The main problem from the original question is that the jwt.KeyFunc is expecting a Go type that represents a public cryptography key, not the JWT's kid.

I had a use case that involved using a remote JWKS hosted via HTTPS, but I was already using this JWT package the question author was using, github.com/dgrijalva/jwt-go, and didn't want to switch.

I read some RFCs and implemented this package to implement a jwt.KeyFunc when using a JWKS. Below are two examples of how these packages work together.

The first example shows how to validate a token with the author's JWKS and JWT. The second example shows how to validate the author's JWT if the JWKS was hosted at an HTTP endpoint at https://keycloak.example.com/auth/realms/example/protocol/openid-connect/certs. Please note the JWT in the original question has not been encoded correctly.

JWKS from JSON

package main

import (
    "encoding/json"
    "log"

    "github.com/dgrijalva/jwt-go"

    "github.com/MicahParks/keyfunc"
)

func main() {

    // Get the JWKS as JSON.
    var jwksJSON json.RawMessage = []byte(`{"keys":[{"kty":"RSA","e":"AQAB","use":"sig","kid":"MjhhMDk2N2M2NGEwMzgzYjk2OTI3YzdmMGVhOGYxNjI2OTc5Y2Y2MQ","alg":"RS256","n":"zZU9xSgK77PbtkjJgD2Vmmv6_QNe8B54eyOV0k5K2UwuSnhv9RyRA3aL7gDN-qkANemHw3H_4Tc5SKIMltVIYdWlOMW_2m3gDBOODjc1bE-WXEWX6nQkLAOkoFrGW3bgW8TFxfuwgZVTlb6cYkSyiwc5ueFV2xNqo96Qf7nm5E7KZ2QDTkSlNMdW-jIVHMKjuEsy_gtYMaEYrwk5N7VoiYwePaF3I0_g4G2tIrKTLb8DvHApsN1h-s7jMCQFBrY4vCf3RBlYULr4Nz7u8G2NL_L9vURSCU2V2A8rYRkoZoZwk3a3AyJiqeC4T_1rmb8XdrgeFHB5bzXZ7EI0TObhlw"}]}`)

    // Create the JWKS from the resource at the given URL.
    jwks, err := keyfunc.New(jwksJSON)
    if err != nil {
        log.Fatalf("Failed to create JWKS from resource at the given URL.\nError: %s", err.Error())
    }

    // Get a JWT to parse.
    jwtB64 := "eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiIsIng1dCI6ImI2TnozUDJwMHg1QWpfWENsUmhrVDFzNlNIQSJ9.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.K1iPtdXiuicuDPaLC6Exw/7UpJVW6Uy1tPpJlfZ29Vqs9M1zR00JpKxvymQMAzbD0GHlXPPsZmhDxOn0WMAPfr1Xi8tiruTLXNbwUPJ/SOovt+zK4JGtrydhc4iv2EROhMUk2uwJUb4DFjqKZRhBvtCW7fRtdtI9yJL4W4OK8Ld90yOb97usPjEPz8S4E4uNrb5lE2rLzIp+EaPwA232lDkhS8gGPIKdlLG1IdEfQ4cFU1VIplvWoHzprF9mGR0ahT2QGgmGE3AcBfkURk8VzIKDG/UcBA9eHu3XGg28j3OvIXWwJhd7Hi+jTqvggi0hplao8ElvjNBw/wNy2UO9WA=="

    // Parse the JWT.
    token, err := jwt.Parse(jwtB64, jwks.KeyFunc)
    if err != nil {
        log.Fatalf("Failed to parse the JWT.\nError: %s", err.Error())
    }

    // Check if the token is valid.
    if !token.Valid {
        log.Fatalf("The token is not valid.")
    }

    log.Println("The token is valid.")
}

JWKS from HTTPS endpoint

package main

import (
    "log"
    "time"

    "github.com/dgrijalva/jwt-go"

    "github.com/MicahParks/keyfunc"
)

func main() {

    // Get the JWKS URL.
    jwksURL := "https://keycloak.example.com/auth/realms/example/protocol/openid-connect/certs"

    // Create the keyfunc options. Refresh the JWKS every hour and log errors.
    refreshInterval := time.Hour
    options := keyfunc.Options{
        RefreshInterval: &refreshInterval,
        RefreshErrorHandler: func(err error) {
            log.Printf("There was an error with the jwt.KeyFunc\nError: %s", err.Error())
        },
    }

