The code is in this question(https://stackoverflow.com/q/20655702/3970355). It's about how to load rsa key and do the sign and unsign.

rsa_encryption.go

package main

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

func main() {
	signer, err := loadPrivateKey("private.pem")
	if err != nil {
		fmt.Errorf("signer is damaged: %v", err)
	}

	toSign := "date: Thu, 05 Jan 2012 21:31:40 GMT"

	signed, err := signer.Sign([]byte(toSign))
	if err != nil {
		fmt.Errorf("could not sign request: %v", err)
	}
	sig := base64.StdEncoding.EncodeToString(signed)
	fmt.Printf("Signature: %v\n", sig)

	parser, perr := loadPublicKey("public.pem")
	if perr != nil {
		fmt.Errorf("could not sign request: %v", err)
	}
	
	err = parser.Unsign([]byte(toSign), signed)
	if err != nil {
		fmt.Errorf("could not sign request: %v", err)
	}
	
	fmt.Printf("Unsign error: %v\n", err)
}

// loadPrivateKey loads an parses a PEM encoded private key file.
func loadPublicKey(path string) (Unsigner, error) {

	return parsePublicKey([]byte(`-----BEGIN PUBLIC KEY-----
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDCFENGw33yGihy92pDjZQhl0C3
6rPJj+CvfSC8+q28hxA161QFNUd13wuCTUcq0Qd2qsBe/2hFyc2DCJJg0h1L78+6
Z4UMR7EOcpfdUE9Hf3m/hs+FUR45uBJeDK1HSFHD8bHKD6kv8FPGfJTotc+2xjJw
oYi+1hqp1fIekaxsyQIDAQAB
-----END PUBLIC KEY-----`))
}

// parsePublicKey parses a PEM encoded private key.
func parsePublicKey(pemBytes []byte) (Unsigner, error) {
	block, _ := pem.Decode(pemBytes)
	if block == nil {
		return nil, errors.New("ssh: no key found")
	}

	var rawkey interface{}
	switch block.Type {
	case "PUBLIC KEY":
		rsa, err := x509.ParsePKIXPublicKey(block.Bytes)
		if err != nil {
			return nil, err
		}
		rawkey = rsa
	default:
		return nil, fmt.Errorf("ssh: unsupported key type %q", block.Type)
	}

	return newUnsignerFromKey(rawkey)
}

// loadPrivateKey loads an parses a PEM encoded private key file.
func loadPrivateKey(path string) (Signer, error) {
	return parsePrivateKey([]byte(`-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----`))
}

// parsePublicKey parses a PEM encoded private key.
func parsePrivateKey(pemBytes []byte) (Signer, error) {
	block, _ := pem.Decode(pemBytes)
	if block == nil {
		return nil, errors.New("ssh: no key found")
	}

	var rawkey interface{}
	switch block.Type {
	case "RSA PRIVATE KEY":
		rsa, err := x509.ParsePKCS1PrivateKey(block.Bytes)
		if err != nil {
			return nil, err
		}
		rawkey = rsa
	default:
		return nil, fmt.Errorf("ssh: unsupported key type %q", block.Type)
	}
	return newSignerFromKey(rawkey)
}

// A Signer is can create signatures that verify against a public key.
type Signer interface {
	// Sign returns raw signature for the given data. This method
	// will apply the hash specified for the keytype to the data.
	Sign(data []byte) ([]byte, error)
}

// A Signer is can create signatures that verify against a public key.
type Unsigner interface {
	// Sign returns raw signature for the given data. This method
	// will apply the hash specified for the keytype to the data.
	Unsign(data[]byte, sig []byte) error
}

func newSignerFromKey(k interface{}) (Signer, error) {
	var sshKey Signer
	switch t := k.(type) {
	case *rsa.PrivateKey:
		sshKey = &rsaPrivateKey{t}
	default:
		return nil, fmt.Errorf("ssh: unsupported key type %T", k)
	}
	return sshKey, nil
}

func newUnsignerFromKey(k interface{}) (Unsigner, error) {
	var sshKey Unsigner
	switch t := k.(type) {
	case *rsa.PublicKey:
		sshKey = &rsaPublicKey{t}
	default:
		return nil, fmt.Errorf("ssh: unsupported key type %T", k)
	}
	return sshKey, nil
}

type rsaPublicKey struct {
	*rsa.PublicKey
}

type rsaPrivateKey struct {
	*rsa.PrivateKey
}

// Sign signs data with rsa-sha256
func (r *rsaPrivateKey) Sign(data []byte) ([]byte, error) {
	h := sha256.New()
	h.Write(data)
	d := h.Sum(nil)
	return rsa.SignPKCS1v15(rand.Reader, r.PrivateKey, crypto.SHA256, d)
}

// Unsign verifies the message using a rsa-sha256 signature
func (r *rsaPublicKey) Unsign(message []byte, sig []byte) error {
	h := sha256.New()
	h.Write(message)
	d := h.Sum(nil)
	return rsa.VerifyPKCS1v15(r.PublicKey, crypto.SHA256, d, sig)
}