CMS 
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CMS (Cryptographic Message Syntax) is a syntax for signing, digesting, and encrypting arbitrary messages. It evolved from PKCS#7 and is the basis for higher level protocols such as S/MIME. This package implements the SignedData CMS content-type, allowing users to digitally sign data as well as verify data signed by others.
Signing and Verifying Data
High level APIs are provided for signing a message with a certificate and key:
msg := []byte("some data")
cert, _ := x509.ParseCertificate(someCertificateData)
key, _ := x509.ParseECPrivateKey(somePrivateKeyData)
der, _ := cms.Sign(msg, []*x509.Certificate{cert}, key)
////
/// At another time, in another place...
//
sd, _ := ParseSignedData(der)
if err, _ := sd.Verify(x509.VerifyOptions{}); err != nil {
panic(err)
}By default, CMS SignedData includes the original message. High level APIs are also available for creating and verifying detached signatures:
msg := []byte("some data")
cert, _ := x509.ParseCertificate(someCertificateData)
key, _ := x509.ParseECPrivateKey(somePrivateKeyData)
der, _ := cms.SignDetached(msg, cert, key)
////
/// At another time, in another place...
//
sd, _ := ParseSignedData(der)
if err, _ := sd.VerifyDetached(msg, x509.VerifyOptions{}); err != nil {
panic(err)
}Timestamping
Because certificates expire and can be revoked, it is may be helpful to attach certified timestamps to signatures, proving that they existed at a given time. RFC3161 timestamps can be added to signatures like so:
signedData, _ := NewSignedData([]byte("Hello, world!"))
signedData.Sign(identity.Chain(), identity.PrivateKey)
signedData.AddTimestamps("http://timestamp.digicert.com")
derEncoded, _ := signedData.ToDER()
io.Copy(os.Stdout, bytes.NewReader(derEncoded))Verification functions implicitly verify timestamps as well. Without a timestamp, verification will fail if the certificate is no longer valid.
