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packages/security-stack/crypto/crypto-rsa/src/keys/serialize-spki.md

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+# `keys/serialize-spki`
+
+## Purpose
+
+Inverse of `parseRsaPublicKeySpki`. Wraps a serialised PKCS#1 `RSAPublicKey`
+in a DER `SubjectPublicKeyInfo` per RFC 5280 §4.1, with the canonical
+`AlgorithmIdentifier { algorithm = 1.2.840.113549.1.1.1, parameters = NULL }`
+inner SEQUENCE.
+
+## Features
+
+- Standard X.509 SPKI emission.
+- RFC 3279 §2.3.1 conformant `NULL` algorithm parameters.
+- Reuses `serializeRsaPublicKeyPkcs1` so the inner body matches the PKCS#1
+  output bit-for-bit.
+
+## Usage: Basic Example
+
+```ts
+import { serializeRsaPublicKeySpki } from "@suaveplan/crypto-rsa";
+
+const spki = serializeRsaPublicKeySpki(key);
+```
+
+## Usage: Advanced Example
+
+```ts
+import { serializeRsaPublicKeySpki } from "@suaveplan/crypto-rsa";
+
+// Compute the SPKI fingerprint downstream consumers expect for cert pinning.
+const spki = serializeRsaPublicKeySpki(key);
+const fingerprint = await crypto.subtle.digest("SHA-256", spki);
+```
+
+## API Reference
+
+### `serializeRsaPublicKeySpki(key: RsaPublicKey): Uint8Array`
+
+Returns the DER-encoded `SubjectPublicKeyInfo`.
+
+## Implementation Notes
+
+The output is the canonical RFC 5280 SPKI; many PKI libraries accept variants
+with implicit-NULL parameters (omitted entirely) but emit the explicit-NULL
+form, so we follow the de-facto majority. The PKCS#1 body inside the
+`BIT STRING` is wrapped with one leading `0x00` "unused bits" byte per
+RFC 5280 §4.1 (handled by `encodeBitString`).