fix: Fetch local fingerprint from SDP

Since firefox does not support `getFingerprints`, we fetch the local
fingerprint from the PeerConnection's `localDescription` field by
parsing the fingerprint line from the SDP.

src/sdp.ts

@@ -7,6 +7,10 @@ import type { HashCode, HashName } from 'multihashes'
 import { inappropriateMultiaddr, invalidArgument, invalidFingerprint, unsupportedHashAlgorithm } from './error.js'
 import { CERTHASH_CODE } from './transport.js'
 
+// import { detect } from 'detect-browser'
+
+// const browser = detect()
+
 const log = logger('libp2p:webrtc:sdp')
 
 /**
@@ -15,6 +19,23 @@ const log = logger('libp2p:webrtc:sdp')
 // @ts-expect-error - Not easy to combine these types.
 export const mbdecoder: any = Object.values(bases).map(b => b.decoder).reduce((d, b) => d.or(b))
 
+export function getLocalFingerprint (pc: RTCPeerConnection): string | undefined {
+  const localDescription = pc.localDescription
+  if (localDescription == null) {
+    return undefined
+  }
+  return getFingerprintFromSdp(localDescription.sdp)
+}
+
+const fingerprintRegex = /^a=fingerprint:(?:\w+-[0-9]+)\s(?<fingerprint>(:?[0-9a-fA-F]{2})+)$/gm
+function getFingerprintFromSdp (sdp: string): string | undefined {
+  const searchResult = fingerprintRegex.exec(sdp)
+  if (searchResult == null) {
+    return ''
+  }
+
+  return searchResult.groups?.fingerprint
+}
 /**
  * Get base2 | identity decoders
  */

src/transport.ts

@@ -211,19 +211,12 @@ export class WebRTCDirectTransport implements Transport {
       throw invalidArgument('no local certificate')
     }
 
-    const localCert = pc.getConfiguration().certificates?.at(0)
-
-    if (localCert === undefined || localCert.getFingerprints().length === 0) {
-      throw invalidArgument('no fingerprint on local certificate')
-    }
-
-    const localFingerprint = localCert.getFingerprints()[0]
-
-    if (localFingerprint.value === undefined) {
-      throw invalidArgument('no fingerprint on local certificate')
+    const localFingerprint = sdp.getLocalFingerprint(pc)
+    if (localFingerprint == null) {
+      throw invalidArgument('no local fingerprint found')
     }
 
-    const localFpString = localFingerprint.value.replace(/:/g, '')
+    const localFpString = localFingerprint.toLowerCase().replaceAll(':', '')
     const localFpArray = uint8arrayFromString(localFpString, 'hex')
     const local = multihashes.encode(localFpArray, hashCode)
     const remote: Uint8Array = sdp.mbdecoder.decode(sdp.certhash(ma))