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43 <div class="section" id="design-goals">
44<span id="id1"></span><h1>2. Design goals</h1>
45<div class="section" id="suitable-for-constrained-devices">
46<span id="scalable"></span><h2>2.1. Suitable for constrained devices</h2>
47<p>The interface is suitable for a vast range of devices: from special-purpose
48cryptographic processors that process data with a built-in key, to constrained
49devices running custom application code, such as microcontrollers, and
50multi-application devices, such as servers. Consequentially, the interface is
51scalable and modular.</p>
52<ul class="simple">
53<li><em>Scalable</em>: devices only need to implement the functionality that they will
54use.</li>
55<li><em>Modular</em>: larger devices implement larger subsets of the same interface,
56rather than different interfaces.</li>
57</ul>
58<p>In this interface, all operations on unbounded amounts of data
59allow <em>multi-part</em> processing, as long as the calculations on the data are
60performed in a streaming manner. This means that the application does not need
61to store the whole message in memory at one time. As a result, this
62specification is suitable for very constrained devices, including those where
63memory is very limited.</p>
64<p>Memory outside the keystore boundary is managed by the application. An
65implementation of the interface is not required to retain any state between
66function calls, apart from the content of the keystore and other data that must
67be kept inside the keystore security boundary.</p>
68<p>The interface does not expose the representation of keys and intermediate data,
69except when required for interchange. This allows each implementation to choose
70optimal data representations. Implementations with multiple components are also
71free to choose which memory area to use for internal data.</p>
72</div>
73<div class="section" id="a-keystore-interface">
74<span id="keystore"></span><h2>2.2. A keystore interface</h2>
75<p>The specification allows cryptographic operations to be performed on a key to
76which the application does not have direct access. Except where required for
77interchange, applications access all keys indirectly, by an identifier. The key
78material corresponding to that identifier can reside inside a security boundary
79that prevents it from being extracted, except as permitted by a policy that is
80defined when the key is created.</p>
81</div>
82<div class="section" id="optional-isolation">
83<span id="isolation"></span><h2>2.3. Optional isolation</h2>
84<p>Implementations can isolate the cryptoprocessor from the calling application,
85and can further isolate multiple calling applications. The interface allows the
86implementation to be separated between a frontend and a backend. In an isolated
87implementation, the frontend is the part of the implementation that is located
88in the same isolation boundary as the application, which the application
89accesses by function calls. The backend is the part of the implementation that
90is located in a different environment, which is protected from the frontend.
91Various technologies can provide protection, for example:</p>
92<ul class="simple">
93<li>Process isolation in an operating system.</li>
94<li>Partition isolation, either with a virtual machine or a partition manager.</li>
95<li>Physical separation between devices.</li>
96</ul>
97<p>Communication between the frontend and backend is beyond the scope of this
98specification.</p>
99<p>In an isolated implementation, the backend can serve more than one
100implementation instance. In this case, a single backend communicates with
101multiple instances of the frontend. The backend must enforce <a class="reference internal" href="../about.html#term-caller-isolation"><span class="term">caller isolation</span></a>:
102it must ensure that assets of one frontend are not visible to any
103other frontend. The mechanism for identifying callers is beyond the scope of this
104specification. An implementation that provides caller isolation must document
105the identification mechanism. An implementation that provides caller isolation must
106document any implementation-specific extension of the API that enables frontend
107instances to share data in any form.</p>
108<p>An isolated implementation that only has a single frontend provides <a class="reference internal" href="../about.html#term-cryptoprocessor-isolation"><span class="term">cryptoprocessor isolation</span></a>.</p>
109<p>In summary, there are three types of implementation:</p>
110<ul class="simple">
111<li><a class="reference internal" href="../about.html#term-no-isolation"><span class="term">No isolation</span></a>: there is no security boundary between the application and the
112cryptoprocessor. For example, a statically or dynamically linked library is
113an implementation with no isolation.</li>
114<li><a class="reference internal" href="../about.html#term-cryptoprocessor-isolation"><span class="term">Cryptoprocessor isolation</span></a>: there is a security boundary between the
115application and the cryptoprocessor, but the cryptoprocessor does not
116communicate with other applications. For example, a cryptoprocessor chip that
117is a companion to an application processor is an implementation with
118cryptoprocessor isolation.</li>
119<li><a class="reference internal" href="../about.html#term-caller-isolation"><span class="term">Caller isolation</span></a>: there are multiple application instances, with a security
120boundary between the application instances among themselves, as well as
121between the cryptoprocessor and the application instances. For example, a
122cryptography service in a multiprocess environment is an implementation with
123caller and cryptoprocessor isolation.</li>
124</ul>
125</div>
126<div class="section" id="choice-of-algorithms">
127<span id="algorithm-agility"></span><h2>2.4. Choice of algorithms</h2>
128<p>The specification defines a low-level cryptographic interface, where the caller
129explicitly chooses which algorithm and which security parameters they use. This
130is necessary to implement protocols that are inescapable in various use cases.
