code: function decryptKey(kms, ciphertextBase64, logger) {
    const params = {CiphertextBlob: new Buffer(ciphertextBase64, 'base64')};
    return decrypt(kms, params, logger).then(result => result.Plaintext && result.Plaintext.toString('utf8'));
  }
docs: Decrypts the given ciphertext in base 64 using the given AWS.KMS instance & returns the decrypted plaintext.
  @param {AWS.KMS} kms - an AWS.KMS instance to use
  @param {string} ciphertextBase64 - the encrypted ciphertext in base 64 encoding
  @param {Logger|console|undefined} [logger] - an optional logger to use (defaults to console if omitted)
  @returns {Promise.<string>} a promise of the decrypted plaintext