING PASSWORDS ........................................... 15 3.6 SUMMARY ..................................................... 16 4. ACKNOWLEDGMENTS............................................... 16 5. REFERENCES..................................................... 16 6. AUTHORS' ADDRESSES............................................. 17 Introduction 1.1 Purpose The protocol referred to as "HTTP/1.0" includes specification for a Basic Access Authentication scheme. This scheme is not considered to be a secure method of user authentication, as the user name and password are passed over the network in an unencrypted form. A specification for a new authentication scheme is needed for future versions of the HTTP protocol. This document provides specification for such a scheme, referred to as "Digest Access Authentication". The Digest Access Authentication scheme is not intended to be a complete answer to the need for security in the World Wide Web. This scheme provides no encryption of object content. The intent is simply to create a weak access authentication method which avoids the most serious flaws of Basic authentication. It is proposed that this access authentication scheme be included in the proposed HTTP/1.1 specification. Franks, et. al. Standards Track [Page 2] RFC 2069 Digest Access Authentication January 1997 1.2 Overall Operation Like Basic Access Authentication, the Digest scheme is based on a simple challenge-response paradigm. The Digest scheme challenges using a nonce value. A valid response contains a checksum (by default the MD5 checksum) of the username, the password, the given nonce value, the HTTP method, and the requested URI. In this way, the password is never sent in the clear. Just as with the Basic scheme, the username and password must be prearranged in some fashion which is not addressed by this document. 1.3 Representation of digest values An optional header allows the server to specify the algorithm used to create the checksum or digest. By default the MD5 algorithm is used and that is the only algorithm described in this document. For the purposes of this document, an MD5 digest of 128 bits is represented as 32 ASCII printable characters. The bits in the 128 bit digest are converted from most significant to least significant bit, four bits at a time to their ASCII presentation as follows. Each four bits is represented by its familiar hexadecimal notation from the characters 0123456789abcdef. That is, binary 0000 gets represented by the character '0', 0001, by '1', and so on up to the representation of 1111 as 'f'. 1.4 Limitations The digest authentication scheme described in this document suffers from many known limitations. It is intended as a replacement for basic authentication and nothing more. It is a password-based system and (on the server side) suffers from all the same problems of any password system. In particular, no provision is made in this protocol for the initial secure arrangement between user and server to establish the user's password. Users and implementors should be aware that this protocol is not as secure as kerberos, and not as secure as any client-side private-key scheme. Nevertheless it is better than nothing, better than what is commonly used with telnet and ftp, and better than Basic authentication. 2. Digest Access Authentication Scheme 2.1 Specification of Digest Headers The Digest Access Authentication scheme is conceptually similar to the Basic scheme. The formats of the modified WWW-Authenticate Franks, et. al. Standards Track [Page 3] RFC 2069 Digest Access Authentication January 1997 header line and the Authorization header line are specified below, using the extended BNF defined in the HTTP/1.1 specification, section 2.1. In addition, a new header, Authentication-info, is specified. 2.1.1 The WWW-Authenticate Response Header If a server receives a request for an access-protected object, and an acceptable Authorization header is not sent, the server responds with a "401 Unauthorized" status code, and a WWW-Authenticate header, which is defined as follows: WWW-Authenticate = "WWW-Authenticate" ":" "Digest" digest-challenge digest-challenge = 1#( realm | [ domain ] | nonce | [ digest-opaque ] |[ stale ] | [ algorithm ] ) realm = "realm" "=" realm-value realm-value = quoted-string domain = "domain" "=" <"> 1#URI <"> nonce = "nonce" "=" nonce-value nonce-value = quoted-string opaque = "opaque" "=" quoted-string stale = "stale" "=" ( "true" | "false" ) algorithm = "algorithm" "=" ( "MD5" | token ) The meanings of the values of the parameters used above are as follows: realm A string to be displayed to users so they know which username and password to use. This string should contain at least the name of the host performing the authentication and might additionally indicate the collection of users who might have access. An example might be "email@example.com". The realm is a "quoted-string" as specified in section 2.2 of the HTTP/1.1 specification . domain A comma-separated list of URIs, as specified for HTTP/1.0. The intent is that the client could use this information to know the set of URIs for which the same authentication information should be sent. The URIs in this list may exist on different servers. If this keyword is omitted or empty, the client should assume that the domain consists of all URIs on the responding server. Franks, et. al. Standards Track [Page 4] RFC 2069 Digest Access Authentication January 1997 nonce A server-specified data string which may be uniquely generated each