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How does RSA encryption and decryption work in TLS cipher suites?

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Transport Layer Security (TLS) is a protocol that provides secure communication over the internet. It uses encryption, authentication, and integrity mechanisms to protect the data exchanged between two parties. One of the key components of TLS is the cipher suite, which defines the algorithms and parameters used for encryption, authentication, and key exchange. In this article, we will explain how RSA encryption and decryption work in TLS cipher suites, and why they are important for security.

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Anudeep Reddy

Associate Lead Software Engineer at Siemens EDA, Security enthusiast
Martin Wellard

[Dis]Assembler of Things.

1 RSA algorithm

RSA is a public-key encryption algorithm, which means that it uses two different keys: a public key and a private key. The public key can be shared with anyone, while the private key must be kept secret. The public key can be used to encrypt a message, but only the private key can decrypt it. Similarly, the private key can be used to sign a message, but only the public key can verify it. RSA is based on the mathematical problem of factoring large numbers, which is very hard to solve.

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Martin Wellard

[Dis]Assembler of Things.
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It might be useful to explain why we need public-key cryptography: it allows two parties to agree on a secret key to use for securing their future communications, without having to have a prior secret and without eavesdroppers being able to discern the key.

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Joey Albert
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I would give a simple graphical representation to show the communications between the 'end points' showing where these keys reside. I would also explain that RSA means Rivest-Shamir-Adleman, (the last names of those who publicly described the algorithm in 1977). That it is one of the oldest still in use.

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2 RSA in TLS handshake

The TLS handshake is the process of establishing a secure connection between two parties. It involves negotiating the cipher suite, exchanging certificates, and generating session keys. RSA can be used in two ways in the TLS handshake: as a key exchange method and as a signature method. As a key exchange method, RSA allows one party to encrypt a random value with the public key of the other party, and send it over. The other party can then decrypt it with their private key, and use it as a session key. As a signature method, RSA allows one party to sign a hash of the handshake messages with their private key, and send it over. The other party can then verify it with their public key, and confirm the identity and integrity of the sender.

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Anudeep Reddy

Associate Lead Software Engineer at Siemens EDA, Security enthusiast
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The use of RSA for Key exchange lacks forward secrecy. RSA uses server's public key to encrypt the session key on the client and the server then uses it's private key to decrypt and use the session key to establish a secure communication channel using symmetric key algorithm as defined in the chosen cipher suite. If the server's private key is compromised then any communication that used that key is essentially compromised. Forward secrecy is a feature that prevents this from happening by making use of temporary keys for each connection.

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Joey Albert
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Who is your target audience? If it is the readers who don't quite understand what encryption is and how it works, it would be helpful to expound on what is a 'cipher' and a 'hash'.

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3 RSA in TLS encryption

Once the TLS handshake is completed, the parties can start exchanging data securely. The data is encrypted and decrypted with the session key, which is derived from the random value exchanged in the handshake. The session key is usually a symmetric key, which means that it is the same for both parties. The symmetric encryption algorithm used depends on the cipher suite, but it can be AES, ChaCha20, or others. RSA is not used for encrypting and decrypting the data, because it is too slow and inefficient for large amounts of data. However, RSA is still important for ensuring the security of the session key, which is the basis of the encryption.

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Joey Albert
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Is there a session 'time-out'? What constitutes when the session is effectively terminated or ended? Again, depending on your audience, you need to explain what the acronyms stand for. Just like in the beginning of this article (TLS - Transport Layer Security)

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4 RSA in TLS authentication

Another aspect of TLS security is authentication, which means verifying the identity of the parties involved. Authentication is usually done with certificates, which are digital documents that contain information about the party, such as their name, domain, and public key. Certificates are issued by trusted authorities, called certificate authorities (CAs), which vouch for the validity and authenticity of the certificates. RSA can be used to sign and verify certificates, as well as to sign and verify the handshake messages. By using RSA signatures, the parties can prove that they own the private keys that correspond to the public keys in their certificates, and that they have not tampered with the handshake messages.

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5 RSA in TLS integrity

The last element of TLS security is integrity, which means ensuring that the data exchanged has not been modified or corrupted by an attacker. Integrity is achieved by using a message authentication code (MAC), which is a value that depends on the data and the session key. The MAC is calculated and appended to each data record before encryption, and verified after decryption. The MAC algorithm used depends on the cipher suite, but it can be HMAC, Poly1305, or others. RSA is not used for calculating or verifying MACs, because it is not a MAC algorithm. However, RSA is still important for ensuring the integrity of the handshake messages, which are used to generate the session key, which is used to calculate and verify the MACs.

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Joey Albert
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I still believe that all acronyms need to be defined. That will give the reader a more thorough understanding and will help the article to be more consistent througout.

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How does RSA encryption and decryption work in TLS cipher suites?

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1 RSA algorithm

2 RSA in TLS handshake

3 RSA in TLS encryption

4 RSA in TLS authentication

5 RSA in TLS integrity

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