131The design of the interface enables applications to implement widely-used
132protocols and data exchange formats, as well as custom ones.</p>
133<p>As a consequence, all cryptographic functionality operates according to the
134precise algorithm specified by the caller. However, this does not apply to
135device-internal functionality, which does not involve any form of
136interoperability, such as random number generation. The specification does not
137include generic higher-level interfaces, where the implementation chooses the
138best algorithm for a purpose. However, higher-level libraries can be built on
139top of the PSA Crypto API.</p>
140<p>Another consequence is that the specification permits the use of algorithms, key
141sizes and other parameters that, while known to be insecure, might be necessary to
142support legacy protocols or legacy data. Where major weaknesses are known, the
143algorithm descriptions give applicable warnings. However, the lack of a warning
144both does not and cannot indicate that an algorithm is secure in all circumstances.
145Application developers need to research the security of the protocols and
146algorithms that they plan to use to determine if these meet their requirements.</p>
147<p>The interface facilitates algorithm agility. As a consequence, cryptographic
148primitives are presented through generic functions with a parameter indicating
149the specific choice of algorithm. For example, there is a single function to
150calculate a message digest, which takes a parameter that identifies the specific
151hash algorithm.</p>
152</div>
153<div class="section" id="ease-of-use">
154<span id="usability"></span><h2>2.5. Ease of use</h2>
155<p>The interface is designed to be as user-friendly as possible, given the
156aforementioned constraints on suitability for various types of devices and on
157the freedom to choose algorithms.</p>
158<p>In particular, the code flows are designed to reduce the risk of dangerous
159misuse. The interface is designed in part to make it harder to misuse. Where
160possible, it is designed so that
161typical mistakes result in test failures, rather than subtle security issues.
162Implementations avoid leaking data when a function is called with invalid
163parameters, to the extent allowed by the C language and by implementation size
164constraints.</p>
165</div>
166<div class="section" id="example-use-cases">
167<h2>2.6. Example use cases</h2>
168<p>This section lists some of the use cases that were considered during the design
169of this API. This list is not exhaustive, nor are all implementations required to
170support all use cases.</p>
171<div class="section" id="network-security-tls">
172<h3>2.6.1. Network Security (TLS)</h3>
173<p>The API provides all of the cryptographic primitives needed to establish TLS
174connections.</p>
175</div>
176<div class="section" id="secure-storage">
177<h3>2.6.2. Secure Storage</h3>
178<p>The API provides all primitives related to storage encryption, block or
179file-based, with master encryption keys stored inside a key store.</p>
180</div>
181<div class="section" id="network-credentials">
182<h3>2.6.3. Network Credentials</h3>
183<p>The API provides network credential management inside a key store, for example,
184for X.509-based authentication or pre-shared keys on enterprise networks.</p>
185</div>
186<div class="section" id="device-pairing">
187<h3>2.6.4. Device Pairing</h3>
188<p>The API provides support for key agreement protocols that are often used for
189secure pairing of devices over wireless channels. For example, the pairing of an
190NFC token or a Bluetooth device might use key agreement protocols upon
191first use.</p>
192</div>
193<div class="section" id="secure-boot">
194<h3>2.6.5. Secure Boot</h3>
195<p>The API provides primitives for use during firmware integrity and authenticity
196validation, during a secure or trusted boot process.</p>
197</div>
198<div class="section" id="attestation">
199<h3>2.6.6. Attestation</h3>
200<p>The API provides primitives used in attestation activities. Attestation is the
201ability for a device to sign an array of bytes with a device private key and
202return the result to the caller. There are several use cases; ranging from attestation