time a 401 response is made. It is recommended that this string be base64 or hexadecimal data. Specifically, since the string is passed in the header lines as a quoted string, the double-quote character is not allowed. The contents of the nonce are implementation dependent. The quality of the implementation depends on a good choice. A recommended nonce would include H(client-IP ":" time-stamp ":" private-key ) Where client-IP is the dotted quad IP address of the client making the request, time-stamp is a server-generated time value, private- key is data known only to the server. With a nonce of this form a server would normally recalculate the nonce after receiving the client authentication header and reject the request if it did not match the nonce from that header. In this way the server can limit the reuse of a nonce to the IP address to which it was issued and limit the time of the nonce's validity. Further discussion of the rationale for nonce construction is in section 3.2 below. An implementation might choose not to accept a previously used nonce or a previously used digest to protect against a replay attack. Or, an implementation might choose to use one-time nonces or digests for POST or PUT requests and a time-stamp for GET requests. For more details on the issues involved see section 3. of this document. The nonce is opaque to the client. opaque A string of data, specified by the server, which should be returned by the client unchanged. It is recommended that this string be base64 or hexadecimal data. This field is a "quoted-string" as specified in section 2.2 of the HTTP/1.1 specification . stale A flag, indicating that the previous request from the client was rejected because the nonce value was stale. If stale is TRUE (in upper or lower case), 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). Franks, et. al. Standards Track [Page 5] RFC 2069 Digest Access Authentication January 1997 algorithm A string indicating a pair of algorithms used to produce the digest and a checksum. If this not present it is assumed to be "MD5". In this document the string obtained by applying the digest algorithm to the data "data" with secret "secret" will be denoted by KD(secret, data), and the string obtained by applying the checksum algorithm to the data "data" will be denoted H(data). For the "MD5" algorithm H(data) = MD5(data) and KD(secret, data) = H(concat(secret, ":", data)) i.e., the digest is the MD5 of the secret concatenated with a colon concatenated with the data. 2.1.2 The Authorization Request Header The client is expected to retry the request, passing an Authorization header line, which is defined as follows. Authorization = "Authorization" ":" "Digest" digest-response digest-response = 1#( username | realm | nonce | digest-uri | response | [ digest ] | [ algorithm ] | opaque ) username = "username" "=" username-value username-value = quoted-string digest-uri = "uri" "=" digest-uri-value digest-uri-value = request-uri ; As specified by HTTP/1.1 response = "response" "=" response-digest digest = "digest" "=" entity-digest response-digest = <"> *LHEX <"> entity-digest = <"> *LHEX <"> LHEX = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" | "a" | "b" | "c" | "d" | "e" | "f" The definitions of response-digest and entity-digest above indicate the encoding for their values. The following definitions show how the value is computed: Franks, et. al. Standards Track [Page 6] RFC 2069 Digest Access Authentication January 1997 response-digest = <"> < KD ( H(A1), unquoted nonce-value ":" H(A2) > <"> A1 = unquoted username-value ":" unquoted realm-value ":" password password = < user's password > A2 = Method ":" digest-uri-value The "username-value" field is a "quoted-string" as specified in section 2.2 of the HTTP/1.1 specification . However, the surrounding quotation marks are removed in forming the string A1. Thus if the Authorization header includes the fields username="Mufasa", realm="firstname.lastname@example.org" and the user Mufasa has password "CircleOfLife" then H(A1) would be H(Mufasa:email@example.com:CircleOfLife) with no quotation marks in the digested string. No white space is allowed in any of the strings to which the digest function H() is applied unless that white space exists in the quoted strings or entity body whose contents make up the string to be digested. For example, the string A1 in the illustrated above must be Mufasa:firstname.lastname@example.org:CircleOfLife with no white space on either side of the colons. Likewise, the other strings digested by H() must not have white space on either side of the colons which delimit their fields unless that white space was in the quoted strings or entity body being digested. "Method" is the HTTP request method as specified in section 5.1 of . The "request-uri" value is the Request-URI from the request line as specified in section 5.1 of . This may be "*", an "absoluteURL" or an "abs_path" as specified in section 5.1.2 of , but it MUST agree with the Request-URI. In particular, it MUST be an "absoluteURL" if the Request-URI is an "absoluteURL". The authenticating server must assure that the document designated by the "uri" parameter is the same as the document served. The purpose of duplicating information from the request URL in this field is to deal with the possibility that an intermediate proxy may alter the client's request. This altered (but presumably semantically equivalent) request would not result in the same digest as that calculated by the client. The optional "digest" field contains a digest of the entity body and some of the associated entity headers. This digest can be useful in both request and response transactions. In a request it can insure the integrity of POST data or data being PUT to the server. In a Franks, et. al. Standards Track [Page 7] RFC 2069 Digest Access Authentication January 1997 response it insures the integrity of the served document. The value of the "digest" field is an <entity-digest> which is defined as follows. entity-digest = <"> KD (H(A1), unquoted nonce-value ":" Method ":" date ":" entity-info ":" H(entity-body)) <"> ; format is <"> *LHEX <"> date = = rfc1123-date ; see section 3.3.1 of  entity-info = H( digest-uri-value ":" media-type ":" ; Content-type, see section 3.7 of  *DIGIT ":" ; Content length, see 10.12 of  content-coding ":" ; Content-encoding, see 3.5 of  last-modified ":" ; last modified date, see 10.25 of  expires ; expiration date; see 10.19 of  ) last-modified = rfc1123-date ; see section 3.3.1 of  expires = rfc1123-date The entity-info elements incorporate the values of the URI used to request the entity as well as the associated entity headers Content- type, Content-length, Content-encoding, Last-modified, and Expires. These headers are all end-to-end headers (see section 13.5.1 of ) which must not be modified by proxy caches. The "entity-body" is as specified by section 10.13 of  or RFC 1864. Note that not all entities will have an associated URI or all of these headers. For example, an entity which is the data of a POST request will typically not have a digest-uri-value or Last-modified or Expires headers. If an entity does not have a digest-uri-value or a header corresponding to one of the entity-info fields, then that field is left empty in the computation of entity-info. All the colons specified above are present, however. For example the value of the entity-info associated with POST data which has content-type "text/plain", no content-encoding and a length of 255 bytes would be H(:text/plain:255:::). Similarly a request may not have a "Date" header. In this case the date field of the entity-digest should be empty. In the entity-info and entity-digest computations, except for the blank after the comma in "rfc1123-date", there must be no white space between "words" and "tspecials", and exactly one blank between "words" (see section 2.2 of ). Franks, et. al. Standards Track [Page 8] RFC 2069 Digest Access Authentication January 1997 Implementors should be aware of how authenticated transactions interact with proxy caches. The HTTP/1.1 protocol specifies that when a shared cache (see section 13.10 of ) has received a request containing an Authorization header and a response from relaying that request, it MUST NOT return that response as a reply to any other request, unless one of two Cache-control (see section 14.9 of ) directives was present in the response. If the original response included the "must-revalidate" Cache-control directive, the cache MAY use the entity of that response in replying to a subsequent request, but MUST first revalidate it with the origin server, using the request headers from the new request to allow the origin server to authenticate the new request. Alternatively, if the original response included the "public" Cache-control directive, the response entity MAY be returned in reply to any subsequent request. 2.1.3 The AuthenticationInfo Header When authentication succeeds, the Server may optionally provide a Authentication-info header indicating that the server wants to communicate some information regarding the successful authentication (such as an entity digest or a new nonce to be used for the next transaction). It has two fields, digest and nextnonce. Both are optional. AuthenticationInfo = "Authentication-info" ":" 1#( digest | nextnonce ) nextnonce = "nextnonce" "=" nonce-value digest = "digest" "=" entity-digest The optional digest allows the client to verify that the body of the response has not been changed en-route. The server would probably only send this when it has the document and can compute it. The server would probably not bother generating this header for CGI output. The value of the "digest" is an <entity-digest> which is computed as described above. The value of the nextnonce parameter is the nonce the server wishes the client to use for the next authentication response. Note that either field is optional. In particular the server may send the Authentication-info header with only the nextnonce field as a means of implementing one-time nonces. If the nextnonce field is present the client is strongly encouraged to use it for the next WWW- Authenticate header. Failure of the client to do so may result in a request to re-authenticate from the server with the "stale=TRUE." Franks, et. al. Standards Track [Page 9] RFC 2069 Digest Access Authentication January 1997 2.2 Digest Operation Upon receiving the Authorization header, the server may check its validity by looking up its known password which corresponds to the submitted username. Then, the server must perform the same MD5 operation performed by the client, and compare the result to the given response-digest. Note that the HTTP server does not actually need to know the user's clear text password. As long as H(A1) is available to the server, the validity of an Authorization header may be verified. A client may remember the username, password and nonce values, so that future requests within the specified <domain> may include the Authorization header preemptively. The