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Add getPublicKey method (#17)
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‎README.md‎

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@@ -17,6 +17,7 @@ Additionally, it proposes to add functions for key encapsulation and decapsulati
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`SubtleCrypto.decapsulateKey` and `SubtleCrypto.decapsulateBits`.
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Finally, it proposes to add a `SubtleCrypto.supports` function,
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for improved algorithm support detection.
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for improved algorithm support detection and a `SubtleCrypto.getPublicKey`
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to be able to derive a public key from its corresponding private key.
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See the [draft specification](https://twiss.github.io/webcrypto-modern-algos/).

‎index.html‎

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@@ -134,6 +134,11 @@ <h2>Introduction</h2>
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<a href="#dfn-SubtleCrypto-method-decapsulateKey">`SubtleCrypto.decapsulateKey`</a> and
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<a href="#dfn-SubtleCrypto-method-decapsulateBits">`SubtleCrypto.decapsulateBits`</a>.
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</p>
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<p>
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Additionally, this proposal also introduces the <a href="#dfn-SubtleCrypto-method-getPublicKey">`SubtleCrypto.getPublicKey`</a> function,
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which provides a convenient way to derive a public key from an asymmetric private key,
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eliminating the need to store both the public and private key separately when only the private key is needed.
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</p>
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<p>
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Finally, this proposal aims to make feature detection easier,
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by adding a <a href="#dfn-SubtleCrypto-method-supports">`SubtleCrypto.supports`</a> function,
@@ -187,6 +192,11 @@ <h2>Partial SubtleCrypto interface</h2>
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BufferSource ciphertext
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);
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Promise&lt;CryptoKey> getPublicKey(
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CryptoKey key,
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sequence&lt;KeyUsage> keyUsages
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);
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static boolean supports(DOMString operation,
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AlgorithmIdentifier algorithm,
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optional unsigned long? length = null);
@@ -720,6 +730,128 @@ <h4>The decapsulateBits method</h4>
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</li>
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</ol>
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</section>
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<section id="SubtleCrypto-method-getPublicKey">
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<h4>The getPublicKey method</h4>
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<p>
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The <dfn id="dfn-SubtleCrypto-method-getPublicKey" data-dfn-for=SubtleCrypto data-lt="getPublicKey">getPublicKey(|key|, |usages|)</dfn>
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method returns the public key corresponding to a given private key.
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It behaves as follows:
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</p>
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<ol>
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<li>
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<p>
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Let |key| and |usages| be the `key` and `keyUsages` parameters passed to the
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{{SubtleCrypto/getPublicKey()}} method,
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respectively.
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</p>
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</li>
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<li>
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<p>
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Let |algorithm| be the <a data-cite="webcrypto#dfn-CryptoKey-slot-algorithm">`[[algorithm]]`</a> internal slot of |key|.
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</p>
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</li>
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<li>
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<p>
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If the cryptographic algorithm identified by |algorithm|
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does not support deriving a public key from a private key,
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then return a Promise rejected with a {{NotSupportedError}}.
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</p>
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</li>
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<li>
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<p>
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Let |realm| be the [= relevant realm =] of [= this =].
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</p>
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</li>
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<li>
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<p>
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Let |promise| be a new Promise.
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</p>
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</li>
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<li>
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<p>
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Return |promise| and perform the remaining steps [= in parallel =].
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</p>
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</li>
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<li>
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<p>
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If the following steps or referenced procedures say to
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[= exception/throw =] an error,
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[= queue a global task =] on the <a data-cite="webcrypto#dfn-crypto-task-source">crypto task source</a>,
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given |realm|'s global object,
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to reject |promise| with the returned error;
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and then <a data-cite="webcrypto#terminate-the-algorithm">terminate the algorithm</a>.
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</p>
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</li>
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<li>
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<p>
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If the <a data-cite="webcrypto#dfn-CryptoKey-slot-type">`[[type]]`</a> internal slot of
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|key| is not {{KeyType/"private"}}, then [= exception/throw =] an {{InvalidAccessError}}.
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</p>
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</li>
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<li>
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<p>
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If |usages| contains an entry which is not supported for a public key
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by the algorithm identified by |algorithm|,
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then [= exception/throw =] a {{SyntaxError}}.
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</p>
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</li>
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<li>
