gotosocial/vendor/github.com/go-jose/go-jose/v4/cipher/ecdh_es.go

87 lines
2.8 KiB
Go

/*-
* Copyright 2014 Square Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package josecipher
import (
"bytes"
"crypto"
"crypto/ecdsa"
"crypto/elliptic"
"encoding/binary"
)
// DeriveECDHES derives a shared encryption key using ECDH/ConcatKDF as described in JWE/JWA.
// It is an error to call this function with a private/public key that are not on the same
// curve. Callers must ensure that the keys are valid before calling this function. Output
// size may be at most 1<<16 bytes (64 KiB).
func DeriveECDHES(alg string, apuData, apvData []byte, priv *ecdsa.PrivateKey, pub *ecdsa.PublicKey, size int) []byte {
if size > 1<<16 {
panic("ECDH-ES output size too large, must be less than or equal to 1<<16")
}
// algId, partyUInfo, partyVInfo inputs must be prefixed with the length
algID := lengthPrefixed([]byte(alg))
ptyUInfo := lengthPrefixed(apuData)
ptyVInfo := lengthPrefixed(apvData)
// suppPubInfo is the encoded length of the output size in bits
supPubInfo := make([]byte, 4)
binary.BigEndian.PutUint32(supPubInfo, uint32(size)*8)
if !priv.PublicKey.Curve.IsOnCurve(pub.X, pub.Y) {
panic("public key not on same curve as private key")
}
z, _ := priv.Curve.ScalarMult(pub.X, pub.Y, priv.D.Bytes())
zBytes := z.Bytes()
// Note that calling z.Bytes() on a big.Int may strip leading zero bytes from
// the returned byte array. This can lead to a problem where zBytes will be
// shorter than expected which breaks the key derivation. Therefore we must pad
// to the full length of the expected coordinate here before calling the KDF.
octSize := dSize(priv.Curve)
if len(zBytes) != octSize {
zBytes = append(bytes.Repeat([]byte{0}, octSize-len(zBytes)), zBytes...)
}
reader := NewConcatKDF(crypto.SHA256, zBytes, algID, ptyUInfo, ptyVInfo, supPubInfo, []byte{})
key := make([]byte, size)
// Read on the KDF will never fail
_, _ = reader.Read(key)
return key
}
// dSize returns the size in octets for a coordinate on a elliptic curve.
func dSize(curve elliptic.Curve) int {
order := curve.Params().P
bitLen := order.BitLen()
size := bitLen / 8
if bitLen%8 != 0 {
size++
}
return size
}
func lengthPrefixed(data []byte) []byte {
out := make([]byte, len(data)+4)
binary.BigEndian.PutUint32(out, uint32(len(data)))
copy(out[4:], data)
return out
}