    // Create the JWKS from the resource at the given URL.
    jwks, err := keyfunc.Get(jwksURL, options)
    if err != nil {
        log.Fatalf("Failed to create JWKS from resource at the given URL.\nError: %s", err.Error())
    }

    // Get a JWT to parse.
    jwtB64 := "eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiIsIng1dCI6ImI2TnozUDJwMHg1QWpfWENsUmhrVDFzNlNIQSJ9.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.K1iPtdXiuicuDPaLC6Exw/7UpJVW6Uy1tPpJlfZ29Vqs9M1zR00JpKxvymQMAzbD0GHlXPPsZmhDxOn0WMAPfr1Xi8tiruTLXNbwUPJ/SOovt+zK4JGtrydhc4iv2EROhMUk2uwJUb4DFjqKZRhBvtCW7fRtdtI9yJL4W4OK8Ld90yOb97usPjEPz8S4E4uNrb5lE2rLzIp+EaPwA232lDkhS8gGPIKdlLG1IdEfQ4cFU1VIplvWoHzprF9mGR0ahT2QGgmGE3AcBfkURk8VzIKDG/UcBA9eHu3XGg28j3OvIXWwJhd7Hi+jTqvggi0hplao8ElvjNBw/wNy2UO9WA=="

    // Parse the JWT.
    token, err := jwt.Parse(jwtB64, jwks.KeyFunc)
    if err != nil {
        log.Fatalf("Failed to parse the JWT.\nError: %s", err.Error())
    }

    // Check if the token is valid.
    if !token.Valid {
        log.Fatalf("The token is not valid.")
    }

    log.Println("The token is valid.")
}

As you might have already noticed, github.com/lestrrat-go/jwx only supports RawURLEncode base64 format. The best solution would be if you could use tokens in that particular format.

As a hacky workaround, you could manually re-encode your token to RawURLEncode format and feed that token to the jwt.ParseString() function:

// token consists of three parts (header, payload, signature) separeted by '.'
stdEncodedParts := strings.Split(tokenStr, ".")
var rawURLEncodedParts []string
for _, part := range stdEncodedParts {
    rawpart, err := base64.StdEncoding.DecodeString(part)
    if err != nil {
        panic(err)
    }
    rawURLEncodedParts = append(rawURLEncodedParts, base64.RawURLEncoding.EncodeToString(rawpart))
}

rawURLEncodedToken := strings.Join(rawURLEncodedParts, ".")
token, err := jwt.ParseString(rawURLEncodedToken)

As for the validation part, it would look something like the code below, although I did not manage to verify your token. Are you sure, you provided the proper verification key? Also note, that you might have to feed a custom fabricated token to jws.VerifyWithJWKSet(), because

1. it cannot parse the signature in the original StdEncoded format

2. but at the same time, the signature should have been made on the first half of the original, StdEncoded string thus you cannot provide the re-encoded string as it is.

const jwksStr = `{"keys":[{"kty":"RSA","e":"AQAB","use":"sig","kid":"MjhhMDk2N2M2NGEwMzgzYjk2OTI3YzdmMGVhOGYxNjI2OTc5Y2Y2MQ","alg":"RS256","n":"zZU9xSgK77PbtkjJgD2Vmmv6_QNe8B54eyOV0k5K2UwuSnhv9RyRA3aL7gDN-qkANemHw3H_4Tc5SKIMltVIYdWlOMW_2m3gDBOODjc1bE-WXEWX6nQkLAOkoFrGW3bgW8TFxfuwgZVTlb6cYkSyiwc5ueFV2xNqo96Qf7nm5E7KZ2QDTkSlNMdW-jIVHMKjuEsy_gtYMaEYrwk5N7VoiYwePaF3I0_g4G2tIrKTLb8DvHApsN1h-s7jMCQFBrY4vCf3RBlYULr4Nz7u8G2NL_L9vURSCU2V2A8rYRkoZoZwk3a3AyJiqeC4T_1rmb8XdrgeFHB5bzXZ7EI0TObhlw"}]}`
customTokenStr := stdEncodedParts[0] + "." + stdEncodedParts[1] + "." + rawURLEncodedParts[2]
jwks, err := jwk.ParseString(jwksStr)
if err != nil {
    panic(err)
}
_, err = jws.VerifyWithJWKSet([]byte(customTokenStr), jwks, nil)