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Encrypt decrypt #2130

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Encrypt decrypt #2130

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a2245a6
add encrypt function
yemmyharry Mar 3, 2024
b9f98b6
made changes to encrypt function
yemmyharry Mar 4, 2024
9bbd1a4
add decrypt function
yemmyharry Mar 4, 2024
1b9b22f
add test for encrypt and decrypt functions
yemmyharry Mar 4, 2024
902f456
btcec: update comment [skip ci]
yemmyharry Apr 18, 2024
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48 changes: 48 additions & 0 deletions btcec/ciphering.go
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Expand Up @@ -5,6 +5,12 @@
package btcec

import (
"bytes"
"crypto/aes"
"crypto/cipher"
"crypto/rand"
"crypto/sha256"
"fmt"
secp "github.com/decred/dcrd/dcrec/secp256k1/v4"
)

Expand All @@ -14,3 +20,45 @@ import (
func GenerateSharedSecret(privkey *PrivateKey, pubkey *PublicKey) []byte {
return secp.GenerateSharedSecret(privkey, pubkey)
}

// Encrypt encrypts data for the target public key using AES-128-GCM
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func Encrypt(pubKey *PublicKey, msg []byte) ([]byte, error) {
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var pt bytes.Buffer

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You want to copy the cipher text into a different variable?

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var pt bytes.Buffer is used to accumulate the encrypted data in a buffer, which is then returned as a single byte slice at the end of the function

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why not return the cipher text itself?

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i didnt return the ciphertext itself because i need some details in the decrypt function. the tag (and other parameters like nonce etc) are appended to the ciphertext in the encrypt function and in the decrypt function the tag is separated from the ciphertext to verify authenticity.

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I think the tag is in the cipher text already

Is there a reason that you did not just do this:

nonce := make([]byte, aead.NonceSize())
ciphertext := make([]byte, 4+len(ephemeralPubKey))
binary.LittleEndian.PutUint32(ciphertext, uint32(len(ephemeralPubKey)))
copy(ciphertext[4:], ephemeralPubKey)
ciphertext = aead.Seal(ciphertext, nonce, plaintext, ephemeralPubKey)

Gotten from here: https://pkg.go.dev/github.com/decred/dcrd/dcrec/secp256k1/v4#section-readme

No need for the intermediary pt bytes in this case.

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Both approaches are valid. I used a buffer as it automatically grows and shrinks as needed, reducing manual memory management.

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IMO, I do not think the buffer is needed at all when the cipher text can just be returned directly unless there is need to copy the cipher text into a different variable. if not maybe we should not use extra space.

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i didnt return the ciphertext itself because i need some details in the decrypt function. the tag (and other parameters like nonce etc) are appended to the ciphertext in the encrypt function and in the decrypt function the tag is separated from the ciphertext to verify authenticity.

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nonce := make([]byte, aead.NonceSize())
ciphertext := make([]byte, 4+len(ephemeralPubKey))
binary.LittleEndian.PutUint32(ciphertext, uint32(len(ephemeralPubKey)))
copy(ciphertext[4:], ephemeralPubKey)
ciphertext = aead.Seal(ciphertext, nonce, plaintext, ephemeralPubKey)

If you write it this way would you still need the intermediary buffer?

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https://github.com/Chinwendu20/btcd/blob/317cdc7a2a2e25f5d0251decaebccc9da2b2fb66/btcec/ciphering.go#L26-L55
This is what I mean, no need for the buffer or any intermediate?

ephemeral, err := NewPrivateKey()
if err != nil {
return nil, fmt.Errorf("failed to generate private key: %v", err)
}

pt.Write(ephemeral.PubKey().SerializeUncompressed())

ecdhKey := GenerateSharedSecret(ephemeral, pubKey)
hashedSecret := sha256.Sum256(ecdhKey)
encryptionKey := hashedSecret[:16]

block, err := aes.NewCipher(encryptionKey)
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Seems like you sliced the shared secret to 16 bytes here

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yes for compatibility with AES-128-GCM encryption

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Ohh thought you said AES-256-GCM here: a2245a6#r1570762508 but looks like the comment has been updated.

if err != nil {
return nil, err
}

nonce := make([]byte, 16)
if _, err := rand.Read(nonce); err != nil {
return nil, err
}

pt.Write(nonce)

gcm, err := cipher.NewGCMWithNonceSize(block, 16)
if err != nil {
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I would like to understand why it has to be a nonce size of 16 instead of the default 12

return nil, err
}

ciphertext := gcm.Seal(nil, nonce, msg, nil)

tag := ciphertext[len(ciphertext)-gcm.NonceSize():]
pt.Write(tag)
ciphertext = ciphertext[:len(ciphertext)-len(tag)]
pt.Write(ciphertext)

return pt.Bytes(), nil
}