OAUTH 2.0 and JWT Token HS256 vs RS256

 

BOTH Algorithm verify signature by trying to regenerate

HS256 gives client secret to client, also stores client secret on server, each time,

it is using client secret to generate and verify signature in JWT token 

Obtain token need client id client secret, like SNOW (SNOW also require username and password not common)

First time user needs to send client id and client secret ("system ensure they match") to obtain token

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Next time API Call  , just token , server verify the signate in token using client secret

Why HS256 with Client Secret

• HS256 (HMAC with SHA-256) is a symmetric algorithm, meaning it relies on a shared secret between the authorization server and the client.
• In the context of OAuth 2.0, the client secret is a shared key used by the client to authenticate itself to the server. The secret is known only to the client and the server.
• When HS256 is used, the client secret acts as the key for both signing and verifying the token. The server validates the signature of the token by using the same client secret.

When RS256 is Used

• RS256 (RSA with SHA-256) is an asymmetric algorithm that uses a private key for signing and a public key for verification.
• RS256 is typically used in situations where the authorization server signs the token with a private key, and the clients validate it with a public key. This setup is commonly seen with JWT-based tokens issued by identity providers (e.g., OpenID Connect).

Key Difference

• HS256 is appropriate when there is a secure and trusted relationship between the client and the server, as both share the same secret.
• RS256 is more secure for scenarios where public-key cryptography is needed, such as in distributed environments, because the private key does not need to be shared.

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RS256 can ask client use client certificates to obtain token

token's signature is signed using private key 

then when API request send token, first validate the claim , expirtaion date etc, then

re generate signature using public key,

beacause of  R256 is symmetric signature generated using public key is same as private key

The public key is used to verify the token instead of the private key because of the way asymmetric cryptography works, and the specific security model it enables. Here's why:

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1. Public-Private Key Pair

• Private Key:
  • Known only to the entity that owns it (e.g., the authorization server).
  • Used for signing the token.
  • Must remain secure and never be shared.
• Public Key:
  • Can be shared publicly with anyone (e.g., API servers or clients).
  • Used for verifying the token's signature.
  • Ensures the signature was created by the corresponding private key.

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2. Separation of Responsibilities

• By keeping the private key secret and using the public key for verification, asymmetric cryptography allows:
• Signing Authority:
  • Only the holder of the private key (e.g., the authorization server) can sign tokens.
  • This ensures the authenticity and integrity of the token.
• Verification by Anyone:
• Anyone with the public key (e.g., resource servers, other systems) can verify that the token was issued by the trusted authority.

IF its one server, you can also use private key

But multiple server u cant

 Why Not Use the Private Key for Verification?

1. Exposure Risk:

   • If the private key were used for verification, it would need to be distributed to every system or API that verifies tokens.
   • This increases the attack surface, making it easier for attackers to compromise the key.

2. Loss of Integrity:

• If multiple systems have the private key, they could potentially sign unauthorized tokens, undermining trust in the system.

Why public and private key token match?

he reason a private key's signature and a public key's verification process match lies in the mathematics of asymmetric cryptography. In asymmetric encryption algorithms like RSA, the private key and public key are mathematically related, forming a key pair. Let’s break this down:

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RS256 private key generate signature

nature:

• Created by signing the base64-encoded header and payload using the private key and the RS256 algorithm.

Signature = RS256(private_key, base64UrlEncode(Header) + "." + base64UrlEncode(Payload))

Public key generate signature

Verify: Signature == RS256(public_key, base64UrlEncode(Header) + "." + base64UrlEncode(Payload))

they will match

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1. Public/Private Key Pair

• Private Key:

  • Kept secret and used to sign data (or decrypt in encryption scenarios).
  • It generates a unique signature for specific input data.

• Public Key:

  • Publicly distributed and used to verify the data’s signature (or encrypt in encryption scenarios).
  • Verifies that the signature was created by the corresponding private key.

The mathematical relationship between the keys ensures that anything signed with the private key can be verified using the public key.

