WWW-Authenticate

WWW-Authenticate: <challenge>

Where a <challenge> is comprised of an <auth-scheme>, followed by an optional <token68> or a comma-separated list of <auth-params>:

challenge = <auth-scheme> <auth-param>, …, <auth-paramN>
challenge = <auth-scheme> <token68>

For example:

WWW-Authenticate: <auth-scheme>
WWW-Authenticate: <auth-scheme> token68
WWW-Authenticate: <auth-scheme> auth-param1=param-token1
WWW-Authenticate: <auth-scheme> auth-param1=param-token1, …, auth-paramN=param-tokenN

The presence of a token68 or authentication parameters depends on the selected <auth-scheme>. For example, Basic authentication requires a <realm>, and allows for optional use of charset key, but does not support a token68:

WWW-Authenticate: Basic realm="Dev", charset="UTF-8"

Multiple challenges can be sent in a comma-separated list

WWW-Authenticate: <challenge>, …, <challengeN>

Multiple headers can also be sent in a single response:

WWW-Authenticate: <challenge>
WWW-Authenticate: <challengeN>

<auth-scheme>

A case-insensitive token indicating the Authentication scheme used. Some of the more common types are Basic, Digest, Negotiate and AWS4-HMAC-SHA256. IANA maintains a list of authentication schemes, but there are other schemes offered by host services.

<auth-param> Optional

An authentication parameter whose format depends on the <auth-scheme>. <realm> is described below as it's a common authentication parameter among many auth schemes.

<realm> Optional

The string realm followed by = and a quoted string describing a protected area, for example realm="staging environment". A realm allows a server to partition the areas it protects (if supported by a scheme that allows such partitioning). Some clients show this value to the user to inform them about which particular credentials are required — though most browsers stopped doing so to counter phishing. The only reliably supported character set for this value is us-ascii. If no realm is specified, clients often display a formatted hostname instead.

<token68> Optional

A token that may be useful for some schemes. The token allows the 66 unreserved URI characters plus a few others. It can hold a base64, base64url, base32, or base16 (hex) encoding, with or without padding, but excluding whitespace. The token68 alternative to auth-param lists is supported for consistency with legacy authentication schemes.

Generally, you will need to check the relevant specifications for the authentication parameters needed for each <auth-scheme>. The following sections describe token and auth parameters for some common auth schemes.

<realm>

A <realm> as described above. Note that the realm is mandatory for Basic authentication.

charset="UTF-8" Optional

Tells the client the server's preferred encoding scheme when submitting a username and password. The only allowed value is the case-insensitive string UTF-8. This does not relate to the encoding of the realm string.

<realm> Optional

A <realm> as described above indicating which username/password to use. Minimally should include the host name, but might indicate the users or group that have access.

domain Optional

A quoted, space-separated list of URI prefixes that define all the locations where the authentication information may be used. If this key is not specified then the authentication information may be used anywhere on the web root.

nonce

A server-specified quoted string that the server can use to control the lifetime in which particular credentials will be considered valid. This must be uniquely generated each time a 401 response is made, and may be regenerated more often (for example, allowing a digest to be used only once). The specification contains advice on possible algorithms for generating this value. The nonce value is opaque to the client.

opaque

A server-specified quoted string that should be returned unchanged in the Authorization. This is opaque to the client. The server is recommended to include Base64 or hexadecimal data.

stale Optional

A case-insensitive flag indicating that the previous request from the client was rejected because the nonce used is too old (stale). If this is true the request can be retried using the same username/password encrypted using the new nonce. If it is any other value then the username/password are invalid and must be re-requested from the user.

algorithm Optional

A string indicating the algorithm used to produce a digest. Valid non-session values are: MD5 (default if algorithm not specified), SHA-256, SHA-512. Valid session values are: MD5-sess, SHA-256-sess, SHA-512-sess.

qop

Quoted string indicating the quality of protection supported by the server. This must be supplied, and unrecognized options must be ignored.

• "auth": Authentication
• "auth-int": Authentication with integrity protection

charset="UTF-8" Optional

Tells the client the server's preferred encoding scheme when submitting a username and password. The only allowed value is the case-insensitive string "UTF-8".

userhash Optional

A server may specify "true" to indicate that it supports username hashing (default is "false")

<challenge>

A set of pairs in the format of <len>:<value> concatenated together to be given to a client. The challenge is made of up a nonce, algorithm, origin, realm, key identifier, and the challenge.

<max-age>

The number of seconds from the time the HTTP response is emitted for which responses to this challenge can be accepted.

<realm> Optional

As above in the directives section.

