Golang

This guide shows you how to decrypt sensitive card details using Go. The decryption process uses Rivest-Shamir-Adleman Optimal Asymmetric Encryption Padding (RSA-OAEP) to unwrap the Advanced Encryption Standard (AES) key. Then Advanced Encryption Standard 256-bit Galois/Counter Mode (AES-256-GCM) is used to decrypt the card data.

Prerequisites

Before you begin, you need:

• Go 1.16 or later installed on your system
• Your RSA private key, which was generated when you created the key pair
• An encrypted response from the View card sensitive details endpoint or the Get card PIN endpoint

Understanding the encrypted response

The View card sensitive details and the Get card PIN endpoints return three base64-encoded values:

Field	Description
encrypted_key	AES-256 key encrypted with your RSA public key
nonce	12-byte initialization vector for AES-GCM decryption
ciphertext	Encrypted card details

Parse the RSA private key

Load your PEM-formatted RSA private key. The function supports both PKCS#1 and PKCS#8 formats:

import (
    "crypto/rsa"
    "crypto/x509"
    "encoding/pem"
    "errors"
)

func parseRSAPrivateKeyPEM(privPEM []byte) (*rsa.PrivateKey, error) {
    block, _ := pem.Decode(privPEM)
    if block == nil {
        return nil, errors.New("invalid private key PEM")
    }

    // Try PKCS#1
    if priv, err := x509.ParsePKCS1PrivateKey(block.Bytes); err == nil {
        return priv, nil
    }

    // Try PKCS#8
    if key, err := x509.ParsePKCS8PrivateKey(block.Bytes); err == nil {
        if rsaKey, ok := key.(*rsa.PrivateKey); ok {
            return rsaKey, nil
        }
        return nil, errors.New("private key is not RSA")
    }

    return nil, errors.New("failed to parse RSA private key PEM")
}

Decode the base64 values

Convert all three base64-encoded values from the API response:

import "encoding/base64"

encryptedKey, err := base64.StdEncoding.DecodeString(payload["encrypted_key"])
nonce, err := base64.StdEncoding.DecodeString(payload["nonce"])
ciphertext, err := base64.StdEncoding.DecodeString(payload["ciphertext"])

Decrypt the AES key

Use your RSA private key to decrypt the AES key. The decryption uses RSA-OAEP with SHA-256:

import (
    "crypto/rand"
    "crypto/rsa"
    "crypto/sha256"
)

aesKey, err := rsa.DecryptOAEP(sha256.New(), rand.Reader, priv, encryptedKey, nil)

Decrypt the card details

Create an AES-GCM cipher and decrypt the card details:

import (
    "crypto/aes"
    "crypto/cipher"
)

block, err := aes.NewCipher(aesKey)
gcm, err := cipher.NewGCM(block)

plaintext, err := gcm.Open(nil, nonce, ciphertext, nil)

Complete decryption code

The following is the complete implementation that combines all the steps. It includes a helper function to parse the RSA private key and the main decryption function:

package main

import (
    "crypto/aes"
    "crypto/cipher"
    "crypto/rand"
    "crypto/rsa"
    "crypto/sha256"
    "crypto/x509"
    "encoding/base64"
    "encoding/pem"
    "errors"
    "fmt"
)

func parseRSAPrivateKeyPEM(privPEM []byte) (*rsa.PrivateKey, error) {
    block, _ := pem.Decode(privPEM)
    if block == nil {
        return nil, errors.New("invalid private key PEM")
    }

    // Try PKCS#1
    if priv, err := x509.ParsePKCS1PrivateKey(block.Bytes); err == nil {
        return priv, nil
    }

    // Try PKCS#8
    if key, err := x509.ParsePKCS8PrivateKey(block.Bytes); err == nil {
        if rsaKey, ok := key.(*rsa.PrivateKey); ok {
            return rsaKey, nil
        }
        return nil, errors.New("private key is not RSA")
    }

    return nil, errors.New("failed to parse RSA private key PEM")
}

func DecryptCardDetails(privPEM []byte, payload map[string]string) (string, error) {
    // Step 1: Parse RSA private key
    priv, err := parseRSAPrivateKeyPEM(privPEM)
    if err != nil {
        return "", fmt.Errorf("parse private key: %w", err)
    }

    // Step 2: Decode base64 values
    encryptedKey, err := base64.StdEncoding.DecodeString(payload["encrypted_key"])
    if err != nil {
        return "", fmt.Errorf("decode encrypted_key: %w", err)
    }

    nonce, err := base64.StdEncoding.DecodeString(payload["nonce"])
    if err != nil {
        return "", fmt.Errorf("decode nonce: %w", err)
    }

    ciphertext, err := base64.StdEncoding.DecodeString(payload["ciphertext"])
    if err != nil {
        return "", fmt.Errorf("decode ciphertext: %w", err)
    }

    // Step 3: Decrypt the AES key using RSA-OAEP
    aesKey, err := rsa.DecryptOAEP(sha256.New(), rand.Reader, priv, encryptedKey, nil)
    if err != nil {
        return "", fmt.Errorf("rsa decrypt aes key: %w", err)
    }

    // Step 4: Decrypt using AES-256-GCM
    block, err := aes.NewCipher(aesKey)
    if err != nil {
        return "", fmt.Errorf("aes cipher: %w", err)
    }

    gcm, err := cipher.NewGCM(block)
    if err != nil {
        return "", fmt.Errorf("aes-gcm: %w", err)
    }

    if len(nonce) != gcm.NonceSize() {
        return "", errors.New("invalid nonce size")
    }

    plaintext, err := gcm.Open(nil, nonce, ciphertext, nil)
    if err != nil {
        return "", fmt.Errorf("gcm decrypt: %w", err)
    }

    return string(plaintext), nil
}

Usage example

The following example shows how to use the DecryptCardDetails function with your private key and an encrypted response:

func main() {
    privateKey := `
-----BEGIN RSA PRIVATE KEY-----
MIIEowIBAAKCAQEA5llQr/KNtRhTn+2LJMwd5H62QUdMZ8Gq25LDpZRaosYbsoxH
... your private key content ...
-----END RSA PRIVATE KEY-----
`

    // Response from the View card sensitive details endpoint
    encryptedPayload := map[string]string{
        "ciphertext":    "kJR3z/bVTMTjiXMIQ8ha/eN/69q5vMr1sm6o1iKnCOixCB98tvp4WNsh9YCFXqNEPWfTkuiQ",
        "encrypted_key": "zmhGe7OkCl2e5lNaSU6rw28u1UvzqWyncHc+4fKfn+GRBua+zleDdaMfHctTPqK0...",
        "nonce":         "W5lMRrL2yKSyWgfq",
    }

    cardDetails, err := DecryptCardDetails([]byte(privateKey), encryptedPayload)
    if err != nil {
        fmt.Printf("Decryption failed: %v\n", err)
        return
    }

    fmt.Printf("Card details: %s\n", cardDetails)
}