server may choose to accept the old Authorization header information, even though the nonce value included might not be fresh. Alternatively, the server could return a 401 response with a new nonce value, causing the client to retry the request. By specifying stale=TRUE with this response, the server hints to the client that the request should be retried with the new nonce, without reprompting the user for a new username and password. The opaque data is useful for transporting state information around. For example, a server could be responsible for authenticating content which actually sits on another server. The first 401 response would include a domain field which includes the URI on the second server, and the opaque field for specifying state information. The client will retry the request, at which time the server may respond with a 301/302 redirection, pointing to the URI on the second server. The client will follow the redirection, and pass the same Authorization header, including the <opaque> data which the second server may require. As with the basic scheme, proxies must be completely transparent in the Digest access authentication scheme. That is, they must forward the WWW-Authenticate, Authentication-info and Authorization headers untouched. If a proxy wants to authenticate a client before a request is forwarded to the server, it can be done using the Proxy- Authenticate and Proxy-Authorization headers described in section 2.5 below. 2.3 Security Protocol Negotiation It is useful for a server to be able to know which security schemes a client is capable of handling. If this proposal is accepted as a required part of the HTTP/1.1 specification, then a server may assume Digest support when a client Franks, et. al. Standards Track [Page 10] RFC 2069 Digest Access Authentication January 1997 identifies itself as HTTP/1.1 compliant. It is possible that a server may want to require Digest as its authentication method, even if the server does not know that the client supports it. A client is encouraged to fail gracefully if the server specifies any authentication scheme it cannot handle. 2.4 Example The following example assumes that an access-protected document is being requested from the server. The URI of the document is "http://www.nowhere.org/dir/index.html". Both client and server know that the username for this document is "Mufasa", and the password is "CircleOfLife". The first time the client requests the document, no Authorization header is sent, so the server responds with: HTTP/1.1 401 Unauthorized WWW-Authenticate: Digest realm="email@example.com", nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093", opaque="5ccc069c403ebaf9f0171e9517f40e41" The client may prompt the user for the username and password, after which it will respond with a new request, including the following Authorization header: Authorization: Digest username="Mufasa", realm="firstname.lastname@example.org", nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093", uri="/dir/index.html", response="e966c932a9242554e42c8ee200cec7f6", opaque="5ccc069c403ebaf9f0171e9517f40e41" 2.5 Proxy-Authentication and Proxy-Authorization The digest authentication scheme may also be used for authenticating users to proxies, proxies to proxies, or proxies to end servers by use of the Proxy-Authenticate and Proxy-Authorization headers. These headers are instances of the general Proxy-Authenticate and Proxy- Authorization headers specified in sections 10.30 and 10.31 of the HTTP/1.1 specification  and their behavior is subject to restrictions described there. The transactions for proxy authentication are very similar to those already described. Upon receiving a request which requires authentication, the proxy/server must issue the "HTTP/1.1 401 Unauthorized" header followed by a "Proxy-Authenticate" header of the form Franks, et. al. Standards Track [Page 11] RFC 2069 Digest Access Authentication January 1997 Proxy-Authentication = "Proxy-Authentication" ":" "Digest" digest-challenge where digest-challenge is as defined above in section 2.1. The client/proxy must then re-issue the request with a Proxy-Authenticate header of the form Proxy-Authorization = "Proxy-Authorization" ":" digest-response where digest-response is as defined above in section 2.1. When authentication succeeds, the Server may optionally provide a Proxy- Authentication-info header of the form Proxy-Authentication-info = "Proxy-Authentication-info" ":" nextnonce where nextnonce has the same semantics as the nextnonce field in the Authentication-info header described above in section 2.1. Note that in principle a client could be asked to authenticate itself to both a proxy and an end-server. It might receive an "HTTP/1.1 401 Unauthorized" header followed by both a WWW-Authenticate and a Proxy-Authenticate header. However, it can never receive more than one Proxy-Authenticate header since such headers are only for immediate connections and must not be passed on by proxies. If the client receives both headers, it must respond with both the Authorization and Proxy-Authorization headers as described above, which will likely involve different combinations of username, password, nonce, etc. 3. Security Considerations Digest Authentication does not provide a strong authentication mechanism. That is not its intent. It is intended solely to replace a much weaker and even more dangerous authentication mechanism: Basic Authentication. An important design constraint is that the new authentication scheme be free