Application layer encryption

Standard Payments APIs support either PGP or JWE for application layer encryption.

PGP encryption

PGP is a standard set of encryption, decryption, and signing algorithms which provide cryptographic privacy and authentication.

When using PGP to encrypt payloads, partners must support:

  • Encrypting and decrypting payloads with multiple PGP keys.
  • Signing payloads with multiple PGP keys.
  • Verifying a payload with multiple signatures, any one of which can be the signature with the key provided by Google.
  • Decryption of Web-safe base64 encoded payloads.

PGP public keys provided to Google must have a subkey used for encryption. The subkey allows for independent rotation from the master key. The master key is used for identity verification. Private keys must be 2048 (or greater) bit RSA keys that expire in one year with a maximum lifetime of two years.

Before beginning development you need to exchange PGP keys with Google. In this step, you generate a PGP public-private key pair, provide the public key to Google, and receive a public key back from Google. During development, you will only need to exchange sandbox keys used for development and testing outside of production. Before production testing and launch, you will need to perform another exchange of production keys.

Generating a new PGP key

Assuming you have a GPG binary in your system path, you can use the following POSIX command to create a new key pair.

$ gpg --full-generate-key

When prompted, select an RSA key with at least 2048 bits of entropy and an expiration of 1-2 years. This command should create both a master key (labeled SC, for 'S'igning and 'C'ertificate generation) and a subkey (labeled E, for 'E'ncryption).

PGP Library Configuration

Sending Payloads

  1. When signing, you should use SHA384 as the digest algorithm; do not use SHA1or MD5
  2. When encrypting, you should use AES256 as the symmetric encryption algorithm; do not use CAST5 or IDEA
  3. When encrypting or signing messages, be sure to select the sub key with the corresponding purpose; use the CAN_SIGN key for signing and the ENCRYPT_COMMS/ENCRYPT_STORAGE key for encrypting

Receiving Payloads

  1. When verifying a payload, make sure your library supports modern hash algorithms like SHA384. Google will begin using it on all new keys as of May 14, 2023.
  2. When decrypting a payload, make sure your library supports modern symmetric encryption algorithms like AES256. Google will begin using it on all new keys as of May 14, 2023.

GPG Payload Encryption Example

The below command is an example of how to select secure options when using GPG. It is expected that this operation is performed in a trusted environment where people do not have access to the private keys or sensitive input files.

gpg --output signed-and-encrypted.pgp \
  --sign --digest-algo SHA384 \
  --encrypt --cipher-algo AES256 \
  --armor \
  --recipient {key_id} \
  input.txt

GPG will automatically select the right key from the bundle for each operation you ask it to perform.

JWE encryption with JWS signing

JSON Web Encryption (JWE) is a standard defined by rfc7516 for encrypting content at the application level. JSON Web Signature (JWS) is a standard defined by rfc7515 for signing content at the application level.

Requests and responses will be JWE tokens encrypted using asymmetric (public key) encryption with the "Compact Serialization" option. The JWE token will contain the signed payload as a JWS token. JWS also uses asymmetric keys; private key for signing and public key for verification.

When sending a payload, sign the payload first and then encrypt it. When receiving a payload, decrypt it first and then verify the signature.

When using JWE, Partners must support the following options:

  • Compact Serialization.
  • Decrypting payloads from one of multiple JWE keys.
  • The RSA-OAEP, RSA-OAEP-256, or ECDH-ES algorithm for key management.
  • The A256GCM, A128GCM, A128CBC-HS256, or A256CBC-HS512 algorithm for content encryption.
    • Populated in the enc header.
  • kid header to identify the public encryption key.
  • Message payloads that use JWE encryption must use the content type application/jose; charset=utf-8.

When using JWS, Partners must support the following options:

  • Compact Serialization.
  • Verifying payloads from one of multiple JWS keys.
  • The HS256, HS384, HS512, RS256, RS384, RS512, ES256, PS256, PS384, or PS512 algorithm for signature creation.
  • kid header to identify the private signing key.

JWE/JWS strings will be encoded as UTF-8 strings and their payloads may be arbitrary bytes.

Private keys must be RSA/ECDH-ES keys that expire in one year with a max lifetime of two years. All private key identities must always stay on the partner's server and, accordingly, all signature values must be calculated on the partner's server.

Before beginning development you need to exchange JWE and JWS keys with Google. Keys should be exchanged in JWK format, as defined in rfc7517. In this step, you generate a public-private key pair, provide the public key to Google, and receive a public key back from Google. During development, you will only need to exchange sandbox keys used for development and testing outside of production. Before production testing and launch, you will need to perform another exchange of production keys.