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In cryptography, the Salted Challenge Response Authentication Mechanism (SCRAM) is a family of modern, password-based challenge–response authentication mechanisms providing authentication of a user to a server. As it is specified for Simple Authentication and Security Layer (SASL), it can be used for password-based logins to services like SMTP and IMAP (e-mail), or XMPP (chat). For XMPP, supporting it is mandatory. == Motivation == Alice wants to log into Bob's server. She needs to prove she is who she claims to be. For solving this authentication problem, Alice and Bob have agreed upon a password, which Alice knows, and which Bob knows how to verify. Now Alice could send her password over an unencrypted connection to Bob in a clear text form, for him to verify. That would however make the password accessible to Mallory, who is wiretapping the line. Alice and Bob could try to bypass this by encrypting the connection. However, Alice doesn't know whether the encryption was set up by Bob, and not by Mallory by doing a man-in-the-middle attack. Therefore, Alice sends a hashed version of her password instead, like in CRAM-MD5 or DIGEST-MD5. As it is a hash, Mallory doesn't get the password itself. And because the hash is salted with a challenge, Mallory could use it only for one login process. However, Alice wants to give some confidential information to Bob, and she wants to be sure it's Bob and not Mallory. For solving this, Bob has registered himself to a Certificate Authority (CA), which signed his certificate. Alice could solely rely on that signature system, but she knows it has weaknesses. To give her additional assurance that there is no man-in-the-middle attack, Bob creates a proof that he knows the password (or a salted hash thereof), and includes his certificate into this proof. This inclusion is called channel binding, as the lower encryption channel is 'bound' to the higher application channel. Alice then has an authentication of Bob, and Bob has authentication of Alice. Taken together, they have mutual authentication. DIGEST-MD5 already enabled mutual authentication, but it was often incorrectly implemented. When Mallory runs a man in the middle attack and forges a CA signature, he could retrieve a hash of the password. But he couldn't impersonate Alice even for a single login session, as Alice included into her hash the encryption key of Mallory, resulting in a login-fail from Bob. To make a fully transparent attack, Mallory would need to know the password used by Alice, or the secret encryption key of Bob. Bob has heard of data breaches of server databases, and he decided that he doesn't want to store the passwords of his users in clear text. He has heard of the CRAM-MD5 and DIGEST-MD5 login schemes, but he knows, for offering these login schemes to his users, he would have to store weakly hashed, un-salted passwords. He doesn't like the idea, and therefore he chooses to demand the passwords in plain text. Then he can hash them with secure hashing schemes like bcrypt, scrypt or PBKDF2, and salt them as he wants. However, then Bob and Alice would still face the problems described above. To solve this problem, they use SCRAM, where Bob can store his password in a salted format, using PBKDF2. During login, Bob sends Alice his salt and the iteration count of the PBKDF2 algorithm, and then Alice uses these to calculate the hashed password that Bob has in his database. All further calculations in SCRAM base on this value which both know. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Salted Challenge Response Authentication Mechanism」の詳細全文を読む スポンサード リンク
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