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<p>
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Let |publicKey| be a new {{CryptoKey}} representing the public key corresponding to
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the private key represented by the <a data-cite="webcrypto#dfn-CryptoKey-slot-handle">`[[handle]]`</a>
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internal slot of |key|.
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</p>
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</li>
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<li>
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<p>
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If an error occurred, then [= exception/throw =] a {{OperationError}}.
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</p>
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</li>
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<li>
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<p>
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Set the <a data-cite="webcrypto#dfn-CryptoKey-slot-type">`[[type]]`</a> internal slot of
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|publicKey| to {{KeyType/"public"}}.
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</p>
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</li>
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<li>
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<p>
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Set the <a data-cite="webcrypto#dfn-CryptoKey-slot-algorithm">`[[algorithm]]`</a> internal
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slot of |publicKey| to |publicKeyAlgorithm|.
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</p>
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</li>
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<li>
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<p>
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Set the <a data-cite="webcrypto#dfn-CryptoKey-slot-extractable">`[[extractable]]`</a> internal
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slot of |publicKey| to true.
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</p>
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</li>
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<li>
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<p>
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Set the <a data-cite="webcrypto#dfn-CryptoKey-slot-usages">`[[usages]]`</a> internal slot of
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|publicKey| to |usages|.
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</p>
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</li>
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<li>
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<p>
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[= Queue a global task =] on the <a data-cite="webcrypto#dfn-crypto-task-source">crypto task source</a>,
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given |realm|'s global object, to perform the remaining steps.
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</p>
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</li>
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<li>
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<p>
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Let |result| be the result of converting |publicKey| to an ECMAScript Object in |realm|, as defined by [[WebIDL]].
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</p>
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</li>
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<li>
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<p>
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Resolve |promise| with
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|result|.
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</p>
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</li>
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</ol>
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</section>
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<section id="SubtleCrypto-method-supports">
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<h4>The supports method</h4>
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<p>
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"`digest`", "`generateKey`", "`deriveKey`", "`deriveBits`",
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"`importKey`", "`exportKey`", "`wrapKey`", "`unwrapKey`",
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"`encapsulateKey`", "`encapsulateBits`",
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"`decapsulateKey`" or "`decapsulateBits`",
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"`decapsulateKey`", "`decapsulateBits`" or "`getPublicKey`",
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return false.
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</p>
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</li>
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"`digest`", "`generateKey`", "`deriveKey`", "`deriveBits`",
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"`importKey`", "`exportKey`", "`wrapKey`", "`unwrapKey`",
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"`encapsulateKey`", "`encapsulateBits`",
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"`decapsulateKey`" or "`decapsulateBits`",
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"`decapsulateKey`", "`decapsulateBits`" or "`getPublicKey`",
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return false.
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</p>
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</li>
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If |op| is "`decapsulateKey`" or "`decapsulateBits`", set |op| to "`decapsulate`".
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</p>
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</li>
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<li>
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<dl class="switch">
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<dt>If |op| is "`getPublicKey`":</dt>
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<dd>
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<ol>
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<li>
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<p>
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Let |normalizedAlgorithm| be the result of
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<a data-cite="webcrypto#dfn-normalize-an-algorithm">normalizing an algorithm</a>, with
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`alg` set to |alg| and `op` set to "`exportKey`".
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</p>
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</li>
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<li>
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<p>
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If an error occurred, return false.
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</p>
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</li>
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<li>
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<p>
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If the cryptographic algorithm identified by |normalizedAlgorithm|
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does not support deriving a public key from a private key,
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then return false.
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</p>
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</li>
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<li>
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<p>
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Otherwise, return true.
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</p>
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</li>
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</ol>
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</dd>
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</dl>
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</li>
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<li>
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<p>
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Let |normalizedAlgorithm| be the result of

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