of patent and export restrictions. Most needs for secure HTTP transactions cannot be met by Digest Authentication. For those needs SSL or SHTTP are more appropriate protocols. In particular digest authentication cannot be used for any transaction requiring encrypted content. Nevertheless many functions remain for which digest authentication is both useful and appropriate. Franks, et. al. Standards Track [Page 12] RFC 2069 Digest Access Authentication January 1997 3.1 Comparison with Basic Authentication Both Digest and Basic Authentication are very much on the weak end of the security strength spectrum. But a comparison between the two points out the utility, even necessity, of replacing Basic by Digest. The greatest threat to the type of transactions for which these protocols are used is network snooping. This kind of transaction might involve, for example, online access to a database whose use is restricted to paying subscribers. With Basic authentication an eavesdropper can obtain the password of the user. This not only permits him to access anything in the database, but, often worse, will permit access to anything else the user protects with the same password. By contrast, with Digest Authentication the eavesdropper only gets access to the transaction in question and not to the user's password. The information gained by the eavesdropper would permit a replay attack, but only with a request for the same document, and even that might be difficult. 3.2 Replay Attacks A replay attack against digest authentication would usually be pointless for a simple GET request since an eavesdropper would already have seen the only document he could obtain with a replay. This is because the URI of the requested document is digested in the client response and the server will only deliver that document. By contrast under Basic Authentication once the eavesdropper has the user's password, any document protected by that password is open to him. A GET request containing form data could only be "replayed" with the identical data. However, this could be problematic if it caused a CGI script to take some action on the server. Thus, for some purposes, it is necessary to protect against replay attacks. A good digest implementation can do this in various ways. The server created "nonce" value is implementation dependent, but if it contains a digest of the client IP, a time-stamp, and a private server key (as recommended above) then a replay attack is not simple. An attacker must convince the server that the request is coming from a false IP address and must cause the server to deliver the document to an IP address different from the address to which it believes it is sending the document. An attack can only succeed in the period before the time-stamp expires. Digesting the client IP and time- stamp in the nonce permits an implementation which does not maintain state between transactions. Franks, et. al. Standards Track [Page 13] RFC 2069 Digest Access Authentication January 1997 For applications where no possibility of replay attack can be tolerated the server can use one-time response digests which will not be honored for a second use. This requires the overhead of the server remembering which digests have been used until the nonce time-stamp (and hence the digest built with it) has expired, but it effectively protects against replay attacks. Instead of maintaining a list of the values of used digests, a server would hash these values and require re-authentication whenever a hash collision occurs. An implementation must give special attention to the possibility of replay attacks with POST and PUT requests. A successful replay attack could result in counterfeit form data or a counterfeit version of a PUT file. The use of one-time digests or one-time nonces is recommended. It is also recommended that the optional <digest> be implemented for use with POST or PUT requests to assure the integrity of the posted data. Alternatively, a server may choose to allow digest authentication only with GET requests. Responsible server implementors will document the risks described here as they pertain to a given implementation. 3.3 Man in the Middle Both Basic and Digest authentication are vulnerable to "man in the middle" attacks, for example, from a hostile or compromised proxy. Clearly, this would present all the problems of eavesdropping. But it could also offer some additional threats. A simple but effective attack would be to replace the Digest challenge with a Basic challenge, to spoof the client into revealing their password. To protect against this attack, clients should remember if a site has used Digest authentication in the past, and warn the user if the site stops using it. It might also be a good idea for the browser to be configured to demand Digest authentication in general, or from specific sites. Or, a hostile proxy might spoof the client into making a request the attacker wanted rather than one the client wanted. Of course, this is still much harder than a comparable attack against Basic Authentication. There are several attacks on the "digest" field in the Authentication-info header. A simple but effective attack is just to remove the field, so that the client will not be able to use it to detect modifications to the response entity. Sensitive applications may wish to allow configuration to require that the digest field be present when appropriate. More subtly, the attacker can alter any of the entity-headers not incorporated in the computation of the digest, The attacker can alter most of the request headers in the client's Franks, et. al. Standards Track [Page 14] RFC 2069 Digest Access Authentication January 1997 request, and can alter any response header in the origin-server's reply, except those headers whose values are incorporated into the "digest" field. Alteration of Accept* or User-Agent request headers can only result in a denial of service attack that returns content in an unacceptable media type or language. Alteration of cache control headers also can only result in denial of service. Alteration of Host will be detected, if the full URL is in the response-digest. Alteration of Referer or From is not important, as these are only hints. 