What is Encryption in Payments?

What Is Encryption?

Encryption is the process of converting readable data -- called plaintext -- into an unreadable format -- called ciphertext -- using a mathematical algorithm and a key. Only someone with the correct decryption key can convert the data back into its original form. Without the key, the encrypted data is meaningless gibberish, even if an attacker manages to intercept it.

In the context of payments, encryption is the fundamental technology that protects card numbers, personal details, and transaction data as they travel across networks and sit in storage. Every time you make an online purchase, tap your card, or pay over the phone through a secure system, encryption is working behind the scenes to keep your data safe.

How Encryption Works

Symmetric Encryption

In symmetric encryption, the same key is used to both encrypt and decrypt the data. Think of it like a padlock where both parties have an identical key. The most widely used symmetric algorithm is AES (Advanced Encryption Standard), which comes in 128-bit, 192-bit, and 256-bit key lengths. AES-256 is the standard for protecting payment card data and is considered unbreakable with current computing technology.

The challenge with symmetric encryption is key distribution -- you need a secure way to share the key with the other party. If the key is intercepted in transit, the encryption is compromised.

Asymmetric Encryption

Asymmetric encryption solves the key distribution problem by using two different but mathematically related keys -- a public key and a private key. Data encrypted with the public key can only be decrypted with the private key, and vice versa. The public key can be shared openly; only the private key must be kept secret.

RSA and elliptic curve cryptography (ECC) are the most common asymmetric algorithms. In practice, asymmetric encryption is typically used to establish a secure channel and exchange a symmetric key, which then handles the bulk of the data encryption. This combination -- used in TLS/SSL -- powers every secure website and payment connection on the internet.

Encryption in Payments

Data in Transit

When card data moves between systems -- from a customer's browser to a payment gateway, from a phone system to a payment processor, or between a merchant and their acquirer -- it must be encrypted. PCI DSS requires that cardholder data transmitted across open or public networks is encrypted using strong cryptography. In practice, this means TLS 1.2 or higher for all payment connections.

For telephone payments, "data in transit" includes the audio channel itself. DTMF tones carrying card digits travel through voice networks, and without protection, those tones can be captured and decoded. DTMF masking addresses this by intercepting and replacing the tones before they reach any unprotected part of the network.

Data at Rest

Card data stored in databases, backup files, or logs must also be encrypted. PCI DSS requires that stored cardholder data is rendered unreadable -- and encryption is the primary method for achieving this. The encryption keys themselves must be managed securely, stored separately from the data they protect, and rotated regularly.

In practice, most businesses avoid storing card data entirely by using tokenisation. The payment processor stores the encrypted card and issues a token to the merchant. The merchant stores the token, which is useless to an attacker because it cannot be reversed back to the card number without access to the processor's secure vault.

End-to-End Encryption (E2EE)

End-to-end encryption means data is encrypted at the point of capture and remains encrypted until it reaches its final destination -- typically the payment processor's secure environment. No intermediate system can access the plaintext data. This is the gold standard for payment security because it eliminates the risk of data being exposed anywhere along the chain.

Point-to-Point Encryption (P2PE)

P2PE is a PCI SSC-validated standard for encrypting card data from the point of interaction (e.g., a card terminal) to the decryption environment (the processor). A validated P2PE solution significantly reduces PCI DSS scope for the merchant because card data is never accessible in their environment in any readable form. The PCI SSC maintains a list of validated P2PE solutions.

Encryption Under PCI DSS

PCI DSS includes several requirements related to encryption:

• Requirement 3 Protect stored account data using encryption, truncation, masking, or hashing
• Requirement 4 Encrypt cardholder data when transmitted over open or public networks using strong cryptography
• Key management PCI DSS v4.0 requires documented key management procedures, including key generation, distribution, storage, rotation, and destruction
• Algorithm standards Only industry-recognised algorithms are accepted -- AES, RSA, ECC, and similar. Proprietary or deprecated algorithms (DES, 3DES after its retirement) do not meet the standard

Encryption vs Tokenisation vs Hashing

These three technologies are often mentioned together but serve different purposes:

• Encryption Reversible. The original data can be recovered with the correct key. Used when you need to read the data later (e.g., a processor needs to decrypt the card number to authorise a payment).
• Tokenisation Replaces data with a random substitute (token) that has no mathematical relationship to the original. The mapping is stored in a secure vault. Used when you need to reference the data without accessing it (e.g., storing a card for future payments).
• Hashing One-way. Converts data into a fixed-length value that cannot be reversed. Used for verification (e.g., checking if a password matches) but not for data retrieval.

Why Encryption Matters for Phone Payments

Telephone payments introduce unique encryption challenges. Voice calls traverse multiple networks -- public telephone networks, VoIP infrastructure, internal PBX systems -- and each hop is a potential exposure point. Traditional voice channels were not designed for secure data transmission.

Solutions like DTMF masking effectively address this by ensuring card data never enters the voice channel in a decodable form. The actual card digits are captured by the secure payment platform at the network level, encrypted, and transmitted directly to the payment processor. The voice channel carries only masked or suppressed tones, so there is nothing of value to intercept.