Multiple challenges may be specified in a single response header:

HTTP/1.1 401 Unauthorized
WWW-Authenticate: challenge1, …, challengeN

Multiple challenges can be sent in separate WWW-Authenticate headers in the same response:

HTTP/1.1 401 Unauthorized
WWW-Authenticate: challenge1
WWW-Authenticate: challengeN

A server that only supports basic authentication might have a WWW-Authenticate response header which looks like this:

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Basic realm="Staging server", charset="UTF-8"

A user-agent receiving this header would first prompt the user for their username and password, and then re-request the resource with the encoded credentials in the Authorization header. The Authorization header might look like this:

Authorization: Basic YWxhZGRpbjpvcGVuc2VzYW1l

For Basic authentication, the credentials are constructed by first combining the username and the password with a colon (aladdin:opensesame), and then by encoding the resulting string in base64 (YWxhZGRpbjpvcGVuc2VzYW1l).

The client attempts to access a document at URI http://www.example.org/dir/index.html that is protected via digest authentication. The username for this document is "Mufasa" and the password is "Circle of Life" (note the single space between each of the words).

The first time the client requests the document, no Authorization header field is sent. Here the server responds with an HTTP 401 message that includes a challenge for each digest algorithm it supports, in its order of preference (SHA256 and then MD5)

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Digest
    realm="http-auth@example.org",
    qop="auth, auth-int",
    algorithm=SHA-256,
    nonce="7ypf/xlj9XXwfDPEoM4URrv/xwf94BcCAzFZH4GiTo0v",
    opaque="FQhe/qaU925kfnzjCev0ciny7QMkPqMAFRtzCUYo5tdS"
WWW-Authenticate: Digest
    realm="http-auth@example.org",
    qop="auth, auth-int",
    algorithm=MD5,
    nonce="7ypf/xlj9XXwfDPEoM4URrv/xwf94BcCAzFZH4GiTo0v",
    opaque="FQhe/qaU925kfnzjCev0ciny7QMkPqMAFRtzCUYo5tdS"

The client prompts the user for their username and password, and then responds with a new request that encodes the credentials in the Authorization header field. If the client chose the MD5 digest the Authorization header field might look as shown below:

Authorization: Digest username="Mufasa",
    realm="http-auth@example.org",
    uri="/dir/index.html",
    algorithm=MD5,
    nonce="7ypf/xlj9XXwfDPEoM4URrv/xwf94BcCAzFZH4GiTo0v",
    nc=00000001,
    cnonce="f2/wE4q74E6zIJEtWaHKaf5wv/H5QzzpXusqGemxURZJ",
    qop=auth,
    response="8ca523f5e9506fed4657c9700eebdbec",
    opaque="FQhe/qaU925kfnzjCev0ciny7QMkPqMAFRtzCUYo5tdS"

If the client chose the SHA-256 digest the Authorization header field might look as shown below:

Authorization: Digest username="Mufasa",
    realm="http-auth@example.org",
    uri="/dir/index.html",
    algorithm=SHA-256,
    nonce="7ypf/xlj9XXwfDPEoM4URrv/xwf94BcCAzFZH4GiTo0v",
    nc=00000001,
    cnonce="f2/wE4q74E6zIJEtWaHKaf5wv/H5QzzpXusqGemxURZJ",
    qop=auth,
    response="753927fa0e85d155564e2e272a28d1802ca10daf449
        6794697cf8db5856cb6c1",
    opaque="FQhe/qaU925kfnzjCev0ciny7QMkPqMAFRtzCUYo5tdS"

A server that supports HOBA authentication might have a WWW-Authenticate response header which looks like this:

HTTP/1.1 401 Unauthorized
WWW-Authenticate: HOBA max-age="180", challenge="16:MTEyMzEyMzEyMw==1:028:https://www.example.com:80800:3:MTI48:NjgxNDdjOTctNDYxYi00MzEwLWJlOWItNGM3MDcyMzdhYjUz"

The to-be-signed blob challenge is made from these parts: www.example.com using port 8080, the nonce is 1123123123, the algorithm for signing is RSA-SHA256, the key identifier is 123, and finally the challenge is 68147c97-461b-4310-be9b-4c707237ab53.

A client would receive this header, extract the challenge, sign it with their private key that corresponds to key identifier 123 in our example using RSA-SHA256, and then send the result in the Authorization header as a dot-separated key id, challenge, nonce, and signature.

Authorization: 123.16:MTEyMzEyMzEyMw==1:028:https://www.example.com:80800:3:MTI48:NjgxNDdjOTctNDYxYi00MzEwLWJlOWItNGM3MDcyMzdhYjUz.1123123123.<signature-of-challenge>

• HTTP authentication
• Authorization
• Proxy-Authorization
• Proxy-Authenticate
• 401, 403, 407