in src/SigV4RequestSigner.ts [129:134]
private async getSignatureKey(dateString: string): Promise<ArrayBuffer> {
const kDate = await SigV4RequestSigner.hmac('AWS4' + this.credentials.secretAccessKey, dateString);
const kRegion = await SigV4RequestSigner.hmac(kDate, this.region);
const kService = await SigV4RequestSigner.hmac(kRegion, this.service);
return await SigV4RequestSigner.hmac(kService, 'aws4_request');
}