How the Matching Works

• Signing:

  • The private key is used to create a signature by hashing the data (e.g., the JWT header and payload) and then applying a cryptographic operation (like modular exponentiation in RSA) with the private key.
  • In RSA, this operation looks something like this: $$\text{Signature} = H(m)^{d} \mod N$$
    • $H(m)$: The hash of the data (e.g., SHA-256).
    • $d$: The private key exponent.
    • $N$: A modulus derived from the key pair.

• Verification:

  • The public key is used to verify the signature by performing the reverse cryptographic operation: $$\text{Decrypted Hash} = \text{Signature}^{e} \mod N$$
    • $e$: The public key exponent.
    • $N$: The same modulus derived from the key pair.
  • The result (Decrypted Hash) is then compared to the hash of the original data. If they match, the signature is valid.

Why They Match:

The mathematical design of RSA (and other asymmetric cryptographic algorithms) ensures that:

$$\text{Signature}^{e} \mod N = H(m)$$

This works because of the mathematical relationship between the public and private keys:

• The private key is used to "sign" data in a way that can only be reversed by the public key.
• The public key can validate the private key's operation without revealing the private key.

-----------GO LANG RSA256

package main

import (

"crypto/rsa"

"fmt"

"github.com/golang-jwt/jwt/v5"

"os"

)

func main() {

// Step 1: Load the RSA private key from a PEM file

privateKeyData, err := os.ReadFile("private_key.pem") // Replace with your key path

if err != nil {

panic(err)

}

privateKey, err := jwt.ParseRSAPrivateKeyFromPEM(privateKeyData)

if err != nil {

panic(err)

}

// Step 2: Create a JWT token

token := jwt.NewWithClaims(jwt.SigningMethodRS256, jwt.MapClaims{

"sub": "1234567890", // Subject

"iat": jwt.NewNumericDate(jwt.TimeFunc()), // Issued at

"exp": jwt.NewNumericDate(jwt.TimeFunc().Add(1 * 60 * 60)), // Expiration time

"role": "admin", // Custom claim

})

// Step 3: Sign the token with the private key

tokenString, err := token.SignedString(privateKey)

if err != nil {

panic(err)

}

fmt.Println("Signed Token:", tokenString)

// Step 4: Verify the token using the corresponding public key

publicKeyData, err := os.ReadFile("public_key.pem") // Replace with your key path

if err != nil {

panic(err)

}

publicKey, err := jwt.ParseRSAPublicKeyFromPEM(publicKeyData)

if err != nil {

panic(err)

}

// Parse and validate the token

parsedToken, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {

// Ensure the signing method is RS256

if _, ok := token.Method.(*jwt.SigningMethodRSA); !ok {

return nil, fmt.Errorf("unexpected signing method: %v", token.Header["alg"])

}

return publicKey, nil

})

if err != nil {

fmt.Println("Error verifying token:", err)

} else if parsedToken.Valid {

fmt.Println("Token is valid:", parsedToken.Claims)

} else {

fmt.Println("Token is invalid!")

}

}

----------------------------

Private key use for verification won't work for GO LANG JWT

No, in Go's JWT libraries (like github.com/golang-jwt/jwt/v5), private key verification will not work if you are using RSA256 (RS256). This is because RS256 is an asymmetric algorithm, and by design, it requires the private key for signing and the public key for verification.

Attempting to verify a token using the private key instead of the public key will result in an error because the library expects the corresponding public key for verification.

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my impelemetnation is ushing jwt.SigningMethodHS256 using private key (same as client secret)

then verification use private key to recreate signature - safe cause I only have private key at all time

1. Use RS256 for signing and distribute the public key to trusted services.

   • This keeps the signing process isolated and secure.
   • Verifying services can validate tokens independently without the ability to forge them.

2. Publish the public key via a JWKS (JSON Web Key Set) endpoint.

   • Trusted services can fetch the public key dynamically to verify tokens.

3. If you must use HS256, limit its use to scenarios where:

   • The private key never leaves the server issuing the token.
   • Verification happens only within the same secure server or environment.