АBOBOА цл€dHHHHHHа цлџџH цл(Р"S   цлИџ-ZџџџPerry The CynicˆfџџBLOM!`-”,џxHHи(џсџтљFG(ќHHи(d'`џџ црP†l €/РŒ† цл8 цхМ ця4†Œ цфА ю8”  цл0[фџџџџџџџџ\Рё[DSETЎ"S"р "S 4h цл цл цл цл ццPдˆ цхЄ';B Рдˆ цх .ƒЛдˆ цхœ"п!pдˆ цхxm  А&+ќ‘ 4^Цл‘ђ V‘ h ` ‘ д‘5§“m“Ÿ““I“p‘ˆы““X“И“ђ‘”v“?“Ћ“Y“b“ъ“P”Y‘d‘5Z:]ф‘я!o!p•!v–!ф—"S—р’ Б&  f Ђ‰  п Їњ  ќ˜   Ы Ђб  S ™^ š_  ƒ ™Š  ™ ™Ї  У ™Ю  д ›ы  5 ™<  л˜ђ  a š b  V˜ h  Њ › Ж š З  • › Є   ›  ) ›8  V ›e  ˜5  § Ј œ7 = œm Јt œŸ ЈЌ œ Ј  œI ЈS œp˜ˆ  А ›Ч  ы Јђ œ Ј œX Јa œИ ЈС œч ё œђ˜ žv Ј~ œ? ЈI œЋ ЈД œY Јc œb Јh œц Ўч œъ Јэ œP žY˜d  Ј ™З  ˜5  n ™Ž  ф˜я Й 2 К : Й ѓ К ќ Й!n  !pŸ!v  !ф Ё€^Tд azд& Жˆдцˆдnћ цкќћэ цкјшЙ цкє ЁЙ цк№ZЬ цкь&E цкшk* цкф•~ цкр цц\" ццX#1 ццTћACLs in CDSA (What The Standard Says) An ACL is an Access Control List. The CDSA 2 standard uses them to control access to databases, keys, and smart cards. We plan to base substantially all our access control on ACLs, directly or indirectly. That means theyеre a very important part of our design. This document describes, to the best of our understanding, how ACLs work in the CDSA framework. It doesnеt say how we are going to use them - for that, see the companion document, Access Control Architecture. ACLs in CDSA In CDSA, each database, key, and smart card login has an ACL associated with it. This is not optional; each such item has an ACL. The internal representation and implementation of these ACLs is opaque to the user of CDSA, and is determined by the plugin module that controls the protected resource (not by CSSM itself). As a result, each such plugin module may pick its own implementation of ACLs, as long as it supports the API interface to their control. In particular, each plugin module will imэplement only a subset of the available features of ACLs. An ACL consists of one or more ACL Entries. Each entry associates a potential subject with a set of authorizations (rights). Essentially, if a caller to a CSSM call matches (satisfies) an entryеs subject conditions, then he is granted the associated rights. An ACL may have any number of entries. When a caller invokes an API function on a controlled CSSM object, he may pass along a set of access credentials as a CSSM_ACCESS_CREDENTIALS structure. This structure may contain a set of certificates and a set of samples. Conceptually, the plugin module tries to match this data against the ACL entries in the protected objectеs ACL. Every entry whose subject matches these credentials confers the associated access rights, and if the collected rights are sufficient for the access requested, the module grants access and proceeds with the call. Otherwise, the call is rejected as having insufficient permissions. In simple terms, you can think of ЙACL entry subjects as вlocksг and of credential samples as вkeysг - if the key presented matches the lock on an entry, it вunlocksг and gives you the access rights of the entry. Donеt take this analogy too far, but itеs a good first approach. The ACL Security Model Be sure to understand who keeps what private from who. ACLs are stored and managed by a CSP or DL plugin module . The module is responsible for keeping and protecting its ACLs from the user and any other modules it doesnеt trust. ACLs are not вself-defendingг - they rely completely on the managing module for protection from alteration and disclosure. How a plugin module accomplishes this is not specified by the standard - it could be by hardware (smart card), software integrity (digital signatures), operating system features (address space separation and file permissions), or any other convenient means. Manipulating ACLs You can retrieve the contents of one or more ACL entries Йwith the CSSM_Get*Acl API functions. These functions translate the internal ACL format (which is specific to the specific plugin module) into CSSM_ACL_ENTRY_INFO structures and return them. Actually, only the вpublicг part of the subjects is returned. This includes the type of subject and nonsensitive information, but it does not (for example) include any cleartext password contained in the entry. There is in fact no way to get the вprivateг part of an ACL slot through the CSSM interfaces. The CSSM_Change*Acl API functions add an entry to an objectеs ACL. To modify or delete an existing ACL entry, you need to call CSSM_Get*Acl followed by CSSM_Change*Acl. The get operation returns a CSSM_ACL_HANDLE for the entry that you then pass back to the change operation. Obviously, you can also delete an existing slot and add a new one, but there is no way to make such a sequence atomic. ACL Entries - The вLockг When you ask for the contents of an AЬCL entry, you get the following components. Remember that how this is stored (and how much is supported) depends on the plugin module that holds the ACLеs object. Tag: an optional ASCII string identifying the entry. Tags do not need to be unique among the entries of an ACL. Subject: describes what or who may use this entry Authorization: an array of rights (capabilities) conveyed to those callers that match the subject Time Range: a start/end time range for the validity of the entire entry Delegation: donеt worry about this yet Credentials - The вKeyг Access credentials are described in the CSSM_ACCESS_CREDENTIALS structure. This structure contains Samples: An array of sample values Callback: A means to generate samples on demand during access evaluation. Entry Tag: If that string is not empty, only ACL entries with this (tag) name are considered. This can be used to request a particular ACL entry match (or set of matches with the same tag), which is importaEnt if evaluating a subject match has side effects (e.g., user interaction or the use of one-time objects, or just plain performance degradation). BaseCerts: Ignore this for now, it is used for delegation Types of Subjects So what kind of subjects, and corresponding samples, can we use? Hereеs a quick rundown of the less obscure ones: Password subjects are matched through the presentation of a secret (password). The password can be stored in the ACL entry itself, or it can be verified through a protected path (e.g. a login server). Login name subjects match a (string) name established somehow, e.g. as the name of the current (UNIX) user. Biometric subjects match by matching a biometric sample of some sort. Again, this can be passed in or obtained through a protected path (say, a hardware fingerprint reader). Public Key subject*s match by the caller proving that he knows a private key. The plugin presents the caller with a random number (вnonceг) and requires it to sign it with its private key. The public key stored in the ACL entry is then used to verify the signature. Hashed subjects contain a cryptographic hash of a sample. This has the advantage that no cleartext secret is stored in the ACL entry . PAM name subjects use the Pluggable Authorization Module system (if available) to validate a subject. There is also a way to make a subject that contains n вsub-subjectsг and requires that k of them be matched by a caller. This can be used to implement an вandг function (n=k), an вorг function (k=1), and the sometimes useful in-between case of вmajority matching.г ACL Owners Actually, each ACL has a single additional entry ~that is called the owner ACL entry. The owner ACL entry controls access to the ACL itself. When a controlled object is created, the creator specifies an ACL entry that becomes both the owner and the (single) initial entry of the ACL itself. The owner (that is, someone who matches the subject of the owner ACL entry) is allowed to change the ACL (including the owner entry itself). Authorization Computation Modules The ACL mechanism described above allows a plugin module to guard data it controls against access by (CSSM) user code. User code can use these mechanisms to protect its own resources from other user code by handing them to a (trusted) plugin module and establishing ACLs to control their use. Sometimes, however, user code wants to keep resources under its own control, either because it doesnеt trust a plugin, or because the resource is not one that CSSM understands (say, a file descriptor or network connection). There is a special type of plugin module, called an Authorization Computation module, that can perform ACL-like security evaluations for its caller. An AC module implements a formalized matching process much like what we described above for ACLs, but it does it on publicly exposed (to its caller) data structures. AC modules do not make security judgments; they merely perform computations for their callers. Thus, they do not protect anyone from anyone, though they still must be trusted1 not to (intentionally?) miscalculate a security outcome. In principle, a plugin module can use an AC module to evaluate its ACLs, but this is not required (and not necessarily a good choice). Delegation Iеll write this section once we tackle this issue. Briefly, if the delegate flag in an ACL entry is set, then someone authorized to use this entry can wrap up an вaccess tokenг in certificate form and give it to someone else, who then passes it in through the BaseCerts field of the authorization structure. At least thatеs how I think it works... but it needs some serious thinking. Q & A How can I have a plaintext password in an ACL entry, if thereеs a call (CSSM_Get*Acl) to retrieve it at will? The CSSM_Get*Acl call only returns the вpublicг part of the ACL entry. The plaintext password is not included. ZџџNдDSETT]]`] ццD ццL ццHд цх”Ѓ Є Ѕ ^ цц@^ An ACL can have zero entries, but that means that nobody can access the object it protects.DSETTА`> цц4 цц< цц8д& цх&В Д Г ? цц0? According to the standard, only CSP and DL plugins have ACL-protected objects under their control. Presumably a different kind of plugin module could provide passthrough (non-standard) interfaces that use ACLs. Note specifically that AC modules, perhaps against expectations, do not manage, control, or contain ACLs.DSETTYY`Y цц$ цц, цц(д цхˆЊ Ж З Z цц Z The в*г stands for Key, Db, or Login, for keys, databases, and CSP logins respectively.DSETTf`f цц цц ццд цд Ћ ­ Ќ g ццg Part of the specification of this subject type is missing in the CDSA standard. 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