Proxy Authentication

I m writing a client to authenticate a proxy  these r the headers 

Server response :-

HTTP/1.1 407 Proxy authentication required

Proxy-Authenticate: Basic realm="www.yahoo.com"

Content-Length: 503

Content-Type: text/html

Client Response  :-

GET http://www.yahoo.com/ HTTP/1.0

Accept: */*

Accept-Language: en-us

Accept-Encoding: gzip,deflate

User-Agent: Mozilla/4.0 (compatible; MSIE 5.0; Windows 98; DigExt)

Host: www.yahoo.com

Proxy-Connection: Keep-Alive

Proxy-Authorization: Basic bWFpbHNjYW46bWFpbHNjYW4=              but it is giving a response : 407 Access  denied

Can any one tell me where it is going wrong ,the password is correct 


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