Pre-shared keys

An EncryptedData message carries content encrypted with a symmetric key that both sides already hold. The message names the cipher but not the key, so the parties agree on the key outside the message.

key := make([]byte, 32) // AES256CBC needs a 32-byte key
if _, err := rand.Read(key); err != nil {
    log.Fatal(err)
}
opts := pkcs7.EncryptOptions{ContentEncryption: pkcs7.AES256CBC}
encrypted, err := pkcs7.EncryptUsingPSK([]byte("shared secret content"), key, opts)
if err != nil {
    log.Fatal(err)
}

p7, err := pkcs7.Parse(encrypted)
if err != nil {
    log.Fatal(err)
}
plaintext, err := p7.DecryptUsingPSK(key, pkcs7.DecryptOptions{})
if err != nil {
    log.Fatal(err)
}
fmt.Printf("%s\n", plaintext)

Keys

The key must have exactly the length the cipher needs:

ContentEncryption	Key length
AES128CBC, the default	16 bytes
AES192CBC	24 bytes
AES256CBC	32 bytes
DESEDE3CBC	24 bytes
DESCBC	8 bytes

EncryptUsingPSK fails with ErrPSKNotProvided for a nil key and with an error such as pkcs7: AES-256-CBC needs a 32-byte key, got 16 bytes for a key of the wrong length. DecryptUsingPSK returns the same length error. Use random key bytes, for example from crypto/rand. The library does not derive keys from passwords.

EncryptOptions.KeyEncryption and OAEPHash have no effect here.

Decryption

DecryptUsingPSK takes the same DecryptOptions as Decrypt, so DES-CBC is refused unless it is listed. It returns ErrNotEncryptedContent for a message that is not EncryptedData and ErrDecryptionFailed when the padding is wrong.

As with EnvelopedData, CBC mode does not authenticate the content. A wrong key or a changed ciphertext usually fails with ErrDecryptionFailed but can also yield garbage. Sign the message when the receiver must detect changes.

OpenSSL

OpenSSL reads and writes these messages with openssl cms -EncryptedData_decrypt and -EncryptedData_encrypt, with the key given in hex through -secretkey. OpenSSL lists the tested commands.