3.4 Spoofing by Counterfeit Servers Basic Authentication is vulnerable to spoofing by counterfeit servers. If a user can be led to believe that she is connecting to a host containing information protected by a password she knows, when in fact she is connecting to a hostile server, then the hostile server can request a password, store it away for later use, and feign an error. This type of attack is more difficult with Digest Authentication -- but the client must know to demand that Digest authentication be used, perhaps using some of the techniques described above to counter "man-in-the-middle" attacks. 3.5 Storing passwords Digest authentication requires that the authenticating agent (usually the server) store some data derived from the user's name and password in a "password file" associated with a given realm. Normally this might contain pairs consisting of username and H(A1), where H(A1) is the digested value of the username, realm, and password as described above. The security implications of this are that if this password file is compromised, then an attacker gains immediate access to documents on the server using this realm. Unlike, say a standard UNIX password file, this information need not be decrypted in order to access documents in the server realm associated with this file. On the other hand, decryption, or more likely a brute force attack, would be necessary to obtain the user's password. This is the reason that the realm is part of the digested data stored in the password file. It means that if one digest authentication password file is compromised, it does not automatically compromise others with the same username and password (though it does expose them to brute force attack). There are two important security consequences of this. First the password file must be protected as if it contained unencrypted passwords, because for the purpose of accessing documents in its realm, it effectively does. Franks, et. al. Standards Track [Page 15] RFC 2069 Digest Access Authentication January 1997 A second consequence of this is that the realm string should be unique among all realms which any single user is likely to use. In particular a realm string should include the name of the host doing the authentication. The inability of the client to authenticate the server is a weakness of Digest Authentication. 3.6 Summary By modern cryptographic standards Digest Authentication is weak. But for a large range of purposes it is valuable as a replacement for Basic Authentication. It remedies many, but not all, weaknesses of Basic Authentication. Its strength may vary depending on the implementation. In particular the structure of the nonce (which is dependent on the server implementation) may affect the ease of mounting a replay attack. A range of server options is appropriate since, for example, some implementations may be willing to accept the server overhead of one-time nonces or digests to eliminate the possibility of replay while others may satisfied with a nonce like the one recommended above restricted to a single IP address and with a limited lifetime. The bottom line is that *any* compliant implementation will be relatively weak by cryptographic standards, but *any* compliant implementation will be far superior to Basic Authentication. 4. Acknowledgments In addition to the authors, valuable discussion instrumental in creating this document has come from Peter J. Churchyard, Ned Freed, and David M. Kristol. 5. References  Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.  Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext Transfer Protocol -- HTTP/1.1" RFC 2068, January 1997.  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. Franks, et. al. Standards Track [Page 16] RFC 2069 Digest Access Authentication January 1997 6. Authors' Addresses John Franks Professor of Mathematics Department of Mathematics Northwestern University Evanston, IL 60208-2730, USA EMail: email@example.com Phillip M. Hallam-Baker European Union Fellow CERN Geneva Switzerland EMail: firstname.lastname@example.org Jeffery L. Hostetler Senior Software Engineer Spyglass, Inc. 3200 Farber Drive Champaign, IL 61821, USA EMail: email@example.com Paul J. Leach Microsoft Corporation 1 Microsoft Way Redmond, WA 98052, USA EMail: firstname.lastname@example.org Ari Luotonen Member of Technical Staff Netscape Communications Corporation 501 East Middlefield Road Mountain View, CA 94043, USA EMail: email@example.com Franks, et. al. Standards Track [Page 17] RFC 2069 Digest Access Authentication January 1997 Eric W. Sink Senior Software Engineer Spyglass, Inc. 3200 Farber Drive Champaign, IL 61821, USA EMail: firstname.lastname@example.org Lawrence C. Stewart Open Market, Inc. 215 First Street Cambridge, MA 02142, USA EMail: stewart@OpenMarket.com Franks, et. al. Standards Track [Page 18]