203of the device state, to the ability to generate a key pair and prove that it has
204been generated inside a secure key store. The API provides access to the
205algorithms commonly used for attestation.</p>
206</div>
207<div class="section" id="factory-provisioning">
208<h3>2.6.7. Factory Provisioning</h3>
209<p>Most IoT devices receive a unique identity during the factory provisioning
210process, or once they have been deployed to the field. This API provides the APIs necessary for
211populating a device with keys that represent that identity.</p>
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222IHI 0086<br/>
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224Version 1.1.0
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227<li class="toctree-l1"><a class="reference internal" href="../about.html">About this document</a></li>
228</ul>
229<ul class="current">
230<li class="toctree-l1"><a class="reference internal" href="intro.html">1. Introduction</a></li>
231<li class="toctree-l1 current"><a class="current reference internal" href="#">2. Design goals</a><ul>
232<li class="toctree-l2"><a class="reference internal" href="#suitable-for-constrained-devices">2.1. Suitable for constrained devices</a></li>
233<li class="toctree-l2"><a class="reference internal" href="#a-keystore-interface">2.2. A keystore interface</a></li>
234<li class="toctree-l2"><a class="reference internal" href="#optional-isolation">2.3. Optional isolation</a></li>
235<li class="toctree-l2"><a class="reference internal" href="#choice-of-algorithms">2.4. Choice of algorithms</a></li>
236<li class="toctree-l2"><a class="reference internal" href="#ease-of-use">2.5. Ease of use</a></li>
237<li class="toctree-l2"><a class="reference internal" href="#example-use-cases">2.6. Example use cases</a><ul>
238<li class="toctree-l3"><a class="reference internal" href="#network-security-tls">2.6.1. Network Security (TLS)</a></li>
239<li class="toctree-l3"><a class="reference internal" href="#secure-storage">2.6.2. Secure Storage</a></li>
240<li class="toctree-l3"><a class="reference internal" href="#network-credentials">2.6.3. Network Credentials</a></li>
241<li class="toctree-l3"><a class="reference internal" href="#device-pairing">2.6.4. Device Pairing</a></li>
242<li class="toctree-l3"><a class="reference internal" href="#secure-boot">2.6.5. Secure Boot</a></li>
243<li class="toctree-l3"><a class="reference internal" href="#attestation">2.6.6. Attestation</a></li>
244<li class="toctree-l3"><a class="reference internal" href="#factory-provisioning">2.6.7. Factory Provisioning</a></li>
245</ul>
246</li>
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248</li>
249<li class="toctree-l1"><a class="reference internal" href="functionality.html">3. Functionality overview</a></li>
250<li class="toctree-l1"><a class="reference internal" href="sample-arch.html">4. Sample architectures</a></li>
251<li class="toctree-l1"><a class="reference internal" href="conventions.html">5. Library conventions</a></li>
252<li class="toctree-l1"><a class="reference internal" href="implementation.html">6. Implementation considerations</a></li>
253<li class="toctree-l1"><a class="reference internal" href="usage.html">7. Usage considerations</a></li>
254<li class="toctree-l1"><a class="reference internal" href="../api/library/index.html">8. Library management reference</a></li>
255<li class="toctree-l1"><a class="reference internal" href="../api/keys/index.html">9. Key management reference</a></li>
256<li class="toctree-l1"><a class="reference internal" href="../api/ops/index.html">10. Cryptographic operation reference</a></li>
257</ul>
258<ul>
259<li class="toctree-l1"><a class="reference internal" href="../appendix/example_header.html">Example header file</a></li>
260<li class="toctree-l1"><a class="reference internal" href="../appendix/encodings.html">Algorithm and key type encoding</a></li>
261<li class="toctree-l1"><a class="reference internal" href="../appendix/specdef_values.html">Example macro implementations</a></li>
262<li class="toctree-l1"><a class="reference internal" href="../appendix/sra.html">Security Risk Assessment</a></li>
263<li class="toctree-l1"><a class="reference internal" href="../appendix/history.html">Changes to the API</a></li